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F L U I D P OW E R W O R L D. CO M

Fluid Power

HANDBOOK

FLUID POWER

2

OVERVIEW

W

elcome to the sixth edition of the Fluid Power Handbook. Every year, our editors strive to add to the great amount of information we’ve already accumulated on hydraulic and pneumatic components and systems. For this year’s edition, we’ve incorporated some frequently asked questions into the majority of the sections, and I think you’ll find this extremely useful. We’ve also updated the graphical look of the issue, and your feedback is always welcome. You will find updated sections on many components and other areas of fluid power interest, some all-new topics, and a fascinating trends piece on where digitalization is taking fluid power systems in the coming years.

Fluid power systems are comprised of components that include pumps, cylinders, valves, hose, fittings, gauges, sensors, filters, seals, and reservoirs. Some components are considered absolute necessities, while others are optional and used to refine the system for more precise operation or to increase the lifespan of the system or its individual parts. Throughout this handbook, we detail many of the more common and widely used components, explaining their operation, their place in the system, and how an engineer should correctly specify them.

While fluid power can be used in most any industry or application, it is commonly seen in markets that include packaging, off-highway, mining, offshore/marine, medical, material handling, construction, aerospace, automation, robotics and entertainment.

And fluid power is an important technology. A December 2012 study by the Department of Energy discovered that between 2.1% and 3.0% of the United States’ energy is transmitted through fluid power equipment. While that is an impressive statistic, the study also found that the efficiency of fluid power systems ranges from 8% to 40%, averaging a mere 22%.

These numbers illustrate how much improvement there is to make. The study concludes that a 5% improvement in efficiency over 5 years is possible by instituting best practices in industry. Or, even better, a 15% improvement over 15 years is possible with a strategic R&D program. Doing this would save the U.S. an amazing $37 billion.

While some say that fluid power is a static, mature technology, there’s still much in store for the technology. We continue to see hydraulic and pneumatic components become more Internet-friendly, wireless and capable of being monitored and controlled from distant locations, taking preventative maintenance to new levels.

One last note: Our online family of websites grew by one this year, with the addition of SealingandContaminationTips.com. This site features needto-know engineering details on everything from seals to filters to hydraulic fluids, as well as articles on contamination issues. So now you can access our library of unique, high-value editorial content there, in addition to our sites FluidPowerWorld.com, PAU L H E N E Y PneumaticTips.com, MobileHydraulicTips.com, E D I TO R I A L D I R E C TO R and HoseAssemblyTips.com.

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INSIDE THE 2017 FLUID POWER HANDBOOK

105

22

123

02 06

Fluid Power Overview Fluid Power and the Age of Digitization

16 Hydraulics Overview 18 Bar Stock 22 Cylinders 28 Filters 32 Filtration Systems 36 Fittings and Flanges 41 Fluids 45 Heat Exchangers 48 Hose 53 Hose Couplings 58 Manifolds 61 Motors 68 Pumps 72 Replacement/Rebuilds 74 Seals 81 Sensing Technologies 84 Valves

86 Pneumatics Overview 88 Air Springs 90 Compact Cylinders - Actuators 92 Cylinders - Pneumatic Actuators 94 Cylinder Slides/Rodless Cylinders 96 FRLs 98 Grippers 102 Miniature Pneumatics 105 Pneumatic Tubing 108 Vacuum Components 112 Valves 115 Gauges 118 Fluid Power Safety 123 Shock Absorbers and Linear Dampers 126 Retaining Rings

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JUNE 2017 • vol 4 no 4 • www.fluidpowerworld.com

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Let Flow Ezy show you how we can keep your hydraulic system running smoothly

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FLUID POWER WORLD does not pass judgment on subjects of controversy nor enter into dispute with or between any individuals or organizations. FLUID POWER WORLD is also an independent forum for the expression of opinions relevant to industry issues. Letters to the editor and by-lined articles express the views of the author and not necessarily of the publisher or the publication. Every effort is made to provide accurate information; however, publisher assumes no responsibility for accuracy of submitted advertising and editorial information. Non-commissioned articles and news releases cannot be acknowledged. Unsolicited materials cannot be returned nor will this organization assume responsibility for their care.

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FLUID POWER WORLD does not endorse any products, programs or services of advertisers or editorial contributors. Copyright© 2017 by WTWH Media, LLC. No part of this publication may be reproduced in any form or by any means, electronic or mechanical, or by recording, or by any information storage or retrieval system, without written permission from the publisher.

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SUBSCRIBER SERVICES: To order a subscription please visit our web site at www.fluidpowerworld.com FLUID POWER WORLD (ISSN 2375-3641) is published eight times a year: in February, April, May, June, August, September, November, December by WTWH Media, LLC; 6555 Carnegie Ave., Suite 300, Cleveland, Ohio 44103. Periodicals postage paid at Cleveland, OH & additional mailing offices.

POSTMASTER: Send address changes to: Fluid Power World, 6555 Carnegie Ave., Suite 300, Cleveland, OH 44103

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FLUID POWER AND THE AGE OF DIGITIZATION

BY MIKE SANTORA

N

ot too many years ago, one could still find a small number of IoT skeptics amongst fluid power professionals. Today, however, it’s become difficult for those last few IoT holdouts to substantiate their apprehension. There are the companies that have embraced IoT potential, and made it a core innovation initiative, and there are those that are at least monitoring their competition’s advances. Either way, from Hannover Fair to the hallway water cooler, IoT and digitization in general, are the talk of fluid power innovation.

What that looks like in practice varies from company to company. Some manufacturers are still focused on data collection, while others have moved full-scale into the realm of predictive maintenance capabilities and even augmented reality integration.

So how do we collect data more efficiently? And more importantly, how do we use that data once we have collected it? Here, we’ve spoken with a couple experts to give us a look at what fluid power IoT looks like at the ground level.

A JOB SITE foreman with a number of machines in operation can now see where all their machines are at all times. No longer does it require that they make multiple phone calls to figure out where they are, and all the subsequent logistics.

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DATA AT THE FOREFRONT To start at the beginning, the core of any IoT solution is the hardware of the telematics devices actually making the connections for information transfer possible. That could be WiFi connecting to a local network or cellular or satellites connecting to the Cloud, the server, or some other storage location. “That has applications for the whole lifecycle of the machine, from initial design concept on through to production and field service,” said Joe Maher, Product Marketing Manager-Electronic Components for Danfoss Power Solutions.

“But how do you get that data? How do you start making it valuable? That's what it's all about. The system is accumulating large sums of data. If I have been accumulating data on several components or machines in different applications in all regions of the world, and I have multiple instances where I can show in the data that I’ve had a component failure, imagine that at that point I look at all of the data behind it and I start detecting patterns. What can I do with that? If I see a typical pattern that repeats itself before these component failures, now maybe I can take that pattern and project it forward. Now maybe I can forecast when one of those failures is going to happen. How much would that be worth?”

...THE CORE OF ANY IoT SOLUTION IS THE HARDWARE OF THE TELEMATICS DEVICES ACTUALLY MAKING THE CONNECTIONS FOR INFORMATION TRANSFER POSSIBLE. This need for data acquisition and pattern recognition has created a call for specialized talent in the work force. Enter the data scientist. This is certainly not a new title or position—just ask the HR managers at companies like Facebook or Amazon. But the need for their skills in the fluid power space is just now being pushed to the forefront.

The data scientist’s job, of course, goes hand in hand with a company’s other team members dealing with physical infrastructure.

Both must work together to secure data and ensure that it's not corrupted. Privacy is also a primary concern. Contractual obligations with customers abound and the industry trend has also manifested itself in terms of communication protocols.

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There are standard communication protocols for this type of data—such that you can have information coming from multiple components, multiple brands, but all collected in the same spot so that you can aggregate all the information and analyze it.

It’s at least possible that fluid power customers could eventually have a machine dashboard that gives them information about the health of their different systems and perhaps even a recommendation on action plans if components reach a suboptimal level. These are just a couple of examples. There are many other potential uses for a machine dashboard like this.

For example, with GPS, if we have a device on a fleet of machines, we can tell where all those machines are. A rental company is an obvious example. For the owner of a fleet or a job site foreman with a number of machines in operation, now they can see where all their machines are at all times. No longer does it require that they make multiple phone calls to figure out where they are, and all the subsequent logistics.

Maher takes it a step further.

“So now that I can tell where all those machines are, what if I could put a virtual fence around some geographic location? What would I do with that? Well, for example, if I'm a rental company and the contract stipulates that the machine that I'm renting will be operated in a certain area, with a different rate if you go outside of that … well, that would have some use for me. Maybe I can have my machine programmed or my information portal programmed so that whenever a machine goes out of that area, I get a note, a text message, an email, that can all be automated. In fact, at our development center in Ames, Iowa, we have this around our garage. The manager of the development center knows whenever a machine is leaving the garage.”

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HANDBOOK

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GIVING MACHINES A VOICE The IoT integration platforms of companies go by many different names. At Parker, they call it Voice of the Machine. This is the phrase Parker uses to classify its digital transformation initiative and it’s a good example of what we can expect from IoT in the fluid power industry down the line. This approach to IoT includes their centralized initiative to standardize IoT technology across their businesses, IoT-empowered products that result from that initiative, and their focus on customer needs. Voice of the Machine establishes a common set of standards, principles and best practices across Parker’s operating groups. As a result, all of the company’s products use the same communication standards, security architecture, and visualize data in the same way. This helps to ensure interoperability and a consistent user experience.

From a technology perspective, Parker says it has focused its efforts on minimizing the challenges that have prevented operators in critical industries from using IoT to solve operating problems, such as downtime and maintenance costs. For engineers who have considered greater IoT integration, challenges like legacy devices that are not IoT-enabled, competing communication protocols used by various suppliers, securing devices and data, and determining what data to collect and how to present it, are certainly familiar.

Jeff Smith, Business Development Manager–IoT at Parker, said simplification of IoT integration is central to their business model.

“As you may know about Parker, we make many discreet things and many different parts. Voice of the Machine is really the umbrella for Parker, around our IoT initiative, and so as we start to empower these discreet elements so there is one simple platform that we use for people to see their data, run reports and get alerts. It’s important for us to have a center initiative so that anybody buying Parker related products, if it was IoT empowered, they only had one software platform to come to,” Smith said. “This really is the main idea behind Voice of the Machine. We have built out these elements of the platform where any of these divisions can really consume that platform.”

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Michele and Pasquale apply a LEGO® approach to automation.

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Flexibility and reliability. They’re the building blocks for IASE’s modular approach to robotic packaging systems. For Michele and Pasquale, choosing the right components is critical. To them, they’re not just building systems; they’re building their customers’ trust. That’s why they rely on the superior automation products and support they receive from Festo.

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Fluid Power

HANDBOOK

THE CORE OF ANY IoT solution is the hardware of the telematics devices actually making the connections for information transfer possible.

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Specifically, at the component level, for example, there is the ability to have an interface to log into for data on a pump. This provides some kind of an overview where a user can get flow rates, temperature, pressure, and discharge pressure, among other data sets.

In another example, let’s say a user has a fleet of pumps in multiple remote locations requiring long drives between sites. Being able to access a centralized dashboard would be quite valuable. Smith continued, “If we can communicate with any location, we can also command and control it—from turning it on and off to changing the speed of the drive or modifying the flow rate. Much like a volume knob, we can turn that up or turn that down remotely. We don't have to be there.”

Additionally, all of these kinds of things are tools a user can log and track. This gives people different access options. If one person is on site, they only have access to things on their site. Or if somebody is a fleet manager, for example, they can see that fleet. This is all in an effort to help improve the continuous operation of these pumps over time. Shrinking the space between geographically separated components, the ones measured in miles as well as meters, is important to the future of IoT. It’s that level of real world application that’s helping bridge the gap between theoretical value and actual business advantage.

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Setting the Standard as a Global Manufacturer of Steel & Stainless Hydraulic Fittings.

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Special thanks to the following companies who helped supply editorial content and images for this edition.

Fluid Power

HANDBOOK

CREDITS

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So much happens between issues of Fluid Power World that even another issue would not be enough to keep up. That’s why it makes sense to visit fluidpowerworld.com and stay on Twitter, Google plus, Facebook and Linkedin.

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

R

fluidpowerworld.com pneumatictips.com mobilehydraulictips.com sealingandcontaminationtips.com hoseassemblytips.com

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Fluid Power

HANDBOOK

F L U I D P OW E R W O R L D. CO M

16

HYDRAULICS

OVERVIEW

H

ydraulic technology has been around in one form or another for thousands of years. Water has been used to irrigate and control water clocks, turn waterwheels to mill flour or grind wood into pulp.

Modern hydraulic systems continue to flourish, as they have one particularly important thing going for them: Power density. No other technology can match the pure brute force of hydraulic oil (often at pressures approaching 3,000 psi, 5,000 psi, or even much higher) and do it in a reasonable amount of space. That’s why we see hydraulics at work in some of today’s most demanding applications, from the Caterpillar equipment building our roads and cities to the John Deere equipment servicing our farmland and the Komatsu equipment harvesting the raw materials from our mines. That’s not to say that hydraulics can’t be precise, however. You will find the technology on passenger airliners and military jets, as well as on machine tools and material handling equipment.

Hydraulics differs from pneumatics in that the medium being used to transmit power is a liquid as opposed to a gas. The liquid is generally hydraulic fluid, which is based on a mineral oil base stock.

In some cases, water can be used—but this requires the use of very specialized components and is not altogether common. Hydraulic fluid has low compressibility (or a high bulk modulus) and generally a good thermal capacity.

Naysayers may argue that hydraulics is a dirty, loud and even an environmentally unfriendly technology. However, that lazy argument doesn’t ring true for fluid power engineers. Those claims merely indicate that the systems being described are improperly designed, installed or maintained.

Understanding the operation of and parameters for the application is critical, as is a good working knowledge of sealing and how to deal with contamination. Even something as basic as adding a new component to a sealed hydraulic system can introduce contamination—something that leads to eventual systemic breakdown.

The bottom line is that, as in any industrial system, smart engineering design and regular maintenance will avoid problems in the future.

FLUID POWER WORLD

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HANDBOOK

BAR STOCK Y

Fluid Power

ou’ll often hear the term “bar stock” used in fluid power settings, but it’s important to realize that it can refer to either one of two quite different things.

It can mean piston rod bar stock, the metal rods used in cylinders, or the metal used for manifolds, subplate mounts, and plumbing. This section examines all of these uses.

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PISTON ROD BAR STOCK Hydraulic cylinders are the essence of fluid power motivation. However, their simplicity often leads us to discount their subtleties of manufacture, often assuming they’re constructed of identical stock. You’d be surprised, then, to discover the devil is in the details, and not all cylinders are fabricated equally. One factor often overlooked is the bar stock used for piston rod construction. To help with the finer points of bar stock, we employed the help of Adam Hart, plant manager at Higginson Equipment in Burlington, Ontario.

Piston rod stock is nearly as varied as what is produced from the steel industry, but some are more common than others.

“The most common bar stock material by far, is 75 kpsi 0.0005in. (1⁄2 thou) chrome plated steel bar,” said Hart. “There are also many other options. With a steel piston rod, you can increase the tensile strength … up to 100 kpsi, and the chrome can be increased to 0.001.” He is describing the tensile strength and the chrome plate thickness of the bar stock, which is important because most cylinders spend half their time pulling.

Additionally, thicker chrome results in superior corrosion resistance.

Other techniques are employed to strengthen the rod stock.

“Most large diameter piston rods are induction hardened, which helps improve impact resistance,” said Hart. “If an end user keeps breaking male rod threads, sometimes this stronger material can help improve the longevity of the cylinder.”

Regarding cylinder finish treatments, in extreme conditions, such as corrosive or salinated fluid exposure, rod stock can be further upgraded to stainless steel.

“Some end users require corrosion resistance for their process, which is where stainless steel steps in,” said Hart. “Most grades of stainless steel can have a chrome finish.” However, stainless steel is not the only finish available. “Aside from chrome, the only other common finish treatment for piston rods is nitride. This is an extremely durable finish. It is a chemical process that hardens and darkens the material, which provides wear and corrosion resistance.”

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Hart revealed what he wishes engineers and end-users would consider when designing and applying a cylinder application.

“I would like engineers and end-users to keep in mind, wherever there may be misalignment issues, a female rod thread with a stud may decrease downtime. If you break the attachment off the end of a rod, it is a relatively simple to replace the stud and attachment without the need of replacing the entire rod,” he said.

20

BAR STOCK FOR MANIFOLDS Bar stock may be used either as a mounting for other valve systems or simply for consolidation of plumbing. The bar stock itself is typically an alloy of either aluminum or ductile iron, and is manufactured in billets suitable for machining finished product. The most popular use is the bar stock manifold, which is a block of varying length drilled with passages, ports and bolt holes for mounting valve systems.

Aluminum is a popular choice for bar stock material when system pressure is 3,000 psi or less. It is easier to work with than ductile iron, and is also lower in physical mass and overall cost. However, when working pressure is higher than 3,000 psi, iron is required to withstand the additional stress. Ductile iron, such as Dura-Bar, is a continuous cast (iron) that is less brittle than standard cast iron and is pressure rated to 6,500 psi. Ductile iron is a compound with a highly controlled microstructure, improving strength and machinability. Although forged steel is another option for bar stock, it is rarely used on less than the most extreme applications.

Whatever name you know them by best—ISO, cetop, NG6, D03—the industry standard modular stackable valves are the most common system of circuit construction, and they all require a manifold to interface with. A manifold for a D03 valve, for example, is around 3 in. tall and 3 in. deep, but can be as long as needed to mount any number of valve stacks. The manifold most often has pressure and tank drillings running its length. Each “station” of the manifold, where the valve mounts with four bolts, has four drillings mating up with the pressure and tank passages, as well as mating up with the work ports, which are drilled on the

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side of the manifold in a vertical arrangement. Bar stock manifolds can be drilled as either parallel or series circuits, depending on the application.

Bar stock can be cut into smaller slices and drilled in similar arrangements to bar manifolds to create subplate mounts. The subplates allow one valve to mount atop, with four ports on each of the four sides. Bottom-ported subplates are also available, but are rarely used, because of their tricky mounting, and ports all on one surface, making plumbing difficult. Bar manifolds have plenty of material to enable the addition of a relief valve cavity, but subplates have no such luxury of real estate. Both manifolds and subplates are available in sizes from D02 to D08, and many manifold accessories are available to help complete the hydraulic circuit, such as tapping plates, cover plates and gauge blocks.

Bar stock can also be used to clean up plumbing on machines by reducing the need for adapters and fittings. By drilling ports into a bar, a header or manifold can provide a junction to common feed or return lines, so that each tube or hose plumbs neatly into the same source.

Manifolds and headers can reduce leak points, but also add a look of professionalism compared to a mess of tees and adapters. Bar stock is great for mounting components, such as test points, transducers or pressure switches. The bar material can also be anodized any color, or even just treated for corrosion resistance by clear anodizing for aluminum or nickel plating for ductile iron. Lastly, because bar stock is so commonly used in various applications, it is readily available through every fluid power distributor in North America.

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M O B I L E H Y D R AU L I C T I P S . CO M

HYDRAULIC

CYLINDERS

H

ydraulic cylinders are at work in both industrial applications (hydraulic presses, cranes, forges and packaging machines) and mobile applications (agricultural machines, construction equipment and marine equipment). When compared with pneumatic, mechanical or electric systems, hydraulics can be simpler, more durable and also offer greater power density. For example, a hydraulic cylinder has about ten times the power density of an electric linear actuator of similar size. Hydraulic cylinders are also available in an impressive array of scales to meet a wide range of application needs.

Selecting the right cylinder for an application is critical to attaining maximum performance and reliability, which means taking into consideration several design and performance parameters.

Fortunately, an assortment of cylinder types, mounting methods and “rules of thumb” are available to help select the appropriate cylinder.

CYLINDER TYPES

The three most common types of cylinders are tie-rod, welded and ram, the latter of which is single acting, meaning it is powered in one direction only.

Tie-rod cylinders can be single acting, although they are most often powered in both directions. They have machined, square caps and heads being forced together against the barrel by high-tensile steel tie rods fastened by nuts, making them easy to disassemble and repair in the field. Welded cylinders employ a steel barrel with a cap welded to the bottom and the end treatment subsequently welded to the cap. The rod and piston assembly then has to be assembled around the head—which uses a buttress thread for strength—and is tightened into the barrel. Finally, the single-acting ram is typically just a rod inside a barrel with a single port and requires either a spring or mass to retract.

For all types of cylinders, the critical measurements include stroke length and bore and rod diameter. Stroke lengths vary from less than an inch to several feet or more, depending on the requirement of the machine. Bore diameters can range from 1 in. up to more than 24 in., and piston rod diameters range from 1⁄2 in. to more than 20 in. In practice, however, the choice of stroke, bore and rod dimensions may be limited by environmental or design conditions.

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OF ESY URT TH O C RO GE IMA CH REX BOS

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CYLINDER MOUNTING METHODS

Mounting methods also play an important role in a cylinder’s performance.

Generally, fixed mounts on the centerline of the cylinder are best for straight line force transfer, ideal column loading and avoiding excessive wear. Pivoting mounts, such as clevis or trunnion, require care in application, because of their capacity to move as the cylinder is stroked, resulting in a possible bent rod or excessive wear.

COMMON TYPES OF MOUNTING INCLUDE:

Flange mounts—Strong and rigid, but have little tolerance for misalignment. It is recommended to use cap end mounts for thrust loads and rod end mounts for loads under tension.

Side-mounted cylinders—Easy to install and service, but the mounts can sometimes create a bending moment as the cylinder applies force to a load, increasing wear and tear. To avoid this, specify a stroke at least as long as the bore size for side mount cylinders (heavy loading tends to make short stroke, large bore cylinders unstable). Side mounts, such as side lugs, need to be well aligned and the load supported and guided.

Centerline lug mounts—Absorb forces on the centerline, but require dowel pins to secure the lugs to prevent movement at higher pressures or as a result of shock loads.

Pivot mounts—Absorb force on the cylinder centerline and let the cylinder change alignment in one plane. Common types include clevises, trunnion mounts and spherical bearings.

Because these mounts allow a cylinder to pivot, they should be used with rod-end attachments that also pivot.

Pivoting mounts are required for many applications, such as booms and buckets, but are also most prone to rod buckling, especially as the rod reaches end of stroke.

H OW DO YO U S P E C I F Y A H Y D R AU L I C C Y L I N D E R ?

Hydraulic cylinders are specified by the force required to be pushed and moved, and they are specified by their method of mounting.

There are subordinate functions to be considered when specifying a hydraulic cylinder, such as fluid type, column strength and material construction. However, 90% of cylinder applications are not exotic, and simply identifying force and mounting requirements will suffice, leaving all else standard.

The first step in specifying a hydraulic cylinder is to calculate the required force. Factors required to determine force are the mass of the load, direction of cylinder travel and angle of the force vector.

The mass is easiest to know, in most cases. The direction of cylinder travel is also easy to determine; will the cylinder push or pull on the load? It is important to consider the difference because a cylinder pulls with less force than it pushes, due to the area taken up on the piston by the rod. The angle at which the cylinder pushes on the load affects the force required, but this part of the discussion requires trigonometry, so we’ll leave it out for now. Let’s just say that if you’re not pushing at ninety degrees to the load, it requires more force.

