Power Transmission Reference Guide 2021

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POWER TRANSMISSION REFERENCE GUIDE

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POWER TRANSMISSION REFERENCE GUIDE Flexible shaft systems that safely and efficiently transmit rotational power.

Flexible Shaft for Construction Flexible shafts are used in a range of construction tools, such as concrete vibrators, power screeds and trowels, drywall sanders, and duct cleaners.

Flexible Shaft & Push/Pull Actuation Systems For thrust reverser actuation, flap and slat actuation, seat adjustment, and passenger door drives.

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THE RESILIENCY OF MECHANICAL POWER TRANSMISSION WHAT

a difference a year makes. Last year at this time, as we were putting the finishing touches on the Power Transmission Reference Guide, much of the country was shut down as the first wave of the coronavirus pandemic swept through the nation and the world. Uncertainty reigned, with businesses and schools shutting down and nobody really sure what the future was going to bring -- how long was this going to last; a few weeks, a month or two? More?

As the country quickly came to learn, keeping stores stocked with food and paper products and cleaning supplies relied on a largely hidden world of manufacturing, transportation, logistics and supply chain management. Ditto for the critical medicines and PPE which medical personnel in hospitals need.

Despite the overall challenging economic climate of 2020 and the shortages that we all experienced, one area that held its own was automation. A report from the Power Transmission Distributors Association (PTDA) out late last year showed a forecasted decline in early 2021 followed by a market rebound for the rest of 2021 and into and through 2022. In fact, other economic indicators are pointing toward a brisk recovery.

For instance, the first few months of 2021 saw increased consumer spending, signaling a healthier economy and the start of a recovery, which is good news for manufacturing.

That, as well as the government stimulus bode well for the nation’s economic outlook.

Mechanical power transmission is a central part of automation and manufacturing.

These are the screws and actuators, the slides and rails and guide ways and the linear bearings that help bear the loads – and the belts, brakes, clutches, couplings, motors and gears, and the components that dampen shock and vibration. This Reference Guide covers some of the basics of these components, with updates on new designs as well.

What’s more, Design World editors have been busy putting together highly focused Motion Design Guides, a series on the basic technologies of motion control, with in-depth coverage of the basics of these critical components and everything you need to know about what they are and how to design with them. You’ll find them online at motioncontroltips.com and linearmotiontips.com, where you’ll also find all of the latest news and information about power transmission and motion control. And be sure to check out our other power transmission and motion-related sites -- bearingtips.com and couplingtips.com as well as our Design World flagship site at designworldonline.com.

MILES BUDIMIR • SENIOR EDITOR

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CONTENTS V O L U M E

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EDITORIAL LINEAR ACTUATORS STEPPER MOTORS ROTARY BEARINGS BELT DRIVES • CHAIN DRIVES BRAKES & CLUTCHES CAM FOLLOWERS COUPLINGS GEARS LINEAR BEARINGS • SLIDES • GUIDES SPECIALTY MOTORS MOTORS RETAINING RINGS SPRINGS AD INDEX

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POWER TRANSMISSION REFERENCE GUIDE

LINEAR ACTUATORS VERSUS STAGES ALTHOUGH

there are no industry standards that define linear actuators and linear stages, generally accepted terminology indicates that: • A linear actuator is typically constructed with an aluminum extrusion or base • A linear stage is typically built on a flat machined steel or granite base.

This distinction implies that linear actuators can provide longer strokes and use a variety of drive mechanisms (belt, screw, rack and pinion) while stages generally have higher rigidity and use high-precision linear guides and drive mechanisms (typically a ballscrew or linear motor) for excellent travel and positioning accuracies.

But one actuator design — the U-shaped linear actuator — defies these specifications ... using an extruded steel base to provide rigidity and travel accuracy

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specifications that rival some linear stages.

The use of a steel (rather than aluminum) profile makes the U-shaped design extremely rigid and allows manufacturers to offer a linear actuator with the high travel and positioning accuracies typically found in more precise (and more expensive) linear stages.

The steel base can also be machined to provide a reference edge for precise alignment with other machine components ... or with other actuators in a multi-axis system.

With their high rigidity, U-shaped linear actuators can be better suited than other designs to applications where the actuator is supported only on one end.

These include two and three-axis Cartesian systems, for example.

In the U-shaped actuator design, the linear guide system is integrated — and there is no guide rail. Instead, the raceways that would normally be found on the guide rail are ground into the inside of the base.

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The carriage or table is analogous to a linear bearing block turned inside-out, with the balls riding on the outside.

This leaves the center portion of the carriage available to accommodate the ballscrew nut. This construction principle makes the entire actuator extremely compact, with a width-to-height ratio of approximately 2:1.

For example, a U-shaped actuator with a width of 60 mm is only 33 mm high.

The most common cross-sections (width x height) are 40 x 20 mm, 50 x 26 mm, 60 x 33 mm, and 86 x 46 mm … although other sizes are offered as well.

Note: Despite their compact dimensions, U-shaped linear actuators have excellent load and moment capacities.

This is because the raceways are spaced relatively far apart ... so the geometry of the carriage is like that of a bearing block much larger than the actuator could accommodate in its standard form. motioncontroltips.com

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LINEAR ACTUATORS Some manufacturers offer U-shaped linear actuators made from extruded aluminum profiles, with steel inserts for the linear guide raceways.

Aluminum versions lack the rigidity of steel designs, but they offer a very compact profile. In addition, they are often dimensionally interchangeable with steel versions where an application might benefit from a lower-cost option.

While steel versions of U-shaped linear actuators use ballscrew drives almost exclusively, aluminum designs are more likely to be offered with both ballscrew and leadscrew drive options.

Originally developed for high-precision applications such as semiconductor wafer handling and medical diagnostic dispensing — for which space constraints don’t allow a typical linear stage — U-shaped linear actuators are now used in a wide variety of industries and applications. These include plasma welding, automated assembly, and optical inspection.

One of the driving factors behind the widespread adoption of U-shaped actuators is that they are the only linear actuator design with dimensional interchangeability between manufacturers.

However, it’s important to note that due to differing guideway and ballscrew designs, technical specifications (such as load capacity, speed, or rigidity) can vary between manufacturers and product lines — even for products with the same cross-sectional size and mounting dimensions.

motioncontroltips.com | designworldonline.com

The use of a steel (rather than aluminum) profile makes the U-shaped design extremely rigid and allows manufacturers to offer a linear actuator with the high travel and positioning accuracies typically found in more precise (and more expensive) linear stages.

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STEPPER MOTORS IN LINEAR APPLICATIONS LINEAR MOTION

is essential in countless automated designs and increasingly in Cartesian robots as well. Many linear-motion axes pair a rotary motor motion with a mechanical device such as a screw, belt and pulley set, or rack and pinion assembly. Some are sold as separate components for the OEM or end user to customize and integrate; others (especially NEMA-sized offerings) are integrated by the motion-component supplier and sold as complete linear actuators.

In fact, linear-motion designs based on leadscrews as well as ballscrews often employ stepper motors. That’s in part because step motors rotate a preset discrete amount for every pulse of electrical power into their windings … a characteristic that’s useful for axes involved in positioning loads. In addition, the fine positioning possible with certain leadscrews and ballscrews complements the stepping resolution of the motors.

Variations of stepper motors for linear actuation include designs that run closed loop and actuator builds that incorporate programmable electronics.

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Consider the NEMA-17 and NEMA-23 motors so essential to the many industries that employ motion designs. These two motor types often drive axes necessitating the conversion of rotary to linear motion. End users can buy step motors as well as couplings and linear screws to execute builds, though it’s increasingly common for OEMs in particular to buy turnkey motion subsystems — including those driven by stepper motors.

NEMA STEPPER ACTUATORS These linear actuators typically include a leadscrew.

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ROTARY BEARING FUNDAMENTALS 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 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.

A thrust and guide bearing used within the reactor coolant pump at a new-build nuclear power plant. courtesy of Michell Bearings

WHAT IS 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 high-speed 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 — consequently, they 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.

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Because plastic generates none of the sounds of metal subcomponents, plastic bearings of various types are useful in medical-device and consumer designs that must be quiet. Their nonmagnetic nature is indispensable in magnetic resonance imaging (MRI) machines.

PLASTIC ROTARY BEARING MATERIALS NOT

all plastic rotary bearings are plain bearings. Some plastic rotary bearings are rolling-element bearings with balls to bear the axis loads. Others are adapted bearing designs that integrate rollers (embedded over an internal working surface) to serve as deep-groove, thrust, angular-contact, and miniature rolling-element rotary bearings.

In the past, some engineers (especially those most familiar with bronze and steel options) hesitated to specify these various types of plastic bearings. However, that’s changed with increased market familiarity with engineered plastics over the last couple decades and exhaustive documentation of industrial-grade plastics’ capabilities. There’s also more engineering support than ever from manufacturers specializing in supplying plastic bearings — so OEMs needn’t start from scratch with mechanical components from injection-molding service providers.

