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Michele and Pasquale apply a LEGO® approach to automation. Festo helps them build customer trust. Flexibility and reliability. They’re the building blocks for IASE’s modular approach to robotic packaging systems. For Michele and Pasquale, choosing the right components is critical. To them, they’re not just building systems; they’re building their customers’ trust. That’s why they rely on the superior automation products and support they receive from Festo. Making customers competitive. That’s what moves Michele and Pasquale. What moves you?
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Visit festo.us/OEE Michele & Pasquale D’Egidio Owners, Industrial Automation Systems Engineering (IASE) © 2017, Festo Group. All rights reserved. LEGO is a trademark of the LEGO Group, which does not sponsor, authorize or endorse these products.
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MOTION SYSTEM
HANDBOOK
Engineers new and experienced:
2017 Design World Motion Handbook is for you Talk
to a recent engineering graduate, and of top concern isn’t landing a job but rather a nagging inability to design something worth something — especially given the openended parameters of real-world projects. Now, early-career jitters and performance anxiety are as old as time and even helpful for minimizing the inevitable mistakes of inexperience. But today’s hype surrounding STEM education and innovation has brought real skill limitations and gaps into sharper focus. U.S. Department of Education estimates now put a four-year engineering degrees at about $160,000, so more are asking: What employable skills do students gain from the increasingly expensive college venture? Clearly, engineering degrees don’t bestow upon recipients the skills to conceptualize and build machines from nothing. Practical learning is core to an engineer’s development — whether in school laboratories and on open-ended design projects; arising from self-guided explorations in mom and dad’s garage or on a discarded computer or appliance; during internships or shadowing programs; or on the job after graduation. One might assume that Millennials at least are immune to such practical concerns. This generation may be partially buffered by attitudes confirmed (with caveats) by studies from Jean Twenge and others measuring higher self-ratings of drive, leadership, and other abilities.
LISA EITEL | SENIOR EDITOR
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That confidence is helpful in many cases; numerous studies suggests that career success often depends more on selfassurance than talent. But success in manufacturing and engineering fields may be less dependent on mindset and “soft skills” and more on abilities related to analytical thinking and intellectual effort. There’s a certain satisfaction to be derived from this point, and it’s one of the things that draw many to this field. After all, measures of ultimate success in engineering are uncompromising — dependent on hard and verifiable results related to design efficiency, output capacity, and serviceability, to give a few examples. No wonder so many superstars in the engineering world were or are notoriously difficult or quirky personalities. Strides have been made to restore the clout of manufacturing in the U.S., and that’s prompted more discourse on what constitutes practical education in technical fields. New modes of production facilitated by digital technologies continually provide new opportunities for pragmatic and rewarding career tracks. What’s more, a proliferation of manufacturer-based programs to facilitate the entry of young folks into the field have become instrumental in ensuring the future of U.S. manufacturing and prosperity. The only caveat for the latter is that company-based training programs — invaluable and heterogeneous offerings — might benefit from more national coordination and centralization of regional partnerships between corporations, community colleges, and universities. Right now, only about 5% of young Americans entering the workforce begin with any internship experience. In contrast, apprenticeship programs of other leading industrialized countries are integrated into all modes of higher education ... and formalized on a national scale. That means that far higher percentages of those just beginning careers start with practical experience. It also tends to harmonize the integration of different career paths in engineering and manufacturing. So those entering the workforce as plant operators, engineering technologists, engineers, and researchers in technological sciences also begin with a more integrated perspective on the field — and a better-honed ability to solve open-ended problems. motioncontroltips.com | designworldonline.com
8/16/17 10:31 AM
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Our own Lee Teschler pneumatic cylinders (and writes extensively on servopneumatics) work. the topic of practical Other technical reviews design skills. Just head to in this 2017 Motion Handbook designworldonline.com detail how electromechanical and search for “Teschler” motion designs proliferate to read his latest, “You as innovations make motion can’t control your way out components increasingly of poor mechanical design” effective. and “What are engineering Still other category technologists, and do we subsections in this Handbook have enough of them?” dive into specific design Nationally formalized challenges and parameters most I L L U S T R AT I O N C O U R T E S Y OF ISTOCK internship tracks of commonly used to quantify countries outside the U.S. also tend to motion-component requirements. These normalize the naturally harrowing process of include items on ground versus roller becoming competent enough to personally ballscrews; the where ad why of oil versus contribute to real engineering and grease; the meaning of accuracy class; manufacturing projects. This lets students minute of arc (MOA) and how it defines move beyond relatively rote plug-and-chug bearings; integrated feedback for lifecycle tasks and into higher-level projects requiring tracking; foil shielding on data cables; critical thinking — all from the safety of when to use M12 and M8 connectors on programs that impose real pressures but motion designs; PLC function blocks and with professional supervision and signoff. IEC 61131-3 classifications; how torsional We at Design World consider ourselves rigidity is measured for couplings; the another venue for real-world experience difference between sinusoidal and fieldto serve those engineers just beginning oriented control (FOC); and much more. their careers. This leads me to this Design As always, we hope these technology World Motion Handbook. For 2017, we’re updates help those of you engineers charting new territory: Peruse each section just starting out — as well as you OEM of this issue, and you’ll see componenttypes aiming to specify and integrate design summaries along with dozens of new new components to improve existing and technical FAQs answered in quantitative next-generation machine builds and end detail. That’s in large part because motion products. We invite your feedback and designs continually evolve ... and we aim requests for technical information always: to resolve the new questions they prompt. Contact me at leitel@wtwhmedia.com or @ In fact, many questions we answer are DW_LisaEitel, @Linear_Motion, and those online readers — including many @Motion_Control. Connect with our Design young engineers — pose in response to World Network Facebook page at big-picture topics. Most surprising is the facebook.com/DesignWorldNetwork and demand for explanations of engineering drop us a line (publicly or privately) on your fundamentals in defining and applying design challenges and ideas. these new technologies — relating to beam Note that motion-technology news deflection, preload, repeatability, and announcements (as well as technical parameters expressed in RMS values, to archives) are also posted daily on our give just a few examples from linear-motion motion tips sites — design engineering. motioncontroltips.com, linearmotiontips. To stay with linear designs for just com, sensortips.com, bearingtips.com, and another moment, consider the lineup of couplingtips.com. One final pitch before new subtopics relating to actuators in you dive into the editorial of this Design this Handbook: On pages 14 to 23, we World Motion Handbook: Please look explore the state of actuator technology for the 2017 Casebook coming to you in today; safety factors for reliable operation; November for quantitative coverage of and realtime communications for electrictechnologies for motion designs in realactuator integration … pneumatic-actuation world application examples — including benefits and uses; where new designs of some stunning photos and illustrations to rodless cylinders excel; and how smart inform your next build.
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DESIGN WORLD — MOTION
8 • 2017
8/16/17 10:31 AM
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HANDBOOK
MOTION SYSTEM 6
Are engineers really becoming less innovative? Pundits
have made headlines in recent years by claiming there is a slow-down in innovation. The evidence they usually cite comes from statistics about patents. The information firm Thomson Reuters, for example, recently used patent volume to conclude that the rate of innovation is slowing. The volume of global patents rose by 3.3% between 2013 and 2014, but this constitutes a slowdown in growth: In 2012, that rate was 20%; in 2013, 17.7%. The implication is that slower growth in the number of patents means less new technology. However, patent information may be misleading. That’s one conclusion to be drawn from work by researchers from the Santa Fe Institute and Arizona State University. Writing in the Journal of the Royal Society Interface, they looked at U.S. patent records back to 1790 to determine whether the development of new technologies or combining existing technologies is more important in driving invention. The researchers were helped along by the fact that the Patent Office classifies technologies responsible for inventive novelty through an elaborate system of technology codes. Examiners assign these codes to better summarize the claimed novelty. Patents can have not just single codes, but combinations of codes. And the codes can be grouped into classes of technologies. For example, the patent for the recombinant DNA technique is described as bringing together 24 distinct technologies of which 20 are drawn from the same class. This classification scheme helped researchers get a handle on how technological innovation has evolved. During the first decades of the 1800s almost every patent represented entirely new technology, not combinations of existing technologies. One reason: The patenting system itself was an innovation, the researchers say, and inventors rushed to patent existing stuff they’d come up with over the years. But if there has been a slow-down in invention, that slow-down happened in the late nineteenth century, at least if you go by the Patent Office’s technology codes. By the late 1800s about 150,000 technological functionalities had accumulated. Since then there have been few additions to the stock of individual codes, researchers say.
DESIGN WORLD — MOTION
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The conclusion from sizing up patents this way is that most patents are new combinations of existing technologies, and this has been the case for the past 200 years. In other words, the process of invention is driven almost entirely by combining existing technologies rather than by coming up with something completely new. Put another way, most patents don’t describe radical new technology. And it has been that way for a long time. But that doesn’t mean inventors aren’t innovative. Viewing patents from the standpoint of technology codes, fears of slowing innovation reduce to a simple question: Given that new technology codes don’t come along too often, how much further can we go by just combining existing technologies? The researchers say this basically boils down to calculating the theoretical bound for how many combinations of codes there can be. Run these numbers and you conclude the occasional introduction of a new code is more than enough to allow patentable innovation to continue for a long time. But there is a rising chorus of commentators who say patents may impede rather than further innovation. Typical of those in this camp is technology journalist Matt Ridley, who points out that the original idea of a patent was not to reward inventors with monopoly profits, but to encourage them to share their inventions. “Most patents are now as much about defending monopoly and deterring rivals as about sharing ideas. And that discourages innovation,” he says. If Ridley is right, then, weirdly, we may need fewer patents, not more of them.
LEE TESCHLER EXECUTIVE EDITOR
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8/18/17 9:11 AM
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EDITORIAL Editorial Director Paul J. Heney pheney@wtwhmedia.com @dw_Editor Managing Editor Leslie Langnau llangnau@wtwhmedia.com @dw_3Dprinting Executive Editor Leland Teschler lteschler@wtwhmedia.com @dw_LeeTeschler Senior Editor Miles Budimir mbudimir@wtwhmedia.com @dw_Motion Senior Editor Mary Gannon mgannon@wtwhmedia.com @dw_MaryGannon Senior Editor Lisa Eitel leitel@wtwhmedia.com @dw_LisaEitel Associate Editor Mike Santora msantora@wtwhmedia.com @dw_MikeSantora
DESIGN & PRODUCTION SERVICES VP Creative Services Mark Rook mrook@wtwhmedia.com @wtwh_graphics Art Director Matthew Claney mclaney@wtwhmedia.com
Traffic Manager Mary Heideloff mheideloff@wtwhmedia.com Production Associate Tracy Powers tpowers@wtwhmedia.com Director, Audience Development Bruce Sprague bsprague@wtwhmedia.com
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THE MOTION SYSTEM HANDBOOK VOLUME 3 NUMBER 3
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HOW DO I KNOW IF I’M TALKING TO AN ENGINEER OR A SALESMAN? Ask Smalley. We have nothing against sales people. But when it comes to differentiating Inconel from Elgiloy or overcoming dimensional variations within a complex assembly, wouldn’t you rather work with an engineer? Our customers would. That’s why they collaborate directly with our world-class team of Smalley engineers—experienced professionals whose only focus is helping you specify or design the ideal wave spring, Spirolox® retaining ring or constant section ring for your precision application.
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INSIDE THE MOTION SYSTEM HANDBOOK VOLUME 3 NUMBER 3
Actuators Electrical ........................................................... 14
81
Pneumatic ......................................................... 18 Rigid Chain . . ..................................................... 22
P2
Ballscrews ................................................................ 24 Bearings Plain .................................................................. 28 Rotary .. ............................................................. 32 Belts & Pulleys . . ....................................................... 35 Cable Management ................................................. 39 Cables . . ...................................................................... 41
ENGINEERS NEW AND EXPERIENCED: 2017 DESIGN WORLD MOTION HANDBOOK IS FOR YOU
Clutches & Brakes ................................................... 46
ARE ENGINEERS REALLY BECOMING LESS INNOVATIVE?
Controllers ............................................................... 52 Conveyors ................................................................ 64
P12
Couplings ................................................................. 70 Drives AC ..................................................................... 80 DC ...................................................................... 81
P6
NIBA AND PTDA COMBINE FORCES FOR JOINT SUMMIT
Encoders .................................................................. 82 Gearing ..................................................................... 88 Gearmotors .............................................................. 99 HMIs .........................................................................102
32
Leadscrews ............................................................ 108 Linear Guide Rail Slides & Ways .. ......................... 110 Locking Devices ..................................................... 118 Lubrication .............................................................. 122 Motors AC .................................................................... 124 DC .................................................................... 126 Integrated .......................................................130 Linear .............................................................. 132 Piezo-based Miniature . . ................................. 138 Servo .............................................................. 140 Stepper ...........................................................144 Networking .............................................................148 Retaining Rings . . ..................................................... 153
154
Shocks & Vibration Damping ................................ 154 Positioning Stages, Gantries & Tables .. ............... 162 Wave Springs .........................................................164
COVER PHOTO COURTESY OF ISTOCK
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DESIGN WORLD — MOTION
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HANDBOOK
MOTION SYSTEM
NIBA and PTDA combine forces for joint summit
NIBA
— the Belting Association and the Power Transmission Distributors Association (PTDA) will be holding a one-time Joint Industry Summit September 27 to 30, 2017 at the Diplomat Beach Resort in Hollywood, Florida. The event is for top executives and those seeking to become leaders among distributors and manufacturers in the belting and power transmission and motion-control industries. The combined meeting is an opportunity for cross-channel and cross-industry networking and collaboration between the two oft-related industries. The two organizations are combining the best of the NIBA Annual Convention with the PTDA Industry Summit to create the NIBA/PTDA Joint Industry Summit. Both associations sought to coordinate an exclusive event that would maximize networking opportunities, enhance educational content, and
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mitigate meeting costs for those members attending both conferences. A key networking program for the event is the Joint MD-IDEX/Expo. This Expo combines PTDA’s Manufacturer-Distributor Idea Exchange (a prescheduled 20-minute business meeting format) with the NIBA Expo — a tradeshow-style networking opportunity where participants browse exhibitor booths to see product demos and more. Both the prescheduled meetings and the product demo booths will be available for participants of the Joint MD-IDEX/Expo this year only. Volunteers from NIBA and PTDA are also working to create a unique networking and education event for members. Event specifics can be found at NIBAPTDA.org. PTDA | W W W .PTDA.ORG motioncontroltips.com | designworldonline.com
8/17/17 12:46 PM
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HANDBOOK
MOTION SYSTEM
PBC Linear offers integrated motor and screw technology. The actuator shown here includes the manufacturer’s Constant Force Technology (CFT) including a constant-force spring to apply uniform pressure to the nut even over variable motion profiles.
linear actuators TYPES OF
AND WHERE THEY EXCEL
Linear
actuators come in myriad configurations to suit almost any application, environment, or industry. They’re primarily categorized by their drive mechanism; then manufacturers use other features such as the type of guide and housing to further differentiate them. Here’s an explanation of the most common linear actuator categories. BELT AND SCREW-DRIVEN ACTUATORS: Although belt and screw drives are different technologies, it makes sense to put them in the same category because they are the two most common types of electromechanical actuators. Most manufacturers of linear actuators offer both belt and screw-driven options. Belt driven actuators can use a variety of guide mechanisms, with plain bearings, camroller guides, and recirculating bearings (riding on a profiled rail or round shaft) being the most common. Because their strengths are high speeds and long strokes, belt-driven systems are often housed in an aluminum extrusion or in an open configuration with no protective housing. Within the screw-driven category, there are two sub-categories — ball-screw driven and lead-screw driven. While ball-screw
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actuators have higher repeatability and thrust forces than lead screw actuators, both provide inherent gearing through the lead (pitch) of the screw. The most common guide system for screw driven actuators is the profiled rail, although lead screw types are sometimes guided by plain bearings. Because screw driven actuators need end bearings that must be rigidly mounted, they are often enclosed in an aluminum extrusion. However, when high travel accuracy is required, ball screw types are commonly offered with a machined steel housing. One design that doesn’t exist is a ball-screw actuator using cam rollers as the guide mechanism. That’s because the strength of cam roller guides is high speed, whereas ball-screw actuators are primarily used for high repeatability and high thrust force, with limited speed capabilities. Using cam roller guides in a ball screw driven actuator is comparable to putting racing slicks on a Hummer ... it can be done, but it wouldn’t make much sense. PNEUMATICALLY DRIVEN ACTUATORS: Although they’re not electromechanical devices like the other actuator types, their prevalence in automated equipment makes pneumatic driven versions an important category of linear actuators. Pneumatic actuators can be further divided into two sub-categories — slidertype and rod-type. In slider-type actuators, the motion is contained within the limits of a housing and the load is mounted to a slider ... also called carriage, saddle, or table. In rod-type actuators, the motion is produced by a rod that extends and retracts from a housing. The motioncontroltips.com | designworldonline.com
8/16/17 12:06 PM
Let’s solve this. Together. Having a linear motion automation issue? Not sure what products to select for your packaging application? You’re not alone. For more than 45 years, Nook Industries has helped solve some of the smallest and biggest linear motion problems, and we can help solve yours. With production and engineering in Northeast Ohio, and support available across the country, we’re close by and ready to partner with you.
together.nookinfo.com 800•321•7800 Nook Industries, Cleveland, OH USA
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HANDBOOK
Electric rod-style actuators as those here deliver more performance, control, and efficiency than fluid-power (pneumatic and hydraulic) counterparts. Electric actuators meet motion objectives from low cost to high performance in everything from food and beverage processing and production to high-force timber processing. Design tip: Fully analyze the linear-motion application and determine exact parameters (such as loads, forces, application footprint, and environment) before specifying an actuator. Careful upfront analysis reduces overall costs and makes for automation systems with higher reliability, better performance, lower energy expenditures, and less maintenance. Courtesy Tolomatic Inc.
MOTION SYSTEM
load may mount to the end of the rod, or the rod can push the load. Here, think of pressing or stamping a label onto a carton, or pushing defective products to a diverter lane along a conveyor. Slider-type pneumatic actuators can be guided by recirculating or plain bearings, depending on the load for which they’re designed. Rod-style versions are not typically designed for axial loads, and use simple plain bearings to provide guidance to the rod, without significantly contributing to load-carrying capacity. RACK-AND-PINION ACTUATORS: For extremely long lengths and robustness against contamination, rackand-pinion drives are often the best choice. However, these characteristics make finding a suitable guide system difficult in some applications. For extremely long
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Electric Actuators — Motion System Handbook 8-17 V3.LE.indd 16
8/21/17 8:06 AM
E L E C T R I C A C T U ATO R S
These PA programmable actuators from Nook Industries have long life and deliver high-speed output. They come as complete systems (including motor, drive, controller, and power supply) and are rated for continuous duty; acme-screw actuators are for lower duty cycles where self-locking is needed; toothed belt drives are for parallel motor mounting. Strokes reach 24 in. with ± 0.004 in. to ± 0.001 in. repeatability; the actuators also have standard motor and gearhead flanges for simplified selection. They have IP54 or IP65 ratings, depending on the motor; optional motors are available.
lengths, joined profiled rails are sometimes used ... but when contamination is a significant concern, metal wheels are usually preferred. A unique feature of rack and pinion types is how they can drive multiple carriages independently. A common application for rack and pinion actuators is the overhead gantry, often found in automotive production. LINEAR-MOTOR ACTUATORS: Linear-motor actuators can also long travel lengths with multiple carriages, but they’re primarily used for high-precision, highly dynamic motion. Complementing the strengths of the linear motor are usually high-precision profiled rails, crossed roller guides, or even air bearings for the actuator’s guidance system. Linear motor types can mount in an extruded housing or on a machined aluminum plate ... but to meet the highest travel accuracy specifications, they can also mount on a machined steel plate or granite base.
Note that while there are very few standards in the linearactuator industry, one thing that manufacturers have done is maintain a distinction between actuators and stages. When the mounting surface is a machined steel or granite base, the term stage is used. The performance difference between stages and actuators has primarily to do with travel accuracy. Stages hold much higher travel accuracies than actuators and use linear motors or very high precision ball screws for extremely smooth, precise positioning. With so many options, choosing the best linear actuator is a complex task, and there’s no one right way to make a selection. But the best place to start is usually with a manufacturer’s sizing software or selection program. Results often include several choices — which engineers usually narrow down with nonquantitative criteria such as ease of maintenance, integration with existing components or systems, and actuator footprint.
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HANDBOOK
actuators
MOTION SYSTEM
PNEUMATIC
18
Pneumatic
cylinders and actuators are sometimes referred to as bang-bang devices, making quick moves from one end of their stroke to the other with limited regulation of force or move profile. On the other hand, electromechanical actuators with servo controls offer high levels of refinement in positioning, force-torque, and accuracy. Generally speaking, pneumatics offers a cost-effective solution for rather crude, point-to-point moves, while electromechanical actuators provide high precision at a higher cost. However, there’s a design range between these two solutions needing a relatively high level of control ... but without the complexity and cost of electromechanical servo driven systems. Bridging this gap are pneumatics that operate in a closedloop system—in other words, servo pneumatics. A servo system is one that uses a feedback device and a controller to monitor and correct the system’s error (in position, speed, or torque-force). So integrating a pneumatic cylinder or actuator with a feedback system and a controller that can issue commands based on that feedback yields a servo pneumatic device. Another key component of a servo pneumatic system is a proportional valve, which precisely regulates air delivery to ensure the axis delivers its commanded position and force. Traditional pneumatics enable rapid, high-force, point-to-point motion. Servo pneumatics provide the same speed and force capabilities, with the advantage of higher accuracy positioning, not only at the ends of the stroke, but also at intermediate points along the travel. In addition to obtaining feedback on position, servo pneumatics also monitor and regulate air pressure, which enables precise control of the force that’s produced. One drawback (real or perceived) to traditional pneumatics is air consumption. Air preparation and delivery costs money, and pneumatics can use a significant amount of air even when they’re not working. Servo pneumatics, on the other hand, control air flow based on the required position and force. This leads to less air consumption than standard pneumatics, by as much as 30%. It’s important to note that servo pneumatics
DESIGN WORLD — MOTION
Pneumatic Actuators — Motion System Handbook 8-17.LE.indd 18
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A complete linear servo system featuring Phase Index ™
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HANDBOOK
MOTION SYSTEM
require higher quality air than standard pneumatics. In addition to industry-standard filtration, a 5-micron filter is typically recommended for servo pneumatic systems. WHY SERVO PNEUMATICS INSTEAD OF ELECTROMECHANICAL ACTUATORS? Of course, electromechanical actuators can provide accurate position and force control, but servo pneumatics have a much higher power density — that is, force capability for a given size. A servo pneumatic cylinder or actuator typically provides many times the force capability of an electromechanical actuator of a similar body size, which is a significant advantage in pressing, inserting, and tightening applications. Servo pneumatics also operate with 24 Vdc power supplies, which allows them to be used in lowpower applications. Lower power also reduces heat generation and thermal build-up, so they perform well in continuous-duty applications and hightemperature environments.
While electromechanical servo systems have been in use for decades, the adoption of servo pneumatics in industrial applications hinged on advancements in controls and software. Air is compressible, and this variable is much more difficult to define and model than the compliance or backlash, in an electromechanical system. Before servo pneumatics could be commercialized, pneumatic control algorithms capable of taking into account this non-linearity had to be created. But the past ten years or so have seen the development and integration of high-response valves and digital signal processors (DSPs) that can perform highspeed computations, making servo pneumatic systems a reality — and capable of providing accurate, highly responsive positioning and force control.
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P N E U M AT I C A C T U ATO R S
WHAT ARE THE BENEFITS OF PNEUMATIC ACTUATORS? Pneumatics is the technology of compressed air. Pressurized gas — generally air that is dry or lubricated — actuates an end effector and does work. End effectors can range from the common cylinder to more application-specific devices such as grippers or air springs. Vacuum systems also in the pneumatic realm use vacuum generators and cups to handle delicate operations ... such as lifting and moving large sheets of glass or delicate objects such as eggs. Pneumatics is common in industries that include medical, packaging, material handling, entertainment and even robotics. By its nature, air is easily compressible, and so pneumatic systems tend to absorb excessive shock — a feature useful in some applications. Most pneumatic systems operate at a pressure of about 100 psi, a small fraction of the 3,000 to 5,000 psi that some hydraulic systems see. As such, pneumatics is generally used when much smaller loads are involved. A pneumatic system generally uses an air compressor to reduce the volume of the air, thereby increasing the pressure of the gas. The pressurized gas travels through pneumatic hoses and is controlled by valves on the way to the actuator. The air supply itself must be filtered and monitored constantly to keep the system operating efficiently and the various components working properly. This also helps extend design life. HOW DO THESE FLUID-POWER ACTUATORS WORK? Many industrial applications require linear motion during their operating sequence. One of the simplest and most cost-effective ways to accomplish this is with a pneumatic actuator, often referred to as an air cylinder. An actuator is a device that translates a source of static power into useful output motion. It can also be used to apply a force. Actuators are typically mechanical devices that take energy and convert it into some kind of motion. That motion can be in any form, such as blocking, clamping or ejecting. Pneumatic actuators are mechanical devices that use compressed air acting on a piston inside a cylinder to move a load along a linear path. Unlike their hydraulic alternatives, the operating fluid in a pneumatic actuator is simply air, so leakage doesn’t drip and contaminate surrounding areas. There are many styles of pneumatic actuators, including diaphragm cylinders, rodless cylinders, telescoping cylinders and through-rod cylinders. The most popular style of pneumatic actuator consists of a piston and rod moving inside a closed cylinder. This actuator style can be sub-divided into two types based on the operating principle: single acting and double acting. Single-acting cylinders use one air port to let compressed air enter the cylinder to move the piston to the desired position, as well as an internal spring to return the piston to the “home” position when the air pressure is removed. Double-acting cylinders have an air port at each end and move the piston forward and back by alternating the port that receives the high pressure air. In a typical application, the actuator body connects to a support frame and the end of the rod is connected to a machine element that is to be moved.
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Pneumatic Actuators — Motion System Handbook 8-17.LE.indd 21
An on/off control valve is used to direct compressed air into the extended port while opening the retract port to atmosphere. The difference in pressure on the two sides of the piston results in a force equal to the pressure differential multiplied by the surface area of the piston. If the load connected to the rod is less than the resultant force, the piston and rod extends and moves the machine element. Reversing the valving and the compressed air flow causes the assembly to retract back to the home position. Pneumatic actuators are at the working end of a fluid power system. Upstream from these units producing the visible work of moving a load are: Compressors and filters • pressure regulators • lubricators • on-off control valves • and flow controls. Connecting these components together is a network of piping or tubing (either rigid or flexible) and fittings. Pressure and flow requirements of actuators in a system must be considered when selecting these upstream system components. Undersized upstream components can cause pneumatic actuators to perform poorly, or even make them incapable of moving their loads.
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MOTION SYSTEM
Rigid-chain actuators FOR BIDIRECTIONAL AXES LARGE AND SMALL
Rigid-chain
actuators pair electric motors with chain sporting shoulders on each link, and the motor output shaft includes a pinion to apply tangential force to the chain. The chain comes out and straightens so the link shoulders lock to form a rigid series. When the motor runs in the opposite direction, the chain shoulders disengage and allow for coiling. Inside the actuator body, reaction plates and guides counter thrust resistance and keep the chain on track. Links travel around the pinion to exit the actuator body along the stroke path. Here, the motor’s torque comes to act as forward thrust via the link shoulder to the rest of the links’ shoulders. The last link in the chain (before the load) has geometry that puts the thrust higher than the articulation axis. This makes a moment that effectively locks the link shoulders. In reverse, pulling force acts along the links’ cross axes. Rigid-chain actuators have the mechanical benefits of conventional chain but can act in horizontal push setups or vertically as jacks. Plus they’re compact. In contrast, traditional chain drives can only pull, so need two drives for bidirectional motion. Traditional screw jacks for vertical power transmission need space for retraction that’s as long as the working stroke itself.
Rigid-chain actuators range from very small versions shown in this skylight application to extremely large setups transporting many tons in AS/ RS installations, for example. Image courtesy VELUX
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If a rigid-chain actuator is appropriate for automating an axis, begin specification by determining total load requiring lifting or pulling, acceleration forces, external environmental forces, and that due to friction — with a coefficient between 0.05 and 0.5 for typical setups. Next, determine what type of actuator body and chain-storage magazine the application can accommodate. Determine whether the chain will need to change direction on its way from the magazine to actuator body. Actuators usually feed chain around 90° or 180° turns. Note that rigid-chain actuators can work alone or in tandem. Twin-chain setups deliver positioning accuracy and stability where loads are large or bulky. Here, a pushing bar acts as a yoke to keep loads steady, with optional hooks for pulling as well. Optimized geometry has the force vector act on the load’s center for balance. If twin-chain setups are impossible, consider adding framework to guide awkward loads. Guides boost stability because they address side and buckling forces. Such guides come in different shapes with different crampons and subcomponents to engage the chain. Where use of chain guides is impossible, most designs run the chain with link shoulders down for moderate stability.
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MOTION SYSTEM
24
Ballscrews: LINEAR ACTUATION MAINSTAY
Ballscrews
are a tried and true method of linear actuation. The basic components of a ballscrew are a nut, a screw with helical grooves, and balls (often made from steel, ceramic, or hard plastic material) that roll between the nut, the screw and the grooves when either the screw or nut rotates. Balls are routed into a ball-return system of the nut and travel in a continuous path to the ball nut’s opposite end. Seals are often used on either side of the nut to prevent debris from compromising smooth motion. Compared to other actuation methods such as leadscrews, ballscrews typically cost a bit more but are generally more accurate. They also boast higher efficiencies, even though they demand more lubrication because of the use of recirculating balls. A number of factors are used to classify ballscrews including lead accuracy, axial play and preload, and life/ load relationship. Lead accuracy refers to the degree to which the shaft’s rotational movements are translated into linear movement. With lead accuracy and axial play determined by the manufacturing method of the ballscrew shaft and the assembly of the nut, high lead accuracy and zero axial play is usually associated with relatively higher-cost precision ground ballscrews, while lower lead accuracy and some axial play is associated with lower cost rolled ballscrews. (See sidebar.) The axial play in a ballscrew is the degree to which a ball nut can be moved in the screw axis direction without any rotation of either nut or screw. Preload is typically applied to eliminate axial play, which also has the effect of removing backlash and increasing stiffness. Several techniques for preloading are common. Some of these methods include oversizing the balls inside the nut housing; using the so-called “double-nut” or “tension nut” method; or by using a manufactured offset in the raceway spiral to change the angle of ball engagement (the “lead shift” method) and deliberately force the balls into a preload condition. Each method has its advantages and disadvantages, but all serve to minimize or eliminate backlash between the nut and screw. One factor affecting precision and repeatability is ball recirculation inside the ball nut. Thus, ball nuts are available with a range of preload options to reduce or
DESIGN WORLD — MOTION
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Ballscrews such as the BSL series from NSK, intended for use in miniature lathes, feature an end-deflector recirculation system adopted for high-speed and low-noise operation.
remove the axial play as they rotate around the screw. Minimal axial play allows better accuracy because no motion is lost from the clearance in the balls as they reengage. The biggest overall benefit of a ballscrew is the high efficiency that can be well over 90%. By contrast, Acme leadscrews average about 50% efficiency or less. There are also minimal thermal effects. Ballscrews also offer smooth movement over the full travel range. The higher cost of ballscrews can be offset by decreased power requirements for similar net performance. Not to be overlooked is lubrication. As a bearing system, ballscrews need some type of lubrication to avoid metal-to-metal contact of the balls in the raceway. While the lubrication choice can be either oil or grease, it’s advisable to avoid solid additives (such as graphite) as they will clog the recirculation system. An NLGI no. 2 type grease is recommended but it should also depend on the application, whether food-grade or another special type of lubrication is required. Ballscrews, especially those used in machine motioncontroltips.com | designworldonline.com
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MOTION SYSTEM
HANDBOOK
tools, generally require lubricants with EP additives to prevent excessive wear. The amount of lubrication will typically be fixed, but the frequency of lubrication will vary depending on factors such as the move cycle characteristics or the level of contamination in the environment. Contaminated lubrication can increase friction. In addition, ballscrews can fail if the balls travel over metal chips or dirt in the ball thread raceway. Using lubricants recommended by machine tool manufacturers can help prevent this effect. Using telescopic covers or bellows can help keep ballscrews clean when used in environments with much contamination.