Once the force requirement is defined, you will have to calculate the required bore of cylinder. The bore is inside diameter of the barrel, but also describes the outside diameter of the piston. The hydraulic fluid acts upon the piston, imparting force energy against it, and the larger the area of the piston or the higher the pressure you exert, the more force is generated. Are and force are calculated as such:

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A = πr² A = Area in square inches π ≈ 3.14 r = Piston radius (1⁄2 diameter) After piston area is calculated, simply multiply its area by the available system pressure: F=PxA F = Force in pounds P = Pressure in psi A = Area in square inches Let’s take an example to help with the math.

We have a 4-in. bore cylinder, and our system pressure is capable of 3,000 psi: A A A A

= = = =

3.14 x r² 3.14 x 2² 3.14 x 4 12.56 in²

F=PxA F = 3,000 psi x 12.56 in² F = 37,680 pounds

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H Y D R AU L I C C Y L I N D E R S

WHAT IS THE MAXIMUM PRESSURE FOR THE APPLICATION?

The hydraulic cylinder must be rated to work within pressure limit of the hydraulic system it is installed on. An excavator, for example, can operate at 4,000 psi or more, so light-duty snap-ring cylinders rated for 2,000 psi should be avoided. Cylinders are designed with safety factors of 2:1 to 4:1, so sometimes running slightly over-limit might be acceptable, but not double.

WHAT STROKE LENGTH WILL BE REQUIRED?

This is fairly straightforward, but it should be ensured that the machine has appropriate clearance, because the longer retracted length of the cylinder should be factored. Also, if stroke is too long, additional support will be required, such as a guided load or stop tube.

WHAT MOUNTING METHOD IS BEING USED?

Flange mounting is often the best solution because the load is transferred along the centerline of the cylinder. Non-centerline mounting calls for additional support to avoid misalignment, but these are required when the mechanism must pivot through an arc, so load calculations must be factored accurately.

PUSH OR PULL OR BOTH?

Any cylinder can be used as single acting, which is powered in one direction only, but this means they can either push or pull, not both. When a cylinder pushes, protection against rod buckling and bending must be ensured, which can be achieved through oversized rod material or with a stop tube to prevent full extension, taking advantage of the load-bearing effect provided by the piston. When a cylinder pulls, there is little concern for buckling, but you should ensure your force calculations factored the smaller rod side of the piston, which experiences reduced force compared to the cap side of the piston. A double acting cylinder is powered in both directions to push and pull.

WHAT PUSH OR PULL TONNAGE IS REQUIRED?

Always assume peak loads will require additional strength. The rule of thumb is to choose a cylinder with a tonnage rating of 20% more than required for the load; however, this is always application-specific, so it’s best to consult a hydraulic professional before you make tonnage assumptions. Cylinder force (lb) is equal to the area of the piston (in.³) times pressure (psi), or F=AxP.

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KEY SPECIFICATIONS:

Fluid Power

HANDBOOK

Operating conditions—Cylinders must meet the requirements of the design specification, such as force, maximum pressure and mounting configuration, but consideration for operating conditions must also be heeded. Cylinders must also withstand extreme temperatures, humidity and even salt water for marine hydraulic systems. Also, when ambient temperatures rise to more than 300° F, standard Buna-N nitrile rubber seals may fail and will instead require synthetic rubber seals, such as Viton. When in doubt, err on the safe side and choose a cylinder design capable of more of than you will ask of it.

MS2 — Foot Side Lugs

MF2 — Cap Rectangular Flange

Fluid type—Most hydraulic systems use a form of mineral oil, but applications using toxic synthetic fluids— such as phosphate esters—require Viton seals, which will not break down or swell in the fluid. Once again, Buna-N seals may not be adequate to handle some synthetic hydraulic fluid, although the gentler synthetics, such as PAO-based stock, will be fine. Hydraulic systems using high water-based fluids may require stainless-steel construction, as well as PTFE (Teflon) seals, especially if no glycol is used in the fluid.

MT1 — Head Trunnion

MT4 — Intermediate Trunnion

Seals—Seals are the most vulnerable component of a hydraulic system. Properly chosen and applied seals can reduce friction and wear, lengthening service life, while the incorrect type of seal can lead to downtime and maintenance headaches as a result of failures. Every manufacturer likes to use a different seal style, so it is important to replace them with a similar type and material when rebuilding.

MS3 — Centerline Lugs

MP1 — Cap Detachable Clevis

MF1 — Head Rectangular Flange

MT2 — Cap Trunnion

Cylinder materials—The type of metal used for cylinder head, cap and bearing can make a significant difference in performance and reliability. Most cylinders use bronze for rod bearings and medium-grade carbon steel for heads and bases, which is adequate for most applications. But stronger materials, such as 65-4512 ductile iron for rod bearings, can provide a sizable performance advantage for tough industrial tasks. The type of piston rod material can be important in wet or high-humidity environments (like marine hydraulics) where stainless steel may be more durable than the standard case-hardened carbon steel with chrome plating used for most piston rods. A new option for rod surface treatment is nitriding, which is an oxidation process to increase the surface hardness of metals, and in the case of cylinders, makes for excellent corrosion resistance.

MS7 — End Lugs

MF6 — Cap Square Flange

There are a variety of NFPA-approved cylinder mounting styles; pictured here are 11 of the most common designs, used especially on mobile machines.

MF5 — Head Square Flange

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HANDBOOK

HYDRAULIC

Fluid Power

FILTERS

M

IMAGE COURTESY OF HY-PRO FILTRATION

28

any fluid power systems fail simply because there is too much contamination in the medium. In fact, some estimate that 75% of all fluid power failures can be attributed to contamination issues. Thus, an engineer who ignores filtration does so at the peril of his/her system.

There are multiple reasons why fluid can become contaminated. Use and wear of the components, poor plumbing and contamination brought in with new fluid (or new components) are just a few of the possibilities. This is why no fluid power system can be complete without the use of a filter.

Hydraulic filters keep the hydraulic fluid contaminant free. Because of the dependence of the fluid power system on a filter, they can be found in many applications. Some of these include construction vehicles, factory equipment and even oil rigs.

There are several types of filters, including bag, screen and magnetic. The bag filter consists of a cloth bag in which the hydraulic fluid is pushed through; the contaminants (being solid) are unable to flow through the bag as easily as the fluid. This is particularly useful in the filtering of dirt, rust and particles introduced into the system by a cylinder rod. Screen filters are constructed with many small wires that are woven together to create a metallic cloth. These filters can be constructed to a precise pore size, which allows an engineer to choose the right size for the expected contaminant size. Finally, there are magnetic filters, which use magnetically charged plates that will attract any metallic contaminants in the system.

The construction of the filter is also an important selection criteria. This covers what is included with the filter and the alignment. When purchasing a filter it is important to know if the system needs a filter with or without the housing. It is also possible to just purchase a filter element and replace that section of the filter. There are a few different alignment options available for filters.

When the inlet, outlet and filter are all lined up, it is called an “in-line” alignment. Another alignment is the “off-line” alignment, which is when the filter

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is not on the main hydraulic system’s loop. There is another configuration which is called “duplex.” This configuration is when two filters are combined together. These filters are useful for maintenance (changing of the filter elements) without disrupting the working system. A “return-line” configuration is designed to catch all contaminants introduced to the system through its various parts. It is thus named because it is the last element in the system and placed in the return line for the fluids to the tank.

A filter’s rating is a measure of its effectiveness. One of these, ISO 4406, is a standard measure of the contaminant level found in a given filter system. An ISO 4406 code is composed of two to three numbers, each of which represents a logarithmic measure of the number of contaminants present at three set size ranges (4µ, 6µ, 14µ[c]) in 1 mL of fluid. For example, a new sample of oil might measure 18/15. This corresponds to 1,300 to 2,500 particles of 4 µm, and 160 to 320 particles greater than 14 µm. A filter with a lower ISO cleanliness rating will therefore more thoroughly remove contaminants and can help to prolong the life of the hydraulic system’s components by 2 to 3 times.

Another measure of filter efficiency is the filter’s beta ratio, which is the ratio of the number of contaminant particles upstream of the filter divided by the number downstream. The beta ratio can be subtracted by 1, divided by the beta ratio and multiplied by 100 to obtain the filter’s percent efficiency for a given contaminant size. When choosing a hydraulic filter, a greater beta ratio is desirable.

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W H E R E S H O U L D YO U L OC AT E H Y D R AU L I C F I L T E R S I N YO U R S Y S T E M ?

Is there an ideal location for placement of hydraulic filters in an operating system, and if so what is the correct type of filter to use? Is one filter sufficient, or should multiple filters be used? Can different types of filtration be combined?

However, in general, the ideal situation is to replace the filter elements as soon as the differential pressure indicator (dirt indicator) indicates the need for a change. Usually, the indicator trips at around 75% of full bypass pressure. This leaves some time for changing the elements.

In many cases, only a single pressure filter (downstream of pump) is used in a hydraulic circuit to protect the equipment and to clean the fluid. If this one filter is responsible for 100% of the work and there is no back-up of any kind the flow can go to bypass when the filter is at capacity. The fluid is no longer filtered which means that the system is no longer protected.

Taking line filtration one step further, the optimum protection for any hydraulic system is to use the three-filter option: one downstream of the pump, a return filter and a kidney loop system on the fluid filter reservoir.

To prevent this from happening, using two filters is the answer, creating a more balanced approach. This is done by adding a return filter to the system. In this configuration, both filters are each taking care of 50% of the work and some protection is always in place even if one filter would begin to experience bypass.

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Hydraulic Filters FPW Handbook 2017 v3.indd 31

Adding a kidney loop system on the fluid reservoir, in addition to the pressure and return filters is the ultimate answer. They continuously work 24/7 with constant flow and are not influenced by pressure and flow variations that are present in a typical hydraulic system. Therefore, the filter works very efficiently in removing particles and ends up in providing about 50% of the work (removing of particles), while the pressure/system filter now handle about 25% each. (More on kidney loop filters can be found in the following article.)

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HYDRAULIC

FILTER SYSTEMS

F

iltration is always on a hydraulic maintenance technician’s mind. But for some machinery or in some plant setups, inline hydraulic filters are not enough to keep a hydraulic system functioning properly. Most hydraulic systems are installed with at least a return filter, which semi-purifies fluid before it is once again welcomed into the reservoir.

However, what if a single return filter is not enough? What if your cleanliness codes are not achieved, even if you’ve upgraded to a finer filter media? A pressure filter is an option, which will keep the components downstream of the element one step cleaner. But what if using a pressure filter is impossible, due to plumbing difficulties or pressure drop considerations?

A solid option to increase filtration effectiveness is with an offline filter system, often called kidney loop filters.

Offline filtration uses a dedicated lower pressure pump (still often a hydraulic pump), which draws fluid from the reservoir and then flows that fluid through a dedicated filter assembly—usually of a high-quality medium—and then right back into the tank.

U R TE SY IM A G E CO

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Sometimes hydraulic power units have dedicated offline filter systems, whose only jobs are to circulate fluid from their reservoirs and filter it as it does so. Because a kidney loop filter neither affects nor is affected by the main hydraulic system, it is a consistent and stable way to keep the oil clean. The pressure drop of often low-micron filter media will never be additive to system pressure drop, especially those related to flow surges in the tank lines of machines with rapid cycle times of cylinders. It is not uncommon to see 5- or even 3-μm offline filters with high beta ratios.

Offline filtration also enables changing of filter elements while the machine is running, as shutting down the kidney loop has no association with machine operation. Some filtration systems employ duplex filters, which are two filter assemblies installed in parallel,

O O FI LT R A TI O F H Y- P R

N

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Electrical Engineering

W H E N S H O U L D YO U F L U S H A NEW OR REBUILT H Y D R AU L I C S Y S T E M ?

Particulate contaminants circulating in fluid power systems cause surface degradation through general mechanical wear, such as abrasion, erosion, and surface fatigue. This wear causes increasing numbers of particles to be formed, the result being that wear increases if this chain reaction of wear is not properly contained (by reducing contamination).

Gaps within components grow larger, leakage oil flows increase in size and operating efficiency (e.g. of pumps, cylinders) decreases. To avoid this, you should always remove particulate contamination in new systems, before start-up, and later when the system is in use.

The overall contamination level of a hydraulic system being built in an assembly plant is generally high.

The total contamination (number of particulates) consists of contamination that already exists in the new fluid, contamination that is already on/ in supplied components and contamination introduced during the build process from the surroundings. The sum of all the contamination can be considered the Initial Contamination Level.

The investment in time and equipment to reduce the Initial Contamination Level is worthwhile because of a reduction of costly warranty claims and for quality control and tracing purposes of products.

If the fluid was not maintained to recommended cleanliness standards during use, a warranty claim could be rejected.

Fluid in the reservoir should be flushed with a filter cart or a kidney loop system. It is recommended to flush the system and all the sub-functions by activating them to allow fluid to circulate and to flush particulates back in to the tank where they can be captured, either by the system filters (e.g. return filters) or by external, off-line filter systems (filter carts, kidney loop systems).

A general rule is to achieve a cleanliness level of the hydraulic fluid in the reservoir that is 1 to 2 ISO Codes below the recommended Target Fluid Cleanliness level for the system.

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H Y D R AU L I C F I L T R AT I O N S Y S T E M S

separated by a three-way ball-valve.

This design allows for live selection of either filter so the other can be replaced. Most filter manufacturers offer a filter system dedicated to the offline filtration market that is highly efficient and offers high dirt holding capacity.

Offline filtration is typically the highest quality in a manufacturer’s product line, which is reflected in the cost of these products. To help justify the purchase of such a system, they are often sold as portable units, small units that can be carried by a handle, or large units requiring a wheeled cart to manage their bulk. These units can be wheeled from machine to machine, where a suction tube is placed into a port of the reservoir and then passes through its own filters before being injected back into the tank. Depending on the size of the tank, the filter system’s flow rate and filter quality, one might leave the filter system running on the machine for hours or perhaps days.

Permanently mounted offline systems are now more commonly used as well. They are often mounted to a panel, either near the reservoir or directly attached to it. Eliminating intermittent filtration of the portable type ensures that fluid is clean from storage to service.

Some of these filter systems are installed with auxiliary electronics, such as particle counters. A particle counter will give you a live reading of the ISO Code of the oil passing through the unit, so you can leave the unit running until the desired code is achieved.

If this type of system seems out of your reach, note that some hydraulic distributors will rent these machines out for a reasonable cost.

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HANDBOOK

H O S E A S S E M B L Y T I P S . CO M

36

HYDRAULIC

FITTINGS & FLANGES H

G OF RIN SY CTU E T A UR UF CO MAN E A G AY IM R-W AI

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ydraulic fittings connect tubes, pipes or hoses to components (pumps, valves, cylinders) or together to create a system through which hydraulic fluid under pressure can be transmitted without leaking. Fitting variations allow designers to change the direction of flow, split flows and change the elevation of lines to accommodate the location of the items they marry.

Fitting varieties include plugs, unions, elbows and crosses.

Care needs to be taken in identifying the threads of the connections, as some connectors from one standard appear to thread into a different standard, but do not have enough thread engagement to be safe. Documents from SAE, NFPA and ISO help identify each standard to confirm diameter and thread type.

Fitting connection types include: welded (socket weld, butt weld, slip on); threaded (NPTF, BSPT [both not recommended but used], SAE straight thread, ISO 6149; BSPP); flanged; barbed; quick-disconnect; push-to-connect; 37˚ flare; 24˚ cone; and inverted flare, among others. When selecting a type of fitting, some important considerations are working pressure, vibration, type of fitting, desired attachment, size of piping, flow, material of the conductor or component, and price. The fluid power industry is trying to transition to fittings with an elastomeric seal—generally O-rings—to prevent leakage. These include, but are not limited to, the SAE straight thread, face seal, ISO 6149, and SAE J518 (Code 61 and Code 62) flanges. Seal construction must be compatible with the type of fluid being used in the system, although very few applications require anything other than Buna Nitrile or Viton.

When selecting a fitting, several considerations are important. Most non-flanged fittings have a gender—called male and female—that are joined together to form a union. Most fittings are sized based on the size of the conductor (size of hose, pipe or tube), and overall dimensions can vary greatly based on fitting type, even for the same size conductors.

Additionally, most fitting types are available in a multitude of materials, including plastic, brass, steel, stainless or specialty metals like Monel. Each are applied in applications based on the fluid medium and ambient conditions, and each has different performance characteristics that allow customization within a fitting type. Often the first choice is to match the fitting to a similar material of the conductor or component that it is connecting to: plastic to plastic, steel to steel and stainless to stainless.

Geometry is also an important consideration, and geometry is typically identified by alphabet letters the fittings resemble. Fittings are available inline to change the direction of flow in various increments (45˚ or 90˚ elbows [L]), or a swivel to allow two joined sections to rotate. They can also split or combine flows with run and branch tees [T], “wahys” [Y] and crosses [+]. Fittings, particularly elbows, are offered in a variety of drop lengths, which is the distance from the centerline of one opening—called a port—to the end of the other port.

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Electrical Engineering

H O S E A S S E M B L Y T I P S . CO M

Fittings are available in various sizes to suit differing flow demands, and connection size is often expressed in dimensionless terms representing 1⁄16 of an inch. For example, a -06 thread is 3⁄8 (6/16), and IMAGES COURTESY OF AIR-WAY MANUFACTURING a “dash” 32 size is a 2-in. (32/16) thread.

A Y-flange may split a 2-in. flow into two The clamp can be whole, but is often split so that reduced 11⁄2-in., instead of creating three 2-in. connections to a quarter of the diameter of the flange head is on more closely match the cross-sectional area. either side of the centerline of the bolt holes to O-ring face seal, SAE straight thread and ISO 6149 fittings help minimize torque on the clamp. The screws have a seal, normally Buna N, contained within a groove to used are tightened to a high torque value to avoid seal the fluid. It is important for the seal to be compatible with problems with fatigue. In many cases, using pipe the fluid and the operating temperature range. An elastomeric or tubing, the flange connections have operated seal greatly reduces the possibility of leakage caused by within their specified working pressure for decades. vibration, thermal cycling and pressure cycling.

Flare fittings, such as the JIC 37˚, are fittings SAE J518 split flange fittings are used on larger line with a conical end face and the seal is formed 1 sizes, starting at ⁄2 in. (-8) but coming into predominance at 2 when this seat is forced against a mating seat, in. (-32) and above. A flange head with an O-ring groove on generally by torquing a swivel nut on one fitting, its face is attached to a conductor (hose, tube or pipe) and is engaging with a threaded portion of the mating secured to the port, which could be a flat-face fitting or a pad fitting. The angle of the seat and face for most JIC on a pump, valve or cylinder, by a clamp with four bolt holes. fittings in the North American market is 37°, and is popular enough that the 24° and 45° versions are rarely used. The fittings can be designed to clamp onto a tube by means of a sleeve or ferrule, and care needs to be taken so that the correct size is used because inch and metric tubing sometimes have sizes that are close to overlapping.

The quick disconnect allows multiple reconnections of the assembly without causing excess wear or concern for thread damage. Some fittings allow disconnection and reconnection under pressure; others do not. Disconnects hold fluid pressure by way of a ball or poppet, which is spring offset to remain closed when the lines are unattached. Upon reattachment, the balls or poppets push against each other, lifting themselves from their seats and allowing fluid flow.

Standard plug and socket configurations, such as the Pioneer coupling, are prone to trapping QUICK -- Thousands Stocked contamination, which was addressed with the INFORMED -- On ISO & SAE committees advent of flat-face couplers, which have no recess DEPENDABLE -- 58 years of service to collect contamination.

Staple and band fittings are low-pressure fittings. Band fittings are attached to the hose by a barbed or beaded end being inserted into a hose and a band clamp securing the connection.

This method is only for extremely low pressures.

Staple fittings have a cylinder with an O-ring and a bead further up on it that slides into a socket.

The connection is secured by a staple that goes www.MAINmfg.com/flg through both sides of the connection behind the bead, although it is still typically used for low Phone: 800.521.7918 pressure or suction lines.

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H OW DO YO U M E A S U R E H Y D R AU L I C F I T T I N G T H R E A D S ?

Electrical Engineering

Before you select a hydraulic fitting, you must first determine if its threads will match the hose you have selected. Measuring and identifying the fittings is easily done but you must have the correct tools. These tools include ID/OD calipers and thread pitch gauges.

Using the thread pitch gauge, as seen in Figure 1, simply place the gauge teeth on the threads until a tight fit is found. Ensure that there is no gap between the teeth of the gauge and the fitting. If FIGURE 1. Thread pitch gauge. Image courtesy of Swagelok. the fitting is not new or is worn, you may have difficulty finding a match so it’s best to always use new fittings. Fitting manufacturers provide thread identification guides either in brochures or on their websites, so you simply need to match the thread measurement to the relevant charts.

To measure the thread’s outer diameter (or male thread), place the ID/OD caliper around the fitting threads for a snug fit, as shown in Figure 2. For the inner diameter (or female thread), insert the caliper into the fitting for the correct measurement and match to the manufacturer’s data charts.

You must also know whether your fitting is tapered or parallel, i.e. either at an angle or measuring the same width from one end of the threads to the other.

Understanding some of the most common connector types will help in identifying and measuring them in thread identification guides. NPTF (National Pipe Tapered Fuel) connectors are used in fluid power applications although not recommended.

Other common designs are NPT, SAE J514 JIC/37°, SAE J1926 straight thread O-ring boss, SAE J512 45°, SAE J1453 O-ring face seal, ISO and DIN connectors, among others.

FIGURE 2. Measuring the OD with an ID/OD caliper.

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H Y D R AU L I C F L U I D S

HYDRAULIC

FLUIDS

H

ydraulic fluid is the energy transfer medium in all hydraulic systems. However, the job of hydraulic fluid goes beyond simple transmission of power.

Although transmitting hydraulic energy is the core purpose of hydraulic fluid, it is useful in four secondary functions—heat transfer, contamination removal, sealing and lubrication.

Hydraulic machines produce a lot of excess heat in normal operation, often caused by inefficiencies of the components themselves, like pumps and motors. Without a way to carry heat away from these components, they could easily overheat with resulting damage of seals and internal components, especially as a result of low local viscosity. As oil returns to the reservoir, it often passes through a cooler to help maintain optimal temperature range before it is pumped back out to the system. Conversely, hydraulic fluid can carry heat into a system during cold starts when needed.

If closed-loop hydraulic systems didn’t bleed off fluid at a controlled rate, contamination would quickly accumulate to critical and damaging levels.

Heat can be considered a form of contamination, but hydraulic fluid also carries particles and water away from sensitive components through filters or other conditioning devices, where it is cleaned up and returned to the circuit.

Oil without impetus would remain within sensitive components, allowing essentially trapped contamination to slowly destroy its surroundings.

Although most believe hydraulic oil is what pieces of hardware—such as O-rings or U-cups—seal against, hydraulic fluid (especially oil) actually provides sealing within the internal components of pumps, valves and motors. A spool valve, for example, has a seal at each end to prevent oil from escaping the valve, but each notch on the spool is sealed from the neighboring cavities by only the tight metal-to-metal tolerances and the oil’s surface tension and resistance to shearing.

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W H Y I S V I S CO S I T Y I M P O R TA N T I N H Y D R AU L I C F L U I D S ?

HANDBOOK

Viscosity, according to Webster’s Dictionary, is the property of fluids and semi-fluids that defines its internal resistance to flow and shear.