MATERIAL-SCIENCE FOUNDATIONS OF PLASTIC BEARINGS Plastic bearings (whether plain or combined with steel or ceramic ball arrays for rolling-element bearing designs) incorporate elements made of a wide range of polymers in various grades as well as hybrid polymer blends. Many proprietary materials (engineered to satisfy specific design objectives related to load and speed ratings as well as heat, chemical, moisture, and even radiation resistance) are typically sold under trademarked names.

Polyacetal — sometimes just called acetal — and polyoxymethylene or POM are all-purpose semicrystalline polymers with excellent chemical, impact, and cold resistance. The opaque white material can be injection molded into even complex shapes with accuracy … and colorized and blended for aesthetic as well as performance objectives. Some thrust washers and flanged plain bearings are made of engineered POM formulations.Polyamides or PAs classified as nylons (a DuPont trademark though now widely used as a generic term) come in various grades and formulas that are particularly useful in rotary-bearing applications. The International Organization for Standardization (ISO) 1043-1 standard defines how base formulations are labeled. For example, PA66-GF40 is a heatstabilized nylon 66 — which indicates a molecular structure having two six-carbon-atom monomers — that’s reinforced by glass fibers motioncontroltips.com | designworldonline.com

constituting 40% of the material by weight. Many PA formulas are a dull greyish color and recognized for their bending stiffness and high tensile strength … even beyond 200 MPa in some instances.

Polyimides or PIs (not to be confused with polyamides) are imide monomers … with imide being a chemistry term indicating two acyl groups (C=O) bound to nitrogen. This chemical makeup is suitable for ball bearings with plastic bodies (and plastic plain bearings) that excel on robotics and other mobile automation. Complicating classifications somewhat are proprietary polymers that are actually polyamide-imide or PAI plastics — so designated for their alternating imide links and amide links. Bearings made of some such PAIs can survive impact loading and temperatures to 250° C though at the design tradeoff of vulnerability to moisture absorption.

Polysulfone (PSU) as well as the newer polyethersulfone (PES or PESU) and polyphenylsulfone (PPSU) polymers are amorphous (noncrystalline) plastics — so clear or pale yellow in common formulations.

Though more brittle and delivering less tensile strength than some other alternatives, bearings made of these thermoformable plastics can also maintain high stiffness even in hot settings to 160° C and beyond.

Polytetrafluoroethylene or PTFE is a synthetic flurocarbon polymer with exceptionally low static and dynamic friction coefficients than only decrease under compressive stress. Sometimes PTFE is compounded with pigmented additives when used in rotary-bearing construction for enhanced wear resistance. Glass-filled PTFE is also common where stiffness is a design objective. One challenge with PTFE is that it can necessitate special cold molding, sintering, or extruding manufacturing processes. Another challenge is temperaturerelated dimensional changes even to 1.5% or more in drastic cases.

Polyether etherketone or PEEK is a thermoplastic that’s colorless until formulated into blends for mechanical applications. It’s most common in rotary ball bearings (with either ceramic or stainless balls) that need to withstand steamy or otherwise hot settings that may damage components made of standard acetal or other materials. Such bearings usually include inner ring, outer ring, and ball-roller cage made of some engineered version of PEEK. Certain high-performance bearings needing high stiffness and load capacity are made of carbonfiber-reinforced PEEK. 5 • 2021

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NEEDLE ROLLER BEARINGS Needle-roller bearings operate in tight spaces such as automotive applications like rocker-arm pivots and transmissions. In short, these are roller bearings with rollers having a length of 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 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).

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 have tapered inner and outer ring raceways with tapered rollers between them. They are angled, so the rollers’ surfaces converge at the bearing’s axis.

Special tapered roller bearings like these from NKE are used in various applications, including industrial and wind turbine gearboxes, generators, and railway applications.

TAPERED ROLLER BEARINGS

LUBRICATION

Tapered roller bearings have tapered inner and outer ring raceways with tapered rollers between them. They are 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.

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 the required torque to make the rotary bearing spin; overly thin oil won’t maintain the protective layer needed to prevent metal-on-metal contact.

Cutaway of a hybrid ceramic bearing ball bearing from Emerson Bearing.

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APPLICATIONS FOR ROTARY BEARINGS Bearings abound in industrial and consumer designs. For example, deep-groove ball bearings often go into small to mediumsized electric motors because they can accommodate high speeds and radial and axial loads. Self-aligning ball bearings, on the other hand, 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 motioncontroltips.com | designworldonline.com

ROTARY BEARINGS

maintaining running accuracy. The only caveat is that they’re one of the most difficult bearings to install correctly.

Tapered roller bearings go in needing support for axial and radial loads — as in a tire hub bearing vehicle weight and the axial loads associated with cornering. These bearings are also 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.

The growing use of plain plastic bearings and increasingly stringent industry standards mean these bearings must often meet FDA, RoHS, and other standards. Shown here: PEEK bearings from SMB Bearings.

SIDE NOTE ON PLAIN BEARINGS Plain bearings are cylindrical sleeves with an array of design elements to cater to specific applications. 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.

The growing use of plain plastic bearings and increasingly stringent industry standards mean 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.

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POWER TRANSMISSION REFERENCE GUIDE

Belt drives are helpful in delicate semiconductor and laboratory-automation applications.

BELT DRIVES AND CHAIN DRIVES INDUSTRIAL

belt drives consist of rubber belts that wrap around drive pulleys, in turn driven by electric motors. In a typical setup, the belt also wraps around one or more idler pulleys that keep the belt taut and on track. The main reasons that engineers pick belt drives over other options is that modern varieties require little if no maintenance; they’re less expensive than chain drives; and they’re quiet and efficient, even up to 95% or more. In addition, the tensile members of today’s belts— cords embedded into the belt rubber that carry most the belt load— are stronger than ever. Made of polyester, aramid, fiberglass or carbon fiber, these tensile cords make today’s belt drives thoroughly modern power-transmission devices.

Manufacturers generally describe belts and pulleys with five main geometries. Pitch diameter is the drive-pulley diameter. Center distance is the distance between the two pulleys’ centers. Minimum wrap angle is a measure of how much the belt wraps around the smallest pulley. Belt length is how long the belt would be if cut and laid flat. Finally, in the case of toothed belts (also called synchronous

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belts) the pitch is the number of teeth per some length—so a 3-mm pitch means that the belt has one tooth every 3 mm.

Some general guidelines are applicable to all timing belts, including miniature and double-sided belts. First, engineers should always design these belt drives with a sufficient safety factor—in other words, with ample reserve horsepower capacity. Tip: Take note of overload service factors. Belt ratings are generally only 1/15 of the belt’s ultimate strength. These ratings are set so the belt will deliver at least 3,000 hours of useful life if the end user properly installs and maintains it. The pulley diameter should never be smaller than the width of the belt.

As mentioned, belts are quieter than other power-transmission drive options, but they’re not silent. Noise frequency increases proportionally with belt speed, and noise amplitude increases with belt tension. Most belt noise arises from the way in which belt teeth entering the pulleys at high speed repeatedly compresses the trapped pockets of air. Other noise arises from belts rubbing against the flange; in some cases, this happens when the shafts aren’t parallel. motioncontroltips.com | designworldonline.com

BELT DRIVES • CHAIN DRIVES Pulleys are metal or plastic, and the most suitable depends on required precision, price, inertia, color, magnetic properties and the engineer’s preference based on experience. Plastic pulleys with metal inserts or metal hubs are a good compromise.

Tip: Make at least one pulley in the belt drive adjustable to allow for belt installation and tensioning. Also note that in a properly designed belt drive, there should be a minimum of six teeth in mesh and at least 60° of belt wrap around the drive pulley. Other tips: Pretension belts with the proper recommended tension. This extends life and prevents belt ratcheting or tooth jumping.

Align shafts and pulleys to prevent belt-tracking forces and belt edge wear. Don’t crimp belts beyond the smallest recommended pulley radius for that belt section. Select the appropriate belt for the design torque.

Select the appropriate belt material for the environment (temperature, chemical, cleaning agents, oils, and weather).

Belt-and-pulley systems are suitable for myriad environments, but some applications need special consideration. Topping this list are environmental factors.

Dusty environments don’t generally present serious problems if particles are fine and dry. But particulate matter can act as an abrasive and accelerates belt and pulley wear. Debris should be prevented from falling into belt drives. Debris caught in the drive is generally either forced through the belt or makes the system stall. In either case, serious damage occurs to the belt and related drive hardware.