RETURN PATH
BALL NUT BALL SCREW
BALL CIRCUIT
The diagram illustrates the main components that make up a ballscrew including the screw, nut, the ball circuit, and return path.
GROUND VS. ROLLED BALLSCREWS: DOES MANUFACTURING METHOD MATTER? There’s much debate among manufacturers, and sometimes among users, regarding the superiority (or lack thereof) of ballscrews with ground threads over those with threads that are formed by rolling. Ground ballscrews have traditionally been the choice for highprecision applications, whereas rolled screws offered an economical solution for general industrial and automation applications. But manufacturing technologies have changed over the last several decades. Here are three facts to keep in mind when determining which to use in your machine or process.
1
DIN/ISO and JIS specifications distinguish between two types of ballscrew accuracy: “P” (precision) and “T” (transport).
The lower the number, the better the accuracy, with precision classes ranging from P0 to P5 and transport classes from T5 to T9 (T10 for JIS). JIS specifications denote accuracy classes with the prefix “C” for precision and “Ct” for transport. A common misconception is that the accuracy class specifies the manufacturing method, but the two are not intertwined. Rolled screws can be made in P5 and even P3 accuracy, and some ground screws only meet T accuracy requirements. The important thing to understand is whether, according to the manufacturer’s specifications, the lead error, v300, accumulates over the length of the screw. P accuracy classes do not allow lead error accumulation, while T accuracy classes do.
2
26
Geometric tolerances are also specified by DIN/ ISO and JIS standards.
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For ground ballscrews, both thread grinding and journal grinding are done using the same reference centers, making it easier to minimize radial run-out and to keep the screw threads and end journals concentric. When screws are manufactured by rolling, the end journals are machined and ground after the threads are rolled, so maintaining concentricity and run-out is more difficult. However, if a ballscrew is manufactured to DIN/ISO or JIS standards, it will not only meet lead accuracy specifications, but also the geometric specifications, regardless of whether it was manufactured by rolling or grinding.
3
The rolling and grinding processes produce different surface finishes.
A rough surface finish is problematic in ballscrew assemblies because it can result in higher friction and more wear on the load-carrying balls. The grinding process produces a smooth surface finish, and in theory, the rolling process does as well. But in reality, screws that are rolled must also be polished in order to remove an oxidation layer that forms during the rolling process. So when considering surface finish, the comparison is not between the ground surface and the rolled surface; it is actually between the ground surface and the polished surface of the rolled screw. The quality of the polishing step, and not the rolling process, determines the surface finish quality of a rolled screw. As with most design criteria, the decision regarding which type of ballscrew to use comes down to performance requirements and cost. Ground screws are necessary when ballscrew accuracy below P5 is required, as the rolling process cannot produce these accuracy classes. But for P5, and in some cases P3, accuracy, both rolled and ground screws can meet the required specifications. And for accuracy classes 7 and 9 (10 by JIS standards), either manufacturing method can produce ballscrews that meet DIN/ISO or JIS standards.
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HANDBOOK
MOTION SYSTEM
plain bearings BASICS OF
Plain
bearings are cylindrical sleeves that bear light to moderate radial loads. They slide radially or axially over shafts to allow rotary motion or linear motion (or sometimes both) of these loads. Plain bearings of all types are compact and lightweight with high strength-to-weight ratio. Plain bearings have none of the moving parts that rollingelement bearings have, so minimize fail points; they’re also cost effective for even fairly rugged applications. Common variations are metallic sleeve bearings (which often ride loads on a hydrodynamic or full film of lubrication) and self-lubricating plastic bearings in an array of geometries for bushing, thrust bearing, and integral-slide applications. Plain-bearing ratings are based in part on test results and its material modulus of elasticity, flexural strength, shore-D hardness, maximum surface pressure and running speed, rotating, and maximum load capacity — with the latter related to the plain
bearing’s material compressive limit. (Here, recall that the compressive limit is the point at which 0.2% permanent deformation occurs.) In addition, a pressure-speed (PV) value expresses plain-bearing load capacity — usually in in psi times the shaft rpm. However, note that PV values are only one to help determine a plain bearing’s overall load capacity — especially where a PV expressions might mislead engineers into thinking that a plain bearing can bear excessively high loads if the speed is very low. In other words, use of PV values requires concurrent consideration of real-world speed and load limits. As mentioned earlier, plain bearings are made of many graphite, bronze, and plastics that include PTFE, nylon, and polyacetal. Material improvements have made plastic plain bearings increasingly common, even in demanding motion applications.
WEAR
FRICTION
EFFECTS ON PLAIN BEARINGS OF SHAFT SURFACE ROUGHNESS
0
32
64
96
0
32
64
96
ROUGHNESS (RMS) Shaft surfaces for mating with plain bearings shouldn’t be too smooth or rough.
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8 • 2017
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SHAFT MATERIAL AND PLAIN-BEARING WEAR The shafts on which plain bearings ride have significant impact on plain-bearing performance and life. One common option is cold-rolled carbon steel. This shaft material makes for a suitable mating surface for plain bearings made of polymers. Ceramic shaft surfaces induce more wear, though are sometimes chosen for their ability to withstand harsh environmental conditions. Though aluminum shafts are lightweight and easy to machine, they also induce accelerated plain-bearing wear. Aluminum shaft made of anodized slightly improves the assembly performance. In fact, shaft surfaces for mating with plain bearings shouldn’t be too smooth or rough. Overly sooth surfaces will cause stick-slip adhesion variations — in turn causing higher friction resistance to bearing movement. More of a disparity between dynamic and static friction will make for faster bearing wear and jerkier motion. In contrast, overly rough shaft surfaces quickly abrade plain bearings. In fact, the rates of wear induced by shaft-bearing interfaces can vary a hundredfold. Some manufacturers recommend shaft-surface finishes to 64 root mean square (rms) for precision applications needing low friction; a smoother shaft with roughness of 20 rms or so is more suitable where long plain-bearing life is a design objective. Recall that the rms expression of surface roughness is derived from measurements of a surface’s microscopic peaks and valleys. Ra is an alternative measure some in industry use to quantify roughness — in this case, as an average roughness of a surface’s peaks and valleys. So the two measures express the same quality, only in different formats. Note that large and outlying peaks or flaws on a shaft surface will affect the RMS value more than its equivalent Ra value.
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Bearings, Plain — Motion System Handbook 8-17 V3.indd 30
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DESIGN WORLD — MOTION
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ANSWERS TO COMMON QUESTIONS
MOTION SYSTEM
HANDBOOK
Rotary bearings: Bearings
are internal machine components that are crucial to motion applications. They reduce friction between moving parts by giving a surface something on which to roll rather than slide. Rotary bearings consist of smooth rollers or metal balls and inner and outer surfaces (races) against which the rollers or balls travel. These rollers or balls carry load carrier and let axes spin freely. Bearings typically encounter radial and axial load. Radial loads are perpendicular to the shaft, and axial loads occur parallel to the shaft. Depending on the application, some bearings must withstand both loads simultaneously. WHAT’S THE DIFFERENCE BETWEEN BALL AND ROLLER BEARINGS? Because the contact area between balls and races is so small, ball bearings excel in light to moderate loads. The small areas of surface contact also minimize friction-generated heat, so ball bearings work well in highspeed applications. In contrast, roller bearings have cylindrical rollers. They’re common in applications such as conveyor belt rollers because their rolling elements make more surface contact with their races — so handle larger loads without deforming. Their shape also allows for a moderate amount of thrust load, as weight is distributed across cylinders instead of spheres. WHAT KINDS OF APPLICATIONS USE NEEDLE ROLLER BEARINGS? Needle-roller bearings operate in tight spaces — for example, in automotive applications such as rockerarm pivots and transmissions. In short, these are roller bearings with rollers having a length at least four times the roller diameter. The large surface area of the needle roller bearing lets them support extremely high radial loads. Usually a cage orients and contains the needle rollers. The outer race is sometimes machined into the a housing interior. Needle-roller bearings come in two different arrangements — a radial arrangement (in which the rollers run parallel to the shaft) and a thrust arrangement (in which the rollers are flat in a radial pattern and run perpendicular to the shaft). Ball bearings’ small areas of surface contact minimize friction-generated heat, so ball bearings work well in highspeed applications. Image courtesy igus
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R O TA RY B E A R I N G S
WHAT ARE THRUST BEARINGS? Thrust roller bearings transmit load from one raceway to the other to resolve radial loads; their self-aligning capability makes them immune to shaft deflection and alignment errors. Thrust ball bearings go in applications with primarily axial loads and handle shaft misalignment. These bearings also work on high-speed axes in the aerospace and automotive industries. TAPERED ROLLER BEARINGS Tapered roller bearings have tapered inner and outer ring raceways with tapered rollers between them — angled so the rollers’ surfaces converge at the bearing’s axis. These bearings are the only bearing type that can concurrently handle large amounts of axial and radial loads. Single-row taper bearings only support high axial loads from one direction. However, installations that put one tapered roller bearing against a second counteract additional load; this allows the bearings to support high radial and axial loads from multiple directions. Other caveats: Tapered roller bearings can only accommodate slight angular misalignment of the inner ring in relation to the outer ring — just a few minutes of arc at most. As with other roller bearings, tapered roller bearings must carry a minimum load, especially in high-speed applications where inertial forces and friction can damage rollers and raceways should they come out of contact.
HOW TO PICK THE RIGHT LUBRICANT FOR A ROTARY BEARING? Lubrication of bearings with rolling elements takes the form of oil or grease; grease usually lasts longer, thanks to thickeners that sustain a layer between raceways and rolling elements. Grease with extreme-pressure additives also extends bearing life under higher forces. Even so, oil is more common for open bearings or those subject to low torque or high speeds. Oil’s lower viscosity imparts less drag than greases as rollers move through the lubricant. Mode of oil delivery, application rpm and temperature, and potential environmental contaminants dictate which oil is most suitable. Case in point: Operating temperature dictates which oil viscosity will work in a given application. Overly thick oil increases required torque to make the rotary bearing spin; overly thin oil won’t maintain the protective layer needed to prevent mental-on-metal contact. APPLICATIONS FOR ROTARY BEARINGS Bearings abound in industrial and consumer designs. For example, deep-groove ball bearings often go into in small to medium-sized electric motors because they can accommodate both high speeds and radial and axial loads.
Needle roller bearings are at the heart of cam followers such as these from Osborn. The compact design carries high loads.
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HANDBOOK
MOTION SYSTEM
Self-aligning ball bearings, on the other hand, are work well in fans. These bearings have two rows of balls with a common raceway in the outer ring. This design allows for angular misalignment while maintaining running accuracy. The only caveat is that they’re one of the most difficult bearings to install correctly. Tapered roller bearings go Alignment is a critical element during in needing support for axial and the installation of rotary bearings. Making radial loads — as in a tire hub bearing vehicle weight and the sure shaft alignment is checked can axial loads associated with cornering. These bearings are also prevent problems before they start. common in gearboxes where they mount with a second bearing of the same type in a face-to-face or back-to-back orientation. They provide rigid shaft support to minimize deflection. This reduced shaft deflection minimizes gear backlash. Tapered bearings also have the advantage of being lightweight but efficient, even while maintaining good overall speed capabilities. In applications where the bearings mount vertically, they typically mount in a face-to-face setup. In horizontal applications, they mount back-to-back. SIDE NOTE ON PLAIN BEARINGS Plain bearings are cylindrical sleeves with an array of design elements to cater to specific applications. For more information, refer to the Motion Handbook section on plain bearings that precedes this one. Some plain bearings go into applications requiring slide plates for straight strokes. Other plain bearings do the same job as roller-based thrust bearings, but use pads arranged in a circle around the cylinder. The pads create wedge-shaped regions of oil to prevent hard contact with the rotating disc supporting the application thrust. Material innovations have made plastic plain bearings more useful than ever, though plain bearings of all types are lightweight and compact and can carry substantial load. Growing use of plain plastic bearings and increasingly stringent industry standards means these bearings must often meet FDA, RoHS, and other standards. Some even meet EU directive 10/2011/EC standards, which holds material manufacturing processes to certain criteria.
Spherical roller bearings like these from Rexnord provide angular misalignment compensation, low friction, cool running temperatures, and high load capacity.
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BELTS & PULLEYS
TODAY’S APPROACHES IN SELECTING
synchronous & V-belt DESIGNS
Power
transmission in linear motion designs is often through rotary-tolinear devices, chain, or belt drives. The earliest belt iteration — and one that’s still useful and economical today — is the friction-based V=belt design. These pair a belt with a pulley (often on an electric motor’s geared output shaft) to provide reliable operation in myriad end-user and industrial designs. Modern V-belts are rubber, urethane synthetic, and neoprene designs with either a V or trapezoidal profile. The latter increases the amount of contact between V belts and pulleys to minimize tension needed to transmit torque. Even so, polyurethane outperforms
rubber thanks to its higher resistance to chemicals and adaptability to specialized profiles. (Polyurethane also boosts the shear strength of the teeth on synchronous belts covered in this article’s next section). A V belt’s most important element — its tension-bearing top — includes fiber cords for strength to bear the actual traction load. Modern tension-member cords are often aramide, polyester, fiberglass, or even steel. Pre-stretched variations help minimize stretch. The cords embed into the main belt material that serves to hold the belt body together and shed heat. The part of most modern friction belts that engages the pulley is a compression section designed to actually wedge into pulley grooves as a way to boost engagement. In many instances, a rubberized fabric cover helps protect the belt and prevent slipping and overheating cords. Though they’re versatile and forgiving, improperly sized frictionbased belt drives can slip (tangentially on the pulley — a form of lost motion) and creep axially. That can make for unreliable speed output. Here are some things to remember if a V-belt drive makes the most sense for a motion axis: Output torque depends on belt resistance to tension and belt-pulley adherence. The latter is why oils and greases must be kept away from belt drives — or threaten drive failure due to slipping. Be prepared to specify V belts by cross section (including the belt’s top width, V angle, and depth) and
Shown here are five pulley designs and a Concentric Maxi Torque bushing attachment system from Custom Machine & Tool Co. Inc.
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Manufacturers of Power Transmission and Motion Control Components
overall pitch length (defined as a circumferential length along a belt’s pitch line). Then suitable V belts are narrowed further by which have sufficient power ratings (determined by rpm and sheave speed) to satisfy design demand of nominal horsepower (to be transmitted or output at the motor) with application of a service factor. Sound complicated? In fact, industry has simplified much of this with references that list fairly specific V-belt service factors that adjust for typical levels of special application demands and losses from variable loads and rpm, heat, environmental conditions, and shock and vibration. Where friction belts are insufficient for a motion design — as on positioning table, conveyor, and printing-machine axes needing true synchronous operation, for example — toothed synchronous belts excel. Such belt drives are also indispensable in compact designs that need power-dense linear drives in awkward or compact design envelopes. As with V belts, be prepared to specify synchronous belts by length and axis power demand. Here, additional factors include the teeth’s maximum shear strength (dictated by their cross section as well as pulley-engagement dynamics). On the topic of teeth engagement, remember that synchronous belt drives need tooth clearances at the engagement with pulley grooves (so teeth can enter and exit channels sans interference). That’s why most synchronous belts exhibit some backlash. In addition, a synchronous belt’s tooth shear strength must be high enough to
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HANDBOOK
MOTION SYSTEM
Embrace the oxymoron: Static motion designs — those that only depend on consistent end-of-move positioning — can tolerate the errors of more basic friction-belt drives. In contrast, dynamic motion designs require axes that move predictably over their complete strokes — even if load varies during operation. So sometimes engineers will work to minimize backlash with toothed belts needing shallower clearance for pulley engagement.
withstand maximum application torque demand. As with V-belt selection, service factors can help engineers pick synchronous belts having shear strengths to withstand the application’s worst expected shocks and loading. Despite the extra considerations, synchronous belts are indispensable in precision motion designs. A mature technology is belting with teeth of a trapezoidal shape (not to be confused with V belts sporting trapezoidal cross sections) — although modified iterations are suitable for very precise positioning. More common in new designs are rounded profiles carry more load than belts with trapezoidal teeth. The belts do this in two ways: 1) They have inherently higher tooth shear strength and they 2) More evenly spread load over the belt’s tensile cords. Generic labels for synchronous belts with round-profile teeth are variations on the term high-torque drive or HTD for short — with the latter a trademark of belt and rubbercomponents manufacturer Gates Corp. In some cases, belts with round-profile teeth can triple horsepower ratings. Another design — belts with curvilinear teeth — help optimize pulleytooth engagement and pressure angles to boost overall power transmission. Many such belts go into automotive applications, which come with tensile cords and in sizes unsuitable for industrial designs.
V BELT AND SYNCHRONOUS BELT GEOMETRY SYNCHRONOUS BELT USES POSITIVE ENGAGEMENT. V BELT USES FRICTION.
CURVILINEAR TEETH
TRAPEZOIDAL TEETH
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CABLE MANAGEMENT
Cable trays are often made of metal, like this aluminum design from Cope, which can be manufactured in ladder, trough and hat styles.
Cable management
CHOICES ENSURE PROPER ROUTING OF CABLE RUNS
Cabling
in a motion control system can run the gamut from a single wire running to a single-axis direct drive, to a complicated system of expensive cables and wiring on multi-axis robots. As a result, it is necessary to consider cable management in your system design. A simple design may only require twist-tie type bundlers but as systems become more complex, with longer, heavier cables, you may need to upgrade to cable trays or cable carriers to prevent sagging and undue strain on cables. CABLE TRAYS FOR STATIC CABLE RUNS Cable trays are best suited to stationary applications. These devices route cables in a system, providing protection from contaminants and necessary ventilation while keeping different cable types separated. Featuring a U-shaped open design, the cables are simply placed into the tray, which can be closed or open to the air. Typically manufactured from stainless steel, aluminum, plastic, or fiberglass, they may be designed ladder style or in solid, ventilated or perforated designs with knockouts for cable exits. It is important to consider the cables that are being routed when choosing a cable tray, including its weight and diameter. On a related note, be aware of the span between supports and the cable run distance, as cable weight can have an impact here. Finally, understand the application where the cables are used; for instance, if they need to be protected from contaminants with a closed design or if they will require ventilation.
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Cable Management — Motion System Handbook 8-17 V5.indd 39
CABLE CARRIERS PREVENT CABLE DAMAGE IN DYNAMIC MOTIONS Cable carriers are advanced components that house cables and hose in motion systems. Available in metal or plastic designs, they help prevent sagging, tangling and crushing of cables in robotics, machine tool, and large off-road and industrial machinery. Unlike cable trays or ladders, these devices are best suited to systems that require movement, such as robotics, machine tool and large off-road and industrial machinery. They can support the heavy weight of numerous cables and hoses, without sagging or putting stress on the cabling. They also prevent cable from tangling and getting caught in moving parts of a machine. Two styles are available: closed or open. Closed carriers protect cables from environmental damage posed by contaminants like metal shavings, while open carriers offer visual and quick and easy access. Plastic cable carriers, made from high-performance polymer or nylon, offer reduced weights while being corrosion resistant in hostile environments. Some polymer designs resist seawater and mineral oils, and other harsh applications. Plastic cable carriers can be easily opened and disassembled. They feature a modular design, for easy configuration to most applications. The easy-to-maintain designs mean damaged or broken links can simply be removed and substituted with a new link. Some plastic carriers can be opened by hand. 8 • 2017
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COMPLETE LINE OF CABLE & HOSE CARRIERS
Steel and other metal alloy carriers are best suited for heavy mechanical loads that run long distances carrying large cables and hose. Steel cable carriers are designed to handle harsh environments—even longterm temperatures above 600° without negative impact. They also offer high resistance to many harsh and corrosive chemicals, so are usually found where maintenance is difficult. These extreme applications include everything from mining to marine and oil and gas to foundries.
· Plastic, Metal, Hybrid Designs · Open & Enclosed-Link Styles · Standard, Modular, & Custom Engineered Carrier Options · Trusted Brands: GORTRAC® / GORTUBE® NYLATRAC® / NYLATUBE®
Plastic cable carriers, such as this model from igus, feature a modular, easy to maintain and lightweight design. They can be provided to users empty or prefilled with complete cable assemblies.
SHOULD YOU LUBRICATE STEEL CABLE CARRIERS? Often used in highly corrosive environments, steel cable carriers provide a high strength-to-weight ratio and maximum unsupported spans. They usually feature a special coating to resist corrosion from chemicals and other abrasive materials. Generally, lubrication isn’t required. Should it be required, certain applications have more abrasive environments where a dry lubrication is required, but in Metal cable carriers are the majority of applications, ideal for harsh environments. lubrication of the steel chain is They require little maintenance not required. and usually no lubrication. For applications that require Image courtesy of Dynatect real lubricity where you need very low friction on a steel track, you can use self-cleaning links. In addition, the manufacturer can install wear discs — often made of nylon — ahead of time, in between the steel, to reduce friction. This offers some added lubrication, if this is required. The obvious downside of lubrication is it’s going to pick up dirt and contaminants. If you lubricate with a W-40 or some other similar lubricant, it’s going to pick up particulate or debris near it, which is not a good thing for the cable carrier or the cables housed inside of it.
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Cable Management — Motion System Handbook 8-17 V5.indd 40
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CABLES Traditional servomotors used in industrial automated equipment require the use of power and feedback cables similar to HELUKABEL’s TOPSERV (orange) and TOPGEBER (green) families. Machines exchanging data with controllers using PROFIBUS protocols require cables designed to transfer this information without failure, such as HELUKABEL’s line of PROFIBUS data cables (purple).
HOW TO OPTIMIZE YOUR MOTION CONTROL
cable selections Industrial
automation equipment, such as multi-axis machines and robots, is in continuous operation. These repetitive motions can stress all the moving parts of the machine. Electrical data, power and signal cables—critical components in these designs—must be considered with care. Standard cables can fail prematurely, so it is necessary to select the right type of high-flex motion control cables for your industrial motion application. These flexible or continuous-flex cables are designed and manufactured to cope with the tight bending radii and physical stress associated with motion applications. These highly flexible cables extend their service life, especially when run inside cable carriers. A regular cable typically manages 50,000 cycles, but a flexible cable can complete between one and three million cycles. Flexible cables fall into two categories — those with conductors stranded in layers inside the cable and those with bundled or braided conductors.
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Cables — Motion System Handbook 8-17 V3.indd 41
Cables with stranded layers are easier to produce so usually less expensive. The cable cores are stranded firmly and left relatively long in several layers around the center and are then enclosed in an extruded tube-shaped jacket. In the case of shielded cables, the cores are wrapped up with fleece or foils. However, this type of construction means that during the bending process the inner radius compresses and the outer radius stretches as the cable core moves. This can work quite well because the elasticity of the material is still sufficient, but material fatigue can set in and cause permanent deformations. The cores move and begin to make their own compressing and stretching zones, which can lead to a corkscrew shape and even core rupture. The other construction technique involves braiding conductors around a tension-proof center instead of layering them. Eliminating multiple layers guarantees a uniform bend radius across each conductor. At any point where the cable flexes, the path of any core moves quickly from the inside to the outside of the cable. The result is that no single core compresses near the inside of the bend or stretches near the outside of the bend, which reduces overall stresses. An outer jacket is still required to prevent the cores from untwisting. A pressure-filled jacket fills all 8 • 2017
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HANDBOOK
Flat cables, like this EZ-Flexx design from Cicoil, are ideal for motion applications because they take up less space and naturally flex in a single plane.
MOTION SYSTEM
DEFINING VOLTAGE AND CURRENT CAPACITIES
the gussets around the cores and ensures that the cores cannot untwist. The resulting flexible cable is often stiffer than a standard cable, but lasts longer in applications where it must constantly flex. FLAT CABLES AS AN ALTERNATIVE An alternative to flexible cabling in some motion applications are flat cables. These cables can incorporate any variety of power, signal, and video conductors in a single compact cable. In addition to every type of electrical conductor, flat cables can also include tubing for air or liquids, and even fiber optics. By incorporating all these elements into a single flat cable, motion equipment can be significantly smaller, quieter, and more energy efficient. Flat cables are best for continuous flexing. Their wire conductors can individually flex in a single plane, which provides optimum flex life. Some motion control systems may encase separate wires, cables, and tubes in a carrier track to contain and manage the separate elements and to constrain their motion. These tracks are usually made of plastic and have a rather large bend radius. These tracks do not add performance to the motion device or machine, as
Cables without a jacket may not be used as a substitute for a jacketed cable or as an extension cable. This also is the case for connecting Class 2 cables to equipment, unless the cables in the construction spec have been defined as cables for extra light duty in a fixed installation. The corresponding VDE and HD regulations shall be observed for the cables used in mining operations, quarrying, and equipment e.g. cranes with spring-loaded reeling devices. VOLTAGE The rated voltage for a cable is the voltage for which a cable is designed and defines the electrical testing requirements. The rated voltage is expressed as the ratio of two values, U0/U, where U0 is the effective value (root mean square or r.m.s.) of the voltage between any insulated conductor and the “ground” (metal covering of the cable or surrounding medium). U is the effective value (r.m.s.) between any two-phase conductors of a multi-conductor cable or system of single conductor cables. In an alternating current (AC) system, the rated voltage of a cable should be at least equal to the nominal voltage given by the U0/U value. In direct current (DC) system, the rated voltage of the system should be no higher than 1.5 times that of the nominal voltage of the cable.
A look at a voltage test machine, which helps determine the rated voltage for a cable. Image courtesy of Helukabel
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Helukabel -- Motion Systems Handbook 8.17.indd 43
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HANDBOOK
MOTION SYSTEM
they are simply cable management devices. Cable tracks can add bulk, mass and inertia to the motion system, and moving this extra mass requires more energy. While certain motion systems such as robotic applications may require this type of cabling design, other designs may not and can use standard flat cabling instead to save weight and cost. Some flat-cable manufacturers offer cables with silicone jacketing. These types of flat cables are durable and need no external armor for protection. They resist abrasion and will even self-heal minor nicks. Silicone encapsulation also provides protection against oils, acids, ozone, steam, and extreme temperatures. SELECTION CRITERIA Selecting the right cable for an application starts with fundamental parameters. First, determine whether the cable will be moving. Does the motion induce cable flexing or twisting? Or does the application induce flexing and torsion? Cables exist for all three situations. If the application is only bending, determine the cable’s worst-case bend radius. Bend radius depends on the cable wire gauge and the kind of conductors in the cable. Cable size is determined by the gauge of wire which in turn is dependent on current
requirements (and the number of conductors the application needs). As a general rule, finer conductor gauge allows tighter bend radii. Flat cables with PTFE jackets can have a larger bend radius than cables with silicone jacketing for a given number of conductors. For cabling in flexing applications, two key factors are wire conductors and the cable jacket. With continuous flexing, conductors containing multiple strands of fine-gauge wire generally last the longest. The most suitable choice of cable material depends on application needs, and can include PVC and halogen-free to Neoprene, rubber, silicone and other materials. Also, do the cables require electrical shielding? Tip: Consider any approvals that the cables may need to meet such as UL, CSA, CE, and RoHS. Chief environmental factors dictating the most suitable cable choice include exposure to harsh conditions such as temperature and humidity and required resistance to environmental contaminants such as oil or corrosive materials. For instance, what is the operating temperature for the application? Will the cables be in cold (freezing and below freezing) or hot environments? Will the cables need to endure exposure to oil? Here, other cables can resist full immersion for days. In the same way, cables also have varying degrees of flame resistance.
DEFINING VOLTAGE AND CURRENT CAPACITIES - CONTINUED Note: The operating voltage of a system may permanently exceed the rated voltage for the cable by 10%. CURRENT CARRYING CAPACITY The nominal cross-section of each chosen conductor should be large enough so that the current carrying capacity is not less than the maximum continuous current that flows through the conductor under normal operating conditions. The limiting temperature with respect to the current carrying capacity should not exceed the cable insulation and jacket required. Regulations define how a cable should be installed along with the amount of current the cable can carry. Other factors may also be included in the values given for the load rating to allow for other conditions, such as:
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1. 2. 3. 4. 5. 6. 7.
cable grouping excess current protection ambient temperature reeled/drummed cables thermal insulation current frequency (if other than 50 Hz) harmonic wave effects
Serious damage can occur if cables are operated for longer periods of time above recommended current ratings and can lead to early failure or considerable deterioration of the cable’s performance. THERMAL INFLUENCES Cables should be selected, located and installed so that the intended heat dissipation is not inhibited and does not present a fire hazard to adjacent materials. The temperature limits for individual cables are specified by the manufacturer, which under no circumstances should these values be exceeded.
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Space Tight?
GO FLAT! Flat cables occupy just 50% the space of comparable round cables. For dense packaging, flat cables are the most space-efficient solution. Cicoil Flat Cable, with its thin profile and extreme flexibility is the ideal choice where space is tight.
u One Cable For Everything: Combine Power, Signal, and Tubing into a single flat cable for the smallest packaging density. u Standard or Custom: A wide variety of standard cables are available, and our engineers can design custom cables for any requirement or industry.
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HANDBOOK
MOTION SYSTEM
COMMON MODES OF INDUSTRIAL
clutch & brake ENGAGEMENT
Key
operating parameters dictate which clutches and brakes are suitable for a given application. The first parameter is the mode of motion and whether the axis needs a brake or clutch. Clutches generally bridge moving machine sections to shape relative motion, while brakes’ main job is to stop and hold rotating elements. Several design variations exist, and a main differentiating factor is the way in which a brake or clutch engages. Some options include mechanical, electric, fluidic, or self-actuation. Brakes and clutches using fluid-power actuation usually include a piston to physically engage friction discs for torque transmission from an input to an output. Hydraulically and pneumatically actuated clutches and brakes are quickresponse units that act on fluid pressure and piston mass, so engagement speed follows control pressure.
Some air-actuated brake and clutch designs run coolly and maintain hold with minimal power. Some caveats though: In some clutch variations, full piston force transmits through a bearing — effectively a wear point — to let the body rotate as the housing stays put. Here, the bearings and high loads also limit operating speed. Brakes using fluid-power actuation don’t need such bearings. Brakes and clutches using mechanical actuation include a lever for mechanical advantage to trigger engagement and disengagement. This levering usually works to squeeze friction discs together for torque transmission. When the mechanism moves, the clutch or brake condition changes from engaged to disengaged or vice versa. Some such brakes and clutches have a locking mechanism to keep the unit as it is until repowering. Such clutches and brakes have no bearings to fail; are fairly speed tolerant;
The FEB0590 by Carlyle Johnson is a spring-set brake for medical applications such as surgical robotic arms. Highly accurate, the brake allows for precision movement and position control.
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Power Transmission and Motion Control Solutions for Industrial Applications
Heavy Duty Clutches and Brakes
Electric Clutches and Brakes
Gear Motors
Overrunning Clutches
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Power Transmission Components Linear Actuators & Controls
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Bluegrass Bourbon &
Brakes
come in optional one-position setups; are unaffected by power failures; and offer automatic overload release during over-torqueing or locking. On the other hand, some mechanically actuated variations don’t automatically disengage during power loss — and may need adjustment to compensate for wear. Clutches and brakes that are electromagnetically engaged (or disengaged, depending on design) use a magnetic coil to generate a magnetic flux — either to move an armature from friction-disc contact or to move the armature for squeezing discs together. That’s because electromagnetic clutches and brakes are either electrically activated or spring-applied-electrically-deactivated setups. The latter does doubleduty as a failsafe during power loss.