Or, in more simple terms, how thick or thin a fluid is. For example, water is less viscous than tar.

(Fluids that have no resistance to shear stress are known as ideal fluids or “superfluids.” Zero viscosity is observed only at extremely low temperatures.)

Fluid Power

Viscosity is a critical property of hydraulic oil, as it affects the performance and efficiency of complete systems as well as the wear rates of individual components like pumps and valves.

A hydraulic fluid’s viscosity is defined primarily by the size and structure of its molecule chains— the larger the molecules, the thicker the fluid. The hydrocarbon molecules in mineral oil vary in size, while synthetic oils have a more-consistently sized make-up. Also, as hydraulic fluid ages or experiences shear stress, oil molecules can break down and that lowers the viscosity.

The most common unit of measure for viscosity is kinematic viscosity, which gages how easily oil flows under the force of gravity. It’s usually shown in spec sheets at temperatures of 40° and 100° C. Viscosity is measured in centistokes or mm2/sec, where 1 cSt = 1 mm2/sec.

Manufacturers of hydraulic fluid provide various products with different viscosities. The fluids are most-often labeled in terms of ISO number or grade, where common grades for hydraulic circuits include ISO VG 32, 46 and 68. They are generally selected based on the oil’s viscosity for use in a Viscosity Grading Systems certain type of equipment operating over a specific temperature range.

Saybolt Viscosities

Kinematic Viscosities cSt cSt @40°C @100°C 2000

70 60

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AGMA SAE SAE Grade Crankcase Gear

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40 30

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1 15W

100 1000

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75W

60 55 50 45

150

Determining the right fluid viscosity is a balancing act. As oil temperature rises, viscosity drops and it flows more easily—to a point. If the oil gets too thin, volumetric efficiency suffers and the system becomes less responsive and can lead to overheating, high wear and shorter component life. At the other extreme, if fluid viscosity is too high, mechanical efficiency is low and that leads to friction during startup, sluggish operation and, in the worst case, cavitation and mechanical failure.

The viscosity of a hydraulic fluid is a critical element in the transfer of hydraulic power. The ideal viscosity range for a fluid—and thus its highest efficiency—is usually between 10 and 100 mm²/sec, depending on the application. If in doubt, always consult the equipment manufacturer’s recommendations.

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Lubrication is required in most hydraulic components to protect internal parts from wearing, or even downright melting, as a result of metal-to-metal friction. Oil provides full-film lubrication between moving parts, such as the slippers and lens plate of a piston pump. Without the lubricating properties of oil, hydraulic systems would be inefficient and unreliable.

These functions of hydraulic fluid are common to all types, except some water-based fluids, which require special design considerations during engineering. However, the majority of machines use refined or synthetic oil, which are formulated and manufactured to specific test standards for important properties like viscosity, pour point and viscosity index, to name a few.

Those three properties are often considered when choosing a fluid for a particular application, which is based on maintaining a specific viscosity throughout a particular set of ambient and machine operating conditions. For example, if ambient temperatures are low, you would choose

an oil with lower rated viscosity and a low pour point, which is the temperature at which oil will still pour. If your machine sees varying temperature ranges, like an all-weather mobile machine, a high viscosity index, which describes an oil’s ability to maintain its viscosity over a wide temperature range, is crucial. It is important to consider the viscosity requirement of the components in your hydraulic system. A piston pump, for example, may require between 16 and 40 centistokes, which is a description of kinematic (measured while flowing) viscosity.

Hydraulic oil is engineered with other important properties, although these tend to be common regardless of brand, viscosity or application. Hydraulic oil has a package of chemical additives to improve the performance of both the oil and the components of the hydraulic system.

These additives can improve the foaming resistance of the oil, its corrosion/rust resistance and the water-retention properties. The additive package of hydraulic fluid is what separates low- and high-quality fluids, and additives also improve the viscous properties of oil. When in doubt, always choose a premium fluid for your application at a viscosity appropriate for your operating conditions.

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H E AT E XC H A N G E R S

HEAT

EXCHANGERS

H

eat is generally not a friend of hydraulic systems. Higher heat can lead to faster degradation of hydraulic fluids, as well as increased wear on components. Hydraulic fluid needs to be maintained at an optimal operating temperature for the system to function properly and have a long life.

In fact, temperatures that are either too high or too low can cause problems in hydraulic systems. When oil temperature drops, the viscosity of the fluid increases, making it more difficult to pump, creating higher pressure drop and increasing the chance of cavitation.

Conversely, as oil temperature increases, the viscosity of the fluid decreases, which reduces lubricity, increases oxidation rate and can cause the fluid to varnish.

Heat exchangers are used in hydraulic systems to control oil temperature—and therefore viscosity—within an optimal range, where the fluid has the best combination of properties useful to the components of the hydraulic system. Although a few hydraulic machines can make do without external cooling, such as small, low-duty or load-sensing systems, most require a device to keep oil in its ideal temperature range. This is where heat exchangers come in.

What a heat exchanger does is self-explanatory. It will use a fluid such as water or air to transfer heat into or away from hydraulic liquid; very simple. However, the nature in which heat exchangers transfer heat can vary vastly. Liquid-to-air and liquid-to-liquid are the two primary types of heat exchangers, and you can imagine they can use air and water, respectively, to remove heat from a hydraulic system.

Liquid-to-air coolers transfer the heat from the hydraulic fluid through radiation and convection. The simplest liquid-to-air coolers are radiators that count on the thermal difference between the hydraulic fluid and the ambient air. The rate in which a heat is removed from the oil is factored only by the temperature difference between the air and the oil (higher differential means more cooling) and by the existence of airflow (which is sometimes likely in a mobile application).

The basic tube and fin cooler is the most economical method of cooling hydraulic fluid, but is for light duty applications, such as low duty cycle or low horsepower applications. They are often very small, such as the type used in a vehicle’s transmission fluid cooler, but in

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BRAZED PLATE heat exchanger. Image courtesy of Thermal Transfer Products, an API Heat Transfer Co.

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hydraulic applications, they can be sometimes paired with light duty fans to improve efficiency.

This type of cooler is small and light enough to be attached to the back of an electric motor to take advantage of the motor’s cooling fan. They’re constructed by forming a copper tube into a snaked web, and then aluminum or copper fins are added to surround the tubes. Heat energy is imparted from the oil, to the tube wall, to the fins and then to the air.

The more efficient liquid-to-air cooler is the plate and bar style. It will use thick and deep channels of rectangular aluminum with spans of aluminum fins separating them. The better coolers will come with a rough internal finish to those channels to add turbulence to the moving liquid.

Although this technique adds pressure drop to the cooler, it removes heat with more efficiency.

The fins on the air side of the cooler can also be crimped for a rough finish, increasing air turbulence and improving the rate of heat transfer to the air.

Regardless of liquid-to-air cooler construction, any design will more rapidly remove heat with the addition of convection. Adding a fan to a cooler will increase cooling exponentially. Cooling fans can be any size, from tiny dc fans used in the computer industry, to high horsepower ac motors used in industrial applications. Extreme duty mobile applications using dc fans can pull upwards of 40 A, which is the upper range of reasonable usage for 12 V applications, and is taxing on the electrical system in the best of cases. When mobile applications are severe, the fan motor can be hydraulic. With hydraulic energy, the fan can be high power while using absolutely no electrical current.

If massive amounts of heat must be removed from a hydraulic system, air coolers aren’t the most efficient option. Although some electric coolers

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can remove over 300 hp worth of heat, they can block a commercial doorway because of their unwieldy size. Liquid-toliquid coolers use water or coolant to remove heat from the hydraulic fluid. Water transfers heat orders of magnitude more efficiently than air, and the same wall of air coolers could be matched in performance by a shell and tube cooler the size of a small bazooka.

The problem, of course, is liquid-to-liquid heat exchangers require water or coolant to do their job.

If you have unlimited supply of fresh water, you can simply use that water to continuously run through your cooler (or control its flow thermostatically).

Tap water can also be used for cooling, although it can be expensive using the municipal water supply.

In large plants or factories, a centralized cooling system can be put in place to supply coolant to the various machines of the plant. This is the most environmentally friendly option, but requires expensive infrastructure, such as complex plumbing and large chiller units exterior to the building.

The shell and tube cooler features a series of copper tubes installed into a larger tube shell. The copper tube can be a single bent tube, or can be many small tubes spanning two plates on either side of the shell. Either method will use a ported shell, itself just a metal barrel, with water flowing into one end and out the other.

Water or coolant passes across the copper BAR AND tubing, transferring heat FIN COO LER from the hydraulic fluid to the

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H E AT E XC H A N G E R S

cooling medium, and then discharging it to wastewater or back to the coolant system.

The most efficient method of cooling hydraulic fluid is with the brazed plate cooler. The same cooling capacity requiring a wall of air coolers or a bazooka tube can be achieved with a plate cooler the size of a textbook. It has long, wide plates brazed together to optimize surface area for heat transfer, and spaces the plates to alternate coolant with hydraulic fluid. Because they are so efficient, the brazed plate cooler can make do with half the water or coolant compared to shell and tube types, although they still need that fresh supply of water or coolant. Some large variants of the plate type cooler can remove a jet engine worth of heat, although those versions do require extraordinary amounts of cooling medium.

It should be noted that any liquid-to-liquid heat exchanger can be used to warm hydraulic fluid as well as cool it. By simply running hot water or coolant through the cooler instead of cold, heat can be transferred to the hydraulic oil. Most often, hydraulic oil is heated electrically, because using warm water requires a more complex cooling system circuit. Regardless, until hydraulic systems have nearly perfect efficiency, heat exchangers will always be a big part of them.

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H O S E A S S E M B L Y T I P S . CO M

HANDBOOK

HYDRAULIC

HOSES

A

IMAGE COURTES TIT CON Y OF

Fluid Power

hydraulic hose is specifically designed to convey hydraulic fluid to or among hydraulic components, valves, actuators and tools. It is used in a variety of industrial applications. It is flexible and usually constructed with several layers of reinforcement because hydraulic systems frequently operate at high pressures.

Dimensions, performance specifications, construction options and features are important parameters to consider when choosing hydraulic hose.

Important dimensions for the selection of hydraulic hose include the inside diameter, outside diameter and minimum bend radius. Hydraulic hose sizes are denoted by the inside and outside diameter of the hose. The inside diameter refers to the inside of the hose tube. The outside diameter is often a nominal specification for hoses of corrugated or pleated construction.

Minimum bend radius is based on a combination of acceptable hose cross-section deformation and mechanical bending limits of the reinforcement.

Construction options for hydraulic hose include reinforced, coiled, corrugated or convoluted. Reinforced hose is constructed with some element of reinforcementâ&#x20AC;&#x201D;styles include spiral wire, textile braid, wire braid, wire helix and other designs in many plies or layer configurations. Coiled hose is designed for flexibility and elasticity. This feature often makes it expandable and easy to store. Corrugated hose contains corrugations, pleats or spiral convolutions to increase flexibility and capacity for compression and elongation. Multi-element hydraulic hoses are constructed of more than one hose formed or adhered together in a flat, ribbon or bundled configuration. Additional features to consider include whether the hose requires integral end connections, anti-static, lay flat, crush-proof and flame-resistance characteristics.

In addition, material considerations include the type of fluid being conveyed and its concentration, as well as substances that may attack the hose cover. Hose selection must ensure compatibility if it is to convey special oils or chemicals. The same holds for hose exposed to harsh environments. Substances such as UV light, ozone, saltwater, chemicals and pollutants can cause degradation and premature failure. For in-depth fluid compatibility data, consult the manufacturer.

While hydraulic hose is usually constructed of multiple materials, the most commonly used primary materials include elastomers, fluoropolymers and silicone, thermoplastics, metal, and composite or laminated structures.

Elastomeric or rubber hydraulic hose are often selected for their flexibility.

Fluoropolymer hose offer good flex life, superior chemical and corrosion resistance and can handle high temperatures. Thermoplastic hydraulic hose

ECH

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Engineers and technicians specifying hoses for hydraulic circuits routinely consider factors like pressure ratings and flow capacity. But in some instances, electrical shock is a potential risk to equipment and operators, and that demands hydraulic hoses that ensure safety when machines operate near highvoltage sources like power lines.

Thus, nonconductive hydraulic hoses are recommended for use in mobile-lift equipment like “cherry pickers,” hydraulic aeriallift platforms, and agricultural and construction machinery.

Nonconductive hoses are also routinely used in steel mills, foundries, mines and shipyards, and in rescue tools.

Users should never assume a hose is electrically nonconductive, especially if it is made of rubber. That’s because rubber compounds can vary widely in their electrical-conductivity characteristics and, therefore, can be electrically conductive, partially conductive or non-conductive. Further, some rubber hoses can be non-conductive at low voltages but conductive at high voltages. Add to that, they often have steel wires for reinforcement. And unless designed and manufactured for specific electrical characteristics, a hose’s electrical properties may change from one production run to the next.

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Nonconductive hose, on the other hand, is specifically built and tested to ensure the proper electrical properties. For example, the SAE J517 standard for 100R7 and 100R8 hoses states that permissible conductivity should be less than 50 micro-amps leakage under 75,000 volts per foot of length.

Such hose is, for example, built with a nylon or polyester inner tube, one or two layers of braided textile or synthetic fiber reinforcement, and cover materials like polyurethane or PVC. These typically run in sizes from 3⁄16 to 1 in.

ID and have maximum operating pressures from around 1,000 to 5,000 psi.

And they often feature an orange or other non-black cover. Some versions with multiple layers of high-tensile, aramid-fiber reinforcement can have maximum pressure ratings of 10,000 psi or higher.

Many reputable manufacturers offer 100R7, 100R8 and other nonconductive hoses to transport petroleum-based, water-based, and synthetic hydraulic fluids without a risk of conducting electricity. And they routinely engineerin other performance characteristics like a tight minimum bend radius, excellent kink resistance, abrasion resistance and suitability for operation in temperatures from about –40° to 212° F (-40° to 100° C).

As a final note of caution: For general hydraulic systems that may contact high-voltage sources, unless a hose is specifically and clearly branded electrically nonconductive, engineers, maintenance technicians and users alike must assume that it is not.

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offer tight minimum bend radii and excellent kink resistance. Metal hoses can handle high temperature flow materials and often can handle higher pressures. They can be either stiff or flexible.

Flexible hoses are easier to route and install, compared with rigid tubing and pipe. They lessen vibration and noise, dampen pressure surges and permit movement between parts.

In addition, increasing demands for higher productivity, efficiency and environmental compatibility are forcing hose manufacturers to improve product integrity—hoses now withstand higher pressures, extreme heat and cold and accommodate a range of fluids including today’s “green” variants.

Most hoses are manufactured to SAE J517 or European Norm (EN) Standards, the latter based on earlier DIN German standards. These standards predominate in the Americas, Europe

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and Australia. Both are also used throughout Asia, though that market is slowly gravitating toward EN specifications primarily because EN-rated hose has a higher pressure rating compared with similar-sized SAE hose. This gives greater safety factors should an application need the highest working pressure.

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H Y D R AU L I C H O S E CO U P L I N G S

HYDRAULIC

HOSE COUPLINGS

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mproper selection or improper use of hoses, tubing, fittings and assemblies can cause failures, personal injury or property damage. Understanding the hose assembly and following the installation instructions provided by manufacturers will reduce these risks.

Engineers looking to specify hose couplings need to consider a few things besides correct size. Will the couplings be reused or permanent? Will they need a locking mechanism to prevent involuntary disconnection? Is one-handed operation required?

Couplings can be two types: permanent and field-attachable (reusable). Permanent couplings are generally more reliable, easier and quicker to attach than fieldattachable couplings, which makes them widely used in industry.

Crimping or swaging equipment (sometimes both) is needed to put a permanent coupling on a hose. Permanent couplings can be pre-assembled (one piece), with a ferrule permanently attached to the stem.

Higher-pressure hoses use field-attachable couplings, as well as permanent couplings. Fieldattachable couplings fit right on the hose using only a wrench and a vise. No special equipment is required. While handy, they do cost more than permanent couplings and take more time to attach.

There are three common types of coupling interfaces used in hydraulics today: thread interface, mated angle and O-ring.

Threaded couplings have two types of threads: male (outside threads) and female (inside threads). The National Pipe Tapered for Fuel (NPTF) has, as the name implies, a tapered thread. When the male and female components are threaded together, the tapered threads deform, applying pressure on one another, and thus making a tight seal.

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Quick-connect couplings are connectors or fittings used to mate fluid lines with equipment that requires repeated connections and disconnections. They are used in both hydraulic and pneumatic applications, and are designed for easy hand operation for use with fitting attachments primarily on mobile machinery.

Their design is simple: a male end—or plug—is inserted into a female end—or socket—to make a secure, leak-tight seal. They are sometimes called push-to-connect because connecting them requires only a quick push.

They usually feature a one-way sleeve to allow for break-away with a tool when a coupling is clamp mounted. Two-way sleeves allow for one-hand disconnection. In two-way designs, twisting and pulling the two ends breaks the connection.

These locking seal designs are created by several designs, including: Ball or bearing type, in which spring-loaded balls lodge in cavities to make the connection. This is the most common type, because they can be disconnected with just one hand.

Flat face designs eliminate the problems of trapped pressure in a line, as well as leakage of hydraulic fluid or air by eliminating the cavity where the fluid or air rests. They can be push-to-connect or threaded, screw-in types. Flat face couplings provide high flow and low pressure drop and their sleeve-locking feature reduces the change of accidental connection.

Non-latching designs are best used when there may be frequent change-outs of the coupling, such as pneumatic ones used in test or medical applications.

They usually come with a self-sealing valve that will hold the fluid in the line to prevent any leakage.

Bayonet couplings are simple to connect. The user plugs the two ends together and once engaged, gives the devices a quarter-turn to push the male end into the female socket. Twisting them a quarter turn the other direction allows them to be easily disengaged and separated. Although used in high-pressure hydraulics, they are more common in pneumatic applications.

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Mated angle couplings form a seal when the male and female threads are screwed together. Two types of mated angle seals are SAE 45° and JIC 37°, but there are others.

The NPSM seal is a mated angle.

Couplings with angle seats for sealing have straight or parallel threads. The threads themselves do not seal fluids as with tapered threads. Instead, the threads function to mechanically bring the two mating angle seats together.

National Pipe Straight Thread Mechanical Joint (NPSM) brings two 30° tapered seats together to make the seal. SAE 45° flare couplings are used on lower pressure applications, such as fuel lines, hot oil lines or refrigerant lines. JIC 37° angle seats are used on medium- and highpressure lines on heavy equipment to join hydraulic hose assemblies to hydraulic system components.

There are three types of O-ring seal designs: O-ring boss, flat-face O-ring seal and O-ring flange. In the boss design, straight threads make the connection while a rubber O-ring makes the seal. Threads pull the O-ring against the port, which has a machined groove for the O-ring, flattening it and making a seal that is excellent for high-pressure applications.

WHAT DO YOU NEED TO SPECIFY HYDRAULIC COUPLINGS? 1. Hose size 2. Seat angle 3. Type of seat (inverted, flat) 4. Thread pitch 5. Maximum temperature 6. Maximum pressure 7. Hose material 8. Flow

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In a flat-face O-ring seal, the O-ring sits in a groove on the male’s face. The seal is made when the O-ring of the male meets the flat face of the female. The solid male O-ring face seal fitting will mate only with a swivel female O-ring face seal fitting.

O-ring flanges make high-pressure, large-diameter connections. A port is bored with a center outlet, surrounded by a smooth flat face, which has four tapped holes and four mounting bolts that tighten down onto flange clamps.

There are no threads on this coupling. The flange itself has the groove for the O-ring.

There are several SAE and ISO standards that cover the performance requirements of hydraulic hose assemblies. Included are the J517, J516 and J343 standards. The three most common ISO standards are ISO A, ISO B and ISO 16028.

The standard for performance testing is ISO 7241-2.

Hydraulic hoses that claim to meet SAE J517 standards (for example, SAE 100R1 and SAE 100R2) need to be designed for, and certified to, the criteria defined by SAE. That criteria includes stringent dimensional tolerances (inside, outside and braid diameters), compound and reinforcement types, length changes, cold flexibility and ozone and heat resistance. There are also burst pressure and impulse requirements in J517. Those requirements are for coupled assemblies, and SAE states that “the general and dimensional standards for hydraulic hose fittings are obtained in SAE J526.”

Hydraulic hose fittings that meet SAE J516 standards are similarly well defined

by SAE as to material type, dimensions, finish and so on. The SAE manual also specifically states that J516 fittings are intended to be used “in conjunction with hydraulic hoses specified in SAE J517 and used in hydraulic systems on mobile and stationary equipment.”

SAE J343 is the standard that establishes “uniform methods of the testing and performance evaluation of the SAE 100R series of hydraulic hose and hose assemblies.” Coupled assemblies are expected to meet or exceed SAE performance if the SAE criteria described above are met.

The integrity of any hose assembly depends upon the components, fittings and hose meeting the rigorous SAE requirements, and then the components being assembled by skilled personnel. This is true regardless of where the components are manufactured. www.fluidpowerworld.com

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MANIFOLDS IN

simplest terms, a manifold is a component from which you attach other things. A slightly less elementary explanation is that it cleans up plumbing—and this is why you should care about this unassuming block of metal that ultimately makes for smoother system design.

A hydraulic manifold is a housing for surface and/or cartridge valves that regulates fluid flow between pumps, actuators and other components in a hydraulic system. It can be compared to a home’s electrical panel. Just as raw electrical power comes to the panel and is distributed to various household circuits to do work (provide light, power the dishwasher, operate the garage door), hydraulic oil under pressure is routed to the manifold by a pump where it is diverted to various circuits within the manifold to do work.

The role of a manifold is to bring the hydraulic circuits to life through the creation of a block machined in a manner consistent with the original circuit design. All valves have a series of orifices to which drilled holes in the manifold must communicate. The configuration of these drilled holes in the manifold is the representation of the defined circuit.

The manifold is the central muscle control of the hydraulic system receiving inputs from switches, manual operations (levers) or electronic feedback systems. These inputs energize various valves mounted on or in the manifold, while specific oil pathways allow oil to flow through hydraulic lines to the appropriate actuator to perform work. The complex matrix of variables can make manifold design and component selection a challenging and rewarding art form, as size, weight, function, performance and operating environment are always part of the design consideration.

In addition to providing a neat and logical layout, consolidating components into a manifold reduces space and pressure drop. This results in fewer fittings, more efficient assembly times and reduced leak points. Manifolds are sometimes viewed as black boxes, as they can be highly complex with upward of 500 holes communicating with each other and

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many valves on a single block. The alternative to manifolding a system is to mount all valving in individual blocks and plumb hoses in a manner consistent with the circuit. This dramatically increases the visual nature of the system, introduces infinitely more leak points and is generally an unacceptable alternative to manifolds. If a system is properly designed and test points provided in key locations, finding a problem becomes much quicker and simpler with a manifolded system. If transducers and other data collection devices are connected to these test points, the data may be linked into the machine controller and operation’s terminal displays.

Manifolds generally operate within 500 to 6,000 psi operating pressures. With additional design considerations, 10,000 psi can be achieved within the scope of steel and stainless-steel manifold designs.

Although not typical in hydraulic application, 50,000 psi can be achieved with special materials and design nuances.

Manifolds come in three basic types. Most common is a solid-block design that contains all drilled passages and valves for an entire system.