Light and occasional contact with water — during occasional washdowns, for example — has little serious effect. However, prolonged contact with constant spray or submersion can significantly reduce tensile strength in fiberglass belts and make aramid belts break down and stretch out. In the same way, occasional contact with oils doesn’t damage synchronous belts. But prolonged contact with oil or lubricants, either directly or airborne, significantly reduces belt service life. Lubricants cause the rubber compound to swell, break down internal adhesion systems and

reduce felt tensile strength. While alternate rubber compounds may provide some marginal improvement in durability, it’s best to prevent oil from contacting synchronous belts.

Ozone can be detrimental to the compounds used in rubber synchronous belts. Ozone degrades belt materials in much the same way as excessive temperatures. Although the bumper materials used in belts are compounded to resist the effects of ozone, eventually chemical breakdown occurs and they become hard and brittle and begin cracking. The amount of degradation depends on the ozone concentration and generation of exposure.

Rubber belts aren’t suitable for cleanrooms, as they risk shedding particles. Instead, use urethane timing belts here, keeping in mind that while urethane belts make significantly less debris, most can carry only light loads. Also, none have static conductive construction to dissipate electrical charges.

Now consider chain drives: Engineers have used chains in motion systems for more than a century. They are versatile and reliable components to drive machinery and convey products. Now, advances in precision and technology let designers use chains in more applications than ever. Remote installations benefit from longlife chain that requires no lubrication, for example.

Chain-based machinery abounds, but the most common industrial designs use roller chain. This type of chain consists of five basic components: pin, bushing, roller, pin link plate and roller link plate. Manufacturers make and assemble each of these subcomponents to precise tolerances and heat treat them to

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5 • 2021

DESIGN WORLD — MOTION

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BELT DRIVES • CHAIN DRIVES optimize performance. More specifically, modern roller chains exhibit high wear resistance, fatigue strength and tensile strength. Roller-chain applications generally fall into two categories: drives and conveyors.

Most typical drive applications use an ASME/ANSI roller chain wrapped around a driver sprocket (connected directly to the motor or reducer) and the driven sprocket (often connected to a machine’s conveyor head-shaft). This portion of the drive lets the designer build a system that’s either faster or slower by simply changing the ratio of teeth between the drive and driven sprocket. The ratio of the teeth determines the reduction in rpm … so to reduce rpm, the driven sprocket must be larger than the driver sprocket. For example, if the driver sprocket has 15 teeth and the driven sprocket has 30 teeth, the ratio is 2:1, so the rpm is halved at the driven sprocket.

The easiest way to select a roller chain is using horsepower charts. First, obtain the motor horsepower and rpm of the small driver sprocket. From this, determine the chain size and number of teeth for the driver sprocket. Where roller chain must drive applications that need long life without contamination, pick chain with selflubricating subcomponents. Where roller chain must drive applications that need high precision, pick chain with precision roller bearings at each link connection.

Conveyor chains come in myriad versions to move product horizontally, vertically or even around curved radii. The most common conveyor chains are ASMEstyle (ANSI-style) attachment chains. These chains include extended pins or plates with tabs onto which parts or product-holding shoes can bolt. Common versions are single-pitch attachment chain, doublepitch attachment chain, hollow-pin chain, curved-attachment chain and plastic-sleeve chain. The attachments let engineers put special fixtures or blocks onto the chain to serve specific conveyor functions.

One subtype of conveyor chain is the accumulating conveyor. These stop discrete products even while the chain is still moving, and they do so with minimal friction and wear. Accumulating conveyors are suitable for applications (such as assembly lines) that have products ride motioncontroltips.com | designworldonline.com

2x PITCH WEAR + 2x PITCH

ELONGATION from pin and bushing wear

INNER WIDTH

ROLLER DIAMETER

through several stations. Tip: Select chain with top rollers or side rollers to let discrete products idle while the conveyor continues to run. Also pick custom attachments or work with manufacturers that make custom fixtures to handle specific parts. Many industries (including the automotive, food and beverage, and consumer-products industries) use custom attachments on their chain-based accumulator conveyors to economically and consistently move product.

The environments of many chain applications are less than ideal. Some require clean operation without the lubrication that can contaminate products.

Others expose chain-driven machinery to weather, water, or chemicals. So, chain manufacturers offer several products to meet these challenges.

Consider roller chain: One critical area where roller chains need lubrication is the pin-bushing contact zone. Selflubricating chains stay cleaner because the exterior of the chain is free of excess lube.

These chains also attract less dust and particulates than regular chains. Such roller chains are useful where oil contamination is a concern, including paper-product or wood-processing industries.

5 • 2021

Roller chains are specified by three main parameters: pitch, roller diameter, and width between inner links. Chains that have identical dimensions in these three dimensions will work on the same sprockets. Elongation of pitch length is caused by wear to the pins and bushings. Choosing a chain with a smaller pitch can reduce the amount of wear and amount of pitch elongation with it.

DESIGN WORLD — MOTION

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FRICTION DISCS

OUTPUT DRIVE CUP

1. TRIGGERED BY ELECTROMAGNETIC ENGAGEMENT ON OFF

Power-on brakes and clutches often employ electromechanical means to actuate ... and springs and other elements to release. That’s in contrast with power-off brakes and clutches (using springs or mechanical means for a defaulting to stopping or disconnecting loads).

SPRINGS

Friction-based clutch and brake technologies abound.

SPRING-SET ELECTROMAGNETICALLY RELEASED FRICTION-DISC CLUTCH BRAKE

ARMATURE

COIL

ELECTROMAGNETIC COIL

Note: Sprung clutches are common in automotive applications for the way they damp the shock of drivetrain engagement through the friction disc.

They integrate springs for this damping function, but aren’t ususally engaged by (other) springs — so aren’t a type of spring-set clutch.

OUTPUT (PULLEY AS EXAMPLE) TORQUE PIN INPUT

SPRING ARRAY

HUB Certain spring-set (power-off) brakes include electromagnetically operated disengagement mechanisms. Not to be confused with electromagnetically actuated brake and clutch components, these use current through the coils to pull an armature away from the rotor’s friction surfaces.

SPRINGS ADJUSTING NUT

Some friction-element-based slip clutches have a cartridge (setscrewed or keyed to an input shaft) and a housing that fixes to an output. Adjustable spring force allows for fine control over the relative motion between in and output.

FRICTION PLATES ON ROTOR

2.

FRICTION DISCS

SPRING-SET ELECTROMAGNETICALLY RELEASED FRICTION-DISC SLIP CLUTCH

TRIGGERED BY SPRING ENGAGEMENT — ALSO CALLED POWER-OFF SAFETY OR FAILSAFE UNITS

ON OFF

Many (electric) motor brakes and clutches are power-off components — which means that by default (through springs and other mechanical elements) they stop the load or disconnect the axis load from the motor. Electrical input or a control command triggers release of this action to let the load be driven once more.

BRAKES AND CLUTCHES BASED ON FRICTION CLUTCH CONTROL HANDLE ENGAGES AND SEPARATES CONES

3. TRIGGERED BY MECHANICAL ENGAGEMENT

BIDIRECTIONAL OUTPUT IS POSSIBLE

FRICTION

brakes and clutches are by far the most common components used to engage and stop rotary-driven loads. Early versions of today’s friction brakes Friction-based clutch and brake types mostly focus on and clutches were rst developed in Pictured the 1800s, drum andfimultiple-disc morphologies. here is a and the basic mode third morphology — that of the cone-shaped of operation remains unchanged: Pairs ofofa surfaces in the form friction-element clutch. It’s one example manually or kinematically engaged option. of shoes, plates, orImage pads areWikimedia (through some mode of pivoting or coutesy Commons • Sweber.de clamping engagement) brought together to:

the pads, discs, plates, and/or shoes that make contact during operation. In fact, brake and clutch manufacturers use thousands of highly engineered composites and metals in these contacting subcomponents that (along with highly engineered additives) allow precise determination of the components’ exact performance SPRING HELPS RETURN MALE CONE TO RELEASED CLUTCH POSITION characteristics.

Friction-based brakes and clutches that use springs (typically CENTRIFUGALLY ENGAGED CLUTCH axial-compression coil springs) for engagement are an exceptionally MALE CONE AXIALLY SLIDES FRICTION MATERIAL ON SPLINED SHAFT IS USUALLY ON FEMALE CONE ON MALE CONE) • Brake a rotating motion axis by converting kinetic energy into (SHOWN HERE important class of components. No wonder their variations often thermal4.energy via the dragging of friction surfaces against a justify unique designations that underscore their very specific and FRICTION PAD TRIGGERED BY PERMANENT-MAGNET ENGAGEMENT COIL in machine designs. These include certain: stationary section of the assembly. Friction force between the essential ROTATING functions ARMATURE This is a permanent-magnet-engaged contact surfaces opposes the axis rotation and generates heat — electromagnetically released friction brake.