Electric brakes are easy to control and cycle, even to a couple thousand cycles per minute.
Yes, Brakes
Electromagnetic clutches and brakes are high-speed tolerant; exhibit long life; and come with self-adjustment mechanisms to compensate for friction-disc wear. But in some cases, there can also be drawbacks with this technology: Some need bearings to support a stationary coil — and the bearings can bear a light load from the radial flux-path use. What’s more, electromagnetic clutch or brake engagement time exceeds that of other designs due to the time needed to generate the coil’s magnetic field.
For 45 years, Mach III has produced made-to-order friction brakes, clutches and torque limiters that meet the exact requirements of our client’s applications. • 2,000+ models • Torque to 60,000 in-lbs. • Quick & reliable lead times • Expert engineering assistance
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Clockwise from top left are a Mach III slip clutch adapted to diameter restriction; a stainless-steel torque limiter coupling to connect square shafts; a water-resistant clutch for shaft-to-output flange connection; and a stainlesssteel tensioning brake. These and other brakes and clutches from Mach III Clutch Inc. come in torque capacities to 62,000 lb-in., bore sizes to 3.625 in., and in myriad mounting configurations — including those for through-shaft, end-ofshaft, flange, NEMA frame, IEC frame, and custom-motor frame mounting.
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Highest Torque in the Smallest Space ... or the largest. Our Maxitorq® clutches and brakes deliver the highest torque in the smallest space, and are customized to meet the exact needs of your installation and application. Land, sea and air – CJM is everywhere.
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HANDBOOK
MOTION SYSTEM
Clutches and brakes called wet-running units contain fluid to work by the effects of a transmissionfluid boundary layer. More specifically, fluid shears between friction discs and drive plates. As the parts come together, the fluid in shear transmits torque. This eliminates direct friction-disc contact during high-speed slip. Just note that oil shear generates heat even in neutral; there’s oil shear until the in and output are almost to equal rpm. But in applications that must rapidly stop, start, reverse, and change speed, sometimes units that use oil-shear technology excel. Heat from the friction dissipates as the fluid circulates to the housing. That’s useful on axes with start-stop cycles or cycle rates to high-cycle setups (to 300 cycles per minute, as a rule of thumb). Here, oil-shear brakes last five to ten times longer than dry clutchbrake offerings. (Just review the application before specification to prevent heat from becoming an issue.) SIZING BRAKES AND CLUTCHES TO TORQUE AND SPEED AND CYCLES The size and type of brake or clutch most suitable for a given design depends on whether the machine needs emergency stops or softer stops that sacrifice brakes to protect transported loads from shock. Then other application criteria (including the machine envelope, thermal capacity, cycle rates, and inspection and repair schedules) dictate final selection. In other machines, it’s more important that brakes prevent shifting loads and misalignment.
Expressed in lb/ft, N/m, or lb/in., static and dynamic torque values express clutch or brake output capability. Applications needing dynamic braking are those in which the brake controls rotating-axis motion by absorbing kinetic-energy changes. Dynamic clutching is that during which a clutch brings a stationary output to the input rpm by assuming the slower axis’ kinetic-energy delta. Static-torque ratings (values that describe clutch and brake behavior when the units aren’t absorbing any kinetic energy) depend largely on torsional load. Reaching static torque operation with a clutch needs a clutch that must engage prior to rotating the input — so the clutch effectively functions as a coupling between in and output. In contrast, reaching static torque operation with a brake just takes holding the output element stationary. Static torque equals clutch holding torque when there’s no relative shaft rpm difference between input and output — or (in the case of a brake) when the shaft is stopped. The point at which a system exceeds a unit’s static torque is aptly called the breakaway torque. This is what a machine assembly must reach before relative motion arises between the shafts (in the case of a clutch) or before the shaft starts turning (in the case of brake). Dynamic torque depends on the rpm delta between in and output (for a clutch) and operating and zero rpm for a brake. Dynamic torque is usually about 50 to 80% of static torque. In friction-based designs, this value depends on the contact surfacres’ friction coefficient. Because that changes slightly (with the portion of
EXAMPLE DYNAMIC TORQUE CURVES FOR CLUTCHES AND BRAKES
TORQUE (LB-IN.)
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Brakes are shaft-to-shaft designs and come with myriad mounting variations to allow attachment of a fixed brake housing to a fixed machine frame. Common clutches come in pulley and shaft-to-shaft variations. For the latter, the clutch output and input are inline. Designers sometimes boost dynamic torque rating (and shorten response time) by selecting larger brakes and clutches — though keep in mind that this also increases torque and shear forces (due to torsion) on mounts to nonrotating machine-frame segments.
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CLUTCHES & BRAKES
operation the brake or clutch is delivering) designers usually employ an average coefficient for design calculations. It’s usually best to size brakes and clutches to the machine axis’ motor torque. But for axes where the brake must stop vertical loads, engineers must account for how motors can temporarily draw higher current to output more than rated torque. Refer to published performance curves from brake and clutch manufacturers to get dynamic torque ratings for operating-speed ranges to properly match brakes and clutches to the axis’ motor output torque. DESIGNING A DYNAMIC CLUTCH OR BRAKE APPLICATION Recall that once there’s relative motion in the assembly, the brake or clutch torque capacity falls. Ultimately the value — called dynamic torque — depends on the relative speed difference. An electric motor driving the load partially dictates clutch or brake size. Axes needing dynamic clutching or braking have a required torque defined by motor horsepower (hp) and output axis’ operational speed. Dynamic torque in lb-ft = (5,250· hp/ rpm) · safety factor. Here rpm = Rotational shaft speed the brake must stop (or the speed difference between a clutch output and input). The safety factor adjusts for the motor type and its typical torsional output during operation. Electric motors have a safety factor near unity, as their output is quite consistent. Notice a major consideration is time needed to change the axis speed. Another parameter is the duty cycle: If there are more than a few incidences of engagement per hour, duty cycle becomes an influencing design factor. We’ll cover this more in a bit, but note that high-cycle applications running to 300 cycles per minute (as in indexing with a clutch brake) each clutch engagement transfers a torque spike to the connection between the motor shaft and input shaft of the clutch brake. High-cycle applications can cause severe hammering on the shaft connection of the motor to the clutch brake. For convenience and ease of assembly, standard C-face connections are most common — but the loose fit can cause torque to transfer through the key and keyway, which hastens failure. Here, clutches and brakes that connect with shrink-fit or clamping couplings for a 360° connection perform better. Note that the dynamic-torque calculation presented above is based on time to stop the load or bring it to speed — not accounting for time needed for clutch or brake actuation. Consider clutch brakes that are electromagnetically engaged: Here a coil must build an electromagnetic field for armature activation and friction-disc engagement — a process that can take 10 to 500 msec depending on unit size.
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clutches & brakes — Motion System Handbook 8-17 V2.LE.indd 51
Cycle rate is how many times per minute a brake or clutch must engage — and the amount of energy a brake dissipates or a clutch absorbs before the next cycle. Except for regenerative setups, energy dissipates as heat — with heat-load maximums published by manufacturers. Note that applications that run fast or hot or are extreme in some other way need extra evaluation. Some tips: Machinery runing beyond a few hundred cycles per minute may need clutches and brakes that are larger than performance curves indicate. Spring-set brakes benefit motion designs that slow loads with the motor before the brake engages; they’re also suitable as holding mechanisms. Brakes using permanent magnets automatically adjust for wear. Industrial designs operating in hot environments may benefit from friction brakes with specialty friction-surface geometries and phenolic resin or ceramic materials. Motion applications that won’t see a lot of maintenance also benefit from longer-life friction materials.
This mayr EAS-reverse is a disengaging torque limiter that withstands high loads. It also reengages automatically when the axis slowly reverses. That’s helpful on drives that don’t allow access for re-engagement.
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controllers: MOTION, SERVO, AND PLC DESIGNS
Motion
controllers in feedback-based designs take an input command, compare it with a feedback signal at the motor, and trigger corrective action to make the output (or actual position) and input (target position) match — ideally, with minimal error. Motion controllers also calculate trajectories for the machine axes’ motors to follow to meet the target commands. These trajectories form motion profiles, which are sequences of position commands (expressed as functions of time) that tell the motors where to position the load and how fast to do it. Common motion profiles are trapezoidal, ramp, triangular and complex polynomial profiles. Each of them satisfies certain motion conditions and tasks. For instance, a trapezoidal profile (with a velocity-versus-time profile in the shape of a trapezoid) delivers constant acceleration, then velocity, then deceleration. Such actions need copious signal processing, so motion controllers typically use digital signal processors (DSPs) to perform the mathematical operations. DSPs handle algorithmic processing better than standard microcontrollers incapable of heavy mathematical processing. To get the target motion profile out of the physical machine, the controller needs to track the profile and (in most cases) reject disturbances. Controllers track commands by commanding motor drives to follow positions, velocities, torques (or forces) and accelerations. Called feedforward control, this compensation relies on accurate machine and motor models. In contrast, disturbancerejecting controls are more active, fixing output to correct for problems with sudden or unexpected loads on the machine or inaccurate feedforward models. The simplest of these is proportional (P) control for constant integer gain. Then programmers can add integral gain (I) and in some cases either derivative (D) gain or velocity (V) gain to make PID or PIV loops to actively reject errors. The integral value integrates error over time and helps to drive it to zero. The derivative value helps to stabilize the P-I system. PID loops are particularly common and powerful algorithms to help machines track commanded trajectories. They work on the error signal (the difference between a commanded value and actual value of an output) and attempt to drive the error to zero while maintaining machine stability.
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IDEC Corp. with Advanced Micro Controls Inc. (AMCI) now offers a line of motion-control stepper products — including controllers, stepper motors, an integrated controller-drive, an integrated drive-stepper motor, and an integrated controller-drive-stepper motor. When combined with an IDEC FC6A PLC and its embedded motioncontrol macro instructions, these simplify quick implementation of single and multi-axis motion.
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8/18/17 7:26 AM
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RJ45 RS232C/RS485 Port RJ45 Ethernet Port
• Modbus TCP and RTU • CAN J1939 CPU
IDEC’s new FC6A MicroSmart PLC provides the power of a PAC in a small form factor, making it an ideal fit for your most demanding applications. The MicroSmart features the industry’s leading price/performance ratio with the capacity to handle 520 I/O including up to 126 analog I/O. 640kB of program memory handles your toughest control applications, and programming is quick and easy with IDEC’s WindLDR PC-based software. Remote access is a snap by creating custom web pages configured for remote monitoring and control using simple drag and drop functionality with no HTML programming required, a feature unique in this class of PLCs. The FC6A MicroSmart provides competitive advantages via simplified programming and web page creation, more cost effective end solutions for your customers, and ease of maintenance via remote access—a set of features unmatched by any other micro PLC. Find out more at: http://FC6A.IDEC.com
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HANDBOOK
MOTION SYSTEM
controllers: FAQ: HOW TO TUNE SERVO VELOCITY-CONTROL LOOPS?
Servo systems typically use a combination of three types of control loops — current, velocity, and position. The velocity control loop lets the system respond to changing velocity commands, and it helps the system resist high-frequency load disturbances. But the velocity loop alone can’t ensure that the system maintains position over long time spans. This is why velocity control loops are often used in conjunction with positioncontrol loops in a cascaded structure. The velocity controller is typically a PI controller using the proportional Kvp and integral Kvi gains; the position controller typically uses only the proportional gain Kp. Current control is almost always set automatically (not manually) but for the sake of understanding let’s review how it works. Recall that proportional gain has a value directly proportional to error; relates to system stiffness; and determines voltage applied to overcome that error. Integral gain accumulates error over time and provides restoring force during move ends to push the system to a point of zero error. In cascaded control loops, the bandwidth of the inner loop should be 5 to 10 times the bandwidth of the outer loop — or else the inner loop will have little effect on the outer. Best practice is to tune the fastest loop first, so in the cascaded position-velocity loops, we start with the (inner) velocity loop. Because the velocity loop is a PI controller, there are only Kvp and Kvi to tune. For the move profile, use a medium-value velocity target (not the lowest or the highest that the application will experience) but keep acceleration rate relatively high. This magnifies the effects of system response, making it easier to tune the system. To begin the tuning sequence, increase velocity proportional gain Kvp to a point just before overshoot occurs. It is sometimes helpful to increase Kvp until overshoot and then back it down until overshoot is eliminated. Now increase the velocity integral gain Kvi until some overshoot (5 to 15%) occurs.
In cascaded control loops, the inner-loop performance affects the performance of the next outer loop. So the inner loop (velocity) acts as a low-pass filter on the outer loop (position). Once the velocity control loop is tuned, the controller should be set to operate in position mode, and the position loop can be tuned. Begin with a low value for the position gain Kp and raise it slowly to a point just before overshoot occurs. With velocity and position gains set and stabilized, increase the target velocity and test the gains again. Adjust them to make the gains suitable across the design’s full velocity range. Note that the output of the position loop is a velocity command. Here’s how it works: The position error detected by the position loop is scaled by position gain Kp to generate a velocity command ... in turn is sent to the velocity loop, which uses it to command more torque ... which the moves the motor to correct the position error. Feed-forward control is for when the following error must be minimized, although it sacrifices system stiffness. If the application requires very low following error, it may need feed-forward control in the tuning. Velocity feed-forward control Vff takes a proactive approach, calculating the velocity needed to meet the desired position and feeding this value directly to the velocity control loop, rather than allowing the control loop to wait for the position error to develop. However, the drawback of using velocity feed-forward is that the position loop proportional gain Kp must be lowered, which reduces system stiffness.
Galil Motion Control Inc. now offers the Galil Design Kit (GDK) — software that facilitates easy communication with Galil motion controllers and PLCs. The GDK includes a Scope (to emulate a traditional digital oscilloscope); Tuner (to optimize system performance); Terminal (to send commands and receive responses from the controller; Editor (to write, save, and execute programs); Viewer (for realtime viewing of the controller’s I/O status, motor position, and more); and Setup (to read, edit, save, and restore all controller memory). All the tools are customizable by editing the XLM.
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8/18/17 7:28 AM
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HANDBOOK
MOTION SYSTEM
controllers: COMPARISON OF OPTIONS FOR MOTION DESIGNS
Modern stages use multiple drives to deliver complex motion sequences — including multi-axis and interdependent motion. Now, Bosch Rexroth SYB 3.0 stage-collision modules keep axes safe from harm and harming. An anticollision system anticipates potential collisions to prevent motion (or halt motion in progress). In fact, the module prevents collisions between drives and fixed structural components in compliance with Safety Integrity Level 3. An anti-collision assistant (to support technical personnel) is much like a modern automobile’s parallel-parking assistant. During programming or in realtime, the assistant visually and audibly warns personnel of potential collisions, with stronger warnings for higher speeds and closer distances.
Electronic controls were first designed about 70 years ago for computer numerically controlled (CNC) machine tools. By the early 1950s, such controls were standard for manufacturing. Then with the advent of transistor logic, solid-state memory, and the ability to program with CRT workstations, all the pieces were available for the creation of programmable controllers in 1968. The Third Industrial Revolution — automation — was about to begin. At about the same time came distributed control systems (DCSs) with unique control requirements for many individual analog controllers employing PID algorithms and sequential control. DCSs or SCADA systems ran chemical processes, nuclear power plants and steel mills. Here, with an array of minicomputers and engineered communications networks, the Texaco Port Arthur petrochemical refinery installed the first modern DCS in 1959. Honeywell launched its DCS product for process automation in the early 1960s. These CNC, PLC and DCS designs were separate disciplines with distinct modes of application modeling. So during this time, their fundamental difference necessitated creation of 56
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unique hardware and software. One controls’ subsystems were inapplicable to the others. Nowadays, industrial-control manufacturers benefit from computer processor power, memory and communications that offer almost boundless capabilities. It’s true that for those without direct experiences of historical control technologies, the existence of today’s disparate range of programming and hardware is confusing. But in reality, any of today’s processors (including those in PC-based controls) are capable of performing CNC, motion, PLC or DCS functions. They are different families of control based on their unique programming and tasking. So it may appear as a distinction without difference, but the difference is there when it comes to the application. DIFFERENCE BETWEEN STANDALONE CONTROLLERS, PC-BASED CONTROLLERS, AND MICROCONTROLLERS Today’s standalone controllers are complete systems (including all electronics, power supplies and external connections) that mount in one physical enclosure. These controllers go into machines to command one application consisting of a single-motion axis or multiple-motion axes. PC-based controllers include a basic personal-computer motherboard or ruggedized industrial PC, as well as PCtype hardware components and a high-speed dedicated bus that transmits information to and from the processor. The latter exists because using a PC for control requires the same inputs and outputs as a basic PC … as well as interfaces to factory-floor devices. Here, typical I/O includes the electric motors and fluid-power devices for actuation, motioncontroltips.com | designworldonline.com
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as well as discrete sensors, pushbuttons, signaling lights and mechanical switches for feedback. One key advantage of PCbased controllers is that they provide a ready-made graphical user interface for easier programming and tuning. PC-based controller software includes some operating system to manage internal processing and resources. OS instability issues are a thing of the past thanks to proliferating realtime operating systems, professional grades of Windows, and Linux application software to trigger the target behavior in a machine or process. Control programming languages include computerscience offerings (such as C+ or Visual Basic) to more application-specific languages — including IEC-61131recognized Ladder Diagram, Instruction List, Function Block Diagram, Structured Texas and Sequential Function Chart languages. Lastly, there are microcontrollers. These are individual integrated chips (ICs) that manufacturers usually design on printed circuit boards with I/O for feedback and driving connected motors. While these controllers are relatively inexpensive and have the advantage of giving designers chip-level access to their systems, the drawback is that some require good programming skills to configure and implement.
Programmable logic controllers (or PLCs) are specialized microprocessor-based controllers to command specific machine or process tasks. They work in automation and manufacturing to control assembly lines and factory-floor machinery as well as mechanical, electrical and electronic equipment in industrial applications. Design engineers have used PLCs for automation since the 1960s, when they began to replace physical relays setup through ladder logic. Basic PLC hardware includes the processor, I/O modules (to handle inputs to the processor and outputs to controlled devices), and a user interface. The latter can be anything from a simple keypad or a touchscreen to an Ethernet connection to a PC for more complex programming. No matter its form, users program the PLC through the user interface. I/O modules bring input signals to the PLC’s CPU and output control signals to devices such as electric motors, sensors, and fluid-power valves and actuators. One key metric of PLC performance is scan time—the time the PLC takes to run through
THE C6 E22 BM IS AN INDUSTRIAL PC (IPC) THAT INSTALLS INTO CONTROL CABINETS. C6 E22 BM (BOOK-MOUNT)
DVI / USB C6 MONITOR ETHERCAT
H6 MULTI AXIS
PM MOTORS
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The KEB America COMBICONTROL C6 E22 industrial PC (IPC) has flexible hardware — so comes in panel, box, and book-mount designs. Shown here is a photo of the panel version and an illustration of a typical installation for a book-mount variation. All C6 E22s have two memories. One is specially protected on the software side for the OS and the runtime components of the KEB automation platform. The other memory contains customer data for control, visualization, and other data relating to automation. With an EtherCAT Master, the C6 E22 can perform conventional SPS and synchronous motion-control tasks.
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8/21/17 8:11 AM
Galil Motion Control
ALL THE RIGHT MOVES...
...for Ultimate Precision Galil Motion Controllers handle virtually any application Between our flexible product offering and our affordable custom solutions, we can accomodate all of your motion and I/O needs. Select one through eight axes. Choose internal multi-axis servo or stepper drives. Increase the number of axes and I/O by daisy-chaining through built-in Ethernet ports. Guaranteed low price per axis with driver included. Galil products are easy to program and use, backed by unparalleled technical support and both ISO 9000 and ISO 13485 certification.
• Process commands in as little as 31 microseconds • Handle all modes of motion, simple to complex • Handle ultra-high resolution systems • Support closed loop stepper motors • Up and running in minutes • Use virtually any encoder • EtherCAT master available
galil.com 1.800.377.6329 Galil 1-17.indd 59
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HANDBOOK
MOTION SYSTEM
Most PC-based controllers from Beckhoff include reliable and common Intel processors; some entry-level controllers integrate ARM-based processors. Core isolation in PC-based controls lets users assign specific tasks to each core within the CPU as a way to optimize processing capabilities.
the program to collect data and update outputs. Scan time usually takes only a few milliseconds, but can take much longer if the program is long or the processor is slow. Faster scan times accommodate processes with more realtime demands. In contrast, fast scans aren’t necessary for traditional applications that run more slowly. REALTIME PERFORMANCE IN PC-BASED CONTROL: AN UPDATE System performance hangs heavy on the minds of every machine builder when commissioning new projects — particularly when it comes to the control platform. That’s with good reason, as the controller is arguably the most important facet of a machine … and often the first place people look when fighting bottlenecks and performance issues. Conventional PLCs have traditionally been the go-to option for engineers, as these devices provide hardened realtime control. But they also come with fairly steep replacement costs, as PLCs often implement proprietary operating systems and software; some also limit flexibility or scalability from platform to platform. So some engineers have begun applying other options for realtime performance in machine control systems. One option is PC-based control — increasingly common thanks in part to its expansive feature set and cost effectiveness. Two major benefits of modern PC-based systems are multi-core processor architecture and the core isolation that PC-based automation software enables. The latter lets designers assign specific control tasks to each core within the CPU as a way to optimize system processing capabilities 60
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while minimizing the times the system runs at maximum capacity. This in turn extends controller life and boosts efficiency, as the processor runs more coolly (thanks to a lower CPU burden). What’s more, PC-based control systems leverage the newest and fastest processors. Case in point: Some advanced PC-based controllers incorporate reliable Intel processors; other entry-level controllers integrate ARM-based processors. That ensures consistent industry-leading performance but at the lowest possible cost for the hardware, software, networking, and total cost of ownership. Other benefits of PC-based control are inherent scalability and flexible migration paths. Should an operation outgrow the performance level or scale of its current control system, migration to a larger system is simple. That’s especially true for PC-based controllers using a Windows OS and automation software to establish realtime control. This lets the PC operate deterministically and in realtime. Plus processor end-of-life doesn’t make system architecture or software obsolete. By incorporating more functionality into software and running that on a standard Industrial PCs, users can simply replace aging equipment with a new IPC without changing to the balance of the control system — especially
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8/18/17 7:33 AM
C6 SMART
USB ETHERCAT COMMUNICATION BUS ETHERNET
ETHERCAT MASTER SERIAL/CAN
The new generation of embedded control from KEB Machine control is easier with the C6 Smart EtherCAT embedded control from KEB. • • • • • • •
Compact DIN rail embedded uses less cabinet space Dual Core ARM Cortex A9 processor 32-bit processor for synchronized motion and camming profiles Integrated EtherCAT E-bus for expandable KEB I/O Standard integrated micro UPS for critical applications Secure remote maintenance features included standard Programs with KEB’s powerful COMBIVIS programming suite (IEC 61131-3 languages supported)
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KEB -- Motion Systems Handbook 8.17.indd 61
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What’s Next? RMC75 1 or 2-Axis
RMC150 Up to 8-Axis
RMC200 Up to 32-Axis
PC-based controllers abound, and many include powerhouse multi-core offerings. Case in point: Some from Beckhoff offer up to 36 processor cores and 2 TB of RAM. Complementing this hardware is TwinCAT software that runs as a realtime extension to the Windows kernel. In fact, TwinCAT software regulates how system resources are allocated. Normally the Windows OS has full control of the CPU, but if TwinCAT is installed (and its realtime kernel is running) it assumes control and allocates space within each tick for TwinCAT operations before handing control back to Windows. Kernel level PLC or C++ code deploy to a larger system via standard storage media; new systems can be setup and run in minutes.
Introducing the new RMC200 Delta’s most capable electrohydraulic motion controller. The RMC200 controls up to 32 axes, and our familiar RMCTools software makes it easy to synchronize every one of them — from simple single-loop position control to complex dual-loop control of positionpressure/force. Make ‘What’s Next’ happen for you. Just call 1-360-2548688 or visit deltamotion.com RMC200 module with door open to the right displaying labels. The terminal blocks feature push-in wiring and latch ejectors to speed wiring and simplify maintenance.
the software. In short, PC-based control systems deliver realtime performance to supplant traditional PLCs. They also enable advanced features and streamline optimization and scalability. || Information on realtime performance from PCbased control is from Eric Reiner, Industrial PC Product Specialist at Beckhoff Automation. COMPLETE CONTROL WITH TODAY’S PACs Programmable automation controllers (PACs) excel in commanding complex automation setups that involve PC-based and HMI functions as well as process control (largely because of the way they handle I/O). PACs are also increasingly common for motion applications in machining or handling discrete product thanks to the flexibility and interoperability they offer machine designs. As mentined, today’s PACs evolved as an option for complex control when microprocessors with significantly more performance became affordable and commonly available. PACs differ from the still-dominant form of control for motion — the programmable logic controller (PLC) — in that all PACs can perform as PLCs but not vice versa. That’s because PACs serve multiple channels of communication; high-data traffic; and coordination with intelligent subsystems. Today’s PLCs are still more appropriate than PACs in standalone applications such as machine axes that run preset sequences. The rule of thumb is that anywhere PAC functions would otherwise go unused, it still makes sense to use the a more economical PLC. Pressure from plant personnel and the enduring value of ladder logic also make PLCs the first choice in many applications.
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CONTROLLERS
But once PLC behavior was proven reliable on a computer, PACs took hold. The aerospace and medical industries are two driving industries here. The FAA and FDA mandate that day, date, and time-tagged data about manufacturing processes are stored for extended periods of time — particularly well run on PACs. Even manufacturers of simple consumer products are finding such information necessary for defending designs in product-liability lawsuits. What’s more, it’s not just product-data logging that leverages the data-tracking functions of PACs; running predictive maintenance and operations monitoring uses data from controls, too. That necessitates more data and complex network interactions — which means PACs will only become increasingly common. Just as PLCs, the controls known as distributed control systems (DCSs), PCs, and remote terminal units (RTUs) include hardware and programming to satisfy specific applications. PAC hardware can run functions as software to replicate legacy forms of these and other pieces of motion-system hardware. DESIGNS WHERE PACs MAKE SENSE PACs handle high bandwidth on the backplane, so in complex applications can share motion data with multiple modules. That boosts performance. High-bandwidth backplanes also makes for more powerful modules ... so depending on the processor core, a motion module here for example may be capable of complex axis-to-axis coordination advanced enough for advanced motion in machine-tool or robotic applications. No matter the permutation, multi-axis machine controllers include hardware and software in a structure organized around the management of complex motion. Consider the task of tracing a circle with two linear axes. Such coordination necessitates fast axis-position updates and comparisons to predicted position — along with error correction to the speed command of each axis-driving motor. This is a very demanding realtime task. The speed of the processor’s execution must be many times faster than the speed of the motor feedback. Given the computing capabilities of today’s processors, PACs (and PC-based controllers) are primed to handle such multi-axis motion control. It’s true that each platform does this in its own way — and it’s common to have myriad independent motion axes in a machine with each operating independently off a start-signal input. But full multi-axis motion usually works to coordinate multiple axes through mathematical definitions. Trends towards standardization continues to address old control issues. PAC vendors now produce general-purpose
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control hardware that integrates into IT networks but is rugged enough for industrial settings — and modular like the PLC form factor. This necessitates relatively advanced controller hardware so the PAC can function in any control application. It also means that PACs are equally capable of managing large data-gathering networks, or running machine control logic, or doing both simultaneously. So when it comes to running multi-axis motion control, today’s computing power delivers. Prompting more standardization on the software side is IEC61131, which dictates the form of much of today’s ladder-logic programming and (in versions 3 and 4) motion extensions for motion programming in the ladder environment. Depending on the motion job, the ladder approach may be fine … but it’s not usually multi-axis. In addition, motion-controller inputs and outputs from homing and limit switches and encoders are high-level inputs that must be dedicated to a specific axis on which they’re setup to work. Sometimes PACs need add-on modules to execute special functions. Remember that PACs are architected mostly as PLCs, but the major difference (from an electronics standpoint) is how PACs have a high-bandwidth backplane that allows integration of various architectures; and allows additional processors to boost the functionality of the primary PAC. So software in the main PAC can manage intelligent peripherals, but it’s often useful add devices to the system architecture when an application needs control functions that exceed the main processor’s capabilities. A common add-on is remote I/O racks using standard offerings implemented with multi-megahertz Ethernet as the communications interface. These setups often use an Ethernet gateway module in a local rack with the main controller … which in turn supports the Ethernet link to the remote I/O rack. Ethernet updates faster than older PLC backplanes ... though in a PAC with high bandwidth, data rates aren’t an issue. Consider the special cases of testing and machine vision. Advanced test setups needing high-speed data acquisition often include timing and triggering functions on their PXI backplanes. Adding a PXI bridge module (or PCI or VME) to a PAC makes the technology available on other bus architectures in the PAC environment. Most machine vision still uses standalone control because the processing burden and speed of vision demand it. But PAC suppliers continue to expand the repertoire of control capabilities by offering high-speed communications bridges to other subsystems. So machine-area networks with machine vision can be demanding — and yet the high-speed communications supported by PACs are an industry-accepted solution.
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Conveying designs FOR NEW MODES OF DISCRETE MATERIAL TRANSPORT
Industry
sometimes classifies synchronous conveyors as those that consistently index workpieces from workstation to workstation. In contrast, asynchronous conveyors are those that make moves as needed to advance workpieces or products in independent transport tasks. Synchronous-conveyor installations run along fixed paths at set speeds — often quick and automated arrangements. The slowest tasks tend to define overall cycle rate. In contrast, asynchronous conveyors service one or just a few workpieces or products at a time over flexible paths to any number of destinations or staging
areas. Usually these conveyors are setup to optimize use of automated machinery and minimize manual workers’ waiting for new product to assemble or otherwise service. Reconsider the special case of synchronous conveyors. These are common for packaging of bottles, boxes, and wrapped bags that ride directly on the conveyor strand. Guides on the conveyor sides as well as friction pads keep discrete items from sliding or going off course. Contrast this with some features often (though not always) more typical of asynchronous conveyors.
iHerb’s newest distribution center relies on the latest conveyor technologies. It uses automated goodsto-person picking for faster order fulfillment and dense product storage. Rockwell Automation OEM Partner Bastian Solutions helped design the new Hebron, Ky. facility to quicken accurate but sometimes labor-intensive processes. Image courtesy Bastian Solutions and iHerb
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CONVEYORS
Here, some designs make discrete products sit in carriers or pallets that go through standardized lift, diversion, rotation, and rerouting stations. Units or other items such as pallets in these kinds of setups often sport datatags for workstation sensors to read. Such connectivity for data collection lets the whole installation more intelligently route products — even letting plant endusers produce or otherwise transport multiple items via one centralized conveyor network. In fact, general motion components as well as dedicated conveyor designs are spurring new levels of material-transport flexibility only recently thought impossible. Consider the proliferating designs of AS/RS installations as well as self-contained vertical transfer units. The latter might make use of spiraled elevators or (increasingly common) elevator units that employ linear actuators for pushing, hoisting, and workpiece (or product or bin) unloading operations. Some variations even employ linear guides on the hoists to steady loads within the elevator cage — especially where the products being lifted are of standard size or collected in bins or pallets. Elsewhere on asynchronous conveyors, motion lifts (usually in the form of pneumatics) can temporarily raise pallets holding workpieces just a bit above the conveyor below; work is then done on the product while the conveyor is protected from damage. Next the workpiece is lowered back onto the conveyor to continue. Alternatively, other relatively new conveyor designs omit such motion lifts and include structural reinforcements to allow assembly and other tasks to occur right on the conveyor surface.
DESIGN WORLD — MOTION
See these conveyors in action! Booth
BACK TO BASICS ON CONVEYORS FOR PRODUCT TRANSPORT Conveyors move bulk material or discrete products from one area to another, and serve as main material-handling arteries to improve efficiency and throughput.