Typical materials for solid-block manifolds are aluminum, steel and ductile iron. Block weight can reach 100,000 lb.

Modular-block, or stackable design, is a subset of the drilled block.

Each modular block usually supports only one or two valves and contains interconnecting passages for these valves as well as flow-through provisions. It normally is connected to a series of similar modular blocks to make up a system. This system is known for its flexibility within a limited range of circuit complexity. Modular block designs are generally

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WHERE ARE MANIFOLDS USED? Manifolds are used in my many fluid power applications, depending upon the application. Manifolds can be used to mount valves or to consolidate plumbing. When used for mounting valves, they are the interface between the valves and the ports to be plumbed into.

With industrial style hydraulic valves, such as ISO valves mounted on D03 or D05 patterns, for example, the valves terminated with plain ports surrounded by O-rings, and cannot be plumbed directly into a hydraulic system. The manifold is a block, or series of adjoining blocks, which has an interface for the valve(s) to mount to, ports for the fluid to travel, and then ports to plumb the manifold to the rest of the circuit. The pressure passage can be parallel to the valve pressure ports, or in series, joining the tank to pressure port of subsequent valves.

The advantage of manifold mounted valve systems is in their modularity; standard valves of various, easily exchangeable iterations can be mounted to a manifold to customize the circuit and its number of actuators. A standard valve series can be mounted to manifolds employing any type of port, such as NPT, Metric or ORB, rather than producing every valve with every version of port, saving manufacturing and inventory costs.

Manifolds can be used outside of valving, as well. A manifold can simply be a chamber with two or more ports joined in series to reduce plumbing. For example, a return line manifold with six smaller ports joining to one large tank port will save the need for a series of expensive tee’d together fittings, reducing both cost and the chance of leakage. A manifold can be used less commonly in pressure lines, cleaning up plumbing and improving appearance.

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held together with tie rods or a system of tapped holes that allows for machine screw connections.

Lastly, laminar manifolds complete the manifold category.

Laminar manifolds are usually made of steel, with passages milled or machined through several plates of metal. These plates are stacked or sandwiched with the various fluid paths determined by the shape of the machined passages. Solidmetal end pieces are added, and the whole stack is brazed together.

Internal passages can be cut to any shape needed, so nearly any flow rate can be accommodated with minimal pressure drop.

Laminar manifolds are always custom-designed. Valves and other connections can be located where appropriate for a specific application. But because of the permanently shaped flow passages and brazed construction, this type of manifold cannot be modified easily if future circuit changes become necessary.

Because there are so many configurations available for manifold design, there are several software packages available to help the engineer design a system. With advances of these design software packages and CNC technology, the installed cost for custom solidblock manifolds, even small runs, is highly competitive to systems using modular blocks or discrete components.

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H Y D R AU L I C M OTO R S

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MOTORS

H

ydraulic motors are used in any application requiring rotational force, also known as torque. A hydraulic motor converts hydrostatic energy into mechanical energy by pushing vanes, gears or pistons attached to a crankshaft.

The power capacity of a hydraulic motor is dictated by its displacement, and the speed in which it rotates.

They are used in industrial applications such as augers, conveyors and mixers, as well as rolling mills, where their robust nature and resistance to heat make them a perfect solution. Hydraulic motors can be used in any application an electric motor can, but require no electricity, and can withstand more environmental extremes, such as submersion and extreme heat. Motors can be sized small enough for high-speed drills, or large enough for injection molding machines.

Hydraulic motors are especially well suited to mobile machinery, where they are the primary drive for most off-highway equipment. Hydrostatic drive systems act to transmit engine power to the drive wheels, with exceptional versatility and reliability. Hydraulic wheel motors are used on wheeled loaders, skid-steer loaders, articulating loaders and backhoe loaders, where the motor’s quick and smooth reversibility make them perfect for the application.

Motors are also used in tracked vehicles, such as excavators of all sizes, bulldozers, and specialty machines, such as bridge-layers or drill rigs. The high power density of hydraulic motors allows them to achieve earth-moving torque in a relatively small package. Hydraulic motors can also be used in hybrid hydraulic vehicle drive systems, where the motor can also absorb energy from the drive wheels to pump fluid into an accumulator, which can subsequently send flow back to that motor for a burst of acceleration from low speed.

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FIXED, AXIAL PISTION MOTOR, with the bent-axis design RADIAL PISTON MOTOR VANE MOTOR

EXTERNAL GEAR MOTOR

Keep in mind that the three different types of motors have different characteristics. Gear motors work best at medium pressures and flows, and are usually the lowest cost. Vane motors, on the other hand, offer medium pressure ratings and high flows, with a mid-range cost. At the most expensive, piston motors offer the highest flow, pressure and efficiency ratings.

Gear motors feature two gears, one being the driven gear— which is attached to the output shaft—and the idler gear.

Their function is simple: high-pressure oil is ported into one side of the gears, where it flows around the gears and housing, to the outlet port and compressed out of the motor. Meshing of the gears is a bi-product of high-pressure inlet flow acting on the gear teeth. What actually prevents fluid from leaking from the low pressure (outlet) side to the high pressure (inlet) side is the pressure differential. With gear motors, you must be concerned with leakage from the inlet to outlet, which reduces motor efficiency and creates heat as well.

In addition to their low cost, gear motors do not fail as quickly or as easily as other styles, because the gears wear down the housing and bushings before a catastrophic failure can occur.

At the medium-pressure and cost range, vane motors feature a housing with an eccentric bore. Vane rotors slide in and out, run by the eccentric bore. The movement of the pressurized fluid causes an unbalanced force, which in turn forces the rotor to turn in one direction.

These designs are available in a variety of different styles, including radial, axial and other less common designs. Radial-piston motors feature pistons arranged perpendicularly to the crankshaft’s axis. As the crankshaft rotates, the pistons are moved linearly by the fluid pressure.

Axial-piston designs feature a number of pistons arranged in a circular pattern inside a housing (cylinder block, rotor or barrel). This housing rotates about its axis by a shaft that is aligned with the pumping pistons. Two designs of axial

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piston motors exist—swashplate and bent axis types.

Swashplate designs feature the pistons and drive shaft in a parallel arrangement. In the bent axis version, the pistons are arranged at an angle to the main drive shaft.

Of the lesser used two designs, roller star motors offer lower friction, higher mechanical efficiency and higher start-up torque than gerotor designs. In addition, they provide smooth, low-speed operation and offer longer life with less wear on the rollers. Gerotors provide continuous fluid-tight sealing throughout their smooth operation. Pressure (outlet) side to the high pressure (inlet) side is the pressure differential. With gear motors, you must be concerned with leakage from the inlet to outlet, which reduces motor efficiency and creates heat as well.

In addition to their low cost, gear motors do not fail as quickly or as easily as other styles, because the gears wear down the housing and bushings before a catastrophic failure can occur.

DUAL-DISPLACEMENT MOTOR, with swashplate design

VARIABLE, AXIAL PISTON MOTOR, with the bent-axis design

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Hydraulic motors can be either fixed- or variabledisplacement and operate either bi-directionally or unidirectionally. Fixed-displacement motors drive a load at a constant speed while a constant input flow is provided.

Variable-displacement motors can offer varying flow rates by changing the displacement. Fixed-displacement motors provide constant torque; variable-displacement designs provide variable torque and speed.

Torque, or the turning and twisting effort of the force of the motor, is expressed in in.-lb or ft-lb (Nm). Three different types of torque exist. First is breakaway torque, which is normally used to define the minimum torque required to start a motor with no load. This torque is based on the internal friction in the motor and describes the initial â&#x20AC;&#x153;breakawayâ&#x20AC;? torque required to start the motor.

Next is running torque, which produces enough torque to keep the load running. Last is starting torque, which is the minimum torque required to start a motor under load and is a combination of energy required to overcome the force of the load and internal motor friction. The ratio of actual torque to theoretical torque gives you the mechanical efficiency of a hydraulic motor.

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Defining a hydraulic motor’s internal volume is done simply by looking at its displacement, thus the oil volume that is introduced into the motor during one output shaft revolution, in either in.3/rev or cc/rev, is the motor’s volume. This can be calculated by adding the volumes of the motor chambers or by rotating the motor’s shaft one turn and collecting the oil manually, then measuring it.

Flow rate is the oil volume that is introduced into the motor per unit of time for a constant output speed, in gallons per minute (gpm) or liter per minute (lpm). This can be calculated by multiplying the motor displacement with the running speed, or simply by gaging with a flowmeter. You can also manually measure by rotating the motor’s shaft one turn and collecting the fluid manually.

WHAT TO KNOW WHEN SPECIFYING MOTORS There are several important things to consider when selecting a hydraulic motor. You must know the maximum operating pressure, speed and torque the motor will need to accommodate. Knowing its displacement and flow requirements within a system is equally important.

Hydraulic motors can use different types of fluids, so you must know the system’s requirements—does it need a bio-based, environmentally-friendly fluid or fireresistant one, for example. In addition, contamination can be a problem, so knowing its resistance levels is important.

Cost is clearly a huge factor in any component selection, but initial cost and expected life are just one part of this. You must also know the motor’s efficiency rating, as this will factor in whether it runs cost-effectively or not. In addition, a component that is easy to repair and maintain or is easily changed out with other brands will reduce overall system costs in the end.

Finally, consider the motor’s size and weight, as this will impact the size and weight of the system or machine with which it is being used.

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ydraulic pumps are used in literally every single hydraulic power transmission system. A hydraulic pump is the device that converts mechanical energy into hydraulic energy, which is a combination of pressure and flow.

A hydraulic pump can be any device that you can input force into to create pressure, which in turn creates flow.

Most hydraulic pumps have a mechanical input from an internal combustion engine or electric motor. These prime movers input their mechanical power to the hydraulic pump in a rotational fashion. The input shaft of the pump will be connected to gears, vanes or pistons of the hydraulic pump, where they will rotate or reciprocate to transfer pressure (force) to the hydraulic fluid.

As long as the force (pressure) created by the pump is high enough, flow will occur at a rate dictated by the displacement volume of the pump and the speed at which it rotates.

These pumps, also called positive displacement pumps, have a small clearance between rotating and stationary parts.

A specific amount of fluid is delivered to the system for each revolution. Positive-displacement pumps can be further divided into two categories: fixed- and variable-displacement.

Fixed-displacement pumps provide a single, specific volume displacement per revolution. In variable-displacement pumps, displacement per cycle can vary from zero to maximum volumetric capacity. Some of the more widely used types of positive-displacement pumps are gear, piston and vane.

Gear pumps can be either internal or external styles.

External gear pumps are one of the most popular types used in modern hydraulic systems. Gear pumps produce flow by using the teeth of two meshing gears to move the fluid. Their simple construction ensures limited purchase costs and servicing. They feature decent mechanical and volumetric efficiency, compact dimensions and low weight/power ratio. Of the three common

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types of positive displacement pumps, gear pumps are the least efficient; their appeal is low cost and simple design.

External gear pumps can be equipped with straight spur (the most common type), helical or herringbone gears. In operation, the drive gear and driven gear rotate, creating a partial vacuum at the pump inlet (where gear teeth unmesh) that draws fluid into gear teeth. Gear teeth mesh at the outlet, forcing fluid out of the pump.

Internal gear pumps contain one internal and one external gear. They pump fluid in the same manner as external spur gear pumps. In the basic design, the internal gear, which drives the outer gear, has one tooth less than the outer gear. As they mesh, the teeth create sliding seal points.

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pressure (the area over the crescent) is relatively long, internal gear pumps can offer lower noise levels than some other types of pumps.

Gears are made of special steel and are often case hardened and quench hardened. Then gears are ground and fine finished. Proper tooth profile design and geometric proportions can reduce pulsation and noise levels during pump operation.

Piston pumps supply high flows at high speed. Two types of piston pumps—axial and radial—are manufactured in both fixed- and variable-displacement versions. Axial-piston pumps contain one or more pistons that convert rotary shaft motion into axial reciprocating motion.

An angled cam (or wobble plate) rotates, causing pistons to reciprocate and take fluid in as they move toward the thin part of the plate. Fluid is expelled as pistons approach the thick end. In the bent-axis design, both pistons and shaft rotate, making a wobble plate unnecessary. Bent-axis pumps use the drive shaft to rotate pistons.

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Precision CNC Turning Precision CNC Milling Plastic Injection Molding

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With the longer sealing paths along the piston walls, piston pump efficiencies tend to be higher than other types of pumps. In addition, variable-displacement pumps can provide savings by only providing the pumping necessary for the function, saving additional energy and costs.

Radial-piston pumps (fixed-displacement) are used especially for high pressure and relatively small flows. Pressures of up to 10,000 psi are common. Variable-displacement is not possible, but sometimes the pump is designed in such a way that the plungers can be switched off one by one, so that a sort of variable-displacement pump is obtained.

Radial-piston pumps are characterized by a radial piston arrangement within a cylinder block. As pistons reciprocate, they convert rotary shaft motion into radial motion. One version has cylindrical pistons, while another uses ball-shaped pistons. Another classification refers to porting: Checkvalve radial-piston pumps use a rotating cam to reciprocate pistons; pintlevalve pumps have a rotating cylinder block, and piston heads contact an eccentric stationary reaction ring.

Rotary vane pumps (fixed and simple adjustable displacement) generally have higher efficiencies and lower noise levels than gear pumps. They can be used for mid pressures of 2,500 psi.

Some types of vane pumps can change the center of the vane body, so that a simple adjustable pump is obtained. These adjustable vane pumps are constant pressure or constant power pumps. Displacement is increased until the required pressure or power is reached and subsequently the displacement or swept volume is decreased until equilibrium is reached.

A critical element in vane pump design is how the vanes are pushed into contact with the pump housing, and how the vane tips are machined at this very point. Several types of “lip” designs are used, and the main objective is to provide a tight seal between the inside of the housing and the vane, and at the same time to minimize wear and metal-to-metal contact. Forcing the vane out of the rotating center and toward the pump housing is accomplished using spring-loaded vanes, or more traditionally, vanes loaded hydrodynamically (by the pressurized system fluid).

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REPAIR, REBUILD

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hen fluid-power components like pumps, motors, valves and cylinders fail, the overriding goal is to get a system up and running again as soon as possible. One option is simply to purchase and install a new replacement part, but a repaired or remanufactured component is often the more economical option.

Depending on the unit, almost every constituent part can be replaced or repaired, provided a suitable replacement is available. As a general rule, a repair makes sense if the cost doesn’t exceed 60 to 70% of the cost of a new component.

Beyond that, the user is typically better off with a new unit. However, if a new product is not readily available and a critical or expensive machine or production line is down, repair is still practical at a higher cost.

Technicians first make a full inspection and diagnose the problem. Sometimes components wear out. But premature failures typically result from contamination, cavitation, overpressurization, and excessive heat. Thus, installing a robust filtration system, keeping components cool, and following a disciplined maintenance program are critical to extending component and machine life. Finally, some components break due to incorrect application, installation or commissioning— failures that are often preventable.

Technically savvy repair shops tear a unit down and try to bring it back to “as new” condition. All critical dimensions and surfaces are inspected and measured. Seals and low-cost consumable parts like springs, washers and shims tend to be replaced. Likewise, bearings will be inspected and possibly replaced.

In more-serious cases, say internal wear due to contamination damage in a piston pump, lapping the surfaces might be suitable if still within acceptable tolerances.

Otherwise, the technician may need to remanufacture or replace rotary barrels, pistons, and other internal parts. That can ultimately extend to replacing other major components like housings, covers, relief valves, controllers and charge pumps. In the worst case, a completely new unit can be built from parts, although that is not the norm.

Another issue is whether to repair/remanufacture a unit with parts sourced from the original hydraulic OEM or with aftermarket replacement parts. The hydraulic repair industry broadly falls into three business sectors. Hydraulic component manufacturers often tend to serve large machine builders directly, with their own parts. But they often leave user service and support to distributors and large, sophisticated repair houses. Some use OEM parts exclusively, some do not. Further removed are smaller shops that can find OEM replacement parts costly and not readily available.

After completing the repair, testing and calibration are also critical to ensure repairs are done right the first time. This is necessary simply because hydraulics has gotten more complex.

Newer components routinely have integrated electronics, digital controls and sophisticated software. Unfortunately, the complexity of today’s hydraulics means pure mechanical aptitude is no longer sufficient to fix many components.

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Components with higher operating pressures and electronics controls also tax the capabilities of test equipment at many repair facilities. Small shops will make repairs and perhaps run basic tests, but they are not capable of performing full-function tests. Larger repair shops have made significant investments in state-of-the-art test stands for qualifying dynamic open and closed-loop systems.

Not surprisingly, the price of a repaired or rebuilt component can vary widely depending on the expertise behind it. Experience and specialized skill is required to correctly diagnose, rebuild and test hydraulically powered equipment. Costs include the expertise for diagnosing the failure, recommending the proper repair, whether to rework or replace a part, and whether to use genuine or aftermarket parts. This ultimately speaks to the overall competency of the repair company and the capabilities of the staff and testing equipment.

The quality of the repair shouldn’t be driven solely by price, either. Companies that shop by price alone are usually disappointed by the outcome, as the repair may not hold up. Consider the time and labor needed to remove a failed pump or cylinder from a machine and install a rebuilt one, plus the cost of the repair itself, as well as the cost of machine downtime and lost productivity, and it quickly adds up. Cutting corners on parts or testing and finding that the repair quickly fails, and forcing the user to start over at square one, gets expensive. That’s why it’s essential to get repairs right the first time.

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Look for a state-of-the-art facility that is esthetically pleasing with clean workshops housing the latest technology, and personnel with the skills to diagnose problems and ensure repaired units meet OEM specifications.

Look for facilities that follow ISO 9001 guidelines. This can help to ensure repeatability with quality.

Check qualifications. Technicians should have training in at least a basics hydraulics course. But today’s complex hydraulics components require technicians that can handle all aspects of inspection, testing and calibration for pumps, servovalves, and closed-loop electronic circuitry.

Testing is also critical to ensure repairs are done right the first time and to as-new specifications. Get to know the test stands at your local repair shop to see if they can run full-function tests on the aforementioned complex electrohydraulic components. Tests could include dynamic open and closed-loop systems and, when needed, charting parameters like frequency response, hysteresis, flow efficiency and mechanical leakage.

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HYDRAULIC

SEALS

S

ealing is a vital factor in the proper function of hydraulic applications, as leakage from the cylinder or across the piston, along with the ingress of unwanted contaminants, can decrease the lifespan and efficiency of the entire application.

MATERIAL OPTIONS Seal material choices are determined by the seal’s environment. Different types of chemicals react differently to different fluids, while some materials have higher pressure and temperature limits. They also must be able to withstand extrusion, so materials are very applicationspecific.

Polyurethane Polyurethane is an organic material whose chemical composition is characterized by a large number of urethane groups. Urethanes belong to the thermoplastic elastomers (TPE) family and close the gap between thermoplastic and elastomeric materials regarding hardness, deforming behavior and consistency. Within certain temperature limits, polyurethane possesses the elastic characteristics of rubber combined with the advantages of a rigid plastic.

The composition of the material is determined by three components: polyol, diisocyanate and a chain extender. The type and amount of these materials used, and the reaction conditions, are decisive in determining the properties of the resulting polyurethane material. In general, polyurethanes possess the following properties: • • • • • •

high mechanical, tensile strength good abrasion resistance modulus of elasticity is variable wide range of hardness values, while retaining good elasticity good resistance to ozone and oxygen outstanding resistance to abrasion and tear

Temperature range for use: -30 to 80° C; high performance types (compounds) up to 110° C in mineral oils (long-term exposure temperature).

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Acrylonitrile-Butadiene-Rubber (NBR) NBR is a polymer of butadiene and acrylonitrile. The acrylonitrile (ACN) component affects the following properties of the NBR: • • • • •

elasticity cold flexibility gas permeability compression set swelling resistance in mineral oils, greases and fuels

An NBR material with low ACN content has very good cold flexibility (down to approximately –45° C) and moderate resistance to oil and fuel. In contrast, a material with very high ACN content with optimum resistance to oil and fuels, may have a cold temperature flexibility only down to –3° C. With rising ACN content, the elasticity and the gas permeability decrease and the compression set becomes worse.

NBR provides: • good resistance to swelling in aliphatic hydrocarbons; greases; fire retardant hydraulic fluids of Groups HFA, HFB and HFC • good resistance to hot water at temperatures up to 100° C (sanitary fittings), inorganic acids and bases at concentrations, and temperatures which are not too high • high swell in aromatic hydrocarbons, chlorinated hydrocarbons, flame retardant hydraulic fluids of the Group HFD, esters www.fluidpowerworld.com

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TRELLEBORG SEALING SOLUTIONS ISO* GROOVE SEAL UNIFORMITY Utilizes consistent ISO specified groove widths and depths compared to competitive products.

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Temperature range for use (depending on the composition of the blend): –40 to 100° C and for short periods up to 130° C (the material hardens at higher temperatures). For special blends, the cold flexibility extends down to –55° C. Fluoro-Rubber (FKM) Copolymers, terpolymers or tetrapolymers with various compositions and with fluorine contents from 65 to 71%, which have varying resistance to surrounding media and varying cold flexibility.

FKM provides: • tough resistance to high heat • excellent resistance to oil, hydraulic fluid and hydrocarbon solvents • good flame retardance • low permeability to gases • high swell in polar solvents, ketones and fire-retardant hydraulic fluids (i.e. Skydrol type) Newly developed materials (cross-lined by peroxides) have good resistance to media, which can only be tolerated to a small extent, if at all, by conventional FKM. Temperature range for use: about –20 to 200° C (for short periods to 230° C). Special grades: –50° to 200° C. Polytetrafluoroethylene (PTFE) PTFE is a polymer of tetrafluoroethylene. This non-elastic material is characterized by: • • •

• • •

slippery surface that repels most media non-toxic at working temperatures up to 200° C low coefficient of friction against most opposing surfaces made of other materials; stiction and friction are almost the same excellent electrical insulating properties (almost independent of frequency, temperature and weathering effects) chemical resistance that exceeds that of all other thermoplastics and elastomers liquid alkali metals and a few fluorine compounds attack PTFE at higher temperatures

The temperature tolerance is between –200° and 260° C; PTFE has some elasticity even at extremely low temperatures; therefore it is used in many extreme cold temperature applications. Most hydraulic applications require the use of a spring or elastomeric component to energize a lip seal configuration because of the low elasticity and tendency to cold flow over time.

SIX COMMON SEAL DESIGNS Following is a list of some of the most common seal designs used in fluid power applications.

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Piston Seals • provide sealing of the piston and barrel, critical to the function of the cylinder • most often a lip-seal design, but can also be O-rings, T-seals, and so on • must provide efficient sealing, but also reasonably low friction • made from various seal materials, depending on application • require system pressure to effectively activate the lip seal

Wipers • provide aggressive wiping force • prevent mud, water, dirt and other contamination rom entering the system • allow lubricating oil film to return to system on inward stroke • protect main sealing elements, thus increasing life of seals • often made from polyurethane, which offers high abrasion resistance • often used as a linkage pin grease seal

• • • • •

Rod Seals prevent system fluid from escaping to atmosphere must provide sealing function at low and high pressure require excellent extrusion and wear resistance should provide good pump-back capability for lubricating oil film often must withstand up to 6,000 psi www.fluidpowerworld.com

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O-RINGS create a leak-tight seal through their initial compression, as seen in this double O-ring design from Trelleborg Sealing Solutions.