Upon removal of electrical power, the armature comes into holding contact with the friction eventually taking the axis to lower or no rpm.

• Spring-set brakes pads for reliable fail-safe brake operation.

Application of electrical power to the coil • Clutch a rotating motion axis by linking friction plates on an • Failsafe brakes repels the armature to release the brake.

5. TRIGGERED BY PNEUMATIC ENGAGEMENT assembly’s drive side to plates or friction surfaces on its driven side.

• Slip clutches • Safety and e-stop brakes STATIONARY Central to the function of these and brakes is the • Holding brakes FRICTIONclutches PLATES ELECTROMAGNETICALLY exact geometry and material makeup of the friction elements — • Servomotor brakes RELEASED ROTATING DISCS FRICTION-DISC BRAKE

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6. TRIGGERED BY HYDRAULIC ENGAGEMENT

PNEUMATIC INLET 5 •and 2021 EXPANDING BLADDER

DESIGN WORLD — MOTION

motioncontroltips.com

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MAGNETIC POLES CALIPER FLUID FROM CLOSED CIRCUIT

BRAKES & CLUTCHES The power-off designation emphasizes how these particular components default to a spring-loaded condition that brings attached loads to a nonmoving state upon removal of release power. That means the mode of disengagement for these units is quite important as well. In most cases, these brakes and clutches are released in one of two ways: • They are electromagnetically released • They are pneumatically released During electromagnetic disengagement, application of a field on an assembly armature counteracts the spring force clamping together the friction elements and separates the friction elements.

During pneumatic disengagement, application of power occurs via air pressure in a piston chamber — which in turn counteracts the spring force clamping together the friction elements and releases the friction-based hold … for independent rotation of the axis output.

Clutches based on spring force and friction-plate action are often used to transmit mechanical power from a constantly rotating motor-output shaft to some end-of-axis process requiring only intermittent rotation.

One of three common variations (in which a clutch hub assembly directly attaches to the motor drive shaft and not the output) includes: • A fixed cylinder or housing with pressure-applying springs and (to apply lateral force for disengagement) either a pneumatic inlet and internal circuit (as well as an exhaust port for quick response) or wiring and a coil for electromagnetic action • A moving (rotating) axis-output sleeve assembly containing friction discs and upon which an output pulley, pinion gear, or sheave bolts • A drive hub enclosed by the housing and sleeve — and having one or more fins (for engagement with the sleeve’s friction elements) as well as an armature of a typical electromagnetically released unit Without any external power, the housing assembly’s springs push together the sleeve’s friction discs and the hub’s fins. Then for disengagement, either electromagnetic or pneumatic-cylinder force compresses the springs to allow the fins and friction discs to disengage.

Brakes based on spring force for DESIGN WORLD — MOTION

23

friction-plate action are by default locked, because in the absence of any external power, mechanical springs hold stationary-side plates, fins, or friction discs in engaged contact

with drive-side friction discs. No wonder the failsafe function of many of these brakes is indispensable in medical diagnostic equipment as well as general automation relayed to

You don’t need to be an expert in clutches, brakes or torque limiters.

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The Mach III Advantage: Rapid Custom Solutions From slight design modifications to unique, clean-sheet creations, we can present you with a customized design solution in about 48 hours.

Built To Order We specialize in high variation, low quantity production. All products are built-to-order, allowing even catalog models to be bored to fit the exact shaft and key size required.

Reliable Ship Dates Every purchase order we receive is confirmed with a ship date that we meet or beat 98% of the time. Lead times for Standard Products: 2–5 weeks and Custom: 3–6 weeks.

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CLUTCH CONTROL HANDLE ENGAGES AND SEPARATES CONES

3. TRIGGERED BY MECHANICAL ENGAGEMENT BIDIRECTIONAL OUTPUT IS POSSIBLE

Friction-based clutch and brake types mostly focus on drum and multiple-disc morphologies. Pictured here is a third morphology — that of the cone-shaped friction-element clutch. It’s one example of a manually or kinematically engaged option.

SPRING HELPS RETURN MALE CONE TO RELEASED CLUTCH POSITION

Image coutesy Wikimedia Commons • Sweber.de

CENTRIFUGALLY ENGAGED CLUTCH MALE CONE AXIALLY SLIDES ON SPLINED SHAFT

4.

FRICTION MATERIAL IS USUALLY ON FEMALE CONE (SHOWN HERE ON MALE CONE) FRICTION PAD

TRIGGERED BY PERMANENT-MAGNET ENGAGEMENT

ROTATING ARMATURE

COIL

5. TRIGGERED BY PNEUMATIC ENGAGEMENT

This is a permanent-magnet-engaged electromagnetically released friction brake.

Upon removal of electrical power, the armature comes into holding contact with the friction pads for reliable fail-safe brake operation.

Application of electrical power to the coil repels the armature to release the brake.

STATIONARY FRICTION PLATES ELECTROMAGNETICALLY RELEASED FRICTION-DISC BRAKE

ROTATING DISCS

6. TRIGGERED BY HYDRAULIC ENGAGEMENT

PNEUMATIC INLET and EXPANDING BLADDER MAGNETIC POLES CALIPER FLUID FROM CLOSED CIRCUIT

PERMANENT MAGNET

PISTON FRICTION PAD HYDRAULICALLY ENGAGED FRICTION DISC BRAKE

7. (WITH OIL-SHEAR ACTION) TRIGGERED BY FLUID-POWER OR SPRING ENGAGEMENT

discrete motion control and servomotor designs such as robotics and mobile equipment complemented by holding brakes.

Strengths: Spring-set brakes are a top choice for emergency-braking applications on the motor-driven axes of robotic arms, vertical axes, and machines that have the capacity to injure personnel should a power failure occur.

That includes escalators, airport-baggage handlers, and elevators. They also benefit motion designs that slow loads with the motor before the brake engages … and they’re suitable as holding mechanisms as well.

There are also countless variants of spring-set brakes and clutches specifically for use on precision motion and servo applications. Many of these have low-backlash hub coupling morphology. That means they

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DESIGN WORLD — MOTION

have a female spline subcomponent on their friction-element assembly that precisely mates with a matching square, D-shaped, polygon, or hexagon-shaped spline on the electric motor output shaft (or in some cases the driven shaft).

Precision-machined splines help minimize (and avoid amplification of unavoidable) radial backlash associated with the internal clearances needed to let the friction elements run free (unengaged) upon application of pneumatic or electromagnetic power. Some units even incorporate diaphragm springs for zero-backlash operation. Otherwise, typical noload backlash (depending on the component diameter) might be 0.2° to 0.8° or so.

Constraints: The force applied by the springs in spring-set units is a factor that limits 5 • 2021

the maximum torque rating. In addition, the maximum force of the disengaging system in a spring-set unit must be well matched to the spring force to be overcome during poweron release situations. More basic spring-set brakes and clutches can also introduce excessive impulse and shock loading on precision operations.

Case in point: Consider an inclined conveyor with regularly spaced on-off cycles.

Here, a power-off brake that is spring-set may suffice to prevent load crashes during power failures. But advanced conveyor installations working to position discrete product of varied size (without jerking) may need multiple deceleration rates via more sophisticated spring-set or other brakes complemented by an advanced motor-and-drive pairing. motioncontroltips.com

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CAM FOLLOWERS

CAM FOLLOWER FUNDAMENTALS CAM FOLLOWERS

are power-transmission components featuring a load bearing rotary bearing core while acting as the interface between independently moving machine sections. Applications include those on turntable conveyors, rotary indexing tables, long-stroke robot transfer units (RTUs), and other types of customized machinery.

Cam followers like the one shown here (with cutaway) from IKO’s CR...VBS Series, are designed for a broad range of applications such as machine tools, industrial robots, packaging and handling, and food processing equipment.

motioncontroltips.com | designworldonline.com

The cam follower bearing assembly’s working face is the outer diameter (OD) — typically made of nylon, steel, urethane, polyamide, or other engineered materials. This OD mates with some machine surface. Most commonly, this was a mechanical cam of some type — like the precision barrel of an indexing table. Such mechanically automated indexing tables have a motion profile cut into a cam drum that engages the followers, which transmits the power to an output. Cam followers are also found in assemblies pairing them with linear tracks and other engineered paths on customized assemblies.

Cam followers are assembled onto machines in one of two ways. Stud-type cam followers include a partially threaded shaft fixed to the follower inner diameter (ID) for assembly onto a machine frame with a nut or similar fastening device. Yoke cam-follower variations (identifiable by their open ID) often mate to machine frames via press fit at a hardened inner race usually held by the follower’s end plates.

Because they’re not a cantilevered design, yoke followers exhibit minimal deflection. But stud cam followers are indispensable in an array of applications — including those that are subject to high loads.