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TRANSFORMING CONVEYOR AUTOMATION
#C-1444
800.397.8664 I www.dornerconveyors.com/dwm 65 Dorner_DesignWrldAd_717.indd 1
conveyors — Motion System Handbook 8-17 V3.indd 65
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MOTION SYSTEM
66
Advances in materials, controls and modular subcomponents have spurred new large conveyors for bulk material transport, miniature conveyors for discrete sorting, and everything in between. During manufacture, myriad products move on conveyors. So conveyors come in an array of shapes and widths of less than 2 in. (for moving extremely small parts) to several feet wide. Once viewed as an afterthought, conveyors have become an integral component in nearly all automated facilities and applications. No matter the conveyor subtype, all conveyors include a carrying medium of steel or plastic belt, fabric belt, chain, or roller series on a steel or aluminum frame. The drive (usually an electric-motor design) advances the substrate. Usually conveyors include legs and framework that sits on the factory floor; but some (including many elevated conveyors) suspend from the ceiling. Of course, the most significant subcomponent is the conveyor’s belt or chain. Optimized setups maximize load carrying and speed to even allow use of a smaller motor drive in some cases — and trim design cost in the process. Select a conveyor by first asking: What types of product is the application moving? Conveyors for material handling of bulk product are more rugged than those for moving discrete product. In contrast, the latter often requires conveyors that can advance product with more precision. How does surrounding equipment interact with the product riding on the conveyor? Conveyor Class 1 includes material-handling uses in which the conveyor serves as an artery to transport bulk or discrete product in a steady stream (with little interaction along the way). Class 2 includes conveyors that act as bridges to take product from one location or machine to another. Class 3 includes conveyors that take materials into or out of machines or stations. Class 4 includes conveyors that run right through machinery without break. The first two classes generally prioritize ruggedness or throughput. The last two classes need positioning and (in many cases) custom workpiece pucks to steady product while machines perform work on the product pieces. What is the maximum weight of the product being moved? Does the conveyor need to operate at a certain speed? Does the application need the conveyor system to have inclines, declines or curves? Look for conveyor features that secure or enclose material or product onto the conveyor. Will moisture be present in the application? Does the application need to be sanitary? Look for rugged or washdown-rated conveyors with open frames. Most conveyors in light to medium-duty discrete-transport applications use belt that’s wrapped around two or more pulleys. A motor powers the pulleys that in turn engage the conveyor belt. Styles and materials abound to meet specific applications. Some belts are low friction, so product can slide a bit for accumulation. In contrast, high-friction belts have more grip to better hold products to the belt. Engineers can design such conveyors to meet exact application specifications. Magnetic conveyors are built with ceramic magnets for applications that need parts to adhere to the belt during processing, or for jobs that need elevation changes. The designer can specify higher magnet strength for use in inverted applications. In contrast, metal-free conveyors have Delrin bedplates (instead of the traditional steel bedplate) under sections where metal-scanning equipment checks product—usually food—for metal shavings. (Delrin is an inflexible polymer that works as a tough, heat-resistant metal substitute.) This lets a device check passing product without getting false readings. DESIGN WORLD — MOTION
conveyors — Motion System Handbook 8-17 V3.indd 66
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8/18/17 8:54 AM
CONVEYORS
conveyors — Motion System Handbook 8-17 V3.indd 67
This is an mk North America KMF-P 2040 curved plastic modular belt conveyor. It’s suitable for maintaining product orientation.
HEAVY-DUTY CONVEYOR PULLEYS Drum Pulleys Wing Pulleys Clean Flight® Wing Pulley Engineering Class Pulleys CEMA C, D, E IDLERS Troughing Idlers Impact Idlers Return Rolls Guide Rollers Live Shaft Idlers Offset Idlers Rubber Disc Idlers Self-Aligning Idlers V-Returns
ma r 81 tinsp 7.2 ro 58 ck .30 et. 00 com
He
av y
-D Pu Co uty lle ys nvey or & Idl ers
Pivot conveyors mount to a pivot base to swing out of the way when workers need to walk through the line. Interlock switches and a timer let the conveyor clear before the gate opens. Some controls can automatically resume product flow after the conveyor returns to the inline position. Servo drives accurately start and stop belt conveyors to provide precise part location. They also let engineers control acceleration and deceleration, so are most suitable for conveyors used in assembly operations. Manufacturers mount encoders to a conveyor’s drive shaft to sense shaft rotation or count pulley revolutions for accurate control of the belt in feeding or indexing applications. One caveat here: Optimized conveyor designs consistently deliver workpieces and products. That demands drives that minimize shock loading and jerking — whether powered drives to steadily advance the actual conveyor belt or chain or VFD-powered motor rollers to enable faster or slower conveyor speed as the application
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HANDBOOK
Conveyors that provide basic machine input and output can be fairly simple.
MOTION SYSTEM
Conveyors that move product through a standalone machine can also use one-way conveyors with the most basic encoder feedback.
Conveyors that move product from one machine to another in a continuous manufacturing line must be precision setups — often with a servodrive and motor running a toothed pulley to (positively) engage a belt.
When a conveyor is a manufacturing line’s main artery, it needs a precision setup as well. Software setup helps facilitate drive sizing to accomodate distance moved, load, and startup inertia.
Conveyors often interact directly with machinery. Here are three variations on those setups.
better products. Aluminum Conveyors
Pallet Conveyor Systems
www.mknorthamerica.com
Stainless Steel Conveyors
Aluminum Framing
better solutions. conveyors — Motion System Handbook 8-17 V3.indd 68
demands. In fact, the latter is paramount where manufacturing plants or distribution warehouses employ conveyors for buffering between stations — to match piece flow through a facility to changing levels of machine or employee output. Traditional conveyors advance product along flat stretches of a single elevation, but new designs maximize the use of space (while boosting throughput) by holding fast to workpieces that dip to the underside of the conveyor; make turns through plants; and lift and lower workpieces and products to different conveyor elevations. Single-drive, multi-belt conveyors serve two or more lanes of product for the sake of efficiency. Here, two or more conveyors run off a single gearmotor on a common drive shaft or coupled shafts. In some arrangements, the belts even mount to a single conveyor frame. Timing-belt conveyors use toothed belts that engage synchronous drive pulleys while serving as the conveyor surface as well. These provide excellent belt-movement control for accurate part or fixture positioning. Vacuum conveyors work with a perforated belt that draws air through grooves in the conveyor bedplate to hold light or flimsy parts on inclines or during especially fast transport. 68
DESIGN WORLD — MOTION
8 • 2017
8/18/17 8:56 AM
Closed Loop Drives and Motors for EtherCAT® Networks
StepSERVOTM Drives and Motors Greater torque, higher acceleration Better throughput in point-to-point motion Higher efficiency, quieter operation EtherCAT networking with CANopen CiA 402 operating modes
StepSERVOTM
Connected. Customized. Closed Loop. StepSERVOTM technology from Applied Motion Products combines the high torque capabilities of step motors with the closed loop dynamic control of servo systems. The result is a motor and drive solution that operates more quietly and efficiently, while providing greater torque and higher acceleration rates than traditional step motor systems.
SS-EC StepSERVOTM Drives and Motors support CANopen over EtherCAT (CoE) control, allowing them to be integrated into new and existing EtherCAT networks. By combining the high-speed networking capabilities of EtherCAT with the motion control performance of StepSERVO, Applied Motion offers yet another feature-packed series of motors and drives designed to help you Make it Move.
800-525-1609 www.Applied-Motion.com Email: sales@applied-motion.com EtherCAT® is a registered trademark and patented technology, licensed by Beckhoff Automation GmbH, Germany
applied motion -- motion control handbook 4.17.indd 69
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MOTION SYSTEM
HANDBOOK
These metal disc-spring couplings from Miki Pulley have ultra-high torsional stiffness to satisfy servomotor applications. Disc-spring designs also impart flexibility in axial, bending, and step-difference directions.
flexible-coupling UPDATE ON
CONSIDERATIONS FOR OEM AND PLANT DESIGNS
Couplings
connect rotating shafts in equipment powered by electric motors and other drives. All transmit torque and angular velocity. Flexible variations compensate for misalignment. Many of the latter even address vibration and improve system dynamics. Design considerations include machine or installation construction and backlash, torsional stiffness, damping, inertia, torque ratings, maximum rpm, size, misalignments, ease of installation, robustness, and cost. For power transmission (as in motors for pumps and large material-handling setups) common choices are gear, disc, elastomeric tire, grid, jaw, and Oldham couplings because of their ruggedness and ability to transmit very large torques. Motion-control applications (as for axes employed in precise positioning of loads, for example) typically employ couplings capable of oft-more modest but far more precise torque transmission. These include curved-jaw, beam (slit), bellows, disc, and other zerobacklash couplings.
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Couplings — Motion System Handbook 8-17 V3.indd 70
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As we’ll explore, any misalignment that couplings accommodate should be what’s otherwise unavoidable even after proper machine-axis squaring and installation adjustments. That’s because misalignment — manifest as parallel, axial, and angular misalignment— degrades efficiency, induces bearing wear, and excites machine natural frequencies. To review, the maximum amount of angular misalignment for which a coupling can compensate is expressed in degrees; parallel misalignment between the shafts a coupling connects is expressed in inches or millimeters. Axial misalignment is also a length value; it’s the maximum permissible spread between coupled shafts — and in fact, a misalignment permutation often most affected by thermal effects. Flexible couplings for motion control are often less forgiving of misalignment than those for more straightforward power transmission, and resolve it with specialty design features. A related phenomenon and a coupling consideration specific to motion-control installations is backlash. In applications for strict power transmission, backlash is far less of a concern than that of efficient torque transmission motioncontroltips.com | designworldonline.com
8/16/17 11:18 AM
THE COUPLING. ABSOLUTE PRECISION 0.1-100,000 NM.
WWW.RW-AMERICA.COM
R+W1 -- Motion Systems Handbook 8.17.indd 71
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TO 7,000 LB-IN.
MORE
ELASTOMERIC INSERT
BEAM STANDARD TO 260 LB-IN.
τ MAX
BEAM COUPLINGS AS THESE FROM HELICAL PRODUCTS CO. ARE SINGLE-PIECE DESIGNS WITH NO ASSEMBLY POINTS TO LOOSEN. NOTE THE ATTACHMENTS AND OTHER CUSTOM VARIATIONS.
RUBBER INSERT
THIS COUPLING DELIVERS TORSIONAL STIFFNESS PLUS DAMPS VIBRATION FOR EASIER SERVO TUNING. THE CENTER MOLDED PORTION IS HNBR. IMAGE COURTESY NBK AMERICA
BELLOWS τ MAX
STANDARD TO 900,000 LB-IN.
APPLICATION EXAMPLE COURTESY GAM ENTERPRISES
DISC ELEMENT τ MAX
STANDARD TO 1,600,000 LB-IN.
ROBA-DS DISC-PACK COUPLINGS HAVE SPLIT CLAMPING HUBS FOR EASY ASSEMBLY. MADE BY MAYR POWER TRANSMISSION, THE COUPLINGS DELIVER HIGH TORQUE.
MORE
Couplings — Motion System Handbook 8-17 V3.indd 72
8 • 2017
τ MAX
TYPICAL TORSIONAL STIFFNESS
DESIGN WORLD — MOTION
STANDARD
JAW (SPIDER)
ALLOWABLE MISALIGNMENT
— and actually a characteristic that (in normal moderate quantities) helps make some couplings in these settings more efficient and forgiving of misalignment. In contrast, couplings on the outputs of steppers and servomotors are designed to prevent the lost motion that can degrade outputproduct quality or overall machine throughput. Note there’s a difference between backlash (which is true mechanical clearance) and the torsional deflection or windup that all loaded rotary components exhibit. Most couplings for motion applications are inherently backlash free or preloaded to eliminate backlash — but they all have different torsional stiffnesses, which is sometimes a tradeoff for lateral flexibility. PITFALLS TO AVOID DURING SELECTION OF COUPLINGS FOR MOTION Design engineers often run into trouble when they neglect to account for environmental effects on couplings — particularly flexible couplings installed in gritty or caustic areas, vacuum environments, or places that are extremely hot or cold. Beyond that and the common design considerations already listed, designers must account for dynamic forces to which a coupling will be subject. Steer clear of using published an axis’ gearset or motor peaktorque values for setting its coupling’s nominal torque rating. That’s because this approach usually makes for an assembly with an oversized coupling and an unnecessary inertial increase. Designers should also avoid the application of a coupling type simply because it’s a familiar technology. For example, beam couplings are extremely well known in industry, and they excel on axes transmitting moderate to light torque — as on leadscrew-driven motorized axes or where there’s a need for attachment of a precision encoder, for example. However, some particularly demanding designs may necessitate a flexible coupling type that maintains higher torsional stiffness. On the other hand, it’s also unadvisable to simply pick a coupling based on high torsional stiffness. Many flexible couplings have an inherent stiffness that exceeds application requirements for servo tuning and motion accuracy. Even in motion designs requiring high stiffness for the shortest possible response time (as in equipment for electronics manufacturing, for example) couplings with good damping characteristics often offer
72
SOME JAW COUPLINGS ARE FOR SIMPLER POWER-TRANSMISSION DESIGNS; LOOK FOR CURVED-JAW AND ZERO-BACKLASH DESIGNATIONS FOR MOTION.
Shown here are some zerobacklash coupling options.
LESS
MOTION SYSTEM
HANDBOOK
FLEXIBLE COUPLINGS FOR MOTION
RIGID
RIGID COUPLINGS ONLY WORK ON EXCEPTIONALLY WELL-ALIGNED AXES.
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PRECISE. ROBUST. AVAILABLE. These new generation CD® Couplings feature zero backlash precision and high torsional stiffness. They answer today’s demanding needs in servo motor applications with high reverse loads and positioning requirements. New clamp style hubs handle increased torque on shafts without using keyways. Manufactured of RoHS compliant materials. Now size, select and see the right CD® Coupling solution for your coupling application with Zero-Max 3D CAD files. Check our FAST deliveries.
www.zero-max.com 800.533.1731
Zero-Max_CD Coupling Zero Max 1-17.indd 73 A1C r4 Design World.indd 1
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MOTION SYSTEM
74
more effective optimization than more torsional stiffness. (Refer to the section, “When motion designs need torsional stiffness — as well as damping” at the end of this section for information on this topic.) That’s because overly stiff couplings of many designs pose an unnecessary risk of fatigue. UNDERSTANDING TORSIONAL RIGIDITY: BELLOWSCOUPLING EXAMPLE Torsional rigidity is an object’s resistance to torsion or twisting under applied torque. Torsional rigidity in couplings is torque per value of angular displacement, and it’s a value that affects overall machine design. Even slight variations degrade positioning accuracy and limit cycle speeds. Consider the case of bellows couplings and the specifics of how their moderately high torsional stiffness is verified and expressed: Torsional stiffness Ct (in N·m/rad) = M/ѱ — where M = Torque, N·m and ѱ = Angular displacement, rad. Applied mass at an industry-set moment arm gives torque M = mgR, where m = mass, kg; R = Moment arm; g = Gravitational acceleration = 9.81 m/sec2 — and m = 0.38 m. Angular displacement Δx from a test-setup dial indicator = Indicator reading (mm); τ = Indicator moment arm (mm); tan (ѱ) = Dx/ τ as noted in the illustration showing a typical fixture for measuring bellowscoupling torsional stiffness. Also — ѱ = arctan (Dx/ τ) in degrees. So torsional stiffness in N·m/rad is Ct = M/ѱ (Nm/deg) = M·180/π · ѱ.
Here’s a common setup for quantifying the stiffness of bellows couplings. One end attaches to a fixed housing; the other coupling end attaches to a freely moving beam moment arm that’s loaded with various weights. Dial indicators on the fixed and free ends measure coupling torsional deflection. The fixture measures overall coupling stiffness, including the end hubs and bellows.
DESIGN WORLD — MOTION
Couplings — Motion System Handbook 8-17 V3.indd 74
8 • 2017
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8/16/17 11:18 AM
WHEN SAFETY IS A REQUIREMENT. 0.1 - 2,800 NM
WWW.RW-AMERICA.COM
R+W -- Motion Systems Handbook 8.17.indd 75
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HANDBOOK
MISALIGNED SHAFTS CAUSE REACTION LOADS OVERHANG
MOTION SYSTEM
REACTION FORCE OFFSET
COUPLING GEARBOX
REACTION FORCE
MOTOR
Misalignment can spur reaction forces and excitation of machine natural frequencies. One resonant frequency to avoid is that of the motor output assembly — defined by shaft diameter, overhang from the bearing, and material, as well as coupling type and weight.
It’s usually unnecessary and impractical to test all components in design, which is why engineers use theoretical system-stiffness values. One caveat here related to couplings is that different manufacturers’ couplingstiffness ratings vary with measurement methods. There can also be differences between published and measured values. Two tips from Mike Parzych of GAM Enterprises on this: Use caution when designing a motion machinery relying heavily on overall stiffness for good design performance. Also, look for evaluations that faithfully model performance characteristics to ensure stiffness and machine-assembly performance. In fact, the most suitable coupling choice ultimately depends on application requirements and machine throughput. If improving axis positioning and cycle time is priority, focus on boosting powertrain stiffness. Selecting couplings with high torsional rigidity (among other things) can minimize lost motion from torsional windup. Shorter couplings or those with reinforced bellows can boost torsional rigidity values ... But keep in mind that while a shorter coupling has higher rigidity (to 60 to 70%) misalignment compensation capabilities also decrease with length. ALIGNMENT: ITS IMPORTANCE CANNOT BE OVERSTATED No coupling — no matter how engineered — can correct for shafts that are excessively misaligned. The nature of flexible couplings occasionally misleads design engineers and assembly personnel (or more often, end users) into believing that they’re a fix-all for compromised or less exacting machine builds. But flexible couplings put into designs with excessive misalignment exhibit material stresses and fatigue and premature failure. Though coupling failures do occasionally originate from couplings themselves, it’s far more common that coupling issues arise as a symptom of other design problems. If a motion design does exhibit coupling
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problems, avoid the temptation to simply upsize or upgrade that coupling. Such upgrades are often unnecessarily expensive and short-lived solutions that actually put system bearings as well as gearing and connected motors at risk of collateral damage. Instead, make a holistic analysis of the design and consult the coupling manufacturer for assistance. Note that when motion systems exhibit coupling issues long after a proper installation and run of service, it’s sometimes a result of some other change in the drive assembly. Even small changes to the motor, drive, or programming can be to blame — especially if a new motion sequence demands higher transmission of motor torque or the elimination of a previously held electronic limitation. WHAT ARE REACTION FORCES? All flexible couplings compensating for misalignment cause reaction forces, and their effect is significant if misalignment is excessive. These often-overlooked reaction forces transmit to connected shafts and support bearings, and can cause damage to motion axes — especially motioncontroltips.com | designworldonline.com
8/16/17 11:20 AM
COUPLINGS
on precision designs with delicate bearings and slender shafts. Though couplings get their compliance from elastomeric deflection, sliding contact, and flexing coupling members, here we focus on the types most common for motion designs employing stepper or servomotors. Ultimately, reaction-force magnitudes depend on the level of misalignment and the coupling type in use. Bellows couplings, socalled membrane couplings such as disc couplings, and beam couplings have thin sections of various designs capable of radial flexing. Resistance to misalignment — a spring-rate reaction defined as a force per unit of deflection — increases proportionally with shaft deflection. Because these couplings bend to accommodate misalignment, reaction force depends on the thickness of the flexible element. But bellows and beam couplings have multiple coils or convolutions, so work as flexible shafts that sweep through complementary bows as the mode of misalignment compensation. Torque transmission is through members in shear, so the convolutions can be thin and keep radial forces low while maximizing torsional stiffness. In contrast, membrane-coupling variations transmit torque via bending members, so need thick members to get high torsional stiffness. Such couplings’ bending (through complementary directions) also compensates for shaft misalignment. The catch is that these torsionally stiff couplings can induce significant radial-reaction forces if excessive misalignment is present. Because radial-force magnitude depends on bend severity, minimizing bending angles reduces the detrimental forces on support bearings (though can reduce misalignment capacity, too). Some membrane couplings address more misalignment with a central member between the flexible members; DESIGN WORLD — MOTION
Couplings — Motion System Handbook 8-17 V3.indd 77
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HANDBOOK
MOTION SYSTEM
the added distance imparts an ability to turn while making shallower bends (and lower radial forces) for a radial shaft offset. In fact, aforementioned beam and bellows couplings with divided flex-element arrays also sometimes leverage more distance between flexure points to get shallower bends for a given radial shaft offset. Short models sometimes connect via an intermediate shaft. In contrast, elastomeric couplings have myriad torsional-damping properties and transmit torque in shear, bending, and compression. Coupling class is key with this design: Some versions exhibit zero backlash while others rated for use in power-transmission applications can exhibit minute inter-hub rotation. Most variations transmit torque (and address misalignment) through compressible elastomeric-insert spiders trapped between jawed halves ... and induce reaction forces when connecting shafts with excessive radial shaft deflection. Jaw designs can accommodate more misalignment (and minimize detrimental forces on the shafts’ support bearings) with softer elastomer spiders, though that sometimes reduces torsional stiffness. Remember that if excessive misalignment is a concern, consult with coupling manufacturers on the design. Their engineers may suggest design improvements; offer coupling types to resolve the misalignment without inducing unacceptable reaction forces; and supply charts of reaction forces that a given coupling is projected to induce under a given set of conditions. WHEN MOTION DESIGNS NEED TORSIONAL STIFFNESS — AS WELL AS DAMPING New technological improvements in servomotors have spurred dramatic improvements in response frequencies. The catch is that vibration (and hunting) tend to arise when designers apply high gain settings to servo systems and use advanced couplings with high torsional stiffness — such as disc or bellows-type couplings, for example. One way to resolve hunting in setups with high gain settings is to use couplings with vibration-damping capabilities. Here, couplings with hydrogenated nitrile butyl rubber (HNBR) center elements are one option to make servo systems for precision automation tasks (as those in semiconductor manufacturing) more responsive. Sometimes called high-gain rubber couplings, these have an integrated structure that includes aluminum hubs on both ends molded with vibration-reducing HNBR to prevent backlash but stay flexible. The rubber-lined claw structure optimizes torsional rigidity as well as damping.
Bode plots show how high-gain rubber couplings increase servomotor gain beyond the capacity of comparable couplings with high torsional stiffness. Gain width between 0 dB and the point at which there’s a phase delay in the Bode plot is -180° — and this is called the gain margin. General guidelines for servo systems recommend gain margins between 10 and 20 dB. As servomotor gain rises, gain margin decreases. When the gain margin falls below 10 dB, hunting tends to occur. Consider the limit gain (the servo gain at which hunting occurs) of assemblies using high-gain rubbertype couplings. The value of 17.40 dB surpasses that of other coupling types. Plus because the gain margin is above 10 dB, the servomotor gain of the rubber-insert couplings effectively shortens stabilization time and increasing throughput. More specifically, a system with a 12-msec limit gain might see improvement to a 3-msec limit gain simply by switching to a rubber-type coupling. This suppresses hunting and minimizes time for stabilization. This information on high-gain rubber couplings is provided by Paulo Castelo, technical solutions supervisor at NBK America. Stay tuned to Design World’s own couplingtips.com in the coming weeks for a detailed feature on servomotor couplings and related stiffness, hunting, damping, and stabilization considerations.
Bellows couplings have a low moment of inertia and suitable for dynamic applications — as those on precision motion axes. Shown here is an example from R+W America. It has balancing bores on the clamping hubs to ensure balanced installation; friction-based clamping and high-grade stainless-steel bellows ensure zero backlash. Some versions of these bellows couplings work on axes running to 10,000 rpm; some finely-balanced variations can run even faster.
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8 • 2017
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8/21/17 8:12 AM
Metal Bellows Transfer Pressure or Temperature into Linear Movement ISO 9001:2008 Certified RoHS Compliant ITAR Certified
SERVOMETER® Electrodeposited Metal Bellows
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Features: • Zero backlash • Thinnest high strength walls • Seamless construction • Premium alloy FlexNIckel® • Diameters as small as 0.020 in (5mm) • Highest cycle life • Design assistance for customization
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servo/bellows -- Motion Systems Handbook 8.17.indd 79
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HANDBOOK
MOTION SYSTEM
ac drives
FUNDAMENTALS OF
An
ac drive controls an ac motor, such as an induction or synchronous motor. The drives convert ac to dc and then using a range of different switching techniques generate variable voltage and frequency outputs to drive the motor. An ac motor’s speed is determined by the number of poles and the frequency. So as frequency is adjusted the motor’s speed can be controlled as well. A common way to control frequency is via pulse width modulation (PWM). A PWM drive outputs a train of dc pulses to a motor and by modulating the pulse width, making it either narrower or wider, delivers an ac current waveform to the motor. One of the most common types of drives is a variable frequency drive (VFD). They operate by switching their output devices, which can be transistors, IGBTs (insulated gate bipolar transistors), or thyristors, on and off. VFDs can be either constant voltage or constant current. Constant voltage types are the most common type of VFD. It uses PWM to control both the frequency and the voltage applied to the motor. Among the benefits of using a VFD for motor speed control is the actual energy savings. Controlling the amount of current drawn by the motor can decrease energy costs because the motor will not run at full load all of the time. This is more important as motor efficiency continues to be a top design priority. For instance, single-phase induction machines (specifically, permanent split-capacitor motors) and universal
motors are managed with simple voltage-control techniques. Contrast this with high-end, high-performance machines where three-phase motors are more common and which are controlled by VFDs. For example, an OEM using a universal motor with simple triac control may now find that a three-phase VFD control will provide better energy efficiency, while OEMs using three-phase/VFD configurations may make the move to technologies like brushless dc motors. VFDs also provide a crucial advantage on motor startup. Without a VFD, an induction motor on start-up has to handle a high initial in-rush current. As the motor speeds up and approaches a constant speed, the current levels off from the peak in-rush values. So with a VFD, the motor’s input starts off with low voltage and a low frequency, avoiding the problem of high in-rush currents. Using a VFD for motor speed control also reduces mechanical wear on the motor components. Eliminating the in-rush currents upon start-up gets rid of the excessive torque on components, increasing the life of the motor and reducing maintenance costs and the need for repair. In addition to reducing wear, mechanical stresses on the entire system are greatly reduced. In many cases, mechanical controls such as throttles, valves, dampers and louvers can be completely removed, thereby reducing mechanical wear and maintenance costs. Further, with reduced mechanical wear, due to precise motor speed control, the system output quality may be improved and production times reduced. There are some drawbacks to using VFDs, however. The main one is the possibility of harmonic distortion which can effect the power quality as well as the operation of other machinery. However, VFD manufacturers have been developing solutions that largely eliminate this problem.
The ACS380 ac drive from ABB is designed to integrate seamlessly into machines and automation systems. A built-in icon-based user interface simplifies programming. Such drives are suitable anywhere constant torque is a requirement, for instance in food and beverage and material handling.
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motioncontroltips.com | designworldonline.com
8/18/17 7:37 AM
DC DRIVES
FUNDAMENTALS OF
dc drives The SV200 series dc drives from Applied Motion Products operate on 24, 48 or higher dc voltages, up to 60 V dc. Their small, compact size makes them suitable for use in space-constrained applications such as automated guided vehicles and other small machines.
A
dc drive is used to control a dc motor. A drive lets operators control a dc motor by starting, stopping, and changing the direction and speed of the motor. A typical drive converts a three-phase ac voltage to an adjustable dc voltage, which is then supplied to the motor armature. A common dc motor control technique is to use a thyristorbased control circuit. These circuits consist of a thyristor bridge that rectifies ac into dc for the motor armature. Varying the voltage to the armature controls the motor’s speed. Thyristors, or silicon-controlled rectifiers (SCRs), provide controllable power output by phase angle control, so called because the firing angle synchronizes with the ac source’s phase rotation. There are two basic types of dc motors; brushed and brushless (or BLDC). Each type of motor has different drive techniques. For instance, one can drive a brushed dc motor by applying a voltage or pulse width modulated (PWM) voltage. The motor will increase in speed (while reducing torque) until the torque and speed match the load. On the other hand, BLDC motors are a bit more complicated to drive compared with brushed dc motors. For example, a typical brushless motor will have three sets of windings connected in a star or “Y” or “Delta” configuration. The motor controller energizes each of the windings to turn the motor. The motor position has to be known to control the windings properly, energizing the correct winding or windings. The controller can be a sensorless type, meaning that it relies on detecting the back EMF (electromotive force) of the windings to detect position and provide the sequence information for the controller. A sensor-less drive allows the use of a motor without Hall effect sensors, making the motor less expensive and requiring fewer connections. The control signal for most BLDC motor controllers is a speed control signal, such as a PWM input signal rather than an analog signal. As for PWM drives, two common types are sinusoidal (or sine) and trapezoidal. Sinusoidal PWM drives increase and decrease the current to each winding to follow a sinusoidal curve in order to smooth the drive power and produce a smoother motor torque. Simple on/ off control of the windings tends to produce an uneven torque through the rotation of the motor and also tends to generate more audible noise due to the uneven torque. Trapezoidal PWM drives are similar to sine drives and will increase the current to each winding in a straight line based on the motor position and then decrease it in a straight line while increasing the current to the next winding. To use a trapezoidal or sinusoidal PWM drive, motor controllers need to know where the motor rotor is to a higher degree of accuracy than a simple
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Drives, DC — Motion System Handbook 8-17 V1.indd 81
Hall effect switch position provides. They do this by monitoring the motor velocity and predicting the position with time. While this won’t be perfectly accurate it is considerably better than a simple on-off drive. Also, a trapezoidal drive will be quieter and smoother than a simple on/off drive but not as smooth or quiet as a sinusoidal drive. In terms of actual usage, dc motors and drives are still among the most common types of motors and drives in many industrial applications including paper-web handling, extruding applications, spindle drives, and material handling. The fact is that tried and true motors like brushed dc motors are capable of high peak torques. Coupled with dc drives, such systems also provide wide speed ranges and have good speed regulation. Also, the fact that they have a linear torque-speed relationship makes control easier, meaning they can be controlled using simple speed controllers and often cost less than other motor and control options. Manufacturers are also finding that using existing dc motors and upgrading the dc drives is often a good option. This is because dc motors are usually well built and can offer many years of reliable service. 8 • 2017
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The ENX EASY from maxon is a sterilizable encoder designed for integration with the company’s brushless dc motors and drives to form a complete integrated motion system, especially in medical applications. The encoders are available in an incremental (1,024 counts) and absolute version (4,096 steps), both designed for 1,000 autoclave cycles.
Encoder
FUNDAMENTALS
Encoders
provide position information for controlling the position of a motion system. They can be classified by the method of sensing (i.e. optical, magnetic, or capacitive sensing) or whether the position output is absolute or incremental. Another parameter is how much force the encoder can handle on the shaft, so there are light duty, medium duty, and heavy-duty encoders. Another quite common way encoders are classified is either rotary or linear, depending on the type of position they measure. Linear encoders typically are composed of a scale such as a coded strip, and a sensing head. Reading the space between the scale coding determines position. The resolution of linear encoders is measured in pulses per inch or millimeter. The scale typically has a fixed resolution with embedded markings that is read by the sensing head. For example, a linear encoder with 100 points per inch resolution would read 100 marks for every inch of movement. Rotary encoders measure resolution in pulses per revolution. Similar to linear encoders, a typical rotary encoder contains an internal coded disk and a sensing head. Think of
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a linear encoder as a type of tape measure while a rotary encoder is more like a measuring wheel. For instance, a rotary encoder with a 100 point per revolution resolution would have 100 marks on its coded disk. ABSOLUTE AND INCREMENTAL ENCODERS Encoders can also be either absolute or incremental. Absolute encoders have a unique code for each shaft position. In other words, every position of an absolute encoder is distinctive. The absolute encoder interprets a system of coded tracks to create position information where no two positions are identical. Another feature is that absolute encoders do not lose position when power is switched off. Because each position is distinctive, the verification of true position is
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HANDBOOK
MOTION SYSTEM
NEW
available as soon as power is switched on without the need for a homing routine. Absolute encoders can be either single-turn or multi-turn. Single-turn encoders are well suited to short-travel motion applications where position verification is needed within a single turn of the encoder shaft. Multi-turn encoders, on the other hand, are better for applications that involve complex or lengthy positioning requirements. There are a number of advantages to absolute encoders. Safety is one. In some applications where a loss of position could lead to operator injury or machine damage, an absolute encoder automatically provides position verification when the power is switched on. Absolute encoders also have good immunity to electrical noise. The device determines position by frequently reading a coded signal. Stray pulses from electrical noise will not build up and accurate position is presented again on the next reading.