O-rings are probably the most common fluid power seals.

These simple seals consist of a donut-shaped ring (technically, a toroid) with a circular cross section. They’re typically made of elastomers like Buna N, Neoprene or silicone, but they also come in plastics like PTFE, metals and other materials.

Sizes range from fractions of an inch in diameter to several meters across.

O-rings seal by mechanical deformation that creates a barrier to a fluid’s potential leak path between two closely mated surfaces. They are typically installed in a groove that’s machined or molded in one of the surfaces to be sealed. Their rubber-like properties let the devices compensate for dimensional variations in the mating parts.

When properly sized, the clearance between the surfaces is less the OD of the O-ring. Thus, as the two surfaces contact, forming a gland, they compress the O-ring, which deforms the round cross section. This diametrically squeezes the seal, and the resulting force ensures surface contact with the inner and outer walls of the gland.

With little or no pressure, the natural resiliency of the elastomer compound provides the seal and keeps fluid from passing by. Increasing the squeeze increases deformation and sealing force, but that can lead to problems in higherpressure dynamic applications.

Applying fluid pressure pushes the O-ring against the groove wall on the lowpressure side, increasing sealing force. Interference between the seal and mating surfaces lets the O-ring continue to operate leak-free. At higher pressures, it deforms to a somewhat “D” shape, and contact area between elastomer and gland

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surfaces may double from initial zero-pressure conditions.

Due to the elastomer’s resiliency, releasing pressure lets the O-ring return its original shape, ready for the next pressure cycle. This also lets them seal in both directions.

Extreme pressures, however, can force elastomer material into the small clearance between the mating surfaces just beyond the groove. Ultimately, the material shears and flows into the so-called extrusion gap, and the seal fails.

Dynamic applications can hasten seal extrusion. But even in static applications, high pressure can stretch assembly bolts and open the extrusion gap sufficiently to permit leakage.

There are a number of design considerations for specifying O-rings. They come in many materials and compounds and variations. Matching the material to the application, however, lets them provide excellent fluid compatibility, withstand various operating environments and handle temperature extremes. Other considerations include static versus dynamic (rotary or axial) conditions, operating pressure, and whether the system sees pressure spikes. These, in turn, let engineers specify design parameters like proper gland dimensions, gland surface finishes, seal cross-section diameter, material hardness, initial compression, clearance gaps, and even how much the seal expands or contracts in relation to its mating surfaces as O-RINGS have a circular cross temperatures change.

section. Image courtesy of Trelleborg Sealing Solutions.

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HANDBOOK

Buffer Seals • must withstand high pressure exposure • protect the rod seal against pressure spikes • feature a pressure-relieving capability that prevents pressure build-up between seals • increase rod seal life • allow for wider extrusion gaps • require high wear resistance

Fluid Power

Wear Bands • prevent contact between metal parts in the cylinder • center rod and piston from housing elements • increase seal life

IMAGE COURTESY OF FREUDENBERG SEALING TECHNOLOGIES

O-rings • most commonly used in static applications and radial or axial deformation to maintain sealing contact force • double-acting, so seal on both sides of a component • can be used as energizing elements or as primary seals • self-acting, so do not require additional system pressure or speed to create the seal

ADVANCED PNEUMATIC VALVES FOR NEXT GENERATION MACHINES Lightweight, energy-efficient, compact and versatile—the AVENTICS AV (Advanced Valve) series feature a unique diagonal design engineered for maximum performance. The savings speak for themselves: up to 40% lighter, 45% less space, and up to 20% less compressed air consumption due to efficient design. Use the AES (Advanced Electronic System) to add bus modules, I/O, or an electro-pneumatic pressure regulator.

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S E N S I N G T E C H N O L OG I E S

PRESSURE

TRANSDUCERS

P

ressure is defined as the force per given area required to stop a fluid expanding.

Pressure transducers, which are a subset of pressure sensors, can be any number of devices that sample and record the pressure in a system. A pressure transducer converts a pressure measurement into an analog electrical output signal, which can be used by sensing instrumentation such as microprocessors and computers. Most often, this is accomplished simply through physical deformation or mechanical deflection. Important criteria to consider when selecting a pressure transducer are the general mechanism type, input and output, and performance specifications.

The most common types of pressure transducers are strain gauge, and thick/thin film.

Strain gauge transducers use the mechanical deformation under pressure of strain-sensitive variable resistors, which may be integrated into measurement circuits such as a wheatstone bridge. In a thick/thin film transducer, a titanium nitride or polysilicon film may be applied to sensing equipment to impart the circuit with piezoelectric sensitivity to pressure.

Almost all pressure transducers require a source of electrical input. The transducer input voltage can vary but typically falls under 10 V, while the output is typically in the hundreds of thousandths of volts. A change in the system’s pressure would cause a change in the transducer’s resistance on the electrical circuit and would result in a change to the output voltage. With the aid of an analog to digital converter (ADC), the transducer’s output signal can be used in systems that require digital signals. For example, a programmable logic controller (PLC) or a programmable automation controller (PAC) can use the digital signal to monitor the pressure and take action if needed.

Some pressure transducers output current rather than voltage, and are then often referred to as transmitters. These values typically fall within tens of thousandths of amps. When choosing the output of a pressure transducer, it is important to keep in mind the input requirements of the device that will be accepting the signal, the distance the signal must travel and possible interference that can be found in the environment around the system.

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POSITION SENSING A variety of technologies exist to provide position feedback or data.

Usually, they monitor a cylinder’s position primarily in mobile machinery, subsea systems or factory automation.

LVITs—Linear Variable Inductive Transducers—are contactless position sensing devices that use eddy currents developed by an inductor in the surface of a conductive movable element to vary the resonant frequency of an L-C tank circuit. The most common form of an LVIT uses a small diameter inductive probe surrounded by a conductive tube called a spoiler that is mechanically coupled to the moving object. Typical LVITs have full ranges from fractions of an inch to 30 in. or more. Modern electronics using microprocessors and small component size makes outstanding performance possible, achieving linearity errors of less than ±0.1% and temperature coefficients of 50 ppm/°F, along with either analog or digital outputs.

In mobile hydraulics, the sensor has a pressure-sealed head and a probe long enough to insert into a gun-drilled hole in the cylinder’s ram.

The ID of this hole in the ram then acts as the spoiler. The sensor head can either be port mounted or embedded into the end cap of the cylinder.

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For subsea cylinder applications involving pumps, chokes, blowout preventers, and ROV-based actuators, the LVIT is designed to withstand the internal and/or external pressures of a PBOF (pressure balanced, oil filled) system. Other technologies commonly used to satisfy these applications require additional hardware like a ring magnet to operate, which adds cost to the machining of the cylinder ram and complexity to the installation.

They are used in factory automation applications because of their contactless design. This reduces the longterm wear of the basic measurement mechanism of an LVIT. Another benefit of LVITs is that they do not have the higher installed cost associated with other contactless technologies.

Hall-effect transducers use a magnet that communicates with internal Hall chips, which then give an output that the microprocessor understands. This is then converted to a signal the user can understand, such as voltage or current output.

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Electrical Engineering

Using Hall-effect technology on linear transducer designs allows manufacturers to make small, compact designs that can be mounted internally and externally onto a cylinder. These devices use a magnet to measure cylinder position. Hall-effect technology also resists shock and vibration. It is used commonly on steering applications.

LVDTs or linear variable-differential transformers are extremely durable and resist shock and vibration while offering high repeatability. These absolute linear position/displacement transducers convert a linear displacement into an analog electrical signal. Their design includes transformer coils wound around non-magnetic coils.

Magnetostrictive sensors measure the distance between a position magnet and the head-end of a sensing rod. The magnet does not touch the sensing rod, so no parts can wear out. The sensing rod mounts along the motion axis to be measured and the position magnet attaches to the member that moves. The head includes an electronics module, which reports the position information to a controller or other receiving device in an analog or digital format. Also within the electronics housing is the electrical connection interface, either an integral connector or cable and visual diagnostic LEDs to ensure proper wiring, power, and magnet positioning.

Encoders are used heavily on mobile machinery. Available with capacitive, optical or magnetic sensing and with incremental or absolute position sensing, these devices accurately determine the stroke of the cylinder. Rotary and linear devices are available, but rotary styles are most common.

These devices measure position and resolution in pulses per revolution.

Some rotary encoder designs use a wire-actuated sensor mounted directly inside the cylinder. Others are mounted externally. Others are based on Hall-effect, magnetostrictive, or inductive technologies.

Motor Feedback Systems for linear and rotative drives

Important performance criteria to consider are the pressure transducer’s operating pressure range, maximum rated pressure, accuracy and operating temperature range. The operating pressure range demarcates the intended pressure bounds at which the transducer has been designed to perform optimally.

The maximum rated pressure is the highest allowable pressure that the pressure transducer is rated to withstand. The accuracy of the transducer is usually represented by suppliers in terms of ASME B40.1 grades: 4A (0.1%), 3A (0.25%), 2A (0.5%), A (1%), B (2%), C (3%) and D (4%) deviance from the true pressure value.

A good pressure transducer is designed to operate independently of temperature; however, the operating temperature specifies a “safe” range; operating outside of this temperature may significantly affect the accuracy of pressure sensing.

For typical industrial applications, select a 0.5% accuracy class. This should be sufficient for most closed-loop systems. Higher accuracy will quickly increase the price. Before making that investment, determine if the rest of your system requires this higher accuracy.

Absolute and incremental magnetic measurement ■ ■ ■ ■ ■

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Accuracy is a constant value found on the data sheet. Unfortunately, most hydraulic systems start cold and get hot, so your actual pressure accuracy will depend on temperature change. The overall accuracy is accuracy class plus error due to temperature change.

The most common output for industrial transducers is 0 to 10 Vdc. Gaining popularity is 0.1 to 10 Vdc, because the control system can detect a transducer fault. If the pressure signal falls below 0.1 Vdc, either the cable has been disconnected or the transducer has failed.

For longer cable runs, a 4 to 20 mA output is preferable. Pressure transmitters reject electrical noise, so the analog signal is clean. The 4 mA offset helps the control system detect sensor faults. However, 4 to 20 mA transmitters have 20% lower resolution, because the 0 to 4 mA is not usable.

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HANDBOOK

M O B I L E H Y D R AU L I C T I P S . CO M

HYDRAULIC

VALVES

H

ydraulic valves, used in conjunction with actuators, are what help make hydraulics unique in its control of force, torque and motion. Valves govern direction, pressure and flow of hydraulic fluid, enabling smooth, safe control of actuators.

Valve use can be as simple as a relief valve to protect your pump and actuator. Conversely, the complexity of a hydraulic circuit can be extensive, using a dozen valves per function as can be seen in manifolds.

DIRECTIONAL CONTROL VALVES

The directional control valve is available in myriad configurations, and is named as such if its primary function is to somehow control the path of fluid flow. Directional control valves manage fluid by blocking, diverting, directing or dumping. Their complexity varies immensely (just like their cost), as does the method of integration. Valve construction runs the gamut from cartridge valves to monoblock valves, or subplate mounted valves to inline valves. Their usage depends on the industry in which they are typically applied.

The most basic directional valve is the check valve; it allows flow into one work port and blocks flow from coming back through the opposite work port. Directional valves can be complex, such as with the pilot-operated valve. A standard spool valve has one directly operated component that controls fluid through the valve. However, as flow increases, the force upon the spool also increases, and these forces can prevent a spool from actuating, most often with electric coils. By using a small pilot valve to control the movement of the larger, main-stage spool, the size (and flow) of the valve are nearly limitless.

Directional valves are often described by the number of “ways” fluid can travel through them, and also by the positions available to be shifted into. The ways are equal to the number of work ports, so a 4-way valve will have Pressure, Tank and A and B work ports.

Positions are equal to the number of positional envelopes.

For example, one would describe a double-acting single monoblock valve as “4-way, 3-position,” or simply a “4/3 valve.”

Directional valves are available in monoblock or sectional valves, common to the mobile-hydraulic industry, as well as subplate mounted industrial type valves such as ISO style D03, D05 and so on. Also common to both mobile and industrial markets are cartridge valves installed into manifold blocks. Cartridge valve manufacturers offer many unique products, and allow high levels of creativity with limitless available valve combinations.

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A pressure valve is any component designed to limit pressure. Most pressure valves are based on a poppet being pushed against a seat with an adjustable spring, although pressure valves can be a simple ball and spring configuration or use spools for high flow circuits. Their operation is simple: a spring pushes the poppet against a seat, and when pressure from the system is strong enough to counteract the force of the spring, the valve will open, bleeding off fluid to limit pressure.

A relief valve controls maximum pressure for either the entire system or a sub-circuit of it, the lowest spring pressure of a system being the one to open up first. Most other pressure valves are based on the relief valve’s simple spring-loaded ball or poppet. Sequence, counterbalance and brake valves are all forms of relief valves with added utility or functionality, such as reverse flow checks or pilot operation built in. The pressurereducing valve differs from the other pressure valves because it limits pressure downstream of itself rather than upstream. It is used in applications where sub-circuit pressures need to be lower, without sacrificing any performance in the rest of the system.

FLOW CONTROL VALVES

Flow control valves control or limit flow in one way or another. They are often just a needle valve, which is just a variable restriction, adjusted by a screw or knob much like pressure valves, to www.fluidpowerworld.com

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H Y D R AU L I C VA L V E S

limit the energy potential to create flow. When installed with reverse flow check valves, we change the name to flow control. Flow control valves can sometimes have multiple ports, such as with a priority flow control. They are able to provide controlled, fixed flow to one part of the circuit (sometimes at the sacrifice of another part) and as long as input flow is high enough for its priority demand.

Flow controls are ideally pressure compensated, which allows the valve to maintain its set flow regardless of load-induced pressure variances.

Pressure compensators are a type of flow control valve available as a single item, often added to other valves in a circuit to provide the same flow setting load-independent accuracy, such as with a proportional valve, for example.

Proportional valves are considered both flow and directional valves, and are able to both meter flow and control the direction flow is metered in. Proportional valves use pulse-width modulation to maintain voltage and control current. Varying the current controls the force of the magnetic field and how far the spool or poppet moves within its body, changing the size of the opening available for fluid to take, which of course limits flow. A simple variable resistor can be used to limit current, but it is inefficient and cannot provide the benefits a PWM controller can.

Firstly, a valve controller can provide adjustable minimum and maximum settings. A minimum current value is needed to move the spool past its overlap where it “starts” to flow. Also, a maximum current value prevents too much electric juice from fatiguing the valve and coil when only a couple amps are required to achieve full flow anyway. Additionally, a proper controller and driver provide a dither signal to the valve, which vibrates the spool so that static friction doesn’t hold it up. The spool movement is unnoticeable, but is enough so that when a change in current is required, the spool responds rapidly without overshooting the desired new position.

IN ADDITION, WHEN CHOOSING A DIRECTIONAL VALVE, CONSIDER THE FOLLOWING QUESTIONS FOR YOUR APPLICATION: • • • •

•

•

•

Is it a parallel or tandem circuit?

Do you need an open- or closedcenter for oil flow?

What is the number of gallons per minute (gpm) from the source?

How many directions do you want the oil to flow (this is referred to as spool type and is usually expressed as 3-way, 4-way)?

How many handle (spool) positions do you want (usually 3-position or 4-position)?

What is the spool action, such as spring center to neutral or various detent selections?

And finally, do you need the valve to be manually or electronicaly controlled or both?

HOW DO YOU SELECT DIRECTIONAL CONTROL VALVES?

A hydraulic system will either have a fixed-displacement pump (gear pump)—which requires an open center valve—or a variable-displacement pressure-compensated pump that limits pressure. In the latter case, you need a closed-center valve. Typically, IMAGE COURTESY OF an open center can be converted to PRINCE MANUFACTURING closed center by adding a closedcenter plug in the outlet section. This will block off the open-center core when the spools are in neutral.

The flow of your source is necessary in determining how small or large of a valve is needed for your application. Typical sizes range from 2 to 40 gpm. If you size your valve too large, you could have trouble shifting the valve and are adding unnecessary cost. If you size your valve too small, you risk overpressurizing your system and building up heat. Excessive heat is detrimental to hydraulic systems.

The number of ways you want the oil to flow is sometimes referred to as the spool type, as mentioned earlier. To expand on that, you may have either a 3-way or 4-way spool. An example of a 3-way would be: pump, return and work port. An example of a 4-way spool would be: pump, return, work port and work port. The spool type is the number of different www.fluidpowerworld.com

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positions for the spool. For example, a 3-position would be forward, neutral and reverse. An example of a 4-position would be forward, neutral, reverse and float. You also have to consider the spool action.

Two common actions are spring center and detent.

Spring center allows the handle to pop back to center when released and detent leaves the handle in place once it is moved.

You can control the shifting of the valve either using a handle (manually) or solenoid (electronically).

If the application is cost-sensitive, manual is the obvious choice. The electronic option is controlled by a solenoid and allows remote electrical on-off control. As systems become more demanding, there has been a push for more electronics to go into directional control valves.

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PNEUMATICS OVERVIEW

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neumatics is the technology of compressed air, although many manufacturers today refer to it as a type of automation control.

Pressurized gas—generally air that may be either dry or lubricated—is used to actuate an end effector and accomplish work. End effectors can range from the traditional cylinder design to more applicationspecific devices such as grippers or air springs. Vacuum systems, also a part of the pneumatic realm, use vacuum generators and cups to handle delicate operations, such as lifting and moving large sheets of glass or delicate objects such as eggs.

Engineers commonly use pneumatics in industries such as medical, packaging, material handling, entertainment and even robotics. What’s more, pneumatics can be useful in very specific applications where hazards are critical—for example, in a mine or on an offshore oil platform—where a single stray spark could mean total disaster and lost lives.

By its nature, air is easily compressible, and so pneumatic systems tend to absorb excessive shock, a feature that can be useful in some applications. Most pneumatic systems operate at a pressure of about 100 psi, a small fraction of the 3,000 psi that many hydraulic systems experience. As such, pneumatics is generally used when much smaller loads are involved.

A pneumatic system generally uses an air compressor to reduce the volume of the air, thereby increasing the pressure of the gas. The pressurized gas travels through pneumatic hoses and is controlled by valves on the way to the actuator. The air supply itself must be filtered and monitored constantly to keep the system operating efficiently and the various components working properly. This also helps to ensure long system life.

In recent years, the control available within pneumatic systems (thanks to advanced electronics and componentry) has increased greatly. Where once pneumatic systems could not compete with many comparable electronic automation systems, the technology today is seeing a renaissance of sorts.

More and more, pneumatics is being used in interesting ways that would have been unthinkable a decade or two ago. Creative applications from soft robotics to pneumatic muscles are consistently making the news, showing not only the creativity of the engineering community, but also the inherent flexibility and adaptability of this important technology.

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HANDBOOK

AIR SPRINGS A

Fluid Power

ir springs ave been used in heavy-duty vehicle suspension systems for nearly a century, where they have been able to provide usefulness by taking advantage of the compressed air required for vehicle braking systems.

Air springs have provided a two-fold advantage over mechanical leaf or coil springs. One advantage with air suspension is the extra comfort provided by being able to vary the air pressure inside the spring, which changes the spring rate, and therefore, ride quality. Additionally, because variable control over air pressure adjusts the deck or trailer height, aligning loading docks to the level of the deck is possible when dock plates are unavailable.

The usefulness of air springs or actuators didn’t go unnoticed in the industrial machine industry, and it was

clear they could offer unique solutions for various applications. Air actuators have seen duty as shock absorbers, linear actuators, vibration isolators and tensioners, to name a few examples. They can be used to absorb shock in material handling applications, such as a saw mill, when logs are dropped onto processing stations. Air springs make some of the best vibration isolators on the market, such as would be used on a vibrating hopper or commercial laundry machine. In summation, air springs are a high-force, low-cost actuator that can operate in a linear fashion or at an angle. They can be stacked to provide longer strokes or greater angular rotation.

As air is directed into air springs, the bladders allow them to expand in a linear fashion. This permits them to be used as force developing actuators—like pneumatic cylinders—and as such, rod attachments are available to mimic the function of them. Most often, however, an air actuator is simply two end plates connected by a bladder, and as they are pressurized, force pushes the plates away from each other. As linear actuators, they can provide

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â&#x20AC;&#x192; AIR SPRINGS

up to 35 tons of force, making them useful in various press applications, such as a forming press or small stamping press. Air actuators are also excellent for constant force applications, such as pulley tensioners or drum roller compression devices. All air springs are single-acting, unless they are coupled together so one extends while the other retracts.

The two major types of air springs are the rolling lobe (sometimes called reversible sleeve) and the convoluted bellow. The rolling lobe air spring uses a single rubber bladder, which folds inward and rolls outward, depending on how far and in which direction it is moved. The rolling lobe air spring is available with high usable stroke lengthâ&#x20AC;&#x201D;but it is limited in strength because of its tendency to bulge, and therefore has limited force capacity. The convoluted bellow type air spring uses one to three shorter bellows, with the multiple units being reinforced by a girdle hoop. Convoluted air springs are capable of ten times the force of a rolling lobe version and twice the life cycle rating, but have less usable stroke to work with.

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P N E U M AT I C T I P S . CO M

COMPACT HANDBOOK

CYLINDERS

C

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Fluid Power

ompact cylinders are ideal in applications where space and weight is limited but high force is still required. Common applications include medical devices, robotics, packaging, and semiconductor, among others. These low-profile components have been shortened relative to standard pneumatic cylinders. They may be up to 50-60% shorter than the normal cylinder, but still maintain the capacity to exert the same force as their larger counterparts. Important parameters for the proper selection of a compact cylinder can be broken up into general, dimensional, performance, material and features.

Originally called the “Pancake cylinder,” these mini cylinders were first invented in 1958 by Al Schmidt, to fill a need for force in a tight, enclosed space. The basic intent was to get the most stroke in a short overall length using common machined parts and seals. Through the years, this design has been further developed, with many features and options to satisfy a variety of customer applications. This round body cylinder has a smooth, clean outside diameter for ease of machinery cleaning. Even though initially used for strokes less than 1 in., manufacturing methods have allowed increased strokes to as much as 4 in. Non-metallic rod bushings and piston bearings can accommodate extreme or unforeseen loads for long-term durability.

Other compact or mini cylinders vary quite a bit. They can be rectangular or square shaped, offer numerous mounting features and can be placed with adjacent cylinders at a close centerto-center dimension. Piston bearings, materials, hard anodized bore and chrome plated rods can enhance cylinder capability for unexpected side loads and long-term durability. Up to 6-in. strokes can be accomplished with extruded body material. Other features may include metric dimensions, extruded sensor mounting and non-rotating styles. They are available in single-acting and double-acting versions.

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When sizing a compact cylinder, it is important to know the following application data points to choose the best component for your application: • • • •

Operating psi, force required (Force = Pressure x Piston Area) Stroke Preferred mounting, foot print Spring return or double acting

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In addition, you should know information about the application where the cylinder will operate to ensure it meets its lifecycle requirements: • • • • •

Ambient temperature Media temperature Environment Excessive loads other than required axial force Load guiding (non-rotating) requirement

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PNEUMATIC ACTUATORS:

CYLINDERS P

neumatic actuators are simple and cost-effective mechanical devices that use compressed air acting on a piston inside a cylinder to move a load along a linear or rotational path. That motion can be in any form, such as blocking, clamping or ejecting. Unlike their hydraulic alternatives, the operating fluid in a pneumatic actuator is simply air, so leakage doesn’t drip and contaminate surrounding areas.