Cam followers most commonly feature needle rollers for carrying high radial loads. Where applications require the axis to run at high speeds, a cage can separate the rollers. For particularly high loads, and high dynamic load capacity requirements for the axis, cam followers can include twin row standard rollers. Though beyond the focus here, some light-load cam followers are even built around simple plain (sleeve) bearings.

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DESIGN WORLD — MOTION

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POWER TRANSMISSION REFERENCE GUIDE

Cam followers from Intech Corp. excel on high-load machines that don’t allow for wearing of the cams or traditional lubrication. Gravity cast nylon-12 bearing surfaces combine with metal hubs and roller bearings for quiet and lubrication-free designs.

It should be noted that cam followers are different than roller-bearings in a few crucial ways. Because the latter are typically interference fit into assemblies, they receive circumferential reinforcement from the surrounding machine frame or housing. In contrast, a cam follower’s outer race must be thick to prevent deformation — especially under the localized line of loading.

Additionally, many cam followers include lubrication ports and more ruggedized surface finishes to withstand exposure to environments during operation — especially those that operate exposed on unprotected machine sections.

Many cam followers have flat outer diameter (OD) profiles, while others (especially those for linear-motion applications) include crowned, edgeflanged, or vee-shaped ODs to engage

tracks and rails that are engineered with mating geometry. Crowned cam followers can compensate for ten times the misalignment that traditional flat-profile cam followers.

Some cam followers serve as track followers by engaging rails to deliver linear motion. These designs are increasingly common in automated storage and retrieval systems (AS/RS), and seventh-axis RTUs mentioned earlier. That’s because camfollower-based linear systems outperform the linear bearings known as profile guides, where compactness and ultra-high accuracy are less important than ruggedness, quick and forgiving installation, high-speed reversals, and long life.

Linear cam-follower (track follower) arrangement. courtesy Güdel US.

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DESIGN WORLD — MOTION

5 • 2021

COUPLINGS

COUPLINGS FOR POWER TRANSMISSION APPLICATIONS FOUND

create a compact and lightweight coupling.

With typical maximum torques to 220,000 lb-in. at their largest, chain couplings wrap lengths of chain around sprockets with clearances to impart flexibility. Power transmission couplings like these are best-suited in high-horsepower applications on axes needing correction of up to 2° and 0.01-in. angular and parallel misalignment.

All couplings transmit drive torque and angular velocity, but applications for power transmission (pumps, material-handling machinery) usually include disc, gear, chain, jaw, grid, and Oldham couplings. Power transmission couplings like this generally handle more torque than couplings for motion control applications. Aditionally, they’re more robust and are designed to hold up to more demanding environments.

These are just some of the coupling types used for power transmission. Others include flexible shafts or Hooke’s joints, also called Cardan, universal, or U-joints.

With maximum torques commonly reaching up to 500,000 lb-in. at their largest, these couplings transmit power through a metal membrane (sometimes of varying thickness).

Diaphragm couplings mitigate the problematic transmission of forces and moments in coupled equipment like bearings, but they are often more costly. Profiles include straight-spoked diaphragms, tapered diaphragms, and convoluted diaphragms assembled in arrays. These correct up to 1° and 0.1-in. angular and parallel misalignment.

in myriad applications, couplings are simple devices that connect two shafts.

Couplings are usually found on rotating equipment such as motors to transmit several motion parameters. These parameters include the transmission of velocity, angular positioning, and torque.

Oldham coupling designs transmits torque through a central disc that slides over tenons on the hubs under controlled preload conditions. courtesy of Huco-Altra Industrial Motion

Like disc couplings, jaw coupling design lends itself to adaptation for both power transmission and backlash-free motion control. courtesy of NBK America

CHAIN COUPLINGS Chain couplings are flexible couplings with a straightforward construction.

They are constructed with the combination of a coupling chain and a pair of coupling sprockets.

Torque is distributed over the entire roller chain and sprocket teeth and is held close to the sprockets’ outer diameter.

This overall configuration can motioncontroltips.com | designworldonline.com

DIAPHRAGM COUPLINGS

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DESIGN WORLD — MOTION

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TOOL-LESS ADJUSTMENT COMPONENTS • Adjustable handles and knobs replace standard hardware and can be used to torque components without tools.

• Levers can be used with Ruland shaft collars for quick installation and adjustment.

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EXPANDED COUPLING LINE • Stainless steel oldham couplings for high temperature and corrosion resistance.

• Controlflex couplings for encoders in single and double insert styles and speeds up to 25,000 RPM. • Slit couplings with bores starting at 1.5mm and speeds up to 70,000 RPM.

MODULAR MOUNTING SYSTEMS • Assortment of components that allow users to build small assemblies for mounting sensors, conveyor rails, machine guards, and more.

UNIVERSAL JOINTS • Friction bearing for high torque.

• Optional pre-designed kits make it easier to select the right system for your application.

• Needle bearing for accuracy and higher RPM. • Single and double styles available.

INDEXING PLUNGERS VIBRATION ISOLATING COMPONENTS • Rubber bumpers are ideal as end stops or mounting feet.

• Vibration isolation mounts can be sandwiched between components to dampen shock loads.

• Spring-loaded indexing plugers with or without lock-out.

• Designed to lock devices in-place for adjustable positioning.

• Both types can have studs or tapped holes.

www.ruland.com I sales@ruland.com

RULANDi)

Carefully Made Shaft Collars and Couplings

COUPLINGS These elastomer jaw servo-couplings have a design that eliminates radial loads and achieves uniform power transmission with an even clamp force and symmetrical arrangement of clamp and screw positions. courtesy of Ringfeder

ELASTOMERIC TIRE COUPLINGS These couplings commonly operate at maximum torques up to 550,000 lb-in. at their largest. They transmit power through a tire-shaped rubber element that bridges the coupling’s two hubs.

These correct up to 1° and 0.2-in. angular and parallel misalignment.

JAW COUPLINGS Jaw couplings have typical maximum torques up to 550,000 lb-in. at their largest. They are found in both curved and straight designs. Like disc couplings, the design lends itself to adaptation for both power transmission and backlash-free motion control.

The coupling hubs have jaws that lock into a spider made of bronze, elastomer, or other material. Power transmission is reliable even through 1° and 0.01 in. angular and parallel misalignment.

OLDHAM COUPLINGS Oldham couplings include a metal or polymer disc with slots on each face 90° offset. They have maximum torques up to approximately 550,000-in. at their largest. Usually, hub fins or tenons engage a slotted disc that slides freely even while transmitting torque. Oldham couplings specified for their accommodation of angular misalignment might transmit through 6° and 0.05-in. Oldham couplings are often selected, primarily, to address parallel misalignment of 0.15-in. or more and around 0.5°.

motioncontroltips.com | designworldonline.com

DISC COUPLINGS With typical max torques up to 5,000,000 lb-in. at their largest, disc couplings are one of a few coupling types used, with variations, for motion-control and power-transmission applications alike. Single thin discs or multi-disc packs (made of metal or engineered composite) bridge the hubs. In certain designs, the discs impart flexibility to transmit torque even while addressing up to 2° and 0.05-in. angular and parallel misalignment.

The Composite Disc pack has high torsional stiffness, and the flex element maintains zero backlash and low bearing loads.

Specially sized and located holes are drilled and positioned on the CD coupling’s hubs to allow the addition of weight during This CD Coupling’s disc pack design final trim balancing on the has high torsional stiffness and also machine. These couplings are handles misalignment while keeping pre-balanced at 4,000 rpm so the reaction loads on the connected that final coupling assembly in bearings and components to a the machine only requires fineminimum, ensuring long life. tune balancing. courtesy of Zero-Max

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DESIGN WORLD — MOTION

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POWER TRANSMISSION REFERENCE GUIDE

This grid coupling’s hub is manufactured with the hub tooth profile designed to permit progressive loading under torsional shock conditions. The surface of the high tensile alloy steel grid spring is shot-peened to impede the propagation of cracks. (Image courtesy of Bibby Turboflex)

GRID COUPLINGS Grid couplings have typical maximum torques up to 5,000,000 lb-in. at their largest. Designs include a heavy spring that weaves between slots on the coupling hubs. Compliant connection damps torsional vibration and shock loading — typically even through 0.3° and 0.30-in. angular and parallel misalignment.

GEAR COUPLINGS Gear couplings include a flexible joint on each hub and have a typical maximum torque of up to 55,000,000 lb-in. at their largest. In most designs, a spindle joins the two. Each joint includes a gearset that mates with a 1:1 ratio. The tooth flanks and external gearing’s outer diameter are crowned to allow rotating-spline action and to accommodate misalignment of approximately 3° and 0.04 to 0.4-in.