Incremental encoders generally supply square-wave signals in two channels, A and B, which are offset (or outof-phase) by 90 degrees. This helps in determining the direction of rotation. The output signals of an incremental encoder only have information on relative position not absolute position like an absolute encoder. In order for the encoder to provide any useful position information, the position of the encoder has to be referenced in some way, traditionally using an index pulse. So the incremental encoder sends incremental position changes to electronic circuits that perform the counting function. A traditional limitation of an incremental encoder is that the number of pulses counted is stored in an external or buffer counter which can be lost if there is an interruption of power. For instance, if a machine with an encoder is turned off, the encoder will not know its position when switched on again. The encoder has to perform a homing routine in order to know its exact position, forcing the motor to move until a home limit switch is activated. Then, a counter or buffer will be zeroed and the system will determine where it is relative
Programmable thru-bore encoder
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ucts
ENCODERS
LOOK TO POSITAL FOR ALL OF YOUR POSITION AND MOTION SENSOR NEEDS
to fixed positional points. One way around this issue of loss of power is to use a battery backup system. Such a solution ensures that the memory is backed up and can store the count information and provide an absolute count once power is restored. ENCODER PERFORMANCE The most critical performance parameter for encoders is their ability to accurately measure position. The right encoder should have the necessary resolution required by the application. However, resolution and accuracy are not the same. For instance, an encoder may have a high resolution but low accuracy, or vice versa. The ideal encoder has both the necessary resolution and is highly accurate. Accuracy depends on a number of factors. For starters, there are differences between optical and magnetic technologies. Generally speaking, magnetic encoders are more resistant to shock and vibration than optical encoders. They’re also better able to withstand environmental contaminants such as dust, grease and moisture. As far as electrical interference goes, optical encoders are fairly immune to it since position measurement is not electrical-based but optical. Also, magnetic technologies may be subject to strong magnetic fields that could impact readings. With more sophisticated communication protocols available, users have a lot more control over encoder operation. For instance, encoder resolution can be programmed through a USB interface via computer. Other parameters can be changed on the fly through simple software commands. Some of
With POSITAL’s outstanding range of configurations and performance characteristics, it’s easy to find sensors that meet your exact requirements.
Magnetic absolute encoders are often used for motion designs in demanding industrial environments or in rough outdoor use. These MAGRES series magnetic absolute encoders from Baumer feature electronics designed and tested for compliance to ECE R10 requirements. They also come with C5-M corrosion resistance and IP67 protection.
Absolute and incremental rotary encoders Kit encoders for motor feedback Inclinometers Linear position sensors
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the most common encoder communication protocols include SSI (Synchronous Serial Interface), BISS (bidirectional serial/ synchronous), ProfiBus, DeviceNet and Ethernet I/P. These protocols are “open”, meaning they are non-proprietary and not tied to a specific manufacturer’s products. Closed or proprietary protocols include Hiperface (High Performance Interface) and EnDat (Encoder Data). FOCUS ON CAPACITIVE ENCODERS Capacitive encoders, a relatively new technology, offer resolution comparable to optical devices, with the ruggedness of magnetic encoders. Currently, there are only a handful of vendors for capacitive
encoders, but their suitability for applications requiring high precision and durability make them a good choice for the semiconductor, electronics, medical, and defense industries. While optical encoders can deliver high resolution, their main components—an optical disk, an LED light source, and photo-detectors—are fragile and highly sensitive to dust, dirt, and other environmental contamination. They can also be damaged by vibrations and require a relatively stable temperature range. Magnetic encoders, on the other hand, are quite robust, but provide lower resolution than optical encoders. They are also sensitive to magnetic interference, which is a notable concern when used with stepper motors. Probably the most important difference between optical and capacitive encoders is that capacitive encoders don’t require an optical disk. This makes capacitive versions more robust, less susceptible to contamination and less influenced by temperature variations than optical encoders are. And with
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ENCODERS
This WITTENSTEIN cyber linear actuator tightly integrates a precision ballscrew, actuator housing, servomotor, and encoder. The absolute single-turn encoder has 12-bit resolution and is available for myriad actuator sizes — even particularly compact ones.
no LED to burn out, capacitive encoders can achieve a much longer life than optical versions. They are also more efficient, with current consumption typically less than 10 mA—as compared to the 20 mA or higher consumption of an optical encoder. This is especially beneficial in applications where power is supplied via battery. Another benefit of capacitive encoders is the ability to change the encoder’s resolution by modifying the line count in the electronics, without changing components. Compared to magnetic encoders, capacitive versions simply provide better resolution in most situations, and can be produced at a lower cost. The basic principle behind capacitive encoders is that they detect changes in capacitance using a highfrequency reference signal. This is accomplished with the three main parts—a stationary transmitter, a rotor, and a stationary receiver. (Capacitive encoders can also be provided in a “two-part” configuration, with a rotor and a combined transmitter/receiver.) The rotor is etched with a sinusoidal pattern, and as it rotates, this pattern modulates the high-frequency signal of the transmitter in a predictable way. The receiver disk reads the modulations, and on-board electronics translate them into increments of rotary motion. The primary concern when using capacitive encoders is their susceptibility to noise and electrical interference. To combat this, the ASIC circuitry must be carefully designed and the algorithms for demodulation must be fine-tuned. Capacitive technology, however, has been used for many decades in digital calipers and is well-proven. Now it is making its way into the encoder product space, where it provides high resolution without sacrificing robustness. 8 • 2017
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DESIGN WORLD — MOTION
Encoders — Motion System Handbook 8-17 V2.indd 87
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GEARING ESSENTIALS
General gear design The
main function of a gear is to mesh with other gears to transmit altered torque and rotation. In fact, gearing can change the speed, torque and direction of motion from a drive source. When two gears with an unequal number of teeth engage, the mechanical advantage makes their rotational speeds and torques different. In the simplest setups, gears are flat with spur teeth (with edges parallel to the shaft) and the input gear’s shaft is parallel to that of the output. Spur gears mostly roll through meshing, so can be 98% or more efficient per reduction stage. However, there is some sliding between tooth surfaces, and initial tooth-to-tooth contact occurs along the whole tooth width at once, causing small shock loads that induce noise and wear. Sometimes lubrication helps mitigate these issues. In slightly more complex setups, parallel-axis gearsets have helical gears that engage at an angle between 90° and 180° for more tooth contact and higher torque capacity. Helical reducers are suitable Spur, helical for higher-horsepower bevel, and worm gear applications where long-term sets abound from operational efficiency is more KHK USA Inc. important than initial cost. Helical gear teeth engage gradually over the tooth faces for quieter and smoother operation than spur gearsets. They also tend to have higher load capacities. One caveat: Angled tooth contact generates thrust that the machine frame must resolve. No matter the subtype, most parallelaxis gearsets have gear teeth with tailored involute profiles—customized versions of the rolled trace off a circle with an imaginary string. Here, mating gears have tangent pitch circles for smooth rolling
DESIGN WORLD — MOTION
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engagement that minimizes slipping. A related value, the pitch point, is where one gear initially contacts its mate’s pitch point. Involute gearsets also have an action path that passes through the pitch point tangent to a base circle. Besides parallel-axis gearsets, there are nonparallel and right-angle gearsets. These have input and output shafts that protrude in different directions to give engineers more mounting and design options. The gear teeth of such gearsets are either bevel (straight, spiral or zerol), worm, hypoid, skew or crossed-axis helical gears. The most common are bevel gearsets with teeth cut on an angular or conical shape. Hypoid gears are much like spiral-bevel gearsets, but the input and output shaft axes don’t intersect, so it’s easier to integrate supports. In contrast, zerol gearsets have curved teeth that align with the shaft to minimize thrust loads.
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GEARING ESSENTIALS
Gearboxes, specialty gearheads & servogear sets
Servo
designs are precision-motion setups with feedback and (in most cases) fairly stringent accuracy demands. So for these designs, engineers should pick servogear reducers with good torsional stiffness, reliable output torque and minimal backlash. OEMs tasked with integrating servo systems should look for quiet reducers that easily mount to the motor and require little or (if possible) no maintenance. In fact, a lot of advanced machinery integrates servogears into application-specific electromechanical arrangements, and several of these arrangements are common enough to have specific labels. Here is a look at some of the most widespread. Gearmotor: This complete motion component is a gear reducer integrated with an ac or dc electric motor. Usually the motor includes the gears on its output (typically in the form of an assembled gearbox) to reduce
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IronHorse shaft-mount gearboxes (speed reducers) from AutomationDirect feature shafts and gears manufactured from the highest-grade steel that’s case-hardened and precision ground to AGMA standards for durability. These gearboxes come in 9:1, 15:1 and 25:1 gear ratios and in frame sizes from 2 to 5.
speed and boost available output torque. Engineers use gearmotors in machines that must move heavy objects. Speed specifications for gearmotors are normal speed and stall-speed torque. Gearbox: This is a contained gear train … a mechanical unit or component consisting of a series of integrated gears. Planetary gears are common in integrated gearboxes. Planetary gears: Particularly common in servo systems, these gearsets consist of one or more outer planet gears that revolve about a central, or sun, gear. motioncontroltips.com | designworldonline.com
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The DieQua Advantage We offer the widest range of servo gearheads and gearboxes, along with the experience and expertise to help you select the best solution for your requirements. Right angle or inline, economy or precision, we have the gearhead to meet your needs!
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MOTION SYSTEM
HANDBOOK
Common gear options
Typically, the planet gears mount on a movable arm or carrier that rotates relative to the sun gear. The sets often use an outer ring gear, or annulus, that meshes with the planet gears. The gear ratio of a planetary set requires calculation, because there are several ways they can convert an input rotation to an output rotation. Typically, one Shown here is a Spinea of these three gear high-precision cycloidal reducer wheels stays stationary; another is an input from DieQua Corp. Spinea TwinSpin that provides power to the system, and the (TS) reducers have a unique last acts as an output that receives power speed-reduction mechanism and from the driving motor. The ratio of input radial-axial output bearings to rotation to output rotation depends on the output precision motion. Thanks number of teeth in each gear and on which to a unique design configuration, component is held stationary. DieQua Spinea reducers are also Planetary gearsets offer several much more compact than cycloidal advantages over other gearsets. These reducers from competitors. In include high power density, the ability to the smaller sizes they compete with harmonic flex spline designs get large reductions from a small volume, providing higher torque density multiple kinematic combinations, pure and rigidity. torsional reactions and coaxial shafting. Another advantage to planetary gearbox arrangements is power-transmission efficiency. Losses are typically less than 3% per stage, so rather than waste energy on mechanical losses inside the gearbox, these gearboxes transmit a high proportion of the energy for productive motion output. Planetary gearbox arrangements distribute load efficiently, too. Multiple planets share transmitted load between them, which greatly increases torque density. The more planets in the system, the greater load ability and the higher the torque density. This arrangement is also very stable due to the even distribution of mass and increased rotational stiffness. Disadvantages include high bearing loads, inaccessibility and design complexity. In servo systems, besides boosting output torque, gearboxes impart another benefit — reducing settling time. Settling time is a problem when motor inertia is low compared to load inertia … an issue that’s the source of constant debate (and regular improvement) in the industry. Gearboxes reduce the reflected inertia at the controls by a factor equal to the gear reduction squared.
Spur gearsets are simple ...
Pitch circle
Reaction force ... but helical gearsets are more efficient. Cross-axis sets are another option.
Planetary gearsets are compact and run to 10,000 rpm. Here, a lightweight Schaeffler differential for a hybrid vehicle has an axial spline to boost efficiency. Zerol bevel gearsets are a special veriation of straight right-angle bevel sets.
Worm gearsets are rugged and don’t let designs backdrive ... which can eliminate the need for brakes. Note there’s some overlap between bevel and worm applications. Case in point: The MS-Graessner DynaGear below is a single-stage bevel gear with a 30:1 ratio.
The ratio of a helical or bevel gearset is simply the number of teeth in the larger gear divided by the number of teeth in the smaller gear. Other gear types such as planetary gears have more complex ratio relationships.
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MOTION SYSTEM
HANDBOOK
GEARING ESSENTIALS
Strain-wave gearing Strain-wave
gearing is a special gear design for speed reduction. It uses the metal elasticity (deflection) of a gear to reduce speed. (Strain-wave gearing sets are also known as Harmonic Drives, a registered trademark term of Harmonic Drive Systems Inc.) Benefits of using strain-wave gearing include zero backlash, high torque, compact size and positional accuracy. A strain-wave gearset consists of three components: wave generator, flexspline and circular spline. The wave generator is an assembly of a bearing and steel disk called a wave generator plug. The outer surface of the wave generator plug has an elliptical shape machined to a precise specification. A specialty ball bearing goes around this plug to conform to the same elliptical shape of the wave generator plug. Designers typically use the wave generator as the input (attached to a servomotor). The flexspline— usually acting as the output—is a thin-walled steel cup. Its geometry makes the cup walls radially compliant but torsionally stiff (because the cup has a large diameter). Manufacturers machine the gear teeth into the outer surface near the open end of the cup (near the brim). The cup has a rigid boss at one end for mounting. The wave generator goes inside the flexspline so the bearing is at the same axial location as the flexspline teeth. The flexspline wall near the brim of the cup conforms to the same elliptical shape of the bearing. This Circular spline conforms the teeth on the outer surface of the flexspline to the elliptical shape. That way, the flexspline effectively has an elliptical gear-pitch diameter on its outer surface. The circular spline is a rigid circular steel ring with teeth on the inside diameter. It is usually attached to the housing and does not rotate. Its teeth mesh with those of the flexspline. The tooth pattern of the flexspline engages the tooth profile of the circular spline along the major axis of the ellipse. This engagement is like an ellipse inscribed concentrically within Flexspline a circle. Mathematically, an inscribed ellipse contacts a circle at two points. However, gear This is a progression of teeth have a finite height, flex-spline tooth engagement with so two regions (instead of circular-spline teeth. The profile of two points) engage. Harmonic Drive gear teeth lets up to 30% of the teeth engage ... for The pressure angle of higher stiffness and torque than the gear teeth transforms gearsets with involute teeth.
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the output torque’s tangential force into a radial force acting on the wave-generator bearing. The teeth of the flexspline and circular spline engage near the ellipse’s major axis and disengage at the ellipse’s minor axis. The flexspline has two less teeth than the circular spline, so every time the wave generator rotates one revolution, the flexspline and circular spline shift by two teeth. The gear ratio is: number of flexspline teeth ÷ (number of flexspline teeth - number of circular spline teeth) The tooth engagement motion (kinematics) of the strain wave gear is different than that of planetary or spur gearing. The teeth engage in a manner that lets up to 30% of the teeth (60 for a 100:1 gear ratio) engage at all times. This contrasts with maybe six teeth for a planetary gear, and one or two teeth for a spur gear. In addition, the kinematics enable the gear teeth to engage on both sides of the tooth flank. Backlash is the difference between the tooth space and tooth width, and this difference is zero in strain-wave gearing. As part of the design, the manufacturer preloads the gear teeth of the flexspline against those of the circular spline at the ellipse’s major axis. The preload is such that the stresses are well below the material’s endurance limit. As the gear teeth wear, this elastic radial deformation acts like a stiff spring to compensate for space between teeth that would otherwise increase in backlash. This lets the performance remain constant over the life of the gear. Strain-wave gearing offers high torque-to-weight and torque-to-volume ratios. Lightweight construction and single-stage gear ratios (to 160:1) let engineers use the gears in applications requiring minimum weight or volume ... especially useful for designs with small motors. Another tooth profile for strain-wave gearing is the S tooth design. This design lets more gear teeth engage for a doubling of torsional stiffness and peak torque rating, as well as longer life. The S tooth form doesn’t use the involute tooth curve of a tooth. Instead, it uses a series of pure convex and concave circular arcs that match the loci of engagement points dictated by theoretical and CAD analysis. The increased root filet radius makes the S tooth much stronger than an involute curve gear tooth. It resists higher bending (tension) loads while maintaining a safe stress margin.
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Consultation, custom gear design & gear analysis Custom
gearboxes are increasingly common, mainly because they’re easier than ever to manufacture to specification. That’s not to say that the design work isn’t challenging. However, modern manufacturing lets some suppliers make gearboxes and components to meet specific application requirements. New supplier approaches to giving engineering support as well as new machine tools, automation and design software now let OEMs and end users get reasonably priced gearing even in modest volumes. When enlisting help from a consultant or manufacturer, an engineer is more likely to get gearing that mounts properly and performs to specification after reviewing the following and answering as many of these questions as possible: • • • •
• This HDP parallel-shaft gearbox from Bonfiglioli USA has gear ratios from 7.1 to 500 and outputs torque from 5,000 to 210,000 Nm depending on the version. It allows foot, flange, or shaft mounting; the cast-iron housing withstands harsh environments and helps maintain precise and vibration-free operation.
•
•
What’s the input speed and horsepower? What’s the gearbox target output speed or output torque? This partially defines the required gear ratio. What are the characteristics of use? How many hours per day will the gearbox run? Will it need to withstand shock and vibration? How overhung is the load? Is there internal overhung load? Remember that bevel gears usually can’t accommodate multiple supports, as their shafts intersect … so one or more gears often overhang. This load can deflect the shaft which misaligns the gears, in turn degrading tooth contact and life. One potential fix here is straddle bearings on each side of the gear. Does the machine need a shaft or hollow-bore input ... or a shaft or hollow-bore output? How will the gearing be oriented? For instance, if specifying a rightangle worm gearbox, does the machine need the worm over or under the wheel? Will the shafts protrude from the machine horizontally or vertically? Does the environment necessitate corrosion-resistant paints or stainlesssteel housing and shafts?
SERVICE FACTOR: The starting point for most gearbox manufacturers is to define a service factor. This adjusts for such concerns as type of input, hours of use per day, and any shock or vibration associated with the application. An application with an irregular shock (a grinding application, for example) needs a higher service factor than one that’s uniformly loaded. Likewise, a gearbox that runs intermittently needs a lower factor than one used 24 hours a day.
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t material of SCM415. carburizing (case hardening), SCM415 is normally used. requirements for harder material, SNCM220 and/or SNCM420 y used. hardness is determined by production condition. pected hardness value is informed in a quotation of gears.
Gear precision grade table : entioned equivalent grades are for reference only. ades completely match each other.
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HANDBOOK
Engineers used Graessner USA’s Gearfox software to develop and optimize the drivetrain on this KUKA KL 100 linear robottransfer unit (RTU). Custom gear designs are increasingly common as software makes specialized builds easier than ever to devise.
CLASS OF SERVICE: Once the engineer determines the service factor, the next step is to define a class of service. A gearbox paired to a plain ac motor driving an evenly loaded, constant-speed conveyor 20 hours per day may have a service class 2, for example. This information comes from charts from gearbox manufacturers that list classes of service. To use these charts, the design engineer must know input horsepower, application type and target ratio. For instance, suppose that an application needs a 2-hp motor with a 15:1 ratio. To use the chart, find the point where 2 hp and 15:1 ratio intersect. In this case, that indicates a size 726 gearbox. According to one manufacturer’s product-number system, size 726 defines a gearbox that has a 2.62 center distance. Such charts also work in reverse, to let engineers confirm the torque or speed of a given gearbox size. OVERHUNG LOAD: After the designer picks a size, the gearbox manufacturer’s catalog or website lists values for the maximum overhung load that is permissible for that sized unit. Tip: If the load in an application exceeds the allowed value, increase the gearbox size to withstand the overhung load. MOUNTING: At this point, the designer or manufacturer has defined the gearbox size and capability. So, the next step is to pick the
EFFICIENCIES OF GEAR CATEGORIES AND SUBTYPES SPUR GEARS SPUR RACK INTERNAL GEARS HELICAL GEARS
PARALLEL-AXIS GEARS
HELICAL RACK
98 TO 99.5%
DOUBLE HELICAL GEARS INTERSECTING-AXIS GEARS
STRAIGHT BEVEL GEARS SPIRAL BEVEL GEARS
98 TO 99%
ZEROL BEVEL GEARS NONPARALLEL AND NONINTERSECTING
98
SCREW GEARS
70 TO 95%
WORM GEARS
30 TO 90%
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mounting. Common mounting configurations abound, and gearbox manufacturers offer myriad options for each unit size. A flanged input with hollow bore for a C-frame motor combined with an output shaft projecting to the left may be the most common mounting, but there are many other choices. Options such as mounting feet for either above or below the body of the gearbox, hollow outputs, and input and output configuration are all possible. All gearbox manufacturers list their mounting options as well as dimensional information in catalogs and websites. Lubricant, seals and motor integration: Most manufacturers can ship gearboxes filled with lubrication. However, most default to shipping units empty to let users fill them on site. For applications where there is a vertical shaft down, some manufacturers recommend a second set of seals. Because many gearboxes eventually mount to a C-frame motor, many manufacturers also offer to integrate motors onto gearboxes and ship assemblies as single units. Work with consultants and even use custom gear designs if the application needs a unique motor-gearbox combination. Some combinations are more efficient. Getting a pre-engineered geamotor ensures that the motor-gearbox combination will perform to specification. Also remember that today’s custom and standard gearing aren’t mutually exclusive. Where fully custom gearboxes aren’t feasible (if quantities aren’t high enough, for example) consider working with manufacturers that sell gearboxes built to order from modular subcomponents. Otherwise, look for manufacturers that leverage the latest CAD and CAM software and machine tools to streamline post-processing work and reduce the cost of one-offs. One final tip: Once the gearmotor has been chosen and installed in the application, perform several test runs in sample environments that replicate typical operating scenarios. If the design exhibits unusually high heat, noise or stress, repeat the gear-selection process or contact the manufacturer.
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This chart (courtesy of KHK USA Inc.) lists 11 gearefficiency ranges. The number of stages, mode of tooth meshing, and (mostly in the case of worm sets) the speed-reduction ratio dictate overall efficiency.
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GEARMOTORS
Gearmotors A
gearmotor is an electromechanical device that integrates a gear reducer and either an ac or dc electric motor into one physical unit. Thanks to its gearset, these components can deliver high torque at low horsepower or low speed. Most industrial gearmotors incorporate fixed-speed ac motors. However, some gearmotors use dc motors. These dc gearmotors are common in automotive applications, where they are commonly used to adjust side-view mirrors and make automatic seat adjustments. Motors and gears can be mixed and matched as needed to fit application requirements. However, housing design, assembly gearings, gear lubrication, and the specific mode of integrating the pinion gear and motor-output shaft all affect gearmotor performance. Motor and gear-reducer combinations abound. For example, right-angle wormgear, planetary and parallel shaft gears can combine with permanent-magnet dc, ac induction, or brushless dc motors to form a gearmotor unit. Though it’s possible to combine many different motors and gearsets, not just any one will work for every application, because certain combinations are more efficient and cost-effective
than others. Knowing the application and getting an accurate estimation of its required torque and operating speeds is the foundation for successfully integrating a gearmotor into a system. COMMON GEARMOTOR TYPES AND VARIATIONS As gearmotors can be built on either ac or dc motors, there are a number of choices for the gear reducer. There are five basic types of gears that can be paired with a motor to form a complete gearmotor. Those are bevel, helical, hypoid, spur and worm types. Another way to classify gearmotors is by the physical arrangement of the final complete unit. So for instance, there are so-called in-line gearmotors where the gear shaft is parallel with the motor shaft, also called a parallel shaft. These can either be offset from the output shaft or completely in line with the output shaft. The other configuration is the right-angle gearmotor, where the output shaft is at a 90-degree angle to the motor shaft. BENEFITS Gearmotors generally perform better than other motor-gear combinations. More importantly, gearmotors simplify design implementation because they save engineers from integrating motors with gears, which in turn reduces engineering costs. If the application requirements are known, engineers can order the right gearmotor from a supplier directly. What’s more, if a gearmotor is sized properly, having the right combination of motor and gearing can prolong operating life and boost overall design efficiency. Another benefit of gearmotors is that they eliminate
These brushless dc electric gearmotors from Brother Gearmotors feature several shaft configurations, including the right-angle and shaft-less versions shown here. Quiet helical and hypoid gearing means higher efficiency, and the lubricated and sealed for life gearmotors make them virtually maintenance free.
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the need for couplings and potential alignment problems that come with those components. Such problems are common when a design includes the connection of a separate motor and gear reducer — which in turn increases the potential for misalignment and bearing failure, and ultimately reduces useful life. GEARMOTOR ACCESSORIES Gearmotors, like non-geared motors, come in versions with many options to satisfy specific application and environmental conditions. For instance, common options include shaft type, bearing choices, seals and lubrication, and mounting and housing options. Housing materials can include stainless steel or aluminum. Some common options for aluminum housings are billet aluminum, sand-cast aluminum, permanent mold aluminum casting, and die-cast aluminum. Other options are die-cast zinc and magnesium injection molding. Despite being a fairly wellestablished technology, advances in gearmotor technology continue. Some of these include the use of new specialty materials, coatings and bearings, and improved gear-tooth designs that reduce noise, increase strength and extend life, all of which boost performance and reduce overall design size.
GEARMOTOR PERFORMANCE CURVE One tool to simplify selection of a gearmotor is the performance curve. These plots relate torque and speed and sometimes efficiency. To help designers select a pre-engineered gearmotor, manufacturers do much of the heavy lifting to ensure that their motor-gearing combinations will work together. Because manufacturers make performance calculations and do testing in advance, gearmotor failures from miscalculations or improper component matching are rare. The speed and torque that a given application needs are critical factors in gearmotor selection. Use speed and torque measurements to identify manufacturer’s performance curves that match the application needs. Gearmotor curves unify information (such as speed, torque and efficiency) to summarize the performance of the motor-gearset combination. If an OEM or end user buys a complete gearmotor assembly from a manufacturer, the latter supplies its performance curve. After identifying gearmotors with performance curves that appear to meet the application needs, review all calculations and use the values to determine which of the chosen gearmotors will cause problems once installed. Remember to consider thermal characteristics, full-load gearbox torque, gearbox input speed, gearbox yield strength and intermittentduty effects. Once the speed and torque requirements are identified, that may not be the end of the story. That is, proper selection of a gearmotor is not all science. The fact is that this is merely a beginning, a starting point. This is because often times the manufacturer’s data may not be derived from empirical testing, so there may be some variation between calculated requirements and the actual application. This is why it’s important to test a sample load under the actual operating conditions of the application.
COMPARING PARALLEL AND RIGHT-ANGLE GEARMOTOR PERFORMANCE A common way to classify gearmotors is by the output shaft orientation, and two of the standard types are parallel and right-angle arrangements. Beyond shaft configuration itself, other factors such as the type of gears used in the gearmotor determine performance. Generally speaking, a right-angle gearmotor is less efficient than a parallel configuration. A typical right-angle gearmotor arrangement includes a worm gear assembly, which serves to translate the rotary motion of the motor by 90 degrees. In parallel-shaft gearmotors, the gear setup can take any number of forms, from helical and spur gears to complex planetary gear systems. In terms of gear efficiency, spiral bevel gears tend to have the greatest efficiency, followed closely by helical and spur gear types, all of which are in the range of 93 to 99%. More demand for compact machine designs means that space considerations are an important design priority. So if your application calls for a right-angle gearmotor as the only one that will fit the design requirements without having to redesign the entire machine, then that’s the one you go with. Even if it sacrifices some efficiency, compactness and fit may be the more relevant design factors.
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+
= $?
What’s the cost? Actually, a single-worm gearmotor costs a lot more than just the gears and motor. You must also add the dollars spent every year in wasted energy. A premium efficient motor may yield 2-3% energy savings, but you still lose 50% or more through an inefficient worm gear. Solution: Use a helical-bevel gearmotor from SEW-EURODRIVE and get 96% gear efficiency. It makes a lot of cents!
seweurodrive.com | 864-439-7537
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Human-machine interfaces (HMIs):
HARDWARE, SOFTWARE, AND NEW MODES OF CONNECTIVITY
Gone
are the days of HMIs merely consolidating switches, pushbuttons, meters, and operator warning lights. Today’s HMIs now control machines and even whole production lines with functions to analyze and optimize operations. Some serve as the processor for I/O signals providing feedback on speeds, temperatures, torque, tension, and more to optimize processes. Here, HMIs provide global insights based on myriad edge devices — those motor-feedback devices, transducers, sensors, and other smart components (such as smart motors or smart bearings) fitted with electronics to communicate current states. Other variations put HMI functionality right on the motion and other automation devices themselves. In fact, HMIs assume many of the tasks associated with legacy distributed control systems or DCSs. Refer to Design World’s literature on controllers (motion, PLC, PC, and PAC) for more on this.
As the effect of Moore’s Law shows influence on the advancement of electronic-display technology, today’s HMIs also assume all processing for the presentation of human-readable information about machines and automated operations. In fact, the task of processing data for human-readable readouts is something industry now takes for granted. Communications with machine controllers are also standard; many hardware iterations have yielded to software for executing communicationsrelated tasks. Low-cost and free drivers abound. Here, the main decision design engineers may face is choosing between integrated options or ever-more-common Ethernet, fieldbus, or simple and cost-effective serial communications. Open-source modules and easily configurable HMIs let designers use programming software for quick setup and customization. Templates allow configuration for collection of networked-machines’ data — and then navigation layouts to allow HMI end users (on the plant floor or elsewhere) to access all of those data streams in a logical way.
Maple Systems now offers the cMT Smart HMI series that enables control and monitoring from Windows PCs as well as and Apple and Android smartphones and tablets. Technological advances in IT, personal electronics, and manufacturing are expanding the role of HMIs in automation. HMIs must network to a central location where managers can monitor operations, receive alerts, and resolve issues quickly. OEMs need the ability to monitor and track in-field equipment, responding without costly onsite visits. Machine operators need freedom to walk the plant and control machines via tablets and smartphones. Maple Systems cMT HMIs enable these functions. Two new Smart HMI models include a powerful displayless HMI (cMT-SVR) and a 15-in. touchscreen HMI (cMT5151) with features for trending, recipes, alarms, and more. Both models support MQTT, OPC UA Server, MySQL, and Modbus Gateway IIoT protocols.
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In fact, HMIs are core components in the industrial IoT, IT/OT convergence, and overall integration of plant operations with enterprise-level management. They can give machine operators detailed information on one running machine cycle; send workers and plant supervisors statistical analysis of throughput; replace printed paper charts on facility bulletin boards; and grant plant managers remote access to global data from smartphones and mobile devices. In fact, many HMIs filter data upfront for its ultimate destination and authorized audience.
Pro-face America now offers an expanded SP5000 series modular HMI family. New options include screens with widths of 15 in. and 19-in. Both screens come with an enhanced glass-top design that boosts visualization and durability. The intuitive P-CAP touch-panel operation detects five touch points at once and features Touch Cancelation to prevent unexpected detections by water or dust. A variety of Windows applications on WES7 can run with the HMIs thanks to an enhanced Open Box rear module. The new SP-5B41 Open Box unit comes standard with Pro-face’s HMI Runtime, and offers twice the CPU computing power, three times the storage space, and twice the data-access speed than its predecessor.