There are many styles of pneumatic actuators including diaphragm cylinders, rodless cylinders, telescoping cylinders and through-rod cylinders.

The most popular style of pneumatic actuator consists of a piston and rod moving inside a closed cylinder. This actuator style can be sub-divided into two types based on the operating principle: single-acting and doubleacting.

Single-acting cylinders use one air port to allow compressed air to enter the cylinder to move the piston to the desired position, as well as an internal spring to return the piston to the “home” position when the air pressure is removed.

Double-acting cylinders have an air port at each end and move the piston forward and back by alternating the port that receives the high-pressure air (Figure 1).

In a typical application, the actuator body is connected to a support frame, and the end of the rod is connected to a machine element that is to be moved. A directional control valve is used to provide a path of compressed air to the extend port while allowing the exhaust air to escape through the valve to the atmosphere. The difference in pressure on the two sides of the piston results in a force equal to the pressure differential multiplied by the surface area of the piston.

If the load connected to the rod is less than the resultant force, the piston and rod will extend and move the machine element.

Reversing the directional control valve will provide compressed air to the retract port, allowing exhaust to escape the extend port, and the cylinder will return back to its home position.

Pneumatic actuators are at the working end of a fluid power system. Upstream of these units, which produce the visible work of moving a load, are compressors, filters, pressure regulators, lubricators, on-off control valves and

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flow controls. Connecting all of these components together is a network of piping or tubing (either rigid or flexible) and fittings.

Pressure and flow requirements of the actuators in a system must be taken into account when selecting these upstream system components to ensure desired performance. Undersized upstream components can cause a pneumatic actuator to perform poorly, or even make it unable to move its load at all.

WHY DO AIR CYLINDERS LEAK? While a leaking pneumatic cylinder does not represent the environmental and safety catastrophe that a leaking hydraulic cylinder does, it’s still a serious situation that you should pay attention to. In addition to the fact that air leakage means wasted energy, it’s also a sign that either the system was designed improperly (say, with high side loads that are damaging the cylinder upon extension), or more likely, that the cylinder is nearing the end of its useful life and has to be replaced.

An air cylinder will generally leak the most at the shaft, at the point where the rod moves in and out—the location of the rod seal. Some technicians recommend putting a small amount of soapy solution (bubbles) in this area to better see if this is the source of the leak. Other areas include welded seams or at the air connection points—where the air lines enter the cylinder body.

Air cylinders generally leak because their seals have worn out, sometimes exacerbated by internal rusting of the metal components. If a piston or rod seal is the culprit, these can be replaced, and seal kits are widely available. Some cylinders are the non-repairable type, and if this is what you are dealing with, the entire cylinder will have to be replaced.

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PNEUMATIC

CYLINDER SLIDES D

esign engineers rely on pneumatic cylinder slides and rodless cylinders (sometimes referred to as guided cylinders) when their systems need to provide power and linear motion while supporting a load. Stand-alone pneumatic cylinders are suitable for providing power and motion, but are not designed to provide support for a side load. Most of these types of cylinders have no way of holding the position of the piston rod, due to the rod’s ability to rotate. Pneumatic slides provide the load capability and a stable, non-rotating platform on which to mount tooling or other actuators. This is especially important when a cylinder is moving in a horizontal direction, or where side load is a major issue—common in automation devices used for picking and placing of parts.

Rodless cylinder slides are popular choices when longer distances of travel are required, or when the overall length must be minimized due to space constraints.

Typical uses for these slides include conveyor stops; part ejection and positioning; opening and closing safety doors, gates or curtains; and multi-axis configurations. In many of these applications, the need for side load capacity and non-rotating capability is critical.

There are several considerations you should be aware of when selecting the best type of pneumatic cylinder slides. These include: • • •

•

Load capacity required. The total payload must be calculated to start the selection process.

Life required from the slide. The bearing system selected will have an impact on the expected life of the unit along with the required speed and payload.

Speed required. The slide speed is a critical component including the ability of the slide to handle the kinetic energy as the load stops at the end of travel. Cylinder shock pads, cylinder cushions or shock absorbers may be required based on the load and speed of the slide.

Deflection needed. The amount of deflection will vary based on the bearing system and the payload being carried. This deflection will affect the positional accuracy of the slide.

Many manufacturers of pneumatic cylinder slides provide specification and sizing software to allow the proper selection of the slide required for various applications.

The idea of applying a load to a linear actuator is common, and there are a number of types of cylinder slides that can be used for these applications.

The first basic style of powered slide is commonly known as a “thruster” or cantilever type

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unit. This type of guided slide is typically powered by a rod-style pneumatic cylinder, which is attached to the body of the slide, or may be integral to the slide. In either case, the cylinder piston rod is attached to a tool plate providing power and motion. The tool plate is supported by a bearing mechanism, and together they are able to carry any loads that are attached, rather than transferring the load to the cylinder rod. This type of slide is designed to carry an overhung load known as a cantilevered load.

The second basic type of cylinder slide is called a saddle slide or base slide. In this case, the pneumatic cylinder is attached to a saddle that supports the bearing system on each end of the slide’s travel. This type of powered slide can be used for longer travels with less deflection based on the bearing system being supported on each end. Like the thruster style slide, the saddle carries the load versus the cylinder’s piston rod.

Another type of slide is a rodless slide. In this case, the bearing system is attached to the rodless cylinder directly on one or both sides of the cylinder. The cylinder’s piston is linked to a carriage mounted upon the bearing system, offering load carrying capability as well as resistance to side loads. Rodless slides offer the most space savings as the cylinder’s travel is contained within its own overall length.

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The types of bearing systems can vary on thruster slides, saddle slides and rodless slides. The most common type of bearing system on cylinder slides uses round shafts with linear bearings. The bearings can be precision reciprocating ball bushings or a variety of composite bushings. The precision ball bushings provide low friction and more than 200 mil in. of travel life.

Composite bushings are typically lower in cost and can be used in harsh environments. These have more friction and do not have the life expectancy of the reciprocating ball bushings. Rodless slides use either profiled rail bearing systems or reciprocating ball bearings, and in some cases, composite bushings. As with thruster or saddle slides, load and life is commensurate with the bearing system.

Other pneumatic cylinder slides use profile rails with reciprocating ball carriage bearings. The profile rail bearing systems provide long life with minimum deflection. These can be incorporated in both thruster and saddle type slides.

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FRLs A

ir leaving a compressor is hot, dirty and wet, which can be damaging to—and also shorten the lives of—downstream components, such as valves and cylinders. Before air can be reliably used, it needs to be filtered, regulated and sometimes lubricated.

An air line filter traps particle and liquid contamination in compressed air. It strains the air to trap solid particles (dust, dirt, rust), and also separates liquids (like water and oil) entrained in the compressed air. Filters are installed in the line upstream of regulators, lubricators, directional control valves and air-driven devices such as cylinders and motors.

Filters remove impurities from the pneumatic system, preventing damage to equipment and reducing production losses due to contaminant-

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related downtime. Downtime in an industrial plant is expensive and is often the result of a contaminated and poorly maintained compressed air system.

There are three types of filters: general purpose, coalescing and vapor removal. General purpose filters are used to remove water and particles, coalescing to remove oil, and vapor removal to evacuate oil vapor and odor.

Pressure regulators reduce and control fluid pressure in compressed air systems. Regulators are also frequently referred to as PRVs (pressure reducing valves). Optimally, a regulator maintains a constant output pressure regardless of variations in the input pressure and downstream flow requirements, so long as upstream pressure doesn’t drop below that of downstream. In practice, output pressure is influenced to some degree by variations in primary pressure and flow.

Pressure regulators are used to control pressure to air tools, impact wrenches, blow guns, air gauging equipment, air cylinders, air bearings, air motors, spraying devices, fluidic systems, air logic valves, aerosol lubrication systems and most other fluid power applications requiring subordinated pressure.

Regulators employ a control-spring acting upon a diaphragm to regulate pressure, and its spring rate

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FRLs

H OW DO YO U S I Z E A N D SELECT FRLs?

determines the range of pressure adjustment.

General purpose regulators are available in relieving or non-relieving types. Relieving regulators can be adjusted over a wide pressure range, and even when downstream flow is blocked at the reducing valve, relieving regulators will allow the excess downstream load- or head-induced pressure to be exhausted.

Non-relieving regulators, when similarly adjusted, will not allow the downstream pressure to escape.

The trapped air will need to be released by some other means; for example, by operating a downstream valve.

A lubricator adds controlled quantities of oil or other lubricant into a compressed air system to reduce the friction of moving components.

Most air tools, cylinders, valves, air motors and other air-driven equipment require lubrication to extend their useful life. The use of an air line lubricator solves the problems of too much or too little lubrication that arise with conventional lubrication methods, such as either grease gun or direct oil application. Once the lubricator is adjusted, an accurately metered quantity of atomized lubricant is supplied to the air operated equipment, and the only maintenance required is a periodic refill of the lubricator reservoir. Adding lubrication to a system also “washes away” compressor oils that travel through the system in vapor form. Mineral oils added to the system prevent synthetic compressor oil build-up on system components.

When lubricators are not used in a system, a coalescing filter should be installed to remove compressor oil aerosols.

IMAGE COURTESY OF CLIPPARD

The selection of an FRL assembly is based mostly on the flow rate required, and the size of the component bodies reflects this.

Other than the required flow rate, construction material and component options are also considered.

Often in pneumatic systems, components are sized to match connection ports on other components. For example, if your valve manifold inlet port is ¾-in. NPT, then choosing ¾-in. ports for your FRL might seem like the easy choice. However, not all filters, regulators and lubricators are created equal, and their selection should be based on the flow characteristics of the assembly, rather than port size alone.

The size of an air filter is selected based on the maximum airflow through the unit, and care should be taken in referencing the manufacturer’s flow characteristics chart. Just like any pneumatic component, inlet and outlet pressure factor into air flow, but you should also consider the pressure drop characteristics of the filter itself, which can be high if the unit is undersized, causing a reduction in flow.

Because the regulator limits and controls downstream pressure, the outlet pressure plays a critical role in the dynamic flow capability of the whole FRL assembly. If outlet pressure rises or is regulated too close to inlet pressure, the FRL will start to reduce flow, or even stop flowing altogether. Similar to filters, manufacturers offer flow characteristic charts for their products to help chose the correct regulator.

Lubricators are sized by downstream flow requirements, and an analysis of air flow use must be made to accurately determine lubrication rate. After determining how much air flow is needed, a lubricator can be chosen, and once again, manufacturers’ curves will be similar.

Like all compressed air components, each element of the FRL will have a pressure loss characteristic. You first need to know the characteristic of the end used (flow static or flow dynamic), the flow, and the minimum required pressure. Then you must know the minimum input pressure from your compressed air system.

Then it is an exercise in mathematics in selecting the components that will result in proper end use pressure. Each component will have a pressure loss curve you can consult to find the pressure loss at your stated flow.

Construction material of the FRL components also plays a role in selection, and some of the metals and plastics in their construction might not be compatible with the ambient air conditions of their installation location. For example, the standard polycarbonate bowl construction of most filters are not appropriate for exposure to aromatic chemicals, and nylon should be used instead.

Finally, FRLs are available with many options, such as pressure gauges, drains, pressure switches and check valves, to name a few. If you’re unsure what you need, contacting the manufacturer will put you in touch with someone to help with your decisions.

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P N E U M AT I C T I P S . CO M

GRIPPERS

HANDBOOK

PNEUMATIC

G

Fluid Power

rippers are used in many pneumatic systems, as a means of grasping or holding parts; grippers may move, insert, stack or orient the parts to achieve a desired step in a manufacturing process. Grippers, which use compressed air to actuate the jaws, come in multiple styles, including parallel and angular, and may use a rack and piston arrangement, a lateral direct coupling or opposing jaws. They may have two, three or four jaws, depending on the type and shape of part that they will be interacting with.

These components can be used in many applications, including: • • • • • • • • •

Oversize loads Heavy loads Part ejection Part seating Part handling Clamping and fixturing Pick and place Material transfer Camming and indexing

IMAGE COURTESY OF BIMBA MFG.

Industries served include robotics, medical device manufacturing, semiconductor manufacturing, laboratory processing, sensor manufacturing, plastic molding/blow and injection, vehicular lighting and trim, pharmaceutical and biotech, and even bearing manufacturing.

Pneumatically operated industrial grippers have been commercially available for only a few decades, and prior to that, most gripping mechanisms were designed and fabricated by machine builders employing a combination of pneumatic cylinders and external tooling for each individual application.

Their most common purpose is to grasp or enclose parts for transfer, insertion or assembly in automated manufacturing and processing systems. Additional uses may include operating in environments that are hazardous for human presence.

Pneumatic grippers provide their motion and actuation by introducing compressed air into a chamber of the

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device and powering a piston or a rolling diaphragm attached to a rod. The resultant motion of that rod is then converted mechanically to some form of gripping force. Grippers are typically available in either an angular or parallel jaw version. The term parallel implies that the gripping surfaces of the jaws remain parallel to one another throughout the gripper’s travel, whereas angular refers to the pivoting motion of the jaws closing on the workpiece, such as would occur if you held your hands out to the side and then clapped out in front of you.

The simplest of the angular design is a basic linkage or toggle mechanism attached to a piston rod, which is then connected to one of the gripper’s jaws.

A directly opposing linkage is attached to another jaw. These two opposing jaws then make up the simple angular gripping device. With the appropriately designed and attached tooling, this type of gripper can provide a fairly large force in an angular fashion. The force is generally directly proportional to both the pressure applied to the piston and the length of the linkage or toggle. The length of the tooling attached to the gripper also affects the force capacity, as the gripping point must be within the specified gripping distance/range.

One of the limitations of this angular style of gripper jaw motion is that lacking sophisticated tooling, it can only efficiently grip a limited size range of parts. A variation of this angular gripper could include a third, or even a fourth jaw, defining a center gripping axis or region.

The parallel gripper design employs one of several different types of mechanisms that convert the piston or its rod motion to a parallel jaw travel, including: • • •

a lateral direct coupling of a pair of pistons to guided opposing jaws moving opposite each other, such as with a wide opening gripper a design where opposing jaws are driven by a component of the piston rod riding on a cam surface portion of the jaws a rack, powered by pistons, driving a pinion that in turn employs a scotch

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P N E U M AT I C G R I P P E R S

•

The Right Gripper for Your Part

yoke cam device to drive opposing jaws a rather complicated mechanism where the piston or one of its members drives a scroll mechanism similar to a machine lathe-chucking device

Robotic end effector solutions

All of these parallel mechanisms may also be designed to incorporate three or more jaws for gripping round or unusual shaped parts. Similar to angular grippers, typical parallel gripper forces are proportional to the applied pressure.

Due to inherent friction within their jaw mechanisms, parallel grippers may also require de-rating factors based on the length of tooling attached to the jaws.

While the parallel gripper may cost slightly more than a comparable angular version, it is considered by some to be advantageous over the angular style because it can often be applied to grip a wider range of part sizes without changing the attached tooling.

Additional features of both angular and parallel pneumatic grippers include the ability to operate as dual-acting cylinders.

This allows them to be used for gripping on either external or internal features of the part. The grippers may also be available with an internal spring, which can offer multiple functions: to use the gripper as a single-acting version with spring return; to prevent the dropping of the workpiece if air pressure is lost; or to provide supplementary gripping force in addition to the pneumatic force.

Material construction of most commercially available pneumatic grippers is aluminum with various surface treatments where additional abrasion characteristics are required. Other materials could include stainless steel or even engineered plastics where corrosion resistance and washdown service is desired.

Angular & parallel, many sizes & options available For handling body panels & various sized parts Superior design & delivery

Series GRT

3-Jaw Design to Self-Center Parts

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Long Jaw Travel, Low Profile Parallel Gripper with Large Moment Capacities

Series EGRK Parallel Electric Gripper

Series GRK

Parallel Pneumatic Gripper

Series GRW

Low Profile Parallel Gripper with Minimal Jaw Play

Series GRR

Heavy Duty Long Jaw Travel Parallel Pneumatic Gripper

Series GRA

Compact Precision Parallel Pneumatic Gripper

Series GRV

Compact Precision Angular Pneumatic Gripper

To order a catalog and see more solutions, visit

phdinc.com/fp617

1-800-624-8511 P.O. Box 9070 • Fort Wayne, IN 46899 USA

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HANDBOOK

Fluid Power

Electrical Engineering

SIZING Pneumatic grippers are available in a wide range of sizes developing grip forces from just a few ounces to several hundred pounds. Gripper force development does not always translate directly into the capacity to carry a specific load. Additional consideration must also be given to the actuator’s ability to withstand moments that are reflected back into the jaws of the gripper from forces developed while in motion. Most manufacturers of today’s grippers provide sizing assistance through their technical sizing manuals, sizing software or both.

Selecting a gripper requires you to first have an understanding of the object shape and path of pick and placement. If the shape and size of the workpiece are consistent, angular grippers will be acceptable for most applications. If the workpieces vary in size or shape somewhat, parallel grippers will be required, as they have a wider range of usable finger width. Some objects, such as a pill bottle, are best suited for three- or four-finger grippers, which can center the object as it is grasped.

When sizing a gripper, you must factor the mass of the object, the friction of the gripper material and the force of the gripper itself. As a rule of thumb, the force of the gripper should be 10 to 20 times the mass of the workpiece, depending on the coefficient of friction between the attachments and that workpiece. If the coefficient of friction (µ) is 0.2 or higher, a multiplier of 10 will suffice. If the coefficient is 0.1 or lower, a multiplier of 20 will be required to ensure the gripper can safely handle the workpiece in most conditions.

The Quality Choice Look to Dura-Bar to perform better than castings and many lowmedium carbon steels for hundreds of applications. Industries using Dura-Bar for their parts include machine tool, fluid power, oil and gas, industrial, defense, mining, agriculture, automotive and more.

Dura-Bar’s dense grain structure, lack of porosity and shrinkage make for a highly consistent materials with superior machinability, thereby reducing production costs. With continuously cast Dura-Bar, no costly patterns are necessary and bars are available as quickly as the next day from an extensive network of distributors throughout North America and in China.

There are many ways to save with Dura-Bar Continuous Cast Iron.

Customers who have made the switch profit most from significant total part cost reduction due to: • Faster Machining Speeds & Feeds • Zero Defect Guarantee • 10% Lighter Weight • Superior Vibration Damping • Pressure Rated to 6,500 psi Contact us to discuss your application at sales@dura-bar.com 800-BAR-MILL (227-6455) or visit www.dura-bar.com

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102

MINIATURE

PNEUMATICS T

he area of miniature pneumatics is a specialized niche that sees a lot of use in applications such as medical/dental instruments, test equipment, analytics, pharmaceuticals, entertainment/animatronics, semiconductor, HVAC systems, aerospace, down-hole oil tools, machine tools, ink-jet printing and process control systems.

Sometimes referred to as precision pneumatics, miniature pneumatics consists of a range of components that have been miniaturized for use in light- and medium-duty applications with lowto-medium pressure ranges. Size and weight constraints matter in these systems and the need for precision is high. System pressures of 20 psi are not uncommon in miniature pneumatic applications.

Miniature pneumatic products encompass a range of scaled down parts, including valves, cylinders, fittings, manifolds and tubing. Specialized components, such as nozzles and screens, are also seen on occasion.

For example, the evolution of medical equipment has expanded beyond the hospital environment and toward the home care and ambulatory environments. This has made portable battery-operated variants of traditional stationary equipment more attractive. In todayâ&#x20AC;&#x2122;s culture, the persistent demand for reliable and innovative products compels companies to integrate new and more advanced technology into smaller packages. The medical market is the major sector for growth in miniature pneumatics right now. Industry experts expect that in the near future, this will remain the case, especially for small solenoids.

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A magnetically latched solenoid valve is suitable for reducing power consumption in applications where conventional, higher power valves have been used in the past. This type of design can be used for compact, batterypowered pneumatic instruments such as portable oxygen delivery systems, environmental gas samplers and other OEM flow switching devices.

Similarly, properly designed miniature solenoid valves can improve patient comfort by reducing actuation noise. A typical solenoid valve has an inherent clicking sound when energized, which is caused by the metal-to-metal contact of the moving armature and stationary core. Quieter operational design found in some miniature pneumatics uses so-called whisper technology to greatly reduce sound levels. These valves are used for medical applications flowing gas or air, such as dialysis machines, patient monitors, ventilators/respirators and other bedside medical devices.

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M I C R O H Y D R AU L I C S.

MACRO

CAPABILITIES. REDUCING THE SIZE AND WEIGHT

OF F L U ID C O NT R O L .

Designing hydraulic systems to perform flawlessly under less-than-ideal conditions is hard enough. But factor in the need to keep components as small and light as possible, and you’ve got a real challenge.

Fortunately, you’ve got a real solution. The Lee Company.

For more than 65 years, we’ve been engineering stateof-the-art microhydraulic components with diameters as small as 0.10 in. and weighing as little as 0.1g, but able to withstand pressures up to 8,000 psi.

And because everyone of our designs originates out of an application need, and is scrutinized with 100% testing and inspection, we’re found in just about every mission-critical fluid control challenge you could imagine – from miles above the earth in satellite positioning systems, to miles below in downhole drilling. Plus many applications in between.

If you require precise fluid control, and absolute reliability, go with the experts.

Contact The Lee Company.

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The Lee Company

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P N E U M AT I C T I P S . CO M

HANDBOOK

Electrical Engineering

P N E U M AT I C CO M P O N E N T R Y T E R M I N O L OG Y The most commonly found types of miniature

R E L I E F VA L V E S provide safety relief, linear or high-pressure operation. They

fluid power components include the following:

cover a wide range of applications.

P L U G S are the most successful method to seal

S H U T T L E VA L V E S T Y L E S include: spring biased with the emergency port

drilled holes for critical applications. More than

normally closed; detented, where the poppet latches to close off either the

100 million are in use where permanent leak-

emergency port or the normal port; selective shuttle valves with a non-biased

proof seals are required.

loose ball design; and inverse shuttle valves, where the lower pressure inlet port is open to common instead of the higher-pressure inlet port.

R E S T R I C TO R S offer small size, precision performance and long life. Single orifice and

S A F E T Y S C R E E N S protect hydraulic components from contamination.

multi-orifice models cover a wide range of

Manufacturers offer intermediate- and high-pressure etched screens; they can

restriction and can be customized to meet

be customized to meet special requirements.

special requirements.

Fluid Power

P I L OT I N G S O L E N O I D VA L V E S are designed for use in aerospace hydraulic F L O W CO N T R O L S including restrictor check

and fuel systems, oil tool applications and other challenging high-pressure

valves, directional flow controls and constant

applications. These are critical for space and weight savings, as well as power

flow valves are available in a wide range of sizes

consumption.

to suit many applications.

P R E S S U R E CO M P O N E N T S include miniature restrictors for damping N O Z Z L E S can produce either a hollow cone of

pressure transients, adjustable pulsation dampers and miniature passive peak

atomized spray or an accurately targeted jet of

pressure recorders.

lubrication fluid.