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DESIGN WORLD — MOTION

GEARS

WAVE & CYCLOIDAL GEARING — INCLUDING PLANETARY SETS

Shown here is an example of a custom planetary-gear set. courtesy of CGI Motion

MANY

of today’s precision applications necessitate gears capable of dramatic speed reductions, power densities, and transmission accuracies. Leading choices in these designs include trochoidal and cycloidal gearing as well as gearsets relying on wave-inducing subcomponents having an elliptical or Reuleaux or other polygonal shape.

Recall from geometry that trochoidal and cycloidal gearing includes elements that rotate and trace curves around some other element.

More specifically, cycloids traced by a point on a rolling element’s circumference include epicycloids (for which the element rolls along the outside of a sun gear or other reference component) and hypocycloids for which the element rolls within a ring or other reference component. In contrast, trochoids (and their subtypes) are traced not by a point on the rolling element’s circumference but rather some point within or without. One particularly common subtype of epicyclic gearing is planetary gearing.

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PLANETARY GEARSETS FOR SERVOSYSTEMS

Particularly common in servo systems, planetary gearsets consist of one or more outer planet gears that revolve about a central sun gear.

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 of these three gear wheels stays stationary; another is an input that provides power to the system, and the last acts as an output that receives power from the driving motor. The ratio of input rotation to output rotation depends on the number of teeth in each gear and on which component is held stationary.

Planetary gearsets offer several advantages over other gearsets.

These include high power density, the ability to get large reductions from a small volume, multiple kinematic combinations, pure torsional

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DESIGN WORLD — MOTION

31

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GEARS 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, 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.

STRAIN-WAVE GEAR-ASSEMBLY SUBCOMPONENTS

Elliptic cam on bearing Ring gear

Thin-walled flex gear

OTHER WAVE AND CYCLOIDAL GEAR TYPES

So we’ve covered planetary gearing. Now we’ll review other variations that are increasingly common in high-end machine tool, aerospace, material handling, and robotic applications requiring precision servo motion.

Consider the latter — in a robotic joint employing a motor (running at a few thousand rpm) fitted with gearing for output speeds to 100 times slower. Gearing in such designs also serves to boost acceleration torque for top power density — a priority design objective for SCARAs and collaborative robotics for which the total assembly is essentially a cantilevered mass.

Conventional gearing used in such designs (including some planetary gearsets) usually exhibit a very small of amount backlash. This is often limited to only a few arc-min. (100ths of a degree) and serves to accommodate lubrication and thermal expansion. However, this backlash can in some cases degrade system accuracy by unacceptable amounts. So let’s take a look at other gear options that avoid the issue.

Strain-wave gearing uses the metal elasticity (deflection) of a gear to reduce speed. Key benefits of strain-wave gearing include zero backlash and high power density 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 a steel disc with an elliptical shape machined to precise specifications. A specialty ball bearing fits around this disc and conforms to the elliptical shape. Most designs attach the wave generator to a servomotor — to serve as the motion input.

The flexspline is a thin-walled steel cup. These cup walls are radially compliant but remain 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 what one might call the brim.

The flexspline usually serves as the output.

The cup has a rigid boss at one end to provide a rugged motioncontroltips.com | designworldonline.com

mounting surface. The wave generator is 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 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 teeth of the flexspline engage the teeth of the circular spline along the major (long) axis of the ellipse. So there are two areas of meshing made by the ellipse inscribed concentrically within the ring. Roughly 30% of the teeth are engaged at all times — in contrast with six or so teeth engaged at any time for an equivalent planetary-gear set, and one or two teeth for an equivalent spur-gear set.

Recall that backlash is the difference between the space to accommodate the teeth and the tooth width ... and this difference is zero in strain-wave gearing. Elastic radial deformation of the strain-wave flexpline (preloaded by the manufacturer against those of the circular spline at the major axis) acts like a very stiff spring to compensate for space between the

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DESIGN WORLD — MOTION

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POWER TRANSMISSION REFERENCE GUIDE

teeth that would otherwise cause backlash. Preload is set to keep stresses well below the material’s endurance limit.

The pressure angle of the gear teeth transforms the output torque’s tangential force into a radial force acting on the wave-generator bearing. The flexspline and circular spline teeth engage near the ellipse’s major axis and disengage at the 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 the number of flexspline teeth ÷ (number of flexspline teeth – number of circular spline teeth).

The lightweight construction and single-stage gear ratios (to 160:1) of strain-wave gearing let engineers use the gears in designs needing minimized weight and volume.

Even small motors can leverage their large mechanical advantage. Certain tooth profiles (of convex and concave arcs) for strain-wave gearing let more teeth engage — for increased torsional stiffness and torque as well as a longer MTBF. Now let’s consider the thrusted-tooth design mentioned earlier. This newer hightorque gearbox offering offers extreme torsional rigidity and zero-backlash operation for applications that need superior precision in output motion. In contrast with other gear offerings that transmit power over lines of contact on gear teeth, meshing gears in this design make almost full-surface contact. This allows for tooth contact that’s up to 6.5 times that of certain types of conventional involute teeth.

How does it work? In short, the gearbox guides a large array of individual teeth along an internal ring gear. The tooth geometry follows a logarithmic spiral that lets multiple teeth transmit power at once through hydrodynamic contact — covering much larger surface areas than traditional line contact. The result is a gearbox with zero backlash even at the zero crossing. The logarithmic spiral path of the teeth allows for synchronization accuracy that outperforms traditional hollow-shaft drives with comparable outer diameters.

The gearbox also boasts up to 91% efficiency — 18 to 29% better than traditional strain-wave and cycloidal geared arrangements.

As part of a drive system, hydrodynamic tooth contact of the gear drive also delivers high overload capability. Emergency stop torque is 150 to 300% better than comparable systems, and torsional rigidity is to 580% higher ... so gearboxes of other designs might need to be up to three times larger to deliver the same torque as a given thrusted-tooth gearbox. The gear design also allows for a very large hollow-shaft diameter in relation to the outer diameter — to 70% larger in some cases.

Wave and cycloidal gear options include (from top to bottom) strain-wave gearing, planocentric and cycloidal gearing, planetary gearing, and dynamic thrusted-tooth gearing.

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Miniature Actuator with Integrated Servo Drive

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The compact RSF-5 miniature actuator with a zero backlash Harmonic Drive® gear delivers high torque with exceptional accuracy and repeatability. The actuator features an integrated servo drive utilizing CANopen® communication. This evolutionary product eliminates the need for an external drive and greatly improves wiring while retaining high-positional accuracy and torsional stiffness in a compact housing. This new miniature actuator is ideal for use in robotics.

• Actuator + Integrated Servo Drive utilizing CANopen communication conforming to DS402 and DS301 • 24VDC Nominal +7 to +30VDC Supply Voltage Range • Single Axis BLDC Motor Controller/Driver with CAN & TTL-UART Interface • Field Oriented Control • Single Cable with only 4 conductors needed: CANH, CANL, +24VDC, 0VDC

• Zero Backlash • Pigtail Connectors with 2 exit options • 14bit (16384 cpr) resolution motor encoder • Control Modes Including Torque, Velocity, and Position Control, CSP, CSV, CST • Harmonic Drive HDL Software for initial tuning, commissioning, and analysis • Homing modes include: single-ended, doubleended, double-ended with 16bit autoscale hardstop homing or teachable position

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POWER TRANSMISSION REFERENCE GUIDE

LINEAR BEARINGS, SLIDES, AND GUIDES EXPLAINED LINEAR BEARINGS

are included in linear actuators and the axes of other motion systems to guide and support machine assemblies and payloads over the linear stroke. All linear bearings fall into four categories:

NORMAL LOAD F

1. A carriage or comparable table rides on a linear rail or track via plain (sliding) elements 2. A carriage rides on a linear rail via wheel-type track rollers 3. A carriage rides on a profiled linear rail via carriagecontained arrays of ball bearings or cylindrical rollers 4. A bushing studded on its inner diameter with rolling elements rides on a round shaft The interrelated functions of these linear-motion components to both support (bear) loads and guide loads is the core reason why they’re called both linear bearings and linear guides — depending on which function is being emphasized by the source. Both terms are so generic that they can refer to any products from the four categories listed above — including such disparate designs as plain linear bearings, ball bushings, and recirculating-roller linear bearings. Confusing matters is the fact that industry makes inconsistent use of even more specific linear-motion terms.

For example, the term slide is often used to refer to the

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SPHERICAL ROLLER LINEAR TRACK (RACEWAY)

2b

2a

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LINEAR BEARINGS • SLIDES • GUIDES carriages of linear bearings based on rolling (not sliding) bearing elements. The term rolling-contact guide is often used to refer to profiled-rail linear bearings even though track-roller linear guides also include rolling contact (at their track wheels).