MORE ON THE SOFTWARE CORE TO TODAY’S HMIS Thanks to HMI software, user interaction is through a graphical user interface (GUI) that facilitates information exchange and communication between two types of HMI — supervisory and machine level. Generally, programmers write HMI software for either machinelevel HMI or supervisory-level HMI, with applications suitable for both types. Such software has high upfront cost, but is inexpensive long-term thanks to the way it reduces redundancies. Case in point: Even lower-tech applications (in which most machine interaction is via switches and pushbuttons) entrylevel HMI offerings are making inroads — as they often reduce interface-part count and simplify controls. More sophisticated applications benefit in a different way: Pharmaceutical and medical machinery use the latest HMI features to differentiate from competitive offerings. But no matter the performance grade, selecting HMI software starts with an analysis of product specifications and features. What kind of GUI will the machine operator need? Will operators need to view diagrams, digital photos and detailed system schematics? Other considerations include system architecture, performance requirements, integration, cost of procurement, and operations. HMI software editors let designers add touchscreen functions and configure control functions for industrial automation. Usually, programming is through Windows-based software or screen104
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editor software. That lets designers quickly edit schematics and set the right communication protocols in a familiar programming environment. Standard HMI software satisfies the needs of simpler machinery that’s not task-intensive. High-performance HMIs run software that lets users customize the interface to meet specific operator requirements. Sometimes, HMI software lets users program advanced control functions as well — for editing servomotor parameters and issuing global commands to other control axes on a machine, for example. (For motion-control applications, the visual GUI can range from simple 4 line x 20 column text displays to color monitors with touchscreen controls.) Such motion functions go well beyond basic HMI tasks observing processes or making very simple changes to some individual variables or parameters or setpoints. As mentioned, one iteration of HMI software that’s increasingly common are programs that let users remotely monitor and control HMIs from smartphones, tablets, or offsite PCs. Traditional setups only let users get to the HMI on the factory floor, but this new cloud-based HMI software gives operators remote access lets them check machines from anywhere. Sometimes called web-based visualization, this is particularly helpful where machines run in hard-toreach places. Related innovations in HMI software even let remote users make on-the-fly changes to machine functions (for variable production output). motioncontroltips.com | designworldonline.com
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WHY DO SO MANY PC CONTROLS INTEGRATE HMI FUNCTIONS? Previous generations of control relied on Cathode Ray Tubes (CRTs) for information displays as part of programming terminals. Those unfamiliar with CRTs should read the Wikipedia articles and check Google Images for representations of how the CRT works. It is an interesting part of the history of control. CRTs use high-voltage power supplies to generate and steer beams of electrons at phosphor dots on the inside of a glass tube. This requires bulky transformers and coils and generates magnetic fields. Because CRTs were relatively large, heavy, and expensive, they weren’t first choice to package into control systems as an operator interface. In systems where program changes or text inputs were frequently required, typically CNCs, a CRT and keyboard would have to be included. Programming terminals for writing control system software are always supplied with a screen interface so programmers can see what they are doing. Because the programming terminal is used for program development, downloading and debugging user code, the connection is only temporary and there is no need for a permanent display with the control system. Control systems based on relays would typically use pushbuttons and pilot lights as the discrete user inputs and status outputs for operation of the machine as the lower cost alternative. Since the advent of low-cost flat screen displays, the cost of packaging displays as part of a control system solution has been reduced dramatically. At the same time, digitaltechnology costs have fallen to the point where even the traditional pushbutton is available with a limited capability digitally programmable display. Given the touchscreen functionality and the ability to display text and pictures, it is a much more cost effective and powerful solution to use the flat-panel display as the operator interface for machinery. Where the HMI and personal computer intersect from a functional standpoint is in the microprocessor itself. The traditional PC has used a centralized processor that hosts an operating system that manages all the resources attached to the PC. Display, memory and external communications are all managed by the same device. It doesn’t cost extra to have the PC module manage the screen, and because both components are flat, it can be very cost effective to simplify things and make them together as one piece of hardware. This arrangement also works to support fault and diagnostic messaging, support for maintenance and repair, and support for network communications. The added functionality and declining cost of combining flat touch screen and computer in one system make a compelling argument for all new control systems to be implemented this way.
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Advances in human machine interface hardware (HMI) Human machine interfaces or HMIs are terminals on machinery to give operators a way to monitor and adjust controls and functions. This interaction is through a graphical user interface that facilitates information exchange and communication between supervisory and machine-level HMIs. Where are HMIs useful? Everywhere. The buttons and knobs of yesterday’s interfaces are yielding to HMIs in even the humblest designs. Consider how even consumer-grade products such as office printers now include touchscreen controls to help users with common paper, ink, and networking tasks. In the same way, HMIs are increasingly common on more sophisticated industrial machinery but with the same userfriendly simplicity. Industrial HMI hardware consists of compact controllers with embedded functions, usually in the form of ruggedized touchscreens — for example, LCDs with tempered glass and castaluminum frames or even sealed enclosures for outdoor applications or indoor plant applications with a lot of oil, dirt, and machining byproducts, for example. Hardware standardization is increasingly common for HMI hardware on opensource and proprietary setups alike. HMI hardware options can also make displays satisfy specific application requirements. Depending on the application’s complexity, myriad I/O options exist — including digital and analog. Advances in HMI technology have quickened with increasingly affordable touchscreens and replacement of resistive displays with capacitive. In fact, these capacitive displays are particularly helpful in medical and food-and-beverage applications that need bezelfree designs, as they’re sleeker and let personnel clean and even sterilize machinery more easily. Solid-glass capacitive touchscreens also last longer than HMI hardware based on resistive technologies, because the screens don’t use pressure points to form circuits, so don’t wear or lose sensitivity over time. What’s more, many HMIs with capacitive displays have the multitouch capabilities of smartphones, which lets OEMs leverage user familiarity to offer intuitive interfaces.
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8/21/17 9:40 AM
M I C R O H Y D R AU L I C S.
MACRO
CAPABILITIES. REDUCING THE SIZE AND WEIGHT
OF F L U ID C O NT R O L . Designing hydraulic systems to perform flawlessly under less-than-ideal conditions is hard enough. But factor in the need to keep components as small and light as possible, and you’ve got a real challenge. Fortunately, you’ve got a real solution. The Lee Company. For more than 65 years, we’ve been engineering stateof-the-art microhydraulic components with diameters as small as 0.10 in. and weighing as little as 0.1g, but able to withstand pressures up to 8,000 psi. And because everyone of our designs originates out of an application need, and is scrutinized with 100% testing and inspection, we’re found in just about every mission-critical fluid control challenge you could imagine – from miles above the earth in satellite positioning systems, to miles below in downhole drilling. Plus many applications in between. If you require precise fluid control, and absolute reliability, go with the experts. Contact The Lee Company.
Innovation in Miniature 2 Pettipaug Rd, Westbrook CT 06498-0424 860-399-6281 | Fax: 860-399-2270 | 1-800-LEE PLUG | www.TheLeeCo.com
The Lee Company
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WESTBROOK•LONDON•PARIS•FRANKFURT•MILAN•STOCKHOLM
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Leadscrews: THE BASICS
A
leadscrew is a threaded rod or bar that translates rotational motion into linear motion. Leadscrews generate sliding rather than rolling friction between a nut and the screw. Sliding generates higher friction which results in lower overall efficiency. For a leadscrew, efficiency is the ability to convert torque to thrust while minimizing mechanical losses. Leadscrews have been a mainstay on machines for a long time. They typically have higher ratings than comparable ballscrews thanks to more contact between the nut and screw load surfaces. However, innovations in materials and helix geometry address old issues associated with leadscrew
friction, bringing it down to better than 0.10 in some cases—good for fast and dynamic applications. In fact, there’s also been an uptick in leadscrew use because of proliferating machines for 3D printing, manufacturing and medical applications. Specific manufacturing processes can determine the performance and cost of the finished leadscrew. For instance, there are three ways leadscrews can be manufactured; by machining, rolling, or grinding. Ground leadscrews are the most expensive and are generally considered to also be the highest performing. The type of thread also determines efficiency. Acme threads
are the simplest to produce and the most inexpensive, but also among the least efficient. Other types include buttress threads and square threads, which generally have the least amount of friction and higher efficiencies. Leadscrews have several advantages over other kinds of linear actuators, including a relatively high load carrying capacity. They’re also compact and simple to design into a system with a minimal number of parts. The motion is also generally smooth and quiet and requires little maintenance. Leadscrews also work well in wash-down environments because the materials used and the lubricant-free operation allows total immersion in water or other fluids.
Leadscrews can be equipped with a variety of different nut designs, depending on application requirements. For instance, shown here are two different anti-backlash nut designs from Haydon Kerk.
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LEADSCREWS
A downside is that leadscrews do not have high efficiencies. Due to lower efficiency ratings they’re not used in applications requiring continuous power transmission. There’s also a high degree of friction on the threads meaning that the threads can wear quickly. Because a leadscrew nut and screw mate with rubbing surfaces they have relatively higher friction and stiction compared to mechanical parts that mate with rolling surfaces and bearings. Several parameters help determine leadscrew performance. These include thrust, speed, accuracy and repeatability. The two most important factors in determining the performance of a leadscrew are the screw pitch and lead. The pitch is the linear distance between the threads while the lead is the linear distance the nut travels. Speed is another critical parameter. Leadscrews have a critical velocity, which is the rotational velocity limit of the screw. Reaching this limit induces vibrations in the leadscrew. Accuracy and repeatability are also critically important. The accuracy of a leadscrew is a measure of how close to a desired end point the assembly can move a load to within a given tolerance. The accuracy of the leadscrew will mostly determine the system’s accuracy. On the other hand, repeatability is a measure of how well a leadscrew assembly can repeatedly move a load to the same position.  APPLICATIONS WHERE LEADSCREWS EXCEL Leadscrews are often regarded as the less expensive alternative to ballscrews – suitable only for light-duty applications with low precision requirements. While it’s true that ballscrews often have the advantage over leadscrews when it comes to load capacity and rigidity, leadscrews’ reliance on sliding, rather than rolling, motion makes them the better choice for some applications. VERTICAL OPERATION Leadscrews are based on sliding contact between the nut and the screw, which makes them less efficient than ballscrews. But in a vertical application, this inefficiency has a benefit; it can prevent the load from backdriving when the motor is powered off. This ability to maintain a vertical load without backdriving is important in applications where the load needs to wait at the top of its stroke while other processes take place, such as tray stacking and parts sorting. In order to use a ballscrew in these applications, a brake or holding device would be needed, whereas a leadscrew can typically hold a vertical load without the use of a brake. CLEANROOM ENVIRONMENTS Lubrication is a liability in cleanroom environments, because oils and greases are major sources of contamination. And while a ballscrew (or any metal-to-metal contact bearing) requires lubrication, a leadscrew made of stainless steel with a self-lubricating polymer nut does not, making it the more cleanroom-friendly option. This is especially important in medical, laboratory, and electronics applications, where not only is it essential to maintain the integrity of the cleanroom, but where airborne lubrication particles would cause damage to delicate parts or processes. motioncontroltips.com | designworldonline.com
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HIGHLY CONTAMINATED ENVIRONMENTS Because their motion relies on sliding friction rather than rolling elements, leadscrews are much more tolerant than ballscrews of dust, chips, and debris. And despite harsh environmental conditions, leadscrews don’t require seals – which add friction and can need frequent replacement – since there’s no lubrication to keep in, and keeping contamination out is not as critical as it is for ballscrews. Applications such as woodworking and paper processing, which produce a considerable amount of dust and chips, can shorten ballscrew life significantly, while leadscrews can withstand these environments with little effect on performance or life. WASHDOWN APPLICATIONS With no lubrication to be flushed out, and a wide variety of materials and coatings for both the screw and the nut, leadscrews are an excellent choice for washdown environments. This is especially beneficial in food and beverage processing and packaging applications, where equipment must be regularly washed and sanitized. Some leadscrews can even operate when completely submerged in water, making them suitable for water-jet applications. LOW-NOISE APPLICATIONS The steel-on-steel rolling elements and recirculation inherent in ballscrews cause them to generate significant noise. Leadscrews, on the other hand, produce relatively little noise, since they rely on sliding motion, with no rolling elements. Because of their low noise level, leadscrews are often used in laboratory automation and medical imaging equipment, where the environment is inherently quiet, and users or patients could be disturbed by excessive noise. Consumer devices, such as printers, scanners, and appliances also require low-noise components, where leadscrews are the preferred choice over ballscrews. Leadscrews have moved beyond the realm of low-precision, basic linear motion devices, aided by anti-backlash nuts for higher precision and materials that can withstand challenging environments. With these advances, designers and OEMs are using leadscrews in areas once thought to be suitable only for high-precision ballscrews. For instance, in laboratory equipment, leadscrews provide virtually noise-free motion.
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Linear-motion rails, guides & systems: BEARING SPEEDS, MAXIMUM STROKES, AND MORE
Intech iCams polymer cam followers are made with the manufacturer’s Powercore material to run for at least 100 million cycles without lubrication. The cam followers work in linear-rail and rotary applications.
Linear-motion
systems go in everything from manually operated industrial drawers to advanced Cartesian robots. Mechanisms that include the former operate without power, using inertia or manual power to move loads. Components to complete the latter include ready-to-install drive and guidance designs … in the form of self-contained actuators or linear-motion machinery subsections. Some designs simply rely on the rotary-to-linear mechanism or actuator structure for total load support. However, most industrial linear designs have pneumatics, linear motors or motor-driven rotary-to-linear mechanisms to advance attached loads, as well as rails that guide and support the loads.
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Here, linear rails, rotary rails, guide rails, linear slides and linear ways are just a few options to facilitate single-axis motion. Their main function is to support and guide load with minimal friction along the way. Typical linear-motion arrangements consist of rails or shafts, carriages and runner blocks, and some type of moving element. Engineers differentiate these systems by the type of surface interaction (sliding or rolling), the type of contact points, and (if applicable) how the design’s rolling-element recirculation works. In fact, slides and rails are more advanced than ever, with advances in materials and lubrication setups (to help designs last longer in harsh applications), innovative rail geometries (to help designs withstand more misalignment and load than ever), and modular guide mounts (to boost load capacity and minimize deflection). motioncontroltips.com | designworldonline.com
8/17/17 12:36 PM
FOR MAINTENANCE-FREE LINEAR WAY PERFORMANCE, HEAD STRAIGHT FOR IKO TECHNOLOGY.
BALL
BALL SPLINE
ROLLER
IKO C-LUBE LINEAR WAY SERIES Our linear motion rolling guide lineup scores big with maintenance-free performance in a wide variety of applications. Maintenance will never slide with our self-lubricating C-Lube maintenancefree rolling guides that run up to five years. Our C-Lube family of linear motion rolling guides offers you great load capacity, high rigidity, superior running accuracy
Lubricant is deposited directly to the surface of the rolling elements.
and smooth motion for machine tools, semi conductor and liquid crystal manufacturing applications. These great performers make different from others, providing superior cost performance for your machines and offer an unending run of long term maintenance-free lubrication for up to 5 years or 20,000 km.
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MOTION SYSTEM
No matter the ultimate installation, linearmotion rails, guides, and ways enable motion along an axis or rail either through sliding or rolling contact. Myriad moving elements can produce either sliding or rolling support: ball bearings, cam roller sliders, dovetail bearings, linear roller bearings, magnetic bearings, fluid bearings, X-Y tables, linear stages and machine slides. One classic rail with sliding contact is a dovetail slide, and one classic rail with rolling contact is a ball Shown here are Rollon Compact Rail rail with a recirculating system. Sliding-contact axes that function as seventh-axis designs bearings are the more straightforward type of to let pick-and-place robots reach storage linear-motion component. These consist of a carriage or slide that areas in an automated warehouse. rides over a surface known as a rail, way or guide. Sliding contact occurs when the moving part directly contacts the rail section. Older versions of these sliding-contact rails generated considerable friction during movement, so were only suitable for basic applications. However, newer versions have self-lubricating sleeves and other features to boost positioning accuracy and repeatability. In contrast, rolling-element linear-motion systems are either recirculating or non-recirculating. Non-recirculating types use rolling elements such as bearing balls, rollers and cam followers for movement. Recirculating types use some type of moving platform that houses a bearing block. This bearing block contains raceways with rolling elements that let the platform move along the rail with little friction. Recirculating types include linear guides and ball-bushing bearings. More specifically, rolling-element linear guides come in two basic versions—those with circular arc grooves and those with Gothic arc grooves. These groove choices are a result of industry evolution that’s enabled new geometries for better load handling. Circular arc grooves contact bearing balls at two points. The Gothic arch contacts the balls at four points for bidirectional load capacity. Another option for rolling-element linear motion is ball bushings that have a bushing nut lined with recirculating bearing balls. This nut rides along a round shaft to allow axial movement. History lesson: In 1946, the manufacturer Thomson introduced ball bushings, and the technology established
FAQ: WHAT LIMITS LINEAR BEARING SPEED? DANIELLE COLLINS • EDITOR FOR LINEARMOTIONTIPS.COM High-speed motion is necessary for high throughput — as in packaging and electronic assembly. Systems here typically use linear motors or steel-reinforced belt drives with recirculating linear bearing guides for stiffness and load capacity. But speed demanded by these applications can be challenging for recirculating linear bearings, with typical ratings to 9.8 ft/sec. Depending on the type of linear guide and manufacturer, a load factor of between 2 and 4 may be recommended when maximum speed exceeds 2 m/sec. When calculating bearing life, the bearing’s dynamic load capacity is divided by the load factor to account for vibration and shock. Recirculating linear ball bearings have good running properties, with rolling contact between highly machined surfaces and minimum friction (when properly lubricated). So why is their maximum speed limited? The answer has to do with acceleration and how Force = Mass x Acceleration. Recall that the balls in recirculating linear bearings change direction as they move from the load-bearing zone to the recirculation zone ... so they must decelerate as they’re guided around the end cap by the recirculation mechanism. This deceleration produces a force on the recirculation elements — especially the bearing block end cap. Higher ball velocity (based on
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the velocity of the bearing block) makes for greater deceleration and forces on the end cap (as again, F = ma). Understanding the principle of recirculation, note there are two ways to reach higher speeds with linear recirculating ball bearings: Use an end cap capable of withstanding higher forces or reduce the mass of the balls. In fact, most linear bearing manufacturers offer recirculating ball bearings with reinforced recirculation mechanisms, including end caps. This is often the design for linear bearings labeled as high speed (and speeds to 5 m/sec). Some manufacturers also offer high-speed linear recirculating bearings with ball chains (also called ball separators, ball spacers, or caged balls) because they eliminate contact between balls (and friction and heat) and ensure each ball gets constant and sufficient lubrication. The alternative is to reduce the balls’ mass to minimize forces on the end caps. To get this, some manufacturers offer linear bearing blocks with ceramic balls. Ceramic is used because it has a low mass-to-strength ratio, and it has good rolling properties when used on a steel surface. Linear bearings with ceramic balls can reach maximum speeds up to 10 m/sec — but their dynamic load capacities are reduced by up to 30% when compared to similar bearings with conventional steel balls.
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HANDBOOK
the basic mechanism of rolling-element linear-motion bearings. In today’s designs, the bushings may also have integral flanges to support axial loads. SLIDING-CONTACT RAIL GEOMETRIES One feature of sliding carriage-and-rail setups is that manufacturers typically incorporate a ground groove in a rectangular track’s geometry (to serve as a working surface). Manufacturers typically build these rails in one of three shapes. Rails with a boxway shape or square shape are simplest. Square rails excel at carrying large loads without a lot of deflection. Manufacturers often preload square rails, and most linear systems based on square rails do not self-align. Square rails often have a smaller envelope size; the boxway rails handle the highest loads in all directions. Rails with a dovetail shape (or twin rail) have male geometry that securely engages female saddle geometry. That boosts stability and load capacity, even in unusual orientations or applications with unsteady loads. Round rails deflect less under load. In addition, systems based on round rails are inherently self-aligning, so are easier to install than other options. No matter the type, rails come in myriad sizes and lengths. ROLLING-CONTACT FUNCTIONS Rolling-element linear systems need little force to initiate motion. In addition, frictionforce variations due to speed are minimal, so these systems can position loads with small and precise steps. The low friction also lets these systems move at high speeds without generating too much heat. That minimizes wear to help machinery maintain a level accuracy for much of the linear system’s operating life. Manufacturers produce rolling-contact guides in several variations. The differences are in rolling element shape (ball or roller); rolling element size; whether the rolling contact is two or four-point; conformity of ball contact; whether the design has two, four, six or some other number of rolling-element rows; contact angle; and how the rolling-element rows are arranged—in an X or O configuration. All these design factors determine load capacity, rigidity
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HANDBOOK
MOTION SYSTEM
and friction. For example, O-shaped arrangements can withstand higher torque than X arrangements. In general, the number of load-bearing rolling-element rows influences load capacity … so more rail rows means more load capacity and rigidity. However, more rows makes systems more complex and costly. Here are more details on these rolling-contact options: Rolling elements are either linear rollers or balls. Because the rolling elements recirculate in recirculating rolling-element guides, they have a nearly infinite stroke length. They are available on flat guide ways and guide way rails. Flat guide ways are available in single or double row rolling elements. Guide way rails are often square rails. LoPro is a guide wheel based linear actuator from Bishop-Wisecarver Corp. featuring 90° steel guide tracks. The steel tracks can be up to 20 feet long and can be readily butt joined for longer lengths. A pair of opposing tracks are mounted to aluminum track plates in sections of about 10 feet long. polyurethane belting provides very long mechanical actuation with good accuracy and very high speeds while exhibiting low noise generation.
Non-recirculating roller type units have limited stroke length. Flat guide ways are dominant here and have either a grooved race compatible with crossed rollers, or non-grooved race, which uses cage and roller-type rolling elements. Recirculating elements (ball or roller bearings) between the rail and the bearing block enable precise linear motion. The coefficient of friction with rollerelement-based systems is much less than with slide based linear motion guides … about 1/50th that of non-recirculating systems. Ball-type rolling element units are also subdivided into recirculating and nonrecirculating types. The flat guide ways here typically use double row recirculating rolling elements. The guide way rail can be either round or square. If the raceway is not grooved, the rolling element is typically a linear ball bushing. If the raceway is grooved, the unit usually uses a ball spline. For square rails, the raceway is usually grooved. For ball-type rolling element units that are non-recirculating, the flat guide ways are grooved and use linear ball guides. The guide ways are round rail, without a grooved raceway, and use stroke bearings. QUICK NOTE ON FLUID-FLOATED BEARINGS Less common types of linear systems include hydrostatic or aerostatic linearmotion bearings. Because these systems have no mechanical contact, they are suitable for applications that need extremely accurate or quiet operation. Here’s how they work: A pressure regulator sends pressurized fluid between the rail and carriage. That lifts the carriage off the guideway by about 0.01 mm or so. Aerostatic versions use air as the fluid; hydrostatic linear bearings use specially formulated hydraulic oil. This type of guide is difficult to manufacture and expensive, but damps vibrations and allows for moves to 120 m/min and 10 g— useful for ultra-precision machines.
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HANDBOOK
MOTION SYSTEM
This one-piece shaft collar works as a spring tensioner.
Locking devices: DESIGNS AND APPLICATIONS
A
locking device is a mechanical component that prevents mated shafts and other machine elements from moving out of position when subject to external forces. They’re critical components, as they’re often core to the safety of entire systems. So when specifying locking devices, engineers should account for conditions such as initial installation error, temperature variations, vibration, and whether a system needs the coupling of multiple components. Designers use shaft collars in myriad moving machinery applications — including designs for aerospace, mechanical, medical, and industrial industries. In electric-motor-driven designs, they’re most common in gearbox and motor assemblies. Shaft collars: • • •
set shaft position space components on shafts limit shaft movement
More specifically, shaft collars often act as mechanical stops on cylinders and actuators, locate motor and gearbox elements, and keep shafts connected to bearings and sprockets. Shaft-collar variations are usually designed for specific tasks and geometries. Setscrew shaft collars are low cost with easy installation, so they’re common (despite the fact that use of clamping collars has grown). Setscrew shaft collars still dominate where assemblies don’t need postinstallation adjustments and where low cost is a design objective. 118
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CLAMP-STYLE SHAFT COLLARS Clamp-style collars use compression to affix components to shafts without marring. These collars distribute clamping force uniformly around the shaft (and not at one point) so users can adjust them without shaft damage. Clamp-style shaft collars are either one-piece or two-piece collars; the two-piece collar most evenly distributes force and creates the strongest hold. While these types of collars are in nearly any industry where power transmission is present, they’re currently spreading quite fast in medical applications. Like other shaft-collar styles, clampstyle shaft collars work as guides and spacers. But only collars with tightly controlled face-to-bore perpendicularity (with a TIR of less than or equal to 0.05 mm) satisfy parameters for critical designs — specifically where the collar functions as a load-bearing face or aligns critical motion components such motioncontroltips.com | designworldonline.com
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as bearings or gears. Here, two-piece collars have more holding power than one-piece designs and install in place. Some shaft collars use fastening hardware that exceeds industry standards for maximum torque and holding. Some stainless-steel shaft collars have hardware that can also resist corrosion and satisfy regulatory standards. Stainless hardware undergoes surface treatment to prevent galling. Other clamp-style shaft collars come in aluminum, titanium, plastic, or steel with a zinc or black-oxide finish. Black oxide on steel resists corrosion and smooths screw installation to boost the transfer of screw clamping forces to the collarshaft interface while preventing stick-slip. Frictional locking devices leverage a coefficient of friction between contacting surfaces. So when installing these locking devices, internal elements expand to fill the gap between machine shaft and hub — to keep components in place with friction. These devices usually take the form of metallic or non-metallic hollow cylinders, often with a slit on one side. Another friction locking device is the ubiquitous nut — using friction on shaft threads and slight bolt tension and compression of parts held together for secure attachment. Frictional locking devices don’t require keying, so aligning keys and key ways is unnecessary ... and requirements to match geometry are forgiving — so these often accommodate over and undersized shafts. Keyless designs also avoid loosely keyed components and their reduction in safety, accuracy, stability, and torque transmission. End users assembling frictional locking devices into a design simply insert the shaft into the locking
Here one-piece shaft collars work as mechanical stops to control the stroke of a linear slide.
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One- and two-piece shaft collars can align sprocket on shafts.
device ... and then the device exerts radial pressure to lock components in place. The backlash-free operation (assuming a proper fit and tolerances) allow for precise adjustments to axial position and angular timing in a system ... and (in the case of axes that need to make reversals) there’s no impact between key and keyway. These design elements make friction locking devices a viable option for many applications. But which situations are best suited for frictional locking devices and which are best to avoid? Generally, engineers should avoid specifying these locking devices in systems with high external centrifugal forces. These situations can cause a drop in the pressure between the components and lead to slipping. Because there is often a small slit in frictional locking devices (to accommodate shafts of varying diameters) these can cause imbalances in certain operating conditions ... usually at higher speeds. In such applications, engineers can use slitless friction locking devices and leverage their stricter machining and application tolerances, or use another type of locking device. Smaller frictional locking devices work where lower torque is needed. Systems delivering high torques may need larger variations or even specialty configurations. Engineers should consult manufacturer documentation and have ensure their design calculations are accurate. Manufacturers usually provide instructions for sizing locking devices, but offer assistance for specialty applications. Custom-manufactured shaft collars can have modified slots, flats, through-holes, and threaded holes ... as well as cams, hinges, mounting holes, and more. These features often reduce the number of parts needed in a system and improve structural integrity, mechanical efficiency, and drive performance.
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HANDBOOK
MOTION SYSTEM
A lubrication TUTORIAL
In
any system with moving parts, lubrication is essential. Lubrication functions to reduce friction, which dissipates heat and protects components from corrosion and wear. Lubricants are typically classified as either one of two kinds; oils or greases. Oil-based lubricants can be made from petroleum sources or newer synthetic oils. Greases have an oil base to which various thickening agents are added. Lubricants are used across a range of applications, from general-purpose industrial uses to special requirements and applications. There are greases designed for high temperatures and low temperatures, as well as for highload applications. Some greases are designed to be biodegradable and there are also food-grade greases for food and beverage production facilities. No matter the application, the most important parameters for evaluating lubricants include operating temperature, load, speed, viscosity and application rate.
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LUBRICATION FOR MOTION CONTROL Particularly in motion control applications, lubrication plays a critical role. Many kinds of components need lubrication, from ballscrews and leadscrews to bearings, gears and motors. Take ballscrews, for instance. Because ballscrews are a bearing system, they require some type of lubrication to avoid metal-tometal contact of the balls in the raceway. While the lubricant can be either oil or grease, it’s advisable to avoid solid additives (such as graphite) as they will clog the recirculation system. An NLGI no. 2 type grease is recommended, but it should also depend on the application, whether food-grade or another special type of lubrication is required. Ballscrews, especially those used in machine motioncontroltips.com | designworldonline.com
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L U B R I C AT I O N
tools, generally require lubricants with EP additives to prevent excessive wear. The frequency of lubrication will vary depending on factors such as the move cycle characteristics or contamination in the environment. Leadscrew mechanisms using bronze nuts also need a lubricant, usually a thick damping grease. Leadscrew assemblies with plastic nuts can run well without lubricant due to internal lubricants in the nut materials, but the use of a gel-type lubricant will help increase allowable loading and extend life by reducing friction. If particulates are present, the screw should be cleaned before reapplying lubricant. Scheduled preventative maintenance should occur when there is no visible film remaining on the flanks of the screw thread. As for grease, it shouldn’t be used in environments with significant particulate or debris that can load the grease and cause it to become an abrasive slurry. In this type of application, dry film lubricant should be used instead. PTFE coating is a dry film that creates a lubrication barrier between a metal substrate and a polymer bushing or lead nut. It’s well suited for use with plastic nuts and stainlesssteel leadscrews. Lubrication maintenance intervals can be eliminated and the coating does not attract particulate like a gel lubricant. BEARING LUBRICATION: OIL OR GREASE? Grease lubrication is generally restricted to relatively slower-speed applications due to reduced capability for frictional heat dissipation as compared with that of oil.
Oil lubricants are most common in high-speed, high-temperature applications that need heat transfer away from working bearing surfaces. Bearing oils are either a natural mineral oil with additives to prevent rust and oxidation or a synthetic oil. In synthetic oils the base is usually polyalphaolefins (PAO), polyalkylene glycols (PAG) and esters. Although similar, synthetic and mineral oils offer different properties and are not interchangeable. Mineral oils are the more common of the two. The most important characteristic when specifying oil for a bearing is viscosity. Viscosity is a measure of a fluid’s internal friction or resistance to flow. Highviscosity fluids are thicker like honey; low-viscosity fluids are thinner like water. Fluid resistance to flow is expressed in Saybolt Universal Seconds (SUS) and centistokes (mm2/sec, cSt). The difference in viscosity at different temperatures is the viscosity index (VI). An oil’s viscosity is correlative to the film thickness it can create. This thickness is crucial to the separation of the rolling and sliding elements in a bearing. Bearings in some applications use oil, but grease is the lubricant of choice for 80 to 90% of bearings. Grease consists of about 85% mineral or synthetic oil with thickeners rounding out the rest of the grease volume. The thickeners are usually lithium, calcium or sodium-based metallic soaps. Formulations for highertemperature applications often include polyurea. The higher viscosity of grease helps contain it within the bearing envelope. The most important considerations when choosing a grease are the base oil viscosity, rustinhibiting capabilities, operating temperature range and load-carrying capabilities.