Fluid Level Indicators

Protect EVERYTHING Fluid Power

CAPS • PLUGS • GRIPS • PROTECTION • FINISHING • MASKING

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Mini Pneumatics FPW Handbook 2017 V3.indd 104

Wide Variety of Sizes & Styles for Metric, SAE, BSP, JIC, NPT Threads & Fittings www.mocap.com • sales@mocap.com

®

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P N E U M AT I C H O S E & T U B I N G

PNEUMATIC

HOSE AND TUBING E

ngineers use pneumatic hose and tubing to convey pressurized air to actuators, valves, tools and other devices.

There are countless types and sizes of tubing and hose on the market, so a designer should consider a number of important factors to select the right one for a given task.

The best place to start is with construction. Tubing manufactured for pneumatic applications may be extruded of a single material or reinforced internally, typically with textile fibers, for higher strength. Air hose generally consists of an inner tube, one or more layers of reinforcing braided or spiral-wound fiber, and an outer protective cover. In broad terms, hose is more rugged than tubing but costs more.

The air supply and application set a baseline for the necessary product performance. Flow requirements help determine hose or tubing size. Tubing is generally specified by OD and wall thickness, while hose is specified by ID. Regardless, choosing too small an inner diameter “chokes” flow and results in pressure losses, inefficiency and excessive fluid velocity that can shorten service life. Too large a diameter, on the other hand, results in higher than necessary weight, size and cost.

Also ensure that products operate below the stated maximum working pressure. Manufacturers generally rate tubing by measuring the burst pressure at 75° F, and then divide it by an appropriate safety factor (typically 3:1 or 4:1) to determine the maximum working pressure.

Keep in mind that published burst-pressure ratings are only for manufacturing test purposes, and in no way indicate that a product can safely handle pressure spikes or otherwise operate above maximum working pressure. Also note that some products

handle vacuum to approximately 28 in.-Hg without collapse.

Thermoplastic tubing is made from several common materials. But manufacturers offer countless variations of polymer formulations to suit specific needs. Typical tubing materials used in pneumatic applications include: •

•

•

•

•

Polyurethane tubing is strong, flexible, kink and abrasion resistant, and it withstands contact with fuels and oils. It’s commonly used in pneumatic actuation and logic systems, robotics and vacuum equipment, and in a variety of semiconductor manufacturing, medical and laboratory applications.

Nylon tubing is tough, light and dimensionally stable. It can be formulated for higher-pressure pneumatics, flexibility for routing in tight spaces, high flexural-fatigue resistance and low water absorption.

Polyethylene tubing is often used in low- pressure pneumatics and pneumatic controls. It has wide resistance to chemicals and solvents, good flexibility and relatively low cost. HDPE tubing comes in semi-rigid versions that resist cuts and physical damage and has a higher burst pressure than polyethylene tubing.

Polyvinyl chloride (PVC) tubing is light and generally more flexible than nylon and polyethylene, offers good chemical resistance and can be repeatedly sterilized. It is suitable for lowpressure medical applications and can be formulated to meet FDA specifications for contact with food and drugs. PVC tubing is typically clear, and thus wellsuited where v isible indication of flow is necessary.

Polypropylene tubing can be formulated for foodcontact applications, resists chemical attack and withstands UV radiation in outdoor applications.

Engineers should weigh the features, benefits and drawbacks of each material. One important factor is compatibility of fluids conveyed through the conduit. For

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instance, in the case of air systems, oil from air lubricators, as well as fumes or other substances ingested by the compressor, could affect the inner tube. Likewise, remember exterior environmental exposure. Hose and tubing assemblies can be attacked by chemicals, ozone, UV radiation, salt water, air pollutants and other substances that lead to degradation and premature failure.

External mechanical influences can also hasten hose and tubing failure. Loads to keep in mind include excessive flexing, twisting, kinking, tensile and side loading, and vibration.

Also protect against abrasive wear, snagging or bending beyond the minimum bend radius, all of which can lead to premature failure. Replace and discard any hose or tube that is cut, worn or otherwise damaged.

Two common physical characteristics to consider when selecting tubing are flexibility and kink resistance. These are often subjective, and it’s best to compare samples rather than rely on manufacturer’s literature. Another physical attribute is color. Tubing comes in a wide range of colors as well as clear, depending on the compound.

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And don’t overlook the effect of temperature and heat on tubing materials—both inside and outside the assembly. Always operate within minimum and maximum temperature limits. In particular, be aware that elevated temperatures will impact working pressure, especially as tubing materials reach their upper temperature limit. For example, one version of nylon tubing with a 0.25-in.

ID has a rated burst pressure of 1,250 psi at 75° F, but only 600 psi at 200° F.

Many fittings are available to build pneumatic tubing and hose assemblies for connection to mating components. These include barbed, push-to-connect, swaged, crimped and reusable versions; and they come in plastic, brass, steel, stainless steel and other materials. First and foremost, designers should ensure the fitting and tubing or hose are compatible with each other, as not every fitting design is suitable for every type of tubing or hose. Other important considerations for fittings include pressure-holding capacity, ease of installation, size and weight, corrosion resistance and, of course, cost.

Tubing is extruded in straight lengths and stored on reels, but it can also be molded into spring-like coils. This lets the tubing extend considerably as

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needed, then retract to a compact configuration for out-of-the-way storage. Tails—short, straight lengths of tubing that extend from each end of the coiled section—facilitate coupling attachment.

Coil diameter, tubing diameter, wall thickness and the type of material affect retractability. Smaller and tighter coils generate more retraction force than do larger coils; polyurethane and nylon generally offer better material-memory characteristics.

These tend to let the product collapse more easily.

In addition to standard products there are many variations. This can include products made for special attributes like high strength, abrasion resistance or compatibility with a specific chemical; characteristics like flame resistance, weld-spatter resistance, and electrical conductivity or nonconductivity; coextruded products that combine the properties of two materials in a single tube; and multiple tubes bonded together in a single assembly or tubes formed into elbows and bends.

Finally, hose and tubing sometimes must meet regulations or standards from agencies like FDA, DoT, UL and ISO. When questions arise regarding product compliance, experts always recommend seeking advice from application engineers from reputable manufacturers.

N

P N E U M AT I C H O S E & T U B I N G

Custom Features: • Integrated linear position sensors

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• Strokes up to 50’ • Operating pressures up to 6,000 PSI

• Internal Cushioning options for deceleration of motion

• Multi-stage telescopic

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P N E U M AT I C T I P S . CO M

VACUUM COMPONENTS IN

the simplest sense, vacuum is pressure that is lower than atmospheric. At sea level, atmospheric pressure is usually 14.7 psia. Therefore, any pressure lower than that constitutes a vacuum.

In a vacuum system, the difference between atmospheric and vacuum pressure creates the ability to lift, hold, move and generally perform work.

The essence of vacuum generation is the reduction of molecular impacts within a system.

In effect, vacuum is the pressure differential produced by evacuating air molecules from a system. There are two types of vacuum applications: sealed, or non-porous, and open, or porous. In a closed system, removing air progressively decreases the air density within the confined space. This causes the absolute pressure of the remaining gas to drop, causing a vacuum. To achieve a vacuum in an open system, a vacuum unit must have the capacity to remove more atmosphere or air molecules than are able to leak back into the system.

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LEVELS OF VACUUM Vacuum is typically divided into three areas of application, dependent upon the level of vacuum required.

Low level vacuum applications are typically those requiring high flows and low force (inches of mercury). These applications are primarily serviced by blowers. Screen printing on cloth is an application that falls into this range.

Industrial vacuum falls within the range of 6 to 29.5 in.-Hg. This is where the largest number of applications occur. Vacuum in the industrial range can consist of anything from pick and place to thermoforming. The largest segment is in the 12 to 21 in.-Hg range.

Scientific or process is an area that encompasses the deepest levels to 29.92 in.-Hg. Vacuum at this level is usually measured in torr. Flow in this range is minimal in transition from viscous to molecular. Examples of applications are ion implantation and space simulation.

The highest level of vacuum achievable on Earth is 29.92 in.-Hg. A perfect vacuum (30 in.-Hg)—a space that contains no molecules or atoms—is purely theoretical. The only possible place where this condition can exist is in space and, even there, a few atoms can be found.

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We make automation better.

PHD offers an extensive line of pneumatic and electric actuators that are known for long life and high quality. Over our 60 years of innovation, PHD has made automation better and more efficient for our customers around the world.

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H OW DO YO U S E L E C T A VAC U U M C U P ?

When it comes to gripping and moving anything from cola cans to car fenders, vacuum cups on end-of-arm tooling are incredibly versatile devices in many automated handling systems. They come in countless types, sizes, constructions and materials to suit widely varying applications. Here are some general engineering considerations for choosing the right one for a particular task.

Construction. Suction cups are available in various geometries, such as simple, circular types for general-purpose handling; extra-deep cups for round or highly curved surfaces; and oval shapes for picking up long and narrow products. These cups often include molded-in structural details like exterior reinforcing ribs for added strength or interior nubs for better contact with a mating part.

Shape. Suction cups come in two general shapes, flat and bellows. Flat vacuum cups are best for handling workpieces with flat or slightly curved surfaces, such as metal and glass plates, plastic sheets and wooden boards. Properly designed, they have good rigidity and stability to handle high shear forces and can withstand forces and accelerations from fast automatedhandling movements.

Bellows suction cups, on the other hand, have one or more accordion-like convolutions. This lets them compensate for varying workpiece heights and handle parts with uneven surfaces. Evacuating the bellows also creates a lifting action which can be useful to lightly grip fragile parts, like electronic parts or even chocolate candy.

Bellows versions are typically used for handling curved parts like car body panels, pipes and tubes, injected molded plastic parts, and nonrigid packaged goods or shrink-wrapped products.

Interior volume. A corollary to cup shape is its interior volume.

That’s important because this is the volume that must be evacuated to create a vacuum—and the holding force—to actually grip and pick up a workpiece. Flat cups have a relatively small inner volume and, thus, evacuate quickly and can grip in a very short time. The total inner volume of all the cups in a system,

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as well as that of tubing and connectors, must be added to determine the total volume of the gripper system. From there, engineers can calculate the evacuation time based on vacuum flow capacity and, ultimately, how quickly the system can pick up a part. Finally, also consider how long it takes to supply air to the cup, to purge the vacuum and release the workpiece.

Materials. Vacuum cup suppliers offer a wide variety of options to suit specific application requirements. Typical examples include nitrile rubber (NBR) that’s economical, offers excellent oil resistance and is a first choice for generalpurpose applications; silicone for food-grade applications; natural rubber for handling wood; polyurethane for excellent wear resistance and high strength; and fluoroelastomers for high chemical and weathering resistance.

Other common materials include options like PVC and EPDM, as well as proprietary formulations from specialty chemical manufacturers. Some come in anti-static versions for handling electronics.

Environment. Depending on the application and setting, suction cups might need to tolerate rough or porous surfaces or withstand ozone, oils, washdown solvents or steam. Operating and workpiece temperatures can be a concern, too. Some silicone products, for example, offer a temperature operating range from around –20 to 350° F (–30 to 180° C) and fluoroelastomers to nearly 400° F (200° C).

Of course, in addition to selecting the type of suction cup, engineers need to determine the holding force based on parameters like cup size, vacuum level, ambient air pressure, leakage rates, product weight, workpiece surface coefficient of friction, and the magnitude and direction of loads and accelerations.

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VAC U U M CO M P O N E N T S

U S E F U L VAC U U M T E R M S

IMAGE COURTESY OF PIAB

The following are the most important terms to be considered and understood in any discussion or application using vacuum.

VACUUM That pressure or any pressure lower than atmospheric. As previously discussed, at sea level atmospheric pressure is usually 14.7 psia; any pressure lower than that constitutes a vacuum.

VACUUM F L OW The rate at which atmospheric pressure is removed from a system, or the amount of outside atmosphere that flows through a pump. Usually, this is measured in standard cubic feet per minute (scfm). The significance of vacuum flow is that it determines the speed of evacuation of a system, or the ability to compensate for leakage in a system. As the level of vacuum (vacuum force) in a system increases, the flow rate decreases because there are fewer molecular impacts.

F RE E A IR CA PAC ITY The amount of outside atmosphere a vacuum pump can displace at 0 in.-Hg, or wide open. This expression is commonly used by vacuum pump manufacturers as an indicator of size and performance. It is confusing in that it does not tell us what type of performance to expect in a given range (level of vacuum force). Free air capacity is analogous to the size of an engine in a car; it is a starting point, but tells us very little about specific performance or efficiency.

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VAC UUM F O R C E This term is most commonly defined as the level of pressure within a system, usually measured in inches of mercury. The amount of vacuum force created usually determines the lifting capacity of a suction cup or measures the amount of atmosphere left in a system. Vacuum force can not overcome porosity in a system, nor can it speed the evacuation time of a given volume.

The higher the level of force, the longer it takes to achieve.

CO M P R E S S E D AI R The energy source that drives an air-driven vacuum pump, measured in scfm. Compressed air is comparable to the electricity that runs a mechanical vacuum pump. Generated by an air compressor, it is supplied through a network of piping at a certain pressure level.

AI R S UP P LY P R E S S UR E The pressure of the compressed-air supplied, usually measured in psi. The measurement used to determine the optimal operating pressure of an air driven vacuum pump. Optimal pressure is obtaining a balance of supply pressure and air consumption to achieve the maximum efficiency level.

E VAC UAT I O N T I M E The amount of time it takes to evacuate a given volume to a desired level of vacuum.

E N E R G Y CO N S UM P T I O N The amount of energy, whether expressed in hp, kW, scfm or any other expression, that a vacuum pump uses to generate a desired amount of vacuum.

LI F T I N G F O R C E The lifting capacity of a suction cup, determined by multiplying pressure times the area.

VO LUM E The total of all area in a vacuum system from the interior of the pump to, and including, the area of application.

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PNEUMATIC

VALVES

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ontrolling pneumatic actuators in compressed air systems requires safe and precise functionality. Although the medium is fluid, just as hydraulic systems, the execution of control is different in many ways than with a liquid. What is shared in the conduction of any fluid power medium is the need for valves to control force, velocity and direction of movement.

AIR PREPARATION

Pressure relief valves will control pressure at their inlet port by exhausting pressure to atmosphere. Relief valves are typically used only in receivers or air storage devices, such as accumulators, as a means to prevent excessive pressurization. As such, relief valves are often called safety valves and are not typically appropriate for use anywhere but the air preparation stage.

Pressure regulators in pneumatic systems limit pressure downstream of the unit by blocking pressure upstream at the inlet. Regulators are used in the air preparation stage, as well as in control of cylinders and motors.

The letter R in the acronym FRL stands for regulator, which is installed downstream of the receiver tank, but before the circuit they are regulating pressure for.

Sometimes multiple stages of pressure reduction are required, especially with a large centralized compressor and receiver feeding various workstations. A regulator can control pressure within the main grid of distribution plumbing, but sometimes air is piped directly to an FRL at each workstation or machine. Pressure at this main header could be 120 psi or more, but a branch circuit could be regulated at 90 psi, for example. Most regulators are capable of relieving downstream pressure, which prevents that downstream pressure from elevating as a result of load-induced pressure or thermal expansion.

Pressure regulators can be had as stand-alone units, but sometimes a filter is attached to kill two birds with one stone. Regulators are most often available as a component of a modular set, with a filter, regulator, lubricator or dryer, and can be assembled in any combination. The regulator will have an inlet port, outlet port and a port for the pressure gauge, with which they are most often included.

Pressure regulators can also be used to control pressure for individual actuators, such as an inline regulator or work-port mounted regulator.

These are typically quite small and included with reverse flow check valves, as would be required for double-acting function of a cylinder, for example. Further still, differential pressure regulators are offered by some manufacturers to maintain a set pressure differential between the two ports, rather than just maintaining downstream pressure. It should be noted that all pressure regulators are adjustable, most often with screws or knobs.

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P N E U M AT I C VA L V E S

FLOW CONTROLS

Also common in pneumatic systems are valves to control flow. There are fewer available types of flow valves compared to pressure or directional valves, but most circuits apply them to make for easy adjustment to cylinder or motor velocity.

Controlling velocity in pneumatic systems is more complex than in a hydraulic system because pressure differential between the work ports of a cylinder plays a larger part.

Flow control valves for pneumatic systems are quite simple, usually available in two configurations used in two different ways. One configuration is merely a variable restriction, with a screw or knob adjustment to open and close a variable orifice, which is also often referred to as a needle or choke valve. The other type introduces a check valve, which allows free flow in one direction and restriction in the opposing direction.

For whatever reason, this valve has hijacked the name flow control all for itself.

Flow control valves are applied in two different ways: meter in or meter out.

Meter in is the method of controlling the rate of airflow as it enters a motor or cylinder.

When metering in, a cylinder will move rapidly with high force and efficiency, but the motion of the piston is prone to spongy and unpredictable movement. When metering out, the cylinder velocity is more stable and repeatable, but efficiency and dynamic force are lost to the energy required to push past the flow control.

Regardless, most pneumatic applications operate using meter-out flow controls because the disadvantages are easy to overcome by increasing upstream pressure.

A method of increasing cylinder velocity, typically for double-acting or springreturn cylinder retraction functions, is to add a quick exhaust valve to the cap side work port. Because cylinders retract faster than they extend as a result of differential air volumes, it is harder to evacuate the cap side air volume without oversized valves

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or plumbing. A quick exhaust valve vents directly to air from the cap side work port and massively reduces the backpressure created upon retraction, permitting rapid piston velocity.

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DIRECTIONAL CONTROL VALVES Pneumatic directional valves are available in many sizes, styles and configurations. At the basic end of the spectrum is the simple check valve, which allows free flow in one direction and prevents flow in the reverse direction. These can be installed anywhere from right after the receiver to within a flow control valve itself.

As directional valves grow in complexity, they are specified under a general naming practice related to the number of positional envelopes of the valve and the number of work ports in the valve, and specifically in the order described.

For example, if it has five ports, port 1 will be for pressure inlet, ports 2 and 4 for work ports, and 3 and 5 for the exhaust ports. A valve with three positions will have a neutral condition, extend condition and retract condition. Putting it all together, this describes a five-way, three-position valve, also referred to as a 5/3 valve. The common configurations seen in pneumatics are 5/3, 5/2, 4/2, 3/2 and sometimes 2/2 valves.

Also part of the description of a directional valve is its method of both operation and positioning. The valve operator is the mechanism providing the force to shift the valve between its positions. The operator can be a manual lever, electric solenoid, air pilot or cam mechanism, to name a few. Some valves are a combination of these, such as a solenoid pilot valve, which is a tiny valve providing pilot energy to move the main-stage valve. Positioning of any valve is achieved by either a spring, such as with a 5/2 spring-offset valve, or with detents, as in 5/2 detented valves.

A 5/2 spring-offset valve will return to its starting position when energy is removed from its operator, like de-energizing the coil, or removing pilot pressure. A 5/2 detented valve will stay in the position it was last activated to until the operator switches it again.

Pneumatic valves are manufactured in various incarnations. Poppet valves are simple, using a spring to push a face of the poppet down on its seat. Construction can be metal-to-metal, rubberto-metal or even with diaphragms. Poppet valves can often flow in one direction, just as a check valve, but need to be energized to flow in reverse.

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They are limited to two- or three-way port configurations, although they can mimic four- or five-way valves when used in parallel. They offer typically high flow conductance for their size, and are generally resistant to contamination.

Spool valves use a notched metal cylinder that slides within a precisely machined body, drilled with three to five ports, or seven ports if the valve is pilot operated. Low-end valves consist of only a spool and body, and are prone to internal leakage. Better valves use seals in the body or spool to prevent leakage between ports. High-end spool valves are constructed with precision, often requiring fine lapping procedures during manufacturing, and with their tight tolerances, often require few seals, improving reliability and longevity. Other forms of high-end valves use a sliding block of metal or ceramic, which is efficient and extremely resistant to contamination.

MOUNTING CONSIDERATIONS

Pneumatic directional valves come in both standard and nonstandard mounting configurations. The non-standard valve’s port layout, operator style and mounting options are unique to each manufacturer’s product. They can be inline, subplate mounted or sectional stacks mounted in a row. Because each manufacturer does mounting differently, it is best to research the product appropriate for your application.

Luckily, most manufacturers have lines of standardized valves suiting one or more specification, such as ISO 5599-1, with its staggered oval ports; this means one manufacturer’s valve will fit the subplate or manifold of another manufacturer’s.

Port and electrical connections are standardized with most valves as well. NPT ports are common, but many new valves come with push lock fittings on the subplate itself. Electrical connectors for standardized valves are frequently DIN, mini-DIN or with fieldbus connection, making the operation of a dozen valves as easy as one connector.

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G AU G E S

GAUGES A

pressure gauge is a fluid intensity measurement device. Pressure gauges are required for the set-up and tuning of fluid power machines, and are indispensable in troubleshooting them. Without pressure gauges, fluid power systems would be both unpredictable and unreliable.

Gauges help to ensure there are no leaks or pressure changes that could affect the operating condition of the hydraulic system.

The hydraulic system is designed to work in a set pressure range so the gauge must be rated for that range.

Hydraulic pressure gauges are available to measure up to 10,000 psi, although maximum hydraulic pressure is typically in the 3,000 to 5,000 psi range. Hydraulic gauges are often installed at or near the pump’s pressure port for indication of system pressure, but can be installed anywhere on the machine where pressure needs to be monitored—especially if sub-circuits operate at a pressure rate different from pump pressure, such as after a reducing valve. Often, pressurereducing valves have a gauge port to tap into, allowing you to directly monitor its downstream pressure setting.

Pressure gauges have been used in fluid power systems for well over a hundred years, so it might be a surprise that pressure gauge designs continue to evolve. The evolution of pressure gauges for fluid power applications has, generally, come about due to an increase in application specific features. For instance, pressure gauges are now more routinely designed with hydraulic friendly pressure connections (such as SAE/Metric straight threads) to prevent system leaks. Analog gauges with custom scales are more common and digital pressure gauges with customizable firmware allow process measurement of pressure-based www.fluidpowerworld.com

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measurement of leaks or other parameters like torque, load, force and hardness.

Pneumatic and compressed air systems are also rife with gauges, as pressure is also measured in many locations throughout the system. Pressure is measured at the receiver(s), as well as at every FRL or stand-alone regulator in the system. Sometimes pressure is measured at pneumatic actuators as well.

Typically, pneumatic pressure gauges are rated for not much more than 300 psi, although typical systems run around 100 psi.

Pressure is measured in three ways—absolute, gauge and vacuum. Absolute pressure is a measure of actual pressure including ambient air, which is zeroreferenced with a perfect vacuum, but can be as high as 14.7 psi at sea level. Absolute pressure readings are considered in applications interacting with ambient air, such as the compression ratio calculation for flow (cfm) requirements. Gauge pressure is zeroreferenced against ambient pressure and is used in 6 • 2017

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most applications operating in, but not with, ambient air, such as in fluid power systems. Disconnected from equipment, gauge pressure will read zero.

Finally vacuum “pressure” is expressed in Torr, or referenced against ambient pressure, as with “in.-Hg” (inches of mercury) units, which measures pressure below ambient.

The hydraulic gauge can withstand different pressure ranges based on what type of gauge style it is and what material it is made out of.

Because of this, the gauge style and the material make up two of the most important selection criteria for gauges.

The pressure range at which the gauge will be working is a primary selection factor for the type of material used to make the gauge. Gauges operating at higher pressures generally tend to be made of materials such as steel; when operating at lower pressures, they tend to be made of bronze.