That said, linear guide often indicates a standalone guide rod, ball slide, or mechanism solely for guiding loads. The term profiled rail nearly always indicates some linear bearing with roller or ball elements. Many manufacturers use the terms linear slide (whether based on rolling or sliding action) and linear rail (whether plain, track-roller, or profile) to indicate a linear-motion guide element that’s incorporated into a build complete with some mechanical drive. Though the terminology surrounding plain linear bearings is probably the most consistent, various manufacturers use plane bearing (as in one dimension in 3D space) instead of plain bearing. While the two terms are

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often used interchangeably, the American Bearing Manufacturers Association encourages use of the term plain bearing.

The term linear stage generally implies a design that has guided elements as well as some mode of mechanical linear actuation and reinforced body — often without inclusion of the motor.

Motorized rails (more commonly called linear actuators) abound — though the distinction here is that there are countless linear actuators sold without any linear-guide element. That’s useful for OEMs aiming to employ some specialized linear guide or omit guides altogether.

While many linear-motion applications absolutely demand either plain linear bearings or rolling-element linear bearings, the majority are well satisfied with either.

Plain linear bearings (on low-friction aluminum or steel shafting or rails) can outperform rolling-element linear bearings on vibrating, hot, or short-stroke axes — and

5 • 2021

on axes needing very low friction coefficients (µ) even down to 0.12.

In contrast, rolling-element linear bearings (with µ down to 0.001) are often indicated on high-speed axes (running to many meters per second) and those involving high moment loads.

For other applications, it’s left to the design engineer to determine which options offer the most advantages (including cost, life, and friction characteristics) for full design optimization.

HERTZ CONTACT STRESSES IN ROLLING ELEMENT LINEAR BEARINGS Linear bearings that use balls or rollers to carry a load (as opposed to plain linear bearings) are subjected to Hertz contact stresses. This is a type of material stress that significantly impacts bearing load capacity and fatigue life.

When two surfaces of different radii

DESIGN WORLD — MOTION

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POWER TRANSMISSION REFERENCE GUIDE

NORMAL LOAD F

L CYLINDRICAL ROLLER

2b d

LINEAR TRACK (RACEWAY)

are in contact and a load is applied (even an extremely small load) a small contact area is formed — and the involved regions experience very high stresses. These stresses are known as Hertz (or Hertzian) contact stresses. In rollingelement linear bearings, Hertz contact stresses occur on the balls (or cylinders) and the raceways.

In theory, the contact between two spheres occurs at a point, and the contact between two cylinders occurs as a line.

In either case — point or line contact — the resulting pressure between the two surfaces would be infinite and the surfaces would experience immediate yielding.

In real-world applications, when two surfaces are pressed together with a force, some elastic deformation occurs at each surface and a contact area forms. The stresses occurring on the two surfaces may still be very high (sufficient to initiate spalling or other forms of failure) but not actually infinite.

Analysis of Hertzian contact stresses relies on four assumptions: • The surfaces are smooth and frictionless • The bodies are isotropic and elastic • The contact area is small relative to the contacting bodies’ sizes • Strains on the bodies are small and within the elastic limit Hertzian stresses are present when any two surfaces with different radii are in contact — even if one surface is flat or if one surface is convex and the other is concave — the case for rolling-element bearings. In this case, the ball or roller is convex, and the raceway is concave. In the analysis of Hertz

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contact stresses, a convex surface (the ball or roller) has a positive radius, and the concave surface (the raceway) has a negative radius. Of course, flat surfaces have an infinite radius.

Because the surfaces have different radii, the contact area between a spherical ball (or a cylindrical roller) and a bearing raceway has an elliptical shape. Under these conditions, the maximum pressure between the two surfaces is given by separate geometry-based equations.

Hertz contact stresses have a significant effect on bearing dynamic load capacity and L10 life. Shear stresses, which cause fatigue — a primary mode of failure of rolling elements — are proportional to the maximum Hertz pressure between the two bodies.

Hertz contact and the resulting deformation of surfaces is also what causes bearings to skid rather than roll. This is because the Hertz contact areas have different diameters than the rolling elements themselves, so the rolling elements slip. Hertz contact has implications for bearing preload as well. Preloading the rolling elements gives them a larger — and finite — Hertz contact area, which increases stiffness. But the increased contact results in high heat generation.

That’s why a preload amount of just 8% is considered high preload for linear bearings, and preload greater than 10 to 15% being extremely rare. Also because Hertz contact is nonlinear, a small amount of preload can provide a significant increase in stiffness without resulting in unacceptable slip, friction, and heat.

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POWER TRANSMISSION REFERENCE GUIDE

ULTRA-EFFICIENT MOTORS IMPROVING CORDLESS CONSUMER-APPLIANCE PERFORMANCE 25

trillion kilowatt hours of electricity are consumed globally per year — of which 53% is consumed by electric motors. So replacing traditional electric motors with ultra-high-efficiency electric motors in all sorts of industries could significantly reduce energy wasted to noise and heat — and increase the performance and cost effectiveness of motor-driven designs in the process.

The motors traditionally used in household appliances might offer the most room for efficiency improvement. That’s why Infinitum Electric, creator of the silicon-stator electric motor, is now developing its first IEh-series motor for a consumerappliance application — more specifically, an energy-efficient cordless vacuum.

IEh-series motors deliver premium performance for quieter and more efficient consumer appliances with greater durability and connectivity. The silicon-based stator produces the same power as a comparable traditional iron-core and copper wire stator assembly … but has a far more compact design. To illustrate: A traditional 15-hp electric motor (sans drive) is about 6,000 in.3 and 300 lb.

Infinitum Electric’s 15-hp motor is only 1,200 in.3 and 70 lb — including the drive. In some cases, total motor weight is 50% lighter than traditional electric motors. In addition, the IEh-series motors feature: • Quiet operation — generating less than 45 dB for some applications. That’s because there’s no cogging torque or magnetic force between the rotor and stator to cause noise.

• High speed and wide operating ranges — from 0 to 110,000 rpm • Durability: The silicon stator has a 10x longer life than conventional coils and uniform temperature distribution prevents thermal stresses on windings.

Infinitum Electric motors will soon make cordless vacuums more efficient and able to run longer on each charge.

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• Ultra high-efficiency: With an efficiency rating of greater than 90% for any power rating, stator core losses are eliminated due to the coreless construction.

• Low vibration: With vibration levels of less than 0.25 mm/sec, the simple structure prevents rotors from shifting due to thermal or mechanical stresses.

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“Manufacturers are exploring new ways to incorporate IoT to deliver ecofriendly designs and support digital transformation goals,” says industry principal at Frost and Sullivan Anand Gnanamoorthy. “Infinitum Electric’s motors are designed with total efficiency in mind and integrated electronics and IoT circuitry pack a punch, making smart home appliances even smarter.”

Bringing everyday household consumer appliances into a new era of sustainability with the ultra-high efficiency IEh motors is exciting, says Infinitum Electric founder and CEO Ben Schuler.

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DESIGN WORLD — MOTION

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POWER TRANSMISSION REFERENCE GUIDE

MOTORS — MOTION MAINSTAYS

AA

A

B+

A

B

B

B

B

B-

B

B-

B

B

A

A

A-

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B

A+

DESIGN WORLD — MOTION

STEPPER MOTOR STATOR WITH WINDINGS

A

42

NEMA ICS 16-2001 standard for frame sizes, which specifies mounting dimensions such as flange size and bolt circle diameter. However, one dimension not covered by the NEMA standard is motor length. And this flexibility in motor length for a given frame size provides manufacturers with another option for increasing the torque production of a particular NEMA size stepper motor — by creating motors with longer stack lengths. For example, double- and triple-stack stepper motors are now common offerings from several manufacturers.

Double- and triple-stack hybrid stepper motors simply have multiple rotors and stators, stacked end-to-end. With multiple rotor and stator sections, the motor can produce more torque without the need to increase the frame size. Only the length of the motor increases.

However in double- and triple-stack (and quad-stack) stepper motor designs, torque falls off faster as speed increases than it does

STEPPER MOTOR STATOR (WITH WINDINGS) AND ROTOR (WITH TWO SETS OF TEETH)

A

Stepper motors can be grouped in three basic designs — permanent magnet (PM), variable reluctance, (VR), and hybrid. Of the three, hybrid stepper motors are arguably the most common in industrial applications, combining the best performance characteristics of permanent magnet and variable reluctance types.

Hybrid stepper motors are constructed with a rotor made of two sections, or cups, with a permanent magnet between them.

This causes the cups to be magnetized axially — with one cup polarized north and the other cup polarized south. The surfaces of the rotor cups have precisely ground teeth (typically 50 or 100 teeth per cup), and the cups are aligned with an offset of ½ tooth pitch between the two sets of teeth.