There are a number of advantages grease has over oil: • Simplifies seal design, acts as a barrier to contamination and is a sealant • Permits pre-lubrication of sealed or shielded bearings • Generally requires less frequent lubrication • Less costly motioncontroltips.com | designworldonline.com
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ac motors OVERVIEW OF
AC
motors are driven by an alternating current (ac), but differ from dc (direct current) motors in that there is no mechanical commutation involved, and can be single or multi-phase. A typical ac motor has two basic parts; a stationary stator with coils energized with alternating current which produce a rotating magnetic field, and a rotor attached to an output shaft which produces a second rotating magnetic field. The magnetic field in the rotor can be generated in several different ways including with dc or ac electrical windings, permanent magnets or via reluctance.
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Because ac motors have no commutators or brushes, they require less maintenance than brush dc motors. With dc motors, control is done by varying voltage and current, while on ac motors the voltage and frequency (along with the number of magnetic poles) are used to control the motor. A common way to classify ac motors is based on the magnetic principle that produces rotation. So there are two fundamental types of ac motors; induction motors and synchronous motors.
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AC MOTORS
AC induction motors, such as the IronHorse line of general-purpose three-phase motors from AutomationDirect, are used across a range of applications from industrial machinery and conveyors to pumps and blowers. These motors also meet industry specifications such as RoHS and Low Voltage Directives, and are CSA and EU approved.
For induction motors, the key idea is the rotating magnetic field. The rotor turns in response to the induction of a rotating magnetic field within the stator. The most common source of this in ac motors is the squirrel cage configuration. The setup is essentially two rings, one at each end of the motor, with bars of aluminum or copper connecting the two ends. Induction motors have properties that make them especially well suited to a number of industrial as well as home appliance applications. For starters, they are simple and rugged motors that are easy to maintain. They also run at constant speed across a wide range of load settings, from zero to full-load. One drawback used to be that it was difficult to control the speed of induction motors. However, the availability of sophisticated variable-frequency drives means that even induction motors, usually three-phase induction motors, can be easily speed controlled as well. The other type of ac motor, a synchronous motor, is so named because it runs synchronously with whatever the frequency of the source is. The motor speed is fixed and doesn’t fluctuate with changes to the load or voltage. These motors are primarily used where the requirement is precise and constant speed. Synchronous motors can be used in motion control applications, but there are some down sides to using these motors. Because of the rotor size, the motor’s response in incrementing applications is typically not good. Also, because acceleration of inertial loads may not be as high as other motor types, these motors may operate at irregular speeds and produce undesirable noise. And generally, synchronous motors are larger and more costly than other motors with the same horsepower rating. Comparing ac and dc brush and brushless motors, all three have I-R power losses. Because dc motors use permanent magnets, none of their energy needs to be used to generate the magnetic field as in ac motors. The energy used by ac motors to create the magnetic field decreases the efficiency of an ac motor in comparison to dc motors. As for brushless dc motors, for the same mechanical work output they will usually be smaller than a brush dc motor, and always smaller than an ac induction motor. The brush dc motor is smaller because its body has less heat to dissipate. Also, brush and brushless dc systems provide flat torque over a wide speed range while ac motors typically lose torque as speed increases. motioncontroltips.com | designworldonline.com
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DC motors BASICS OF
Brushless dc motors, such as this EC-i 52-mm motor from maxon, are suited for applications such as rotary actuation as well as robotic joints. The 8-pole pair, iron-wound internal rotor motor features high torque density with low cogging torque.
The
main characteristic of direct current (dc) motors is that they generate a magnetic field, either via electromagnetic windings or permanent magnets. The most common dc motor types are brush, brushless, and permanent magnet (PM) motors. Some engineers call brushed dc motors wound-field motors, because a wound and lacquered coil of copper wire makes the electromagnetic field. There are permanent magnet, shunt, series, and compound-wound brushed dc motors. In a brush dc motor, the magnet acts as the stator. The armature is integrated onto the rotor and a commutator switches the current flow. Brush dc motors use commutators and brushes to pass
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current to the rotating rotor’s copper-wire windings. Speed is controlled by changing rotor voltage (and current with it) or by changing the magnetic flux between rotor and stator through adjustments of the field-winding current. This makes them suitable for applications that need simple and costeffective torque and speed control. In contrast, a brushless dc (BLDC) motor is essentially a dc motor without the mechanical commutation of the brush dc motor. BLDC motors are powered by direct current and have electronic commutation systems instead of the mechanical brushes and commutators used in brushed dc motors, eliminating mechanical wear issues. In BLDC motors, the permanent magnet is housed in the rotor and the coils are placed in the stator. The coil windings produce a rotating magnetic field because they’re separated from each other
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electrically, which enables them to be turned on and off. The BLDC’s commutator does not bring the current to the rotor. Instead, the rotor’s permanent magnet field trails the rotating stator field, producing the rotor field. Also, having precise rotor position data is important for proper commutation. This is achieved via magnetic sensing with a Hall Effect sensor, which also allows for tracking of speed and torque. WHERE BRUSH DC MOTORS STILL MAKE SENSE So with more brushless motors and an ever-increasing array of controls for all motor types, why do engineers still use brush motors? After all, it’s common knowledge that brush motors exhibit wear at their brush-and-commutator interface. The truth is that brush dc motors output high peak torques and can run on simple speed controllers. They often cost less than other options, especially in large volumes. Plus they can have a linear torque-speed relationship, which simplifies controls. These performance characteristics are most helpful in automotive designs, consumer appliances, and personal home goods and toys. PERMANENT MAGNET BRUSH DC MOTORS These motors were developed in the 1950s for power-tool applications. Here’s how they work: Permanent magnets line the inner surface of the stator to create a field flux. The field has high reluctance that minimizes armature interaction, so is steady for consistent output no matter the speed-torque requirement. Varying voltage to the armature (rotor) changes rotary output speed. In closed-loop applications, feedback from encoders, tachometers, or back-electromotive-force monitors tracks speed to let controls keep the motor on the application’s momentary target speed. Today, permanent-magnet brush dc motors are by far most common in electric-motor applications in passenger vehicles — including windshield wipers, power windows, and power seat positioners. That’s partly because permanentmagnet brush dc motors lack armature interaction so generate high starting and acceleration torques, even to 10 times rated torque. Permanent-magnet brush dc motors are also common in welding equipment, battery-powered pumps, mobile medical equipment, X-ray and tomography machines, and industrial designs that need momentary power bursts. WOUND-FIELD DC MOTORS Also called universal motors or ac series motors because they accept dc or ac input, these motors have a wound field (stator) coil in series with the armature (rotor) through a commutator. When they run on ac, current in field and armature coils, and the resultant magnetic fields, alternate or reverse polarity synchronously with the supply. Mechanical output is always in one direction (and a function of commutator and field-coil polarity) no matter the input voltage’s direction. So, wound-field dc motors have high starting torque, are compact and light, and can run at high speed, even to 10,000 rpm. Wound-field dc motors are common in small household appliances such as vacuum cleaners, cordless power tools, and food mixers. The only drawbacks are that they’re noisy and exhibit wear at the brushes so don’t work well in applications with high duty cycles.
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Even though integrated motor systems have been around for a while, development on new models and capabilities continues. Case in point: a new joint venture between HEIDENHAIN and Applied Motion Products. The effort has resulted in the StepSERVO™ series of integrated motors aimed at demanding motion control applications. The motors also incorporate a RENCO rotary encoder, resulting in a motor that combines the high torque capabilities of step motors with the closed-loop dynamic control of servo systems.
Integrated motors: COMPACT, ALL-IN-ONE MOTION SOLUTIONS
Integrated
motors, sometimes called smart motors, include a motor rotor and stator (in a brushless dc, servomotor or stepper motor form) paired with other motion components. A typical integrated motor can contain a number of common components including the motor itself, a controller or drive, feedback devices such as encoders, and power for drive and control electronics including the necessary cabling. As a result, what constitutes an integrated motor can vary. For instance, the most basic type may consist of a motor and encoder or a motor and a drive and controller along with communication ports. Integrated motors are said to offer greater reliability mainly because there are fewer parts to connect together. Also, fewer external connections mean less cabling and wiring. Less cabling and wiring reduces costs, as does the fact that the components that one would usually purchase separately such as the motion controller and the drive are integrated into one physical unit. On the down side, an integrated motor may reduce configuration options due to its lack of customizability. There is also the issue of potential vendor lock-in. Integrated motors are designed to be programmed quickly and easily, which can help reduce development times. Communication options range from simple serial
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communication links such as RS232 or RS485 to more advanced network topologies suited to complex motion control tasks such as CANopen, DeviceNet, or Ethernet protocols. For machine builders, designing with integrated motors has a number of benefits including helping to reduce machine size, cost and complexity. In some cases, integrated motors can also eliminate external controllers such as PLCs. Such integrated systems can significantly reduce the amount of space required for a machine by consolidating components, eliminating cabling, and possibly the need for entire enclosures. With the increased use of decentralized motioncontrol architectures, applications using integrated motors have only proliferated. As an alternative to centralized motion control, decentralized architectures distribute motion control to a number of individual motion axes (in this case, to individual integrated motors), eliminating the need for a central controller. This means that individual motors can execute the control closer to the actual axis of motion or load, thereby taking the computational burden off of a central controller and distributing it to individual integrated motors. motioncontroltips.com | designworldonline.com
8/17/17 12:16 PM
WITTENSTEIN alpha
Since our origination as Alpha Gear in 1984, WITTENSTEIN alpha has set the bar for excellence in motion control systems—right from the heart of the Midwest. Today our North American headquarters sits on a six-acre campus in Illinois, where we exceed customer expectations daily: •
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Linear-motor BASICS: IRON-CORE LINEAR MOTORS AND MORE
Linear
motors include a stationary platform that industry calls a platen or secondary (with electromagnetic windings) and a moving forcer or primary that sometimes includes permanent magnets. Linear motors can make fast and precise moves for positioning or move slowly and steadily for material processing. Visit linearmotiontips.com/linear-motors for more on this. Linear-motor speeds range from a few inches to thousands of inches per second. The motors deliver unlimited strokes and (with an encoder) accuracy to ±1 μm/100 mm. Myriad inspection, medical and material-handling applications use linear motors to boost throughput. As is the case with their rotary counterparts, linear motors use common drives and motion controllers. Unlike rotary motors (which need mechanical rotary-to-linear devices to get straight strokes) linear motors are direct drive.
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So linear motors avoid the drawbacks of a rotary motor with a belt and pulley for translation — lower thrust because of tensile-strength limits; longer settling times; belt stretching, backlash and mechanical windup; and typical speeds to 15 ft/sec or slower. They also avoid the gradual wear of traditional rack-and-pinion sets ... lead and ballscrew efficiencies (usually around 50 and 90%) and whip and vibration. Plus they don’t force engineers to sacrifice speed (with higher pitches) for lower resolution. In linear motors, magnetic flux through the airgap area is proportional to magnetic and electrical loading ... the vector quantity of flux lines between platen and forcer. Engineers express this value in Tesla or Gauss. Typical airgap flux densities range from fractions to a few Tesla. Linear stepper motors are an established design with a toothed forcer of laminated steel cores wound with coils. Toothed platens mount end-to-end for unlimited travel. Thrust originates from reluctance force. Linear steppers
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LINEAR MOTORS
deliver speeds to 70 in./sec — useful in inspection machines or parts transfer. Some manufacturers pair twin linear steppers off a common forcer to work as X-Y stages. These stages mount in any orientation and have high stiffness and flatness to a few μm for every hundred mm to output accurate movement. Now consider hybrid linear motors which usually have ferromagnetic platens. Those with steel platens move to 3 m/sec; those with laminated platens move faster. Much like linear steppers, they vary magnetic saturation to shape opposition to magnetic flow ... so thrust originates from reluctance force. Feedback and controls make for servo performance. Key to hybrid linear-motor performance is a yoke on the platen that makes paths through which flux travels and closes flux loops between platen teeth and forcer. Hybrid-motor drawbacks are limited output and cogging from reluctance coupling between the forcer and platen. Two setups are phase-teeth offset or driving to get partial saturation of platen teeth and sections of forcer teeth. Here, the drive only magnetically saturates working teeth sections. Some hybrid-core motors also use external cooling to get more output during continuous operation. Linear ac induction motors that run to 2,000 in./sec work for people movers, rollercoasters and large aerospace applications. General-purpose types can move a few inches to 150 ft/sec or faster. Linear ac synchronous motors are either iron-core or ironless-core motors. Ironless-core linear motors have an epoxy forcer plate holding copper coils. This forcer moves in a U-shaped magnetic platen to output up to 3,000 N and speeds exceeding 230 in./sec. These cog-free linear motors are lighter motors with potentially unlimited travel and quick acceleration — though their main benefit is smooth output. Their speed is helpful in flying-shear applications and long-stroke pick-and place machines in semiconductor fabrication and elsewhere. Other applications exist for waterjet and laser cutting and robotics tasks. Iron-core motors have slotted steel lamination stacks (insulated to reduce Eddy currents) to output 7,000 N or more. The forcer coil setup includes these steel laminations and windings in a single or three-phase configuration. This allows for control directly from a line or through a drive. Some such linear motors use water cooling to boost force output — for heavy uses as on baggage handling and amusement-ride axes. Iron-core motors are suitable for some machine-tool applications as well. Cylindrical linear motors are sometimes called tubular linear motors. These have steel rods and a moving coil or rods filled with stacked magnets. With the same footprint as a lot of linear actuators, these offer high stiffness and other advantages over other linear-motor designs … and work in myriad machines that need quick and accurate strokes.
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Aerotech’s new mechantronic ThermoComp system addresses up to 90% of thermal-related errors in linear positioning stages — including those exhibited by linear-motor and ballscrew-based linear stages. In fact, temperature changes and thermal effects are some of the largest error sources in precision machines. That’s because it’s often too complicated and costly to build thermally insensitive machines. The most frequent approach to reducing thermal errors is to regulate the environment in which the machine operates. While that minimizes thermal effects, it’s also expensive and unreliable. Plus the best-case tolerance of ± 1°C is usually insufficient anyway. In contrast, Aerotech’s ThermoComp is a complete mechatronic setup that uses integrated hardware and sensors with a proprietary compensation algorithm running on Aerotech’s A3200 controller software. Elimination of up to 90% of thermally induces errors are eliminated — regardless of stage travel and temperature fluctuations.
FAQ: HOW DO IRON-CORE LINEAR MOTORS WORK? Linear motors are often classified as either ironless or iron core, referring to how their primary parts are constructed. Ironless linear motors have a primary of windings embedded in epoxy resin. In iron-core linear motors, the windings are mounted in an iron lamination stack. For both motor types, the number and length of the windings determine how much force the motor can produce. Iron-core motors have a force density (force per working area) up to twice that of ironless motors. So to produce a given continuous force, an ironless motor must be twice as large as a comparable iron-core design. Force here is the product of current through the windings and the flux density in the primary: F = I x B. Ironless linear motors have a primary that’s embedded in resin, so there’s no magnetic attraction between the primary and secondary ... so forces these motors can produce are smaller than those produced by ironcore designs. For iron-core linear motors, magnetic attraction between the primary’s iron and secondary’s permanent magnets allows high force output — though this attractive force also creates cogging. Cogging is detent force that the motor experiences when the steel laminations of the coil cross the magnets of the secondary. This force degrades the smoothness of movement and can be significant ... making iron-core motors less desirable for applications that need extremely 8 • 2017
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smooth motion. That said, some manufacturers have developed methods to reduce the effects of cogging — by skewing the magnets of the secondary part (which eases the change in attractive forces as the primary moves across the magnets) or by using feedback and controls to compensate for the effects of cogging. In addition to their ability to produce very high thrust forces, iron-core motors also dissipate heat thanks in part to their relatively open design. But this design also leaves them susceptible contamination — particularly metallic chips or flakes that attract to the permanent magnets and cause damage. Another factor that contributes to the iron-core linear motor’s good heat dissipation is the fact that their primary is mostly metal (as opposed to the epoxy enclosure of an ironless design) which acts as a heat sink. When designing a linear system using iron-core motors, account for the attractive force between the primary and secondary parts, as that’s essential to properly sizing the support bearings. Linear profiled rails are the most common guide systems used with linear motors, although air bearing systems are sometimes used. Because air bearings need a preload, the attractive force between the windings and the magnets is beneficial for an iron-core motor using air-bearing guides. With high continuous forces and good heat dissipation, ironcore linear motors are suitable for pressing, molding, and machining applications. They also excel at high-speed testing that requires the application of high forces or pressures.
This LinMot PR01 linear-rotary motor comes in myriad sizes for different force and torque outputs. The motor integrates a linear motor with an attached rotary torque motor. Controls independently command the two.
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piezo elements and piezo motors? WHAT ARE
Piezo
elements are ceramics that change shape when subject to electricity or (when subject to mechanical loading) output electricity. Manufacturers process and press plumbum, zirconate, titanate (PZT) powder together and fire it into a quartz. Next they integrate the ceramic with ferroelectric material to make electrodes. Then the manufacturers apply electromagnetic fields to the piezo elements to align and polarize the material. Discovered by Pierre and Jacques Curie in the late 1800s, piezo elements are named for the Greek word for press — piezein. The Curies found that compressing crystals of potassium sodium tartrate (also called Rochelle salt after the place of its original formulation) create electric voltage … and that electrifying piezo crystal makes it deform. Today, other piezo crystals immune to humidity are used in engineered systems … but the shape-changing capacity is what all piezo technologies use. TYPES OF PIEZO MOTORS Piezo elements go into myriad transducers and sensors, and that use is perhaps the best known. But some arrangements of piezo elements act as solid-state piezo motors to move loads over short distances. Basic piezo motors generate motion by expanding proportionally to voltage. Stacked, shear, and tube piezo motors are three common options here. The former generate high forces but only to 20 µm or so. The latter (common in medical and dispensing applications) can move several millimeters but with less force. Slightly more complex are flexure-guided piezo motors with motion amplifiers for long and straight moves. Motion is proportional to the drive voltage; multi-axis stages move up items a couple millimeters or more. Ultrasonic-friction piezo motors have oscillating plates that act as the stator to advance a slide or rotor via friction. These output unlimited motion quickly, sometimes within 0.8 msec from the input command — though resolution is only 60 nm or so. Note that some sources call ultrasonic piezo motors standing-wave piezo motors for the way in which they output motion. The only catch with ultrasonic piezo motors is that their reliance on friction to advance a slide or rotor essentially limits resolution to about 35 nm. For stepping piezo motors, manufacturers gang multiple piezo elements together to get actuator setups that can move more than 100 lb. The motors make longer strokes than most other options, and they do it quickly — within less than a millisecond in many cases. Piezo stepping motors can also get picometer resolution under direct piezo actuation under what’s called dithering.
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P I E Z O - B A S E D M I N I AT U R E M O TO R S
Advancing controls are getting increasingly stringent performance from piezo-flexure stages that output highbandwidth actuation over multiple degrees of freedom. These drive advanced microscopies that burst the Rayleigh limit; semiconductor lithography that tracks Moore’s Law down to the atomic scale; silicon photonics manufacturing; and genomics analyzers, to name a few applications. MORE ABOUT ULTRASONIC PIEZO MOTORS As mentioned, in ultrasonic piezoelectric motors the ceramic material vibrates over a few nanometers at a time to output linear or rotary motion to 550 mm/sec or more. Electricity excites a piezoceramic plate (that acts as the motor stator) through electrodes. That induces oscillations at the material’s natural frequency at some number of kilohertz. A plate has a friction pusher that takes it along an inclined path at an eigen frequency. With each oscillatory cycle, the motor advances. Two main benefits of ultrasonic piezoelectric motors are that they are efficient substitutes for miniature electric-motor spindles; and they make moves with resolution to 50 nm.
The leading piezo-motor option for longer strokes are steppers. These have an array of individual piezo elements that perform a sequence of expansions and contractions. Each cycle only moves the attached load a few micrometers … but at an average of 500 to 3,000 Hertz, the steps makes the motor reach 12 mm/sec. Useful for scanning applications, stepper piezo motors can also withstand external magnetic fields. Rotary step piezo motors cycle frame-affixed crystals through states. One crystal set locks the rotor while a second set moves to advance a third. Controls then release the first crystal set and retract the third. Then both locking sets return to home. In contrast, linear step piezo motors cycle frame-affixed crystal sets through locked and motive settings. Usually one set is moving while the other two lock. That advances an output stage plate even to 1,100 mm/sec.
Multi-Axis Precision Motion PI Hexapod 6-axis parallel positioning systems with user-programmable pivot point speed up alignment, complex positioning, and motion simulation tasks. With load capacities from 1kg to 2,000kg and precision from nanometers to microns, Hexapod success stories include photonics, medical, and automotive applications since 1993.
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PI designs and manufactures precision motion systems at locations in the USA, Europe, and Asia. With over 40 years of experience developing standard and custom products based on piezoceramic and electromagnetic drives and more than 1,000 employees in 13 countries, PI can quickly provide a solution for your positioning and automation projects in industry and research. Physik Instrumente www.pi-usa.us 508-832-3456 (East) 949-679-9191 (West) 100mm base
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PRECISION | SPEED | STABILITY - MOTION CONTROL & POSITIONING SOLUTIONS motors, piezo-based mini — Motion System Handbook 8-17 V3.LE.indd 139
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The advantage of directdrive servomotors is the ability to attach a load directly to the servomotor, allowing a full range of motion control without the need for a shaft coupling or gearhead. The SGM7F line of direct-drive servomotors from Yaskawa deliver from 4 to 35 Nm of torque in a device as small as 2.1-in. high and 5.3-in. in diameter (53 x 135 mm).
servomotor? WHAT IS A
The
hallmark of a servo system is that the system’s error (in position, speed, or torque) is corrected through the use of a feedback device that allows a controller to compare the system’s actual performance with its commanded performance. What makes a motor a servomotor, then, has less to do with its construction and operation, and more to do with whether it incorporates feedback in a closed-loop system. As for servomotors themselves, their greatest benefit is that they provide precise control of torque, speed or position using closed-loop feedback. They also have the
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ability to operate at zero speed while maintaining enough torque to maintain a load in a given position. Servomotors have several distinct advantages over other types of motors. For starters, they offer more precise control of motion. This means they can accommodate complex motion patterns and profiles more readily. Also, because the level of precision is high, the position error is greatly reduced. So what type of motors can be used in servo systems? That’s not an easy question to answer, since there are many ways to categorize motors and manufacturers often use different terminology to describe the same type of motor. This can make deciphering motor classifications highly confusing. To clear things up, here is a guide to motor terminology and a brief explanation of which types are most commonly used in servo systems.
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There are three main ways to classify motors: by their current—ac or dc; by the way in which they achieve commutation—brushless or brushed; and by the speed of their rotating field (rotor)—synchronous or asynchronous. AC VS DC The most basic classification of a motor is whether it is an ac or a dc motor, based on the type of current used. From a performance standpoint, the primary difference between ac and dc motors is in the ability to control their speed. In a dc motor, speed is directly proportional to the supply voltage (given constant load, or torque, on the motor). An ac motor’s speed is determined by the frequency of the applied voltage and the number of magnetic poles. While both ac and dc motors can be used in servo systems, ac motors can withstand higher current and are more commonly used in industrial servo applications. BRUSHED VS. BRUSHLESS When discussing dc motors, the next point of differentiation is whether the motor is commutated mechanically, with brushes, or electronically, without brushes. Brushed motors are generally less expensive and simpler to operate, while brushless designs are more reliable, have higher efficiency, and are less noisy. Brushed dc motors are further sub-divided according to the construction of their stators: series-wound, shuntwound, compound-wound, or permanent magnet. While the majority of motors used in servo systems are brushless designs, brushed permanent magnet dc motors are sometimes employed as servo motors for their simplicity and low cost. The most common type of brushed dc motor used in servo applications is the permanent magnet dc (PMDC) motor. Brushless dc motors replace the physical brushes and commutator with an electronic means of achieving commutation, typically through the use of Hall effect sensors or an encoder. Brushless dc (BLDC) motors are also used in servo systems. Generally, ac motors are brushless, although there are some designs—such as the universal motor, which can run on either ac or dc power—that do have brushes and 142
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SERVOMOTORS
The Tolomatic IMA electric linear actuator is an integrated servomotor rod actuator available with a food-grade white epoxy coating and stainless steel components for washdown applications in the food and beverage industry. The actuator integrates a servomotor and a ball or roller screw and features a hollow-core rotor design that allows the nut of the screw to pass inside the rotor, creating a compact package with an IP67 protection rating.
are mechanically commutated. Terminology for brushless ac motors can be quite confusing, as they are also referred to as permanent magnet ac motors or permanent magnet synchronous motors.
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SYNCHRONOUS VS. ASYNCHRONOUS While dc motors are generally categorized as brushed or brushless, ac motors are more frequently differentiated by the speed of their rotating field— i.e. synchronous or asynchronous. In an ac motor, speed is determined by the frequency of the supply voltage and the number of magnetic poles. This speed is referred to as the synchronous speed. In a synchronous motor, the rotor rotates at the same speed as the stator’s rotating magnetic field. In an asynchronous motor, normally referred to as an induction motor, the rotor rotates at a speed slower than the stator’s rotating magnetic field. When induction motors are paired with variable frequency drives, they can achieve speed control and performance similar to servo motors, although they typically don’t incorporate feedback, and therefore, are not true servo devices.
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This SC3518 series stepper motor from Nanotec is representative of modern stepper motor design. With a 1.8° step angle and a connector for simple cable adaptation for user-specific applications, the stepper motors have an optional shaft end for open-loop or sensorless control or are available with an integrated encoder.
Stepper motor FUNDAMENTALS
The
hallmark of a stepper motor is the ability to be accurately controlled down to fractions of a degree without the use of any feedback devices such as encoders or resolvers. The control is open loop as opposed to closed loop like many other motor types, including servomotors. Stepper motors are typically classified by the number of steps they can be commanded to move. For instance, a 1.8 degree step motor is capable of 200 steps/revolution (1.8 x 200 = 360 degrees, or one full revolution) in full-step mode. If operated in half-step mode, each step becomes 0.9 degrees and the motor can then turn 400 steps/revolution. Another mode called microstepping subdivides the degrees per step even further, allowing for extremely precise movements. There are several different types of stepper motors including permanent magnet (PM) motors, variable reluctance (VR), and hybrid types. Stepper motors operate in a fairly straightforward way. Traditional VR stepper motors have a large number of electromagnets arranged around a central
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gear-shaped piece of iron. When any individual electromagnet is energized, the geared iron tooth closest to that electromagnet will align with it. This makes them slightly offset from the next electromagnet so when it is turned on and the other switched off, the gear moves slightly to realign. This continues with the energizing and de-energizing of individual electromagnets, thus creating the individual steps of motion. Hybrid steppers combine the best features of both PM and VR type stepper motors. The rotor is multi-toothed like the variable reluctance motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide a path to help guide the magnetic flux to preferred locations in the airgap. This further increases the detent, holding, and dynamic torque characteristics of the motor when compared with both the variable reluctance and permanent magnet motor. Hybrid stepper motors are usually more expensive than PM stepper motors but can provide better performance with respect to step resolution, torque and speed. motioncontroltips.com | designworldonline.com
8/17/17 12:27 PM
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HANDBOOK
MOTION SYSTEM
Control techniques such as half-stepping and microstepping let designers get even finer movements of rotation, which make for more exact output than that from VR stepper motors (which can’t usually be microstepped). Hybrid stepper motors also have higher torque-to-size ratios and higher output speeds than other stepper-motor types, and are also quieter than VR stepper motors. When the current changes, the rotor can turn a small amount—an improvement over basic PM motors and VR motors. Though stepper motors are capable of producing high torque at low speeds, they generally are well suited for lower power applications, not for applications requiring lots of torque. A few other drawbacks include the possibility that not properly controlling the motor can produce undesired resonance in the system. Also, stepper motors are generally not easy to operate at extremely high speeds, and as the motor speed increases, torque decreases. There are two basic kinds of winding structures for two-phase stepper motors; unipolor and bipolar. A unipolar arrangement uses 6 wires but current can only flow in one direction. These types of motors also require a unipolar driver. A bipolar winding uses 4 wires and current can flow in 2 directions and it requires a bipolar drive. Bipolar motors are generally more efficient and can provide more torque than unipolar models, although they can heat up faster than unipolar motors. A stepper motor’s low-speed torque varies directly with current. How quickly the torque falls off at higher speeds depends on a number of factors such as the winding inductance and drive circuitry including the drive voltage. Steppers are generally sized according to torque curves, which are typically specified by the manufacturer. Sufficient documentation of a stepper motor includes a torque-curve that shows both pull-in and pullout torque, two critical stepper motor parameters. Lastly, before basing design specifications on a stepper motor torque curve alone, look for data on the following important criteria to ensure the best design fit; power input (which impacts current draw and the possibility of overheating), the drive type, as well as the relation between step angle and torque. REQUIREMENTS FOR STEPPER MOTOR ACCELERATION Another important consideration is the acceleration when starting a stepper motor. Simply programming the controller to advance the motor a given number of steps isn’t sufficient for reliable operation. Programming must also account for steppermotor construction and operation. Inertia ratio is critical to stepper motor acceleration. Too great a difference in inertia ratio between system and motor limits rates of acceleration and deceleration, increasing the likelihood of missed steps. So when starting a stepper motor, acceleration and deceleration should happen through pulses to the motor that start slowly and gradually quicken in a process called ramping. Another consideration when accelerating a stepper motor is current supply. Too little current and too high an acceleration means that the motor won’t have enough power to accelerate both itself and the driven load. It may stall if this condition persists. On the other hand, every system has an upper limit of maximum allowable current supply. Algorithms for determining the proper ramping method and subsequent acceleration are complicated, but simplified algorithms exist to aid in design and implementation. Whatever algorithm is used, it should work well enough to ensure that there are no lost steps or stalls.
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HANDBOOK
MOTION SYSTEM
Networking for motion: FIVE REALTIME ETHERNET-BASED PROTOCOLS
Industrial
communication protocols — including traditional fieldbuses and permutations based on Industrial Ethernet — are systems of rules for messaging between automation components. Variations abound, and there’s no dominant setup with which all devices work. What is clear is that communications offering more connectivity than current-loop (4-to-20-mA) analog communications continue proliferation into even once-simple motion applications. Serial-networking hardware and proprietary protocols endure, and fieldbus networks (especially PROFIBUS) persist and grow. IEC 61158-defined fieldbuses work in setups for which they were developed and beyond, put to use by thousands of components sold and in use today. But while some fieldbuses can still be more reliable than nondeterministic Ethernet setups, Ethernet speeds have come to lead. No wonder then that Industrial Ethernet-based networks continue to spread faster than other options — so that they’re now part of nearly
40% of all industrial communication networks. Other reasons include Ethernet’s ubiquitous and standardized design; widespread support; economies of scale from the worlds of IT, consumer electronics, and commercial off-the-shelf (CotS) Ethernet; ease of use and familiarity; allowance of multiple protocols on a network; and (as it’s often built-in) elimination of traditional networkinterface cards. As a reminder on USB, RS-232, RS-485, and Ethernet hardware, connections complete communication
ETHERNET-BASED PROTOCOLS CONTINUE GROWTH ETHERNET/IP OTHER ETHERNET
SERCOS
CC-LINK
ETHERCAT
PROFINET
OTHER FIELDBUS
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buses, but aren’t protocols and don’t constitute complete setups. To illustrate, Ethernet is merely the basis for the protocol of POWERLINK. Controls hardware often dictates the fieldbus communication a design will use. One benefit here is that offerings using traditional device and fieldbus networks can have multiple physical layers (including RS-232, RS-485, and CAN) but Ethernet networks run on one physical layer adherent to IEEE standard 802.3. What’s more, even though older variations such as 802.3c is slower than 802.3ab (at 1 Gbit/sec — the version called GbE) all use Cat. 5e cable. That makes all permutations of Industrial Ethernet implementation easier. In fact, many fieldbus components can connect to blocks that support Ethernet connectivity. Once on Ethernet, they connect via Ethernet RJ45 connectors and Cat. 5E cable. Ethernet standardization also facilitates IIoT functions, as there’s commonality between systems for networking to enterprise-level systems, connected machines, I/O, and devicelevel functions and components. These unified IIoT architectures even encompass wireless variations and Internet connectivity to systems on the Cloud. That makes for communications that combine myriad data streams related to: • • • • •
Motion control and communications with controls and I/O Dedicated safety functions to protect personnel and equipment Plant energy management and machine health monitoring and diagnostics Production output functions such as product tracing Tracking of plant-wide statistics and the transmission of manufacturing data to plant managers
So now let’s look at five leading realtime protocols that capitalize on the advantages and provide deterministic networking as well: CC-Link, EtherCAT, EtherNet/IP, PROFINET, and SERCOS III. CC-LINK: LEADING ETHERNET-BASED PROTOCOL CC-Link of the CC-Link Partner Association (CLPA) is a protocol that includes field-device, controller, and information networking. CC-Link versions support various physical layer layers and includes CC-Link, CC-Link/LT, CC-Link Safety, and CC-Link Industrial Ethernet (IE) Control and Field. Also encompassed by the standard is Seamless Message Protocol (SLMP), a software protocol to let Ethernet-enabled devices work on CC-Link IE networks. There’s also a new version of a description-language specification (Control and Communication System Profile or CSP+) to simplify integration of engineering tools and device profiles. More specifically, CSP+ is a specification that outlines necessary data for setup, operation, and maintenance of CC-Link-compatible devices. 150
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Though some still perceive the network to be proprietary, CC-Link is an open protocol that myriad equipment manufacturers use to impart networking functionality to PCs, PLCs, robots, servomotors, drives, digital and analog I/O modules, and various controllers. The protocol has speeds to 10 Mbps at 100 m and good speeds over longer runs. Reconsider CC-Link IE Field, which supports the demands of advanced automation. It leverages GbE, so can handle IIoT data-stream quantities and speeds. The network links field-level devices to controllers; it also connects controllers to other controllers and proves absolute deterministic communications without the use of switches or detailed knowledge of Ethernet networking. CC-Link IE Field is IEC approved for safety communications; it also supports integrated motion control and energy management. In fact, it’s a rare Industrial Ethernet implementation actually designed from its inception for industrial environments with a token-based network (in which all networked devices keep control of the network for set times) for inherently deterministic control. RISING TIDE OF ETHERCAT EtherCAT is short for Ethernet for Control Automation Technology. It uses Ethernet and twisted-pair or coaxial cables with BNC adapters over distances to 1,000 m and optical fiber cable beyond that. Transfer rates are those of Ethernet … and EtherCAT can control up to 65,535 nodes. Developed by Beckhoff, it’s fast and deterministic — processing data using dedicated software and hardware. It accommodates any topology and uses a full duplex master-slave configuration. It handles 1,000 I/O points in 30 μsec and communicates with 100 servo axes in 100 μsec. Axes get set values and control data and report actual position and status. A distributed clock technique that’s a simple version of IEEE 1588 synchronizes the axes with less than 1 μsec of jitter. The EtherCAT protocol delivers fast throughput because hardware processes motioncontroltips.com | designworldonline.com
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messages before forwarding to the next slave. Each slave reads data relevant only to it as the data frame passes, and inserts new data into that data stream on the fly. This procedure doesn’t depend on the runtime of the protocol stack, so processing delays are typically just a few nsec. ETHERNET/IP Ethernet/IP (short for Ethernet Industrial Protocol) began as Rockwell Automation’s adoption of Common Industrial Protocol (CIP) standards. EtherNet/IP is an industrial application-layer protocol for communication between industrial controls and components such as programmable automation controllers, PLCs, and I/O. As an implementation of CIP, the protocol uses standard TCP/IP (IEEE 802.3) and communications on existing network infrastructure. Ethernet hardware pairs with TCP and UDP ports. A main Ethernet/IP advantage (actually common to all the protocols we profile here) is an inerrant progress of physical Ethernet from 10 Mbits/sec to 10/100 Mbits/sec to GbE and more. Consider the specific case of Ethernet/IP CIP Motion — a deterministic offering from the Open DeviceNet Vendors Association (ODVA) to support multi-axis distributed motion control. In short, application profiles let users set position, speed, and torque loops in a drive. This plus ODVA CIP Sync technology (an IEEE-1588-compliant Clock Synchronization mapped into CIP object models) allows synchronized multi-axis motion control. Contrast this with event-based synchronization that uses hard-scheduled deliveries of time-critical data across the network. Such synchronization necessitates jitter of less than 1 µsec for cyclic data to get precision positioning and speed control … which traditional Ethernet’s Carrier Sense Multiple Access/Collision Detection (CSMA/CD) data layer can’t deliver. ODVA EtherNet/IP overcomes the limitation without changing standard Ethernet’s four lower layers. Here, Ethernet and TCP/UDP/IP remain unmodified, and obligation for determinism is put to networked devices. These use upfront timing data, tightly synchronized device clocks, and timestamped message information to handle realtime demands ... So EtherNet/IP with CIP Motion has clock synchronization to than 200 nsec to satisfy even demanding motion-control applications. SERCOS: REALTIME IS ITS MIDDLE NAME SERCOS (short for Serial Realtime Communications System) is an open protocol for high-speed serial communication of standard closed-loop realtime data
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over a noise-immune fiber-optic rings (in the case of SERCOS I and II) or Industrial Ethernet cable in the case of SERCOS III. It’s a digital bus for networking motion controllers, drives, I/O, sensors and actuators for numerically controlled machines and systems. SERCOS interfaces close servo loops in the drive to reduce computational load on the motion controller (and synchronize more motion axes than otherwise possible). Closing servo loops this way also minimizes the effect of transport delays between motion controls and drives. SERCOS III is an IEC-standard version. It combines Ethernet and previous SERCOS design benefits for deterministic bidirectional realtime motion and control. It also enables rich I/O communication and efficient transmission of all conventional protocols over the same Ethernet network — in parallel — with SERCOS realtime communication. SERCOS III has cycle times down to 31.25 μsec. It supports 511 slave devices in each network, with multiple networks possible in an installation. PROFINET PROFIBUS (by umbrella organization PROFIBUS & PROFINET Intl. or PI) is short for Process Field Bus. It’s the most common classic serial fieldbus, with use on nearly 20% of all networked industrial installations. The design uses slave-slave, master-slave, and mastermaster communication via twisted-pair or optical-fiber cable. Contract that with PROFINET from the same organization, which uses Industrial Ethernet to get more bandwidth and speed. Like PROFIBUS, it uses GSD files to define connected-device hardware (though PROFIBUS GSD files are ASCII while PROFINET GSD files are XML.) PROFIBUS users benefit from familiarity with the design when migrating to PROFINET. As with the other protocols, PROFINET’s basis in Ethernet leverages larger message sizes, better bandwidth, and unlimited address space (though with the latter, controllers impose processor and memorybased limits.) Another reason PROFINET speeds are fast is because of provider-consumer setups that (unlike more familiar master-slave setups) don’t designate a network-controlling device. The arrangements let nodes on a network communicate as needed. Because PROFINET leverages switched Ethernet networks that don’t exhibit collisions, it gets deterministic cycle times for all speeds using both realtime and isochronous realtime (IRT). Realtime makes for update times down to 250 µsec, while PROFINET IRT has them to 31.25 µsec for synchronizing all networked components.
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R E TA I N I N G R I N G S
retaining rings THE BASICS OF
Retaining
rings are fasteners that hold components together on a shaft or in a housing when engaging a groove. Consider some variations ... Tapered-section rings typically have decreasing thickness from the center of the ring out to the ends. They mount either axially or radially. The taper ensures full contact with the groove when installed. Constant-section retaining rings have a constant width around the circumference of the ring. When installed, these rings do not maintain uniform contact with the entire component. They take on an elliptical shape and make contact with the groove at three points. Spiral retaining rings are installed into the housing or onto the shaft, making full contact with the groove and component. Their grooves are relatively shallow, so their load bearing capability is reduced. Spiral rings are often selected when full contact with the retained component or a lower axial WHAT IS EDGE MARGIN? profile is required. Spiral rings have no This is the distance from the groove (for the retaining ring) protruding ears to interfere to the end of a shaft or housing. with mating components in Edge margin depends partially an assembly. The ring has a on the groove’s depth. Rule of uniform cross-section and no thumb: When edge margin is about triple (or more) the groove gap or lugs for a functional depth, the groove can withstand and aesthetically pleasing ring. the same level of thrust load as Unlike traditional fasteners, the mating ring. retaining rings eliminate
BOWED PRELOADING RING
•
RADIAL RETAINING RING
machining and threading, reducing costs and weight. Spiral retaining rings do not require special tools for removal and come with standard with removal notches for easy extraction from grooves. When selecting a retaining ring for an application, several factors dictate which is most suitable. What kind of assembly does the application require? Is it a housing assembly or shaft assembly? Next, determine the main critical dimensions. These include the housing or shaft diameter, groove diameter and the groove width. Also, what is the rotational speed (usually in rpm) of the assembly? Next, determine the maximum thrust applied to the ring. Generally speaking, designers define this thrust as either a light, medium or heavy-duty load. It’s important that the design engineer define the maximum thrust because its value also helps determine if groove deformation or ring shear could be a problem. Basically, groove deformation occurs because the groove material is soft, which in turn limits maximum capacity. Ring shear, on the other hand, occurs when the groove material is hardened but the load exceeds the ring’s maximum capacity. Other factors, such as the temperature as well as the presence of any corrosive media, also dictate the most suitable ring material.
•
INTERNALLY MOUNTED AXIAL RING
Bowed retaining rings eliminate play in assemblies. Radial rings widen during installation for an interference fit once in place. They only wrap a portion of the groove circumference so withstand lower forces than alternatives, but are easy to install. Axial rings slide into internally machined grooves.
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Shock and vibration damping technologies safely decelerate loads in motion systems. Pictured left to right are ACE Controls’ heavy industrial shock absorber, TUBUS profile damper, PET shock absorber, MAGNUM industrial shock absorber, and FYN series rotary damper.
shock and vibration damping TYPES OF
TECHNOLOGIES AND THEIR USES
Industrial
automation systems almost always require heavy-duty motion and high speeds. But when these components must decelerate and stop, damage to the load and the machine itself can result. This comes from the release of kinetic energy, which causes long-term vibration and fatigue and sudden shock in a system. This is why it’s necessary to decelerate a system smoothly through the use of shock and vibration attenuation components.
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Based on the type of inputs present in the application, vibration and shock attenuation components can be comprised of shock absorbers, linear dampers, wire rope or spring isolators, elastomeric isolators, air springs, or structural damping treatments. These devices help manufacturers reduce equipment downtime and costly cycle time limitations. These products can be used in a broad range of applications, from the rate control mechanisms that slow the motion of overhead luggage bins or seat recline on commercial aircraft, to the isolators that keep GPS systems from losing signal or becoming damaged on farm and construction equipment as they harvest crops or pave roadways.
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SHOCK AND VIBRATION DAMPING Most shock absorbers achieve their damping characteristics through the use of hydraulic fluids. The fluid is pushed by a piston and rod through small orifice holes to create damping, and this action compresses some type of gas. This in turn creates a spring force to return the rod back to its starting position when the load is removed. Shock absorbers and dampers are generally made of high-strength steel to handle the pressures from the internal hydraulic forces. Elastomeric seals prevent the fluid from leaking out of the cylinder, and special plating and coatings keep the units protected from harsh operating environments. Recent and ongoing developments in sealing technologies and in the internal designs of shock absorbers and dampers have allowed for longer service life and more compact designs. Ongoing research in the field of noise attenuation (high frequency, low amplitude vibration) has led to an increased effectiveness in noise reduction technologies.
A unique application for these types of hydraulic damping devices has come with the increased awareness for seismic and environmental protection of our infrastructure (buildings and bridges, for example). By adding damping to these critical structures, energy is absorbed by the hydraulic devices instead of damaging the structure. VIBRATION ISOLATION Vibration isolation products rely generally on mechanical designs to achieve their isolation characteristics. A spring function provides support for the mounted equipment, while decoupling it from the vibration source. Friction and elastomeric material properties give the isolators their damping characteristics. Isolators can be made from a variety of materials. Wire rope and spring isolators can be made from carbon steel, stainless steel or aluminum. Elastomeric isolators generally have metallic components that function as mounting brackets, separated by an elastomeric material that provides the stiffness and damping desired. Common elastomeric compounds include natural rubber, neoprene
Vibration damping material pads, such as these custom designs from Sorbothane, are available in any size, shape and durometer to isolate vibrations. Sorbothane provides vibration and acoustic damping and isolation.
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Gas springs, also called gas dampers, tension values — for rod extension and rod retraction — at springs and gas-pressure springs depending on normal ambient temperature and with the piston the setup and context, are compressed-air or oil rod pointing downward. (Note that typical ranges cylinders that install in motion designs to damp are only those most common; some gas-spring forces and return kinematic linkages and more applications in heavy industries use gas springs complicated assemblies to default positions. Gas delivering several hundred-thousand Newtons springs work through a piston on the end of a cases.) Other gas-spring definitions include two rod that protrudes from a steel cylinder body; pull-in forces — at rod extension and rod retraction usually compressed gas (often nitrogen) within the — and overall friction force. These values depend cylinder exerts force on this piston to reassume on the gas spring’s gas and damping-oil volumes. and maintain set positions. Nitrogen is common Various nozzle orifices and oil quantity allow control here because it’s inert and nonflammable. In such of push-out and push-in speed. designs, oil or grease between the piston and other If design parameters are unknown, look for contacting parts minimize friction. In fact, the small manufacturers capable of prototyping — especially amount of oil in these gas springs serves another for designs requiring an exact force that’s hard to function — to further damp and gently decelerate pre-estimate — as in lifting a frame in a set time, for gas springs during full extension or compression. example. Here, some manufacturers sell prefilled Some setups even include a fine hole in the piston cylinders sporting bleed valves. Then installers can for damping that’s still slower than with other bleed gas from the cylinders after system setup to designs; such slow-damper springs are common on get the correct force-acceleration actuation profile. safety gates and doors. The only caveat here is that if too much gas is bled, In contrast, extended-reach gas springs the assembly will need a new spring. That’s why usually leverage telescoping mechanisms pairing OEM-level quantities of gas springs justify premultiple cylinders on one rod; then the smaller engineered cylinders with preset pressurization. Or cylinder extends from within the larger cylinder. gas springs can offer full in-design adjustability via Consider one particularly long-stroke application: bleed valves and movable-endstop pressurization Passive heave compensators — systems on ships mechanisms, Bowden cables, knobs, and more. or offshore oil-rig systems that reduce the effect of Some emergency-use gas springs also employ gaswaves on engineered structures — use gas springs generator cartridges that resemble those in airbags. with strokes to many meters long. Still other gas-spring applications include those for GAS-SPRING FORCES AND STROKES medical beds and hoists; industrial equipment such as machinetool presses; off-highway and automotive equipment for hatches, hoods, and covers; office equipment F₄ and furniture; and general strut and N S IO N G EXTE IN R P support applications. Fast-acting G A S -S F₃ F₂ S IO N MPRES gas springs find use in weaponry O C G R IN G A S -S P and aerospace design. Specific F₁ variations include gas springs with standard or fixed-height cylinders; spindle-only designs; and cable, STROKE, MM return, adjustable auto-return, F1 (and F₂) = Extension forces with extended (compressed) piston rod F₃ (and F₄) = Pull-in force with extended (compressed) piston rod nonrotating, stage, and multi-mode cylinders. No matter the iteration, These are force-output parameters to define the capabilities of gas-spring extension force — a gas springs. Note that if the design is setup as a tension spring, then value that usually ranges from 1 what’s classified here as compression (retraction) is the extension to 5,000 N — depends on pistonposition, and vice versa. The value difference between extension and rod cross-section multiplied by fill compression forces for a given rod position is equal to the gas spring’s pressure. Manufacturers commonly friction force. express extension force with two FORCE, N
MOTION SYSTEM
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S H O C K & V I B R AT I O N DA M P I N G
Tow bar snubbers (left) reduce shock by managing deceleration and acceleration. Industrial gas springs (middle) safely lift and lower loads in a controlled fashion, while hydraulic dampers (right), allow for precise retraction and extension speeds, allowing controlled movement speed, synchronization to be regulated in both directions and compensated pivoting loads. Image courtesy of ACE Controls.
and silicone; however, a vast selection of compounds and compound blends can be used to achieve different characteristics specific to the application. Air springs are comprised of metallic end fittings coupled by a composite elastomeric-based bladder that contains the compressed air used to provide isolation. These single-acting designs are comprised of a pressurized bladder and two end plates. As air is directed into the air bladders, they are expanded linearly. All of these reusable designs are self-contained, offering a number of advantages over any other technology that may require outside componentry. For example, hydraulic systems may require plumbing while electrical systems may require wiring and power. Energy or power dissipation is key when selecting a damper or shock-absorbing device. The size and characteristics of the device are based on these inputs, so it is generally the first consideration to make. Dynamic spring rate and damping are the two biggest considerations when selecting an isolator. These characteristics will define the natural frequency (sometimes referred to as resonant frequency) of the isolation system and are important in achieving the desired performance. ELASTOMER, RUBBER PADS FOR VIBRATION, SHOCK REDUCING Elastomer and other synthetic and rubber pads can also damp vibration and isolate shock loads. They are available in a number of shapes, including tubes, bushings, blocks, pads 8 • 2017
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and washers. These components are commonly used in heavy-duty applications to create strong Often manufactured of stainless cushioning plates or foundations steel, aluminum or carbon steel, wire in heavy machinery such as rope isolators resist corrosion in harsh cranes, presses, and also for environments, helping to reduce fatigue vibration reduction in lab and caused by vibration in machinery. testing equipment, aerospace, and for pipelines and bridges. The rubber-like materials with which they are designed allow these padding materials to meet specific requirements, such as natural frequency, load, and area. And because IMPACT CAN DESTROY AN OBJECT they are soft, they are forgiving in most OR A SINGLE ELEMENT OF THAT OBJECT environments. Predicting the natural frequency of an application lets material manufacturers target known disturbance frequencies to dissipate energy. The lower the ratio of natural system frequency to disturbance frequency, the more it’s possible to isolate problem vibrations. These cushioning plates can protect machinery subsystems against impacts and isolate vibration and structure-borne noise. For example, PAD plates from ACE Controls withstand compressive loads to 10,000 psi (69 N/mm2) depending on plate form and size. SHOCK & VIBRATION Another custom product called Sorbothane (from a company with the same name) is a thermoset that attenuates shock with near-faultless memory. That means its deformation is elastic and not plastic, so pads of the material reliably return to their original Visit sorbothane.com shape. Custom pieces of the material work for Design Guide and MADE IN for vibration damping, acoustic damping THE U.S.A. Technical Data and isolation. Sorbothane works by turning mechanical energy into heat as the material is 800.838.3906 deformed. Molecular friction generates heat energy that translates perpendicularly away from the axis of incidence.
INNOVATING SOLUTIONS
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Aerotech’s PRO series industrial linear motor and ballscrew positioning stages come in multiple versions. Noncontact linear encoders enable minimum incremental motion to 5 nm and sub-mm repeatability. Optional HALAR factory calibration improves positioning accuracy to ±0.75 μm. Travel is 50 to 1,500 mm for standard models to 2 m/sec.
Positioning stages AND TABLES
X-Y
tables and positioning stages are integrated systems consisting of motors and mechanical power-transmission devices or linear or arc-shaped motors and actuators — complete with encoders, sensors and controllers. Most common are linear stages that combine actuators in X-Y-Z sysytems. Where serial kinematic designs are too bulky, integrated setups (in Cartesian or hexapod and Stewart platforms) are increasingly common. These output more accurate motion with no mechanical error accumulation. Better mechanical components and feedback and control options are enabling stages capable of motion that’s more accurate than ever. So, positioning stages today can execute tasks with tighter synchronization than in the past — useful for complicated axis commands. One new option that leverages the speed of today’s motion controllers is positioning stages that fit a finer piezo-based axis to a coarser and more traditional motion axis based on a rotary-to-linear mechanical device and electric motor. Such tandem axes are useful for fast execution of tasks that need super-fine movements once workpieces (such as medical devices or semiconductor chips) are in place. Another example of leading stage technology is air bearings. These are useful on stages that must deliver high reliability and accuracy. Air bearings support a load with a thin film of pressurized air between the fixed and moving elements. Sometimes called aerostatic bearings, the source of pressure the air film. Unlike mechanical bearings, the surfaces of an air bearing make no physical contact, so don’t need lubrication. Their surfaces don’t wear, so don’t generate particulates ... making the supports suitable for cleanroom applications. In fact, with a clean and filtered air supply, these bearings operate for years without fail.
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This is an example of a simple application-specific (dispensing) Cartesian robot from machine builder Dispense Works Inc. It reaches 36 x 20 in. with preloaded leadscrews, ball slides, and brushless servomotors and a ground machine bed surface for reliability in fixturing. Parts remain stationary while a dispense head moves overhead on a gantry. Users can use preprogrammed moves or import patterns from CAD files.
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8/18/17 8:21 AM
The Standard in High-Performance Linear Motion Just Got Better Aerotech’s new and improved PRO-Series stages
• Performance specifications improved by as much as 98% • New linear-motor stage sizes – PRO115LM and PRO190LM • Linear motor stages now with an absolute encoder option • Ball-screw stages now available with a linear encoder option • Direct mounting to English or metric optical tables Proven linear motor and ball-screw positioning stages now with new sizes, features, and improved performance. Call or email an Aerotech Applications Engineer today to discuss your requirements, or go to www.aerotech.com to request a quote.
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wave-spring WHAT LIMITS
WORKING HEIGHT AND AXIAL FORCE?
Wave
springs are core WAVE SPRING VERSUS COIL SPRING to myriad gear, actuator, clutch, and consumer-grade motion assemblies. They are load bearing — included in designs to address play or compensate for dimensional variations. Wave springs apply load in axially — so are specified by working height WAVE SPRINGS REQUIRE SHORTER (and other parameters). WORKING HEIGHTS THAN COIL SPRINGS. All springs are either for applying load in tension, compression, or torsion. Tension springs work for Even at various work heights, wave springs assemblies where applications apply consistent loads to within tight tolerances. rely on a stretched spring to pull the kinematic system That lets OEMs specify wave springs to very specific application requirements. Note how wave bac to some setting. Compression springs work for springs require less space than coil springs. assemblies where applications rely on a shortened spring to re-expand. Torsion springs work for assemblies needing application of torque. Compression-type wave springs are by far most metal crystallites having long axes tending towards an common. Single-turn wave springs can have overlapping orientation parallel to the spring’s curvature. ends to save axial space; nested wave springs deliver higher axial forces; multiple-turn wave springs are up to HOW TO SELECT A WAVE SPRING? 50% shorter than a coil spring of otherwise comparable A basic formula helps design engineers specify wave performance. What’s more, multiple-turn wave springs springs and identify whether a design can use a standard prevent torsional moves during compression — which is an spring — or require a custom wave spring. The dominant issue with coil springs. parameter is load to which the application subjects the Material science looms large for wave-spring design. wave spring. This load is the axial force the spring must Some manufacturers tout the high strength and reliable deliver at its work height within the assembly. performance of their springs. Many of these characteristics Some applications require multiple working heights so are quantified in terms of tensile strength, shear strength, have critical loads at two or more wave-spring conditions. modulus of elasticity, and more. The material from which It’s often sufficient to use minimum and maximum loads the spring is made as well as the mode of wave-spring for calculations, especially in motion assemblies requiring manufacture influence many of these values. Case in compensation for tolerance stackup. point: Some edge-wound springs made of flat wire can Wave springs install onto shafts or into areas of the outperform springs stamped out of sheet metal, as their assembly machined as working cavities (internal bores).
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WAVE SPRING TECHNOLOGY ADVANCES TO SATISFY TODAY’S APPLICATIONS
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HANDBOOK
Piloting grooves or other geometry helps the wave spring stays in place. Distance between surfaces inducing load defines axial working cavity — and working spring height. This is where the spring’s material cross-section affects overall design. More specifically, the factors dictating suitable wavespring design include application load; bore geometry, shaft, According to Sanjeev Rivera of Lee ID, and OD; working height; and spring material to withstand Spring, there are some industry environmental factors. Other general guidelines are that the moves towards design miniaturization. wave-spring number of turns must be between two and 20; Wave springs excel here, thanks to express the number of waves per turn in half-turn increments; the way they deliver a given spring minimum radial wall is three times wire thickness; and force in significantly reduced space. maximum radial wall is 10 times the wire thickness. So designers can reduce finished Reconsider working height for a moment: Designs must assemblies’ size and weight through use avoid situations that compress a wave spring to solid. That’s of wave springs. because as a wave spring approaches a height at which it Wave springs have also benefitted from new availability of raw materials becomes solid (with the various turns contacting or wave turns and end configurations. Design flattened) loads increase exponentially and even become engineers should consider these designs unpredictable. These loads in turn can induce the spring wherever applications are subject to to take a set or relax. Magnitude of relaxation depends on weight or space restrictions. Designers load and other application conditions. In any case, material should understand that wave springs in deformation wll degrade spring performance. a sense act as stacked wave washers in More specifically, if a spring is going into a static series configuration — but without any application, ensure that percent stress at working height is less worries about maintaining alignment. than 100% because springs can take a set or lose length loss That’s because the wave-spring design under high stress. maintains alignment on its own. Unique wave-spring variations such as nested If a spring is going into a dynamic application, ensure and interlaced wave springs also exist percent stress at working height is less than 80% because again to incorporate advanced fatigue and — springs can take a set if subject to higher stress. If work load-rating characteristics. Application height per turn is less than twice the wire thickness, the spring of these geometries really depends on operates in a nonlinear range … and actual loads may exceed the assembly at hand. calculated loads. Most manufacturers only test standard wave at the loads and work heights specified in literature. FORMULAS FOR SPECIFYING A WAVE SPRING Usually they publish work heights that are the lowest recommended Here are basic formulas to narrow down the wave spring type, shape and size that best suit an application. Note that wave springs are helping today’s push toward design miniaturization. Smaller designs need shorter and smaller for a given spring … though diameter wave springs. That’s spurred some developers to make wave springs less than 0.250 in. in diameter. other heights maybe possible. But contact the manufacturer for Fatigue stress ratio x = (s - s1/(s - s2) 2 2 Operating stress S = (3πPDm)/4bt N analysis of spring characteristics at Where: Where: alternate heights. Manufacturers s = Material tensile strength S = Operating stress P = Load, lb s1 = Calculated operating stress at lower can sometimes help OEMs design Dm = Mean diameter, in. working height (not to exceed s) wave springs for applications b = Radial width of material, in. s2 = Calculated operating stress at t = Thickness of material, in. that cannot adhere to normal upper working height N = Number of waves per turn guidelines. Deflection f = ((PKZDm3)/(Ebt3N4)) x (ID/OD) Spring rate R = (P/f) = (Ebt3N4)/(KZDm3) x (ID/OD) Where: f = Deflection, in. P = Load, lb K = Multiple wave factor Dm = Mean diameter, in. Z = Number of turns E = Modulus of elasticity b = Radial width of material, in. t = Thickness of material, in. N = Number of waves per turn
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: Where: R = Spring rate, lb/in. P = load, lb f = Deflection, in. b = Radial width of material, in. t = Thickness of material, in. N = Number of waves per turn K = Multiple wave factor Dm = Mean diameter, in. Z = Number of turns
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Here are some formulas to identify the most suitable wavespring type, size, and shape for an application. In fact, smaller designs and miniature wave springs (with diameters to less than 0.250 in.) along with material advancements are driving new applications for wave-springs that are more advanced than ever.
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AD INDEX THE MOTION SYSTEM HANDBOOK
ACE Controls Inc. ........................................................................................... 157 Aerotech . . ........................................................................................................ 163 All Motion ............................................................................................................4 Altra Industrial Motion Corp. ......................................................................... 47 Applied Motion Products ............................................................................... 69 AutomationDirect .. ......................................................................................... 145 Bansbach Easylift . . ......................................................................................... 159 Beckhoff Automation ......................................................................................57 Bison Gear & Engineering Corp. .................................................................. IBC Bodine Electric Company .............................................................................. 93 Bosch Rexroth ..................................................................................................55 Brogan & Patrick ............................................................................................. 30 C-Flex Bearing Company ................................................................................33 Carlyle Johnson Machine Company . . ........................................................... 49 CC Link .............................................................................................................. 151 Centritec Seals - A Carlyle Johnson Company ........................................... 31 CGI ........................................................................................................................5 Chieftek Precision USA ................................................................. 133, 135, 137 Cicoil . . ................................................................................................................ 45 Clippard............................................................................................................ BC Custom Machine and Tool Co. Inc. . . ............................................................. 36 Del-tron Precision, Inc. ................................................................................... 115 Delta Products Corp. ...................................................................................... 62 Deublin . . ............................................................................................................. 13 DIEQUA Corporation ....................................................................................... 91 Dorner Manufacturing Corp. ......................................................................... 65 Dunkermotoren .............................................................................................. 143 Dynatect Manufacturing, Inc. . . ...................................................................... 40 Encoder Products Company ......................................................................... 84 FESTO ...................................................................................................................1 Galil Motion Control, Inc. . . .............................................................................. 59 GAM Gear ............................................................................................................3 Harmonic Drive ................................................................................................ 89 HEIDENHAIN Corporation . . .............................................................................83 Helical Products Company ...............................................................................7 Helukabel, USA . . .............................................................................................. 43 Hengli America ............................................................................................... 134 Hiwin Corporation ............................................................................................27 IDEC . . ..................................................................................................................53 igus, inc. . . .......................................................................................................... 29 IKO ...................................................................................................................... 111 Intech ................................................................................................................ 116 ITT Enidine .. ..................................................................................................... 155 KEB America, Inc. ............................................................................................. 61
KHK USA Inc. . . .................................................................................................. 97 Lee Spring Company ..................................................................................... 165 Lenze Americas ............................................................................................... 121 Lin Engineering . . ............................................................................................. 147 Mach III Clutch Inc. .......................................................................................... 48 Maple Systems ............................................................................................... 105 Martin Sprocket ............................................................................................... 67 maxon precision motors ............................................................................... 127 MICROMO ............................................................................................Cover, 129 Mitsubishi . . ...........................................................................................................9 mk North America, Inc. .................................................................................. 68 MW Industries Aerospace Solutions . . .......................................................... 79 NBK America LLC .............................................................................................77 Neugart . . ........................................................................................................... 95 Nippon Pulse America Inc. .. .......................................................................... 142 NOOK INDUSTRIES .......................................................................................... 15 NSK Americas ...................................................................................................25 On Semiconductor ......................................................................................... 149 Osborn .............................................................................................................. 114 PBC Linear ....................................................................................................16, 17 Physik Insturmente ........................................................................................ 139 POSITAL-FRABA Inc. ...................................................................................... 85 Pro-Face America .......................................................................................... 103 R+W America ..............................................................................................71, 75 Renishaw .......................................................................................................... 86 Rollon Corporation . . ........................................................................................ 113 Rotor Clip .. ....................................................................................................... 167 Ruland Manufacturing .................................................................................... 119 Schneeberger .................................................................................................. 117 Schneider Electric IMS . . ................................................................................. 136 Serapid Inc. .......................................................................................................23 SEW Eurodrive .. ...............................................................................................101 Smalley Steel Ring Company.......................................................................... 10 Sorbothane .....................................................................................................160 Stock Drive Products/Sterling Instrument ...................................................37 The Lee Company .......................................................................................... 107 THK America, Inc. . . ......................................................................................... IFC Tolomatic, Inc. ................................................................................................. 20 Ultra Motion ...................................................................................................... 19 US Digital . . ........................................................................................................ 87 WITTENSTEIN .................................................................................................. 131 Yawkawa America, Inc. .................................................................................. 141 Zero-Max, Inc. ...................................................................................................73
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