In a hybrid stepper motor, the stator poles are also toothed, and when pulses are delivered to the stator by the stepper drive, these poles are magnetized, causing the rotor to turn so that the teeth of the rotor and stator align (N-S or S-N).

This hybrid design — with teeth on both the rotor and stator — allows the motor to optimize magnetic flux, and therefore, produce higher torque than permanent magnet or variable reluctance designs.

Hybrid stepper motors can also achieve step angles as small as 0.72 degrees in full-step mode and operate at higher speeds than other designs.

However, 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-tosize ratios and higher output speeds than other stepper-motor types. They are also quieter than VR stepper motors.

One caveat is that hybrid stepper motors are more expensive than other step motors. So, designers should weigh the higher cost against the advantages of quiet operation, smaller steps, and torque output before making a final step-motor selection.

Although proprietary designs and production methods allow manufacturers to optimize the torque output (as well as step accuracy and speed characteristics) of their hybrid stepper motors, torque production is still closely tied to the frame size of the motor.

Stepper motors generally adhere to the

A

motors are central to mechanical power transmission, as they are the main motive force behind motion. The range of motors is broad, including ac motors that power conveyor lines to dc motors, both brushed and brushless, to servo and stepper motors for precision motion. With stepper motors, their compact size and high torque as well as greater controllability are common advantages.

A+

ELECTRIC

ROTOR WITH TWO SETS OF TEETH B+

Shown here are the rotor and stator of a hybrid stepper motor. Two soft iron cups sport teeth to guide the flux from a permanent magnet to the rotor-stator air gap.

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POWER TRANSMISSION REFERENCE GUIDE in single-stack designs. This is because the added rotor and stator sections also increase the motor’s inductance.

And higher inductance means the electrical time constant of the motor — the amount of time it takes the current in the windings to reach 63 percent of its maximum value — is also increased. When a stepper motor operates at high speeds, a high electrical time constant means there isn’t enough time for the current (and, therefore, torque) to reach its maximum value at each motor step, resulting in a torque drop-off as speed increases.

Another way to increase the torque from a stepper motor — without increasing the NEMA frame size — is to use a gearbox with the motor. The addition of a gearbox not only increases the torque delivered from the motor to the load, it can also provide better inertia matching between the motor and the load. And when connected to a gearbox, the motor can operate at higher speeds, which helps reduce or avoid resonance and oscillations.

This graph from Applied Motion Product shows the torque speed curves for a NEMA 17 stepper motor in single-(blue), double-(red), triple-(green), and quad-stack (purple) designs. Note that as stack length increases, so does torque. However, in the longer stack length versions, torque drops more quickly as speed increases.

It’s not a web page, it’s an industry information site So much happens between issues of R&D World that even another issue would not be enough to keep up. That’s why it makes sense to visit rdworldonline.com and stay on Twitter, Facebook and Linkedin.

It’s updated regularly with relevant technical information and other significant news for the design engineering community.

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RETAINING RINGS

SOME RETAINING RING BASICS AS

with other joining hardware such as cotter pins, screws, and bolts, retaining rings prevent mating components from excessive moving. In short, they create a removable shoulder preventing components from migrating out of proper position during operation.

Specifically, retaining rings are fasteners that hold components together on a shaft or in a housing when engaging a groove. Three main types of retaining rings include tapered section, constant section and spiral.

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 is there to ensure 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 profile is required.

Spiral rings have no protruding ears to interfere with mating components in an assembly. The ring has a uniform cross-section and no gap or lugs for a functional and aesthetically pleasing ring. Unlike traditional fasteners, retaining rings eliminate machining and threading, reducing costs and weight. Spiral retaining rings do not require special tools for removal and are supplied standard with removal notches for easy extraction from a groove.

Die stamping is a traditional method of producing retaining rings, along with newer methods such as coiling. Retaining ring materials can include stainless steel, various alloys, carbon tempered steels, or

bronze, among others. Other factors specific to an application, such as the temperature as well as the presence of any corrosive media, can also impact the choice of ring material.

Radial retaining ring

SELECTING A RETAINING RING

When selecting a retaining ring for an application, several factors dictate Bowed preloading ring which is most suitable. The first question to ask: 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 Internally mounted axial ring speaking, designers define this thrust as either a light, medium or heavySpiral ring duty load. It’s important that the 360° contact design engineer define the maximum thrust because its value also helps Constant-section determine if groove deformation ring points of contact or ring shear could be a problem.

Tapered-section ring Basically, groove deformation occurs uniform contact 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.

Another factor involves edge margin. This is the distance from Axially assembled Standard housing ring the groove (for the retaining ring) to inverted housing ring the end of a shaft or housing. Edge margin depends partially on the groove’s depth. Rule of thumb: When edge margin is about triple (or more) the groove depth, the groove can withstand the same level of thrust load as the mating ring.

Standard shaft ring A related consideration is the Axially assembled material finish. This can include an reinforced shaft ring oil dipped finish (for carbon steel rings), vapor degreased and ultrasonic cleaned (for stainless steel rings), or black oxide or a phosphate coat, Axially assembled among other choices. Also, does shaft teeth ring the ring need to meet any special Housing teeth ring standards, such as MIL or Aerospace Inverted shaft ring standards?

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5 • 2021

DESIGN WORLD — MOTION

45

NEW IDEA. NEW GEAR.

BETTER MOTION

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SPRINGS

SPRING BASICS – COMPRESSION AND WAVE SPRINGS ALL SPRINGS

are either for applying load in tension, compression, or torsion.

Tension springs work for assemblies where applications rely on a stretched spring to pull the kinematic system back to some setting.

Compression springs work for assemblies where applications rely on a shortened spring to re-expand. Lastly, torsion springs work for assemblies needing application of torque.

Compression-type wave springs are by far most common. Single-turn wave springs can have overlapping ends to save axial space; nested wave springs deliver higher axial forces; multiple-turn wave springs are up to 50% shorter than a coil spring of otherwise comparable performance. What’s more, multiple-turn wave springs prevent torsional moves during compression — which is an issue with coil springs.

Wave springs are central to many 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 axially — so are specified by working height (and other parameters).

Wave springs are essential to an evergrowing lineup of consumer products, especially in electronics, safety, and automotive

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designs. They’re also in nearly all motioncontrol applications — including gearboxes, actuators, rotary unions, and clutches.

Wave springs operate as load bearing devices. They suit applications with special space needs to take up play or compensate for dimensional variations within assemblies.

They also work in designs that need loads to move either gradually or abruptly to reach a predetermined working height.

A wave spring always applies load in an axial direction. Wave springs also apply consistent loads within a small tolerance range at different work heights. These capabilities let design engineers adjust the application to meet given requirements when needed.

One common type of wave spring is the single-turn. Versions with overlapping ends save axial space so there’s more space for travel. Here, the spring clings to the bore. In contrast, nested wave springs suit applications requiring higher forces to meet safety regulations, such as those in government or military applications. A nested-wave spring provides a higher load than a single-turn wave spring (a stamped wave washer) and uses the same radial space as a single-turn design.

Use of multiple-turn wave springs can save 50% in axial space compared to a traditional coil spring. What’s more, these springs eliminate the risk of torsional movements during compression to work height.

5 • 2021

COMPRESSION SPRINGS

Compression springs are typically used to resist linear compressing forces (such as a push force) where space is limited, where uniform bearing pressures are required, or to reduce buckling. In the automotive industry, they are common in seats, pedals, transmission springs and windshield wipers.

Building-automation applications for compression springs include industrial doors, fasteners, and dampers. In electric-distribution applications, they are indispensable for the function of contact switches and other electromechanical components. Medical uses for compression springs are also common — in simple mechanical equipment, such as beds, to those in advanced actuators for precision medical devices. In consumer devices, they are found in major appliances, lawn mowers, even cell phones.

The most common compression spring, the straight metal coil spring, bends at the same diameter for its entire length, so has a cylindrical shape. Cone-shaped metal springs are distinct in that diameter changes gradually from a large end to a small end; in other words, they bend at a tighter radius at one end. Cone-shaped springs generally go into applications that need low solid height (the total height when compressed) and higher resistance to surging. Whether cylindrical or cone shaped, helical compression springs often go over a rod or fit inside a hole that controls the spring’s movement. Other configuration types include hourglass (concave), barrel (convex), and magazine (in which the wire coils into a rectangular helix).

Most compression springs have squared and ground ends. Ground ends provide flat planes and stability under load travel.

Squareness is a characteristic that influences how the axis force produced by the spring can be transferred to adjacent parts.

Although open ends may be suitable in some applications, closed ends afford a greater degree of squareness. Squared and ground end compression springs are useful for applications that specify high-duty springs; unusually close tolerances on load or rate; minimized solid height; accurate seating and uniform bearing pressures; and minimized buckling.

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AD INDEX POWER TRANSMISSION REFERENCE GUIDE 2021

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