Most pressure gauges in North America come with a 1⁄4-in. NPT male, but SAE thread is gaining popularity. The use of test-point adapters at various locations on the hydraulic system allows for measurement during troubleshooting without having to purchase dozens of pressure gauges.

The test-point fitting attaches to the gauge, which can be screwed onto the test points throughout the circuit, allowing you to connect under pressure to measure at various points in the system. Most gauges are 21⁄2 in. in diameter, and can be either top-mount or panel-mount styles, but gauges are available in every size, material and construction imaginable.

H OW DO G AU G E S W O R K ?

There are many types of gauge styles, the most common being Bourdon tubes and bellow gauges. Bourdon tubes function by taking the pressure and converting it into mechanical energy. This energy moves a dial in the gauge, displaying the current amount of pressure in the system. Bourdon tube gauges are currently some of the most common gauges and have different configurations such as curved, helical and spiral. The different style of tubing, the size of the tube and the material it is made out of all vary based on the pressure range.

One important characteristic to note is the cross section of the tubing changes with increasing pressure. Generally, as the working pressure of the gauge increases, the shape of the cross section of the tube’s design will gradually change from an oval shape to a circular shape.

ILLUSTRATION SHOWS how Bourdon tube pressure gauges work, with a curved tube closed at one end, which is attached to a spring-loaded mechanism that is attached to the gauge’s needle.

Image courtesy of Noshok.

Bourdon tube operation is simple. They consist of a semicircular and flat tube of metal, fixed at one end and attached to a sensitive lever mechanism at the other. As pressure increases inside the tube, the force of the fluid attempts to straighten out the curved tube. The tube then pulls away from the lever, which being connected to the needle on the display, shows the pressure at the fluid port.

While bellow gauges function similarly to Bourdon tubes, they differ in the fact that they use a spring to judge the amount of energy to push the dial. The spring is expanded and compressed by the pressure in the tubes and the energy created by that movement is transferred into gears that move the pressure dial.

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Balluff_

â&#x20AC;&#x192; G AU G E S

Whether used for testing equipment or operating machinery, the right pressure gauge helps reduce costly downtime. In mechanical gauge applications for hydraulic systems, the common threats to gauge reliability are vibration, pulsation and pressure spikes. Therefore, itâ&#x20AC;&#x2122;s best to look for gauges designed specifically for hydraulic applications.

These features include: a forged brass case to prevent resonant frequencies from destroying internal components; a liquid-filled case to protect the gauge from vibration and extreme pressure cycles; and a restrictor to prevent damage to the gauge from pressure spikes.

Although the liquid used in the gauge varies from application to application, glycerin is commonly used and performs well in many conditions. The higher the viscosity of the liquid, the more it dampens the vibrations. When choosing between a dry, water- or glycerin-filled gauge, it is also important to consider the following: temperature range, needle response time required, changes in pressure and the amount of vibration expected from the application.

Finally, depending on the demands of the application, gauge accessories, such as specialized restrictors, piston snubbers or even diaphragm seals, may be needed to prevent premature gauge failure.

A SYSTEM MODEL of a Bourdon tube pressure gauge. Image courtesy of Noshok.

Imagine a 90% reduction in size and weight.

The patented Cyclone Hydraulic Reservoir is designed to rapidly remove air from hydraulic fluid. The result?

Weight and size reduction, along with corresponding savings in fluid expense.

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FLUID POWER

Fluid Power

SAFETY

U

nderstanding where stored energy exists in any fluid power system is critical to safe machines and personnel. It is important to keep components plumbed properly, but also have the correct levels of machine safeguarding in place—from properly labeled lockout/tagout systems to safety valves to ensure redundancy and safe shutdown. It is critical to evaluate the entire system and its complete schematics, including the electrical portion, to minimize exposure to unnecessary risk. Systems are rated based on the weakest link in the control chain.

Several standards (including ISO 138491:2006, ANSI/ASSE Z244.1-2003 [R2008] and ANSI/PMMI B155.1-2011) define the control system as including not only input, sensing and interlock devices, but also output devices such as pneumatic and hydraulic valves.

The function of a fluid control valve mimics that of an electrical-control relay and, therefore, is subject to the same rules for classifying safety integrity. Thus, properly specified machine safeguarding systems include provisions for pneumatic valves, including: • must be functionally redundant • must be monitored for faults (including ENSURING SAFE CYLINDER MOTION is critical in fluid power systems. Double valves, like this Cross Mirror CM Series Safe Cylinder Return double valve are “control reliable,” which means they are capable of stopping or preventing initiation of hazardous motion if there is a single component failure within the device or system. Image courtesy of Ross Controls.

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•

•

•

diminished performance faults, which may create the loss of redundancy), without depending on external machine controls or safety circuitry must return to a safe position in the event of a loss of pressure or other such event must be able to inhibit further operation upon detection of a fault condition until such condition is corrected should have a dedicated, specific function-reset input and should prohibit the ability to perform a reset by simply removing or re-applying pneumatic or hydraulic power, and must not automatically reset.

Providing control reliability with fluid power is not quite the same as with electrical controls, however. For instance, plain redundancy in a safety circuit requires the equivalent function of four valve elements, not just two. Two of the four valve elements handle the inlet function while the other two elements handle the stop function (energy release). Many self-designed systems risk having hidden, potential flaws, which can lead to unsafe conditions because they are unseen, unexpected and, therefore, excluded from design and safety reviews. A good example is the spool cross-over conditions or ghost positions of a valve, which are usually not shown on schematics.

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H OW C A N YO U E N S U R E H Y D R AU L I C S A F E T Y ?

The two most critical considerations in hydraulic safety are with control of flow and pressure. Safe control of loads being manipulated by hydraulics is often primary.

However, the safe containment of pressurized fluid often means loads are also stable, as a catastrophic leak of fluid could cause a failure of the system to control the load.

Safe control of a hydraulic load involves ensuring fluid is accurately metered. The potential for pressure and/or load-induced pressure to impart energy into oil is high.

As such, the oil wants to give up that energy to the ambient surroundings, but is prevented from doing so by only the integrity of the components and conduits of the hydraulic system.

The pressure either wants to escape past flow control or directional valves, or it attempts to exit to atmosphere through seals or plumbing failure points.

A leak in the cylinder hose or tube can also cause catastrophic failure and personal harm. When fluid is free to exit the actuator at a broken conduit, it can no longer hold a load up. The counterbalance valve prevents the load from dropping should there be a conduit failure. A pilot-operated check valve will provide the same safety function of a counterbalance valve, but is designed for static load-holding. It will hold a load indefinitely, but is not as smooth at controlling load-induced movement as the latter valve is.

Pressure is the very essence of a hydraulic system, and it is required to provide the force density that makes a hydraulic system so effective.

But pressure can easily rise through intensification, load spikes, “water-hammer,” and thermal expansion, to name a few. If pressure isn’t controlled and limited, components can fail, seals can give way and either example can render a machine unsafe. For this reason, hydraulics uses many types of pressure control valves.

When fluid is well controlled through some sort of metering EMERGENCY STOPS, such as Doering's manually-operated, three-way cartridge valve, can improve safety of device, such as a flow control, industrial machinery. counterbalance or proportional valve, the actuators and loads are prevented from running away. For example, when a cylinder is installed rod down, Relief valves are used to limit pressure, preventing and loads are typically in tension, it is often installed damage and limiting power requirements. A relief with a meter-out configuration to prevent the load from valve is often used to control main system pressure or running away with the cylinder. Although it is a safe isolated sub-circuits. Some circumstances require submethod to prevent a runaway load, rod-side pressure circuits to operate at different pressures than others, and intensification can be a risk. If intensification blows out the pressure-reducing valve can achieve this. It limits piston seals, then the load can drop regardless, which is pressure downstream of itself, and in some cases, can counterproductive to the meter-out philosophy. be a reducing-relieving valve as well. This type of valve can control downstream pressure spikes or thermal To avoid the pitfalls of metering out, a counterbalance expansion, where heated fluid increases in pressure. valve can be used instead. The counterbalance valve is Some hydraulic systems can use a combination of various considered a pressure valve, but in reality, it controls pressure valves to ensure circuit pressure is safely limited the speed of an actuator. These valves ensure a cylinder at every part of the machine. will only move as rapidly as pump flow is dictating, regardless of load-induced pressure or intensification. If a So by controlling both pressure and flow in a hydraulic pilot signal is not seen from the opposite work port, the system, best practices of safety can be observed. A valve remains shut, preventing the load from dropping. machine failure is unsafe in the best scenarios, so Often mounted directly to the cylinder work port, the ensuring a hydraulic circuit is designed for safety will counterbalance valve also prevents dropping loads when prevent injury to operators. hose or tube failures occur.

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â&#x20AC;&#x192; FLUID POWER SAFETY

Two general abnormal conditions can affect valve safety. The first is similar to an electrical-control fault, such as when a relay might be stuck in the open or closed position. The second abnormal condition is when a valve develops diminished performance, such as when it becomes sticky or sluggish.

In such cases, the valve reaches the proper position, but slower shifting affects safe stopping distances or precise timing. The ANSI B11.19-2010 standard mandates a monitoring system that detects these conditions for critical applications and the ANSI/PMMI B155.12011 standard requires diminished performance monitoring if stopping time can be affected. An easy solution is to use a self-monitoring, Category-3 or -4 valve, designed to detect both conditions.

The use of double valves remained relatively unheard of for many years, except in a few select industries, such as stamping presses, which first initiated control reliability requirements. Double valves provide dual internal functions (redundancy) so that an abnormal function of one side of the valve does not interfere with the overall normal operation. At the same time, the double valves sense abnormal operation on either side of the valve and then inhibit further operation until the problem has been corrected and the valve deliberately reset. This sensing and inhibiting function is commonly referred to as monitoring.

Two standard air valves, whether in parallel or in series, cannot perform the same safeguarding function as a double valve providing this critical function. By simply incorporating two standard air valves into the circuit, no provision is made to sense the abnormal operation of one side of the valve or, even more preferable, diminished performance such as slow shifting.

In addition, there is no provision for inhibiting further operation of the circuit until the valve is repaired. If one valve

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DOUBLE VALVES, such as this DM2 series C double from from Ross Controls, retain a lockout condition to prevent unintentional reset with removal of air or electricity.

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actuates abnormally, the second one continues to function and redundancy is lost. The circuit doesnâ&#x20AC;&#x2122;t recognize lost redundancy, nor would it halt operations as a warning that redundancy has been compromised. Then, if the second valve also actuates abnormally, there is no back up, and control integrity no longer exists.

Double valves are appropriate for pneumatic and hydraulic equipment anytime reliability is an issue. Typical applications include e-stop, two-hand-control, light curtains, safety gates, pneumatic locking devices for safety gates, hydraulic brakes, air brakes, amusement rides, hoists, elevators, pinch-point applications, or any other application where control system integrity depends on valve operation.

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S H OC K A B S O R B E R S

INDUSTRIAL

SHOCK ABSORBERS

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achine builders are always on the lookout for ways to run equipment faster and increase throughput and productivity. However, components moving at high speeds often must decelerate and stop without damaging the equipment or payload. Otherwise, the consequences are excessive loads, vibration and noise that can compromise safety and machine reliability.

Engineers can sometimes dampen motion with products like inexpensive elastomeric bumpers, simple air cushions or gas-spring linear dampers. But these typically have a limited ability to absorb energy and decelerate objects.

Shock absorbers, in contrast, provide controlled deceleration by converting kinetic energy to thermal energy. In action, motion applied to a hydraulic shock absorber’s piston forces pressurized fluid through specially designed internal orifices. That restricts flow and generates heat which, in turn, transfers to the metal body and dissipates to the environment.

After impact, a spring typically returns the piston rod to the starting location. Shocks are used in a wide range of applications, from automotive manufacturing and lumber processing to robots, cranes and packaging equipment.

When choosing a shock absorber, one must specify the stroke length, compressed length, extended length, cylinder (body) diameter and rod diameter. The stroke length is the distance between the compressed and extended length. The cylinder diameter is an important factor in determining whether the cylinder will fit into the desired location, and how the shock absorber will be affixed to the adjacent structure.

RO LS O F AC E CO NT

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Shock Absorbers FPW Hdbk 2017 V3.indd 123

HOW DO YOU SIZE A SHOCK ABSORBER? Sizing a shock absorber is relatively straightforward.

Several reputable manufacturers offer online calculators, but here are a few guidelines to quickly come up with suitable products for a given task.

Manufacturers’ web sites and data sheets typically list products by parameters like stroke, usable velocity range, maximum amount of energy that can be absorbed per cycle, maximum force capacity, and the maximum propelling force it can handle, as well as dimensions and other relevant details. Before sizing a shock absorber, however, users first need to determine the relevant operating conditions, including the weight and velocity of the moving mass and how frequently the shock is loaded. For simplicity, let’s look at a linearmotion application and use Imperial units for the calculations.

Determine kinetic energy in the system from:

Ek = W/(722)(V2) where Ek = kinetic energy, lb-in.; W = weight of moving mass, lb; and V = velocity of moving mass, in./sec. This equation represents the amount of kinetic energy that the shock absorber will convert to thermal energy on each impact.

Next calculate the work energy in the application, defined as the amount of energy an external device generates to move the load:

Ew = Fd(S) where Ew = work or drive energy, lb- in.; Fd = drive force, lb; and S = stroke of the shock absorber, in. Note that Fd should not exceed the unit’s maximum rated propelling force.

If it does, select a larger size and recalculate the work energy.

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The next step is to calculate the total energy, Et (lb-in.) per cycle, shown as:

Et = Ek + Ew Again, if this exceeds the model’s energy-absorbing capacity, select a larger unit and recalculate the work energy. Otherwise, the shock’s temperature may rise beyond rated limits and critical internal components like hydraulic seals could fail.

If the application uses more than one shock absorber, divide the total energy Et by the number of shocks to determine the total energy per shock.

Then determine the total energy a unit must convert in one hour.

That’s because even though a shock might absorb an acceptable amount of energy in a single impact, it might not be able to dissipate the generated heat if the cycle rate is too fast. Here, multiply Et by C, the total number of cycles per hour: Etc = Et(C)

The device’s hourly capacity must exceed this calculated amount.

If not, choose a larger absorber (and recalculate Ew if the stroke changes) or, possibly, add an external oil tank or a cooling device to help dissipate the heat.

Finally, consider the shock force, Fp (lb) in the application.

Shock force, in essence, is the resistive force required by the shock absorber to stop the moving load:

Fp = Et/(Sη)

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where S = the stroke of the shock absorber and η is the unit’s damping efficiency. While the efficiency can vary with the type and model, 85% efficiency is a good baseline for typical industrial shocks.

This is important when selecting a suitable shock absorber because the machine structure and mounting must have the necessary strength and rigidity to withstand the transmitted force.

The efficiency of various units is measured by evaluating how much of the shock’s stroke is used for actual damping of the motion.

Shock absorber efficiency increases as more energy dissipates over the stroke, and more-efficient products typically yield the lowest shock forces for a given stroke.

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UNLESS OTHERWISE SPECIFIED

SIGNATURES

DATE

00 O 2.

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24AUG2016

DSGN

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24AUG2016

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25AUG2016

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LDP MES 24AUG2016

23435 INDUSTRIAL PARK DR. FARMINGTON, MI 48335

TITLE:

INSTALLATION

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ALL INFORMATION DISCLOSED BY THIS DOCUMENT IS CONSIDERED CONFIDENTIAL AND PROPRIETARY BY ACE CONTROLS, INC. & DESIGN, MANUFACTURE, USE, REPRODUCTION & SALE RIGHTS ARE RESERVED BY ACE CONTROLS.

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The above calculations help ensure that a given shock absorber meets all operating parameters. Again, make certain2.7that the selected model matches 4 2X 2-1/2-12 UN-2A 69.6 mm FLATS or exceeds requirements for energy absorbed per cycle and per hour, as well as the shock force. Otherwise, it will likely cause damage or fail prematurely.

1.00 25.4 mm FLATS

Shock absorbers may be made from aluminum, steel and stainless O steel, or thermoplastic. Steel is used when high strength is required. The other materials provide varying balance between strength, weight, corrosion resistance and cost.

Additionally, the rods can be treated with chrome to provide corrosion resistance and increase surface hardness. Nitride will increase the hardness by introducing nitrogen into the outer surface of the rod.

There are also a number of important shock absorber features to consider. Adjustable shock absorbers allow the stiffness of the response to be monitored and fine-tuned. This is usually accomplished by adding or removing hydro/pneumatic medium from the shock absorber by way of a valve. Locking ALL INFORMATION DISCLOSED BY THIS DOCUMENT IS capability allows the position of the rod to beCONSIDERED fixedCONFIDENTIAL at a given position.

AND PROPRIETARY

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Download any product for free from our CAD database.

ALL INFORMATION DISCLOSED BY THIS DOCUMENT IS CONSIDERED CONFIDENTIAL AND PROPRIETARY BY ACE CONTROLS, INC. & DESIGN, MANUFACTURE, USE, REPRODUCTION & SALE RIGHTS ARE RESERVED BY ACE CONTROLS.

2.74 UNLESS OTHERWISE SPECIFIED 69.6 mm FLATS

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Get assistance selecting the right product for your needs.

IMAGE COURTESY OF ACE CONTROLS

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.37 9.4 mm

FLUID POWER WORLD

2.74 69.6 mm FLATS

125

2X 2-1/2-12 UN-2A

SIZE

C

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SCALE:

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800-521-3320 www.acecontrols.com

O UNLESS OTHERWISE SPECIFIED

DIMENSIONS ARE IN INCHES DIMENSIONS ARE NOMINAL

03 2X 1.26.

2 mm

TITLE:

TREATMENT:

75 O .19.

0 mm 6/21/17 6:09 PM

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RETAINING

RINGS

R

etaining rings are engineered components used to hold many types of assemblies together. They are precision engineered to accurately position, locate and retain parts on shafts or in bores.

The rings are installed into a groove, and all the other components of the assembly sit against and are retained by the ring. They help reduce costs by eliminating threading and other machining and offer reduced weights and sizes.

Retaining ring designs include spiral, tapered and constant section. Spiral rings do not have ears or lugs to interfere within the assembly like standard stamp rings. Tapered section rings feature compressible lugs to give them a circular shape in the groove, allowing them to grip tightly along the edge. Constant section retaining rings are best suited for heavy-duty applications.

They feature a uniform, constant section with no change in width throughout their entire circumference. Some specialty designs exist as well, including ones that feature a shallow groove for use on thinwalled sections of components.

In hydraulic systems, spiral retaining rings are used most often on the cylinder, particularly to retain the seal packing in cylinders.

Retaining rings replace machined “steps” to retain the packing.

Additionally, retaining rings are also found in hydraulic couplers, hydraulic pumps and other hydraulic components that need secure fastening. In pneumatic systems, they can be found in actuators, compressors, couplers and so on, to help fasten components as needed.

Standard materials for retaining rings include carbon steel, carbon spring steel, 302 stainless steel and 316 stainless steel. Other available materials include phosphor bronze, beryllium copper, Inconel, Elgiloy and Hastelloy.

A different type of ring design, called a “scraper ring,” is also used in hydraulic cylinders.

These rings help keep debris out of the seal portion of the cylinder to extend the life. They are typically made out of a soft material, such as beryllium copper, so they can hone to the shaft.

HOSE ASSEMBLIES use retaining rings for secure fastening. Images courtesy of Smalley Steel Ring

Specifying rings for fluid power systems

The most important detail to know when selecting a retaining ring for your application is to know if the groove for the ring will be located on a shaft (external) or in a bore (internal) and then specify the diameter of that shaft or bore. Additionally, if the ring will be subjected to axial thrust loads, the ring specifications need to be checked to determine whether a light-, medium- or heavy-duty ring is required. Finally, the material needs to be selected based on what type of corrosive media the ring will be subjected to.

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HOW DO I KNOW IF I’M TALKING TO AN ENGINEER OR A SALESMAN? Ask Smalley. We have nothing against sales people. But when it comes to differentiating Inconel from Elgiloy or overcoming dimensional variations within a complex assembly, wouldn’t you rather work with an engineer?

Our customers would. That’s why they collaborate directly with our world-class team of Smalley engineers—experienced professionals whose only focus is helping you specify or design the ideal wave spring, Spirolox® retaining ring or constant section ring for your precision application.

Smalley wave springs reduce spring operating height by 50%, saving space and weight, fitting tight radial and axial spaces. We offer more than 4,000 stock sizes in carbon and stainless steel.

Visit smalley.com for your no-charge test samples.

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Coil Spring

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ad index ACE Controls Inc....................................................... 125 Aggressive Hydraulic .................................................. 23 AIGNEP USA ......................................................... 51, 66 AMETEK ......................................................................30 Anchor Fluid Power .................................................... 56 Anderson Metals Corporation, Inc............................. 35 AutomationDirect.......................................................... 1 AVENTICS Corporation ..............................................80 Balluff ......................................................................... 122 Beswick Engineering ................................................ 113 Brennan Industries ...................................................... 37 Brogan & Patrick Manufacturing ............................... 71 C.matic......................................................................... 25 CEJN Industrial............................................................ 47 Clippard Instrument Laboratory, Inc......................... BC Compact Automation Products (ITT Enidine) .......... 89 Daman Products .........................................................60 Deublin Company....................................................... 77 Dura-Bar.....................................................................100 Eaton Hydraulics.......................................................... 65 Elesa U.S.A. Corp. ...................................................... 73 Essentra Components .................................................. 9 FABCO-AIR, Inc........................................................... 21 Festo Corporation ...................................................... 11 Flaretite, Inc................................................................... 8 Flow Ezy FIlters ............................................................. 5 FluiDyne Fluid Power.................................................. 67 Freudenberg Sealing Technologies .......................... 79 Gefran .......................................................................... 93 HAWE Hydrauliks ....................................................... 69 Hengli America Co. .................................................... 27 Hercules Sealing Products.......................................... 78 Higginson Equipment ................................................ 87 Hy-Pro Filtration.......................................................... 34 HYDAC International .................................................. 17

Hyde Tools, Inc. .......................................................... 25 HydraForce................................................................ IBC J.W. Winco, Inc. ........................................................ 104 Kawasaki Precision Machinery (U.S.A) Inc............... 101 Kraft Fluid Systems, Inc. ............................................. 44 Kuriyama of America, Inc............................................50 Kurt Hydraulics ............................................................ 19 Lillbacka USA, Inc........................................................ 49 Main Manufacturing Products.................................... 38 Midland Metal Manufacturing ................................... 55 MOCAP ..................................................................... 104 MP Filtri USA Inc. ........................................................ 29 Murrelektronic ............................................................ 12 NOSHOK, Inc...............................................................10 Peninsular Cylinder Co............................................... 95 Permco, Inc.................................................................. 64 PHD, Inc .............................................................. 99, 109 Price Engineering ............................................... 70, 117 Prince Manufacturing Corporation.......................... 107 ROSS Controls........................................................... 119 Rota Engineering Ltd. ................................................ 83 RYCO Hydraulics, Inc. ................................................ 52 Servo Kinetics ............................................................. 91 SIKO Products ............................................................. 82 Smalley Steel Ring Company................................... 127 Stauff ............................................................................ 33 Super Swivels............................................................... 57 Taimi Hydraulics ........................................................ 121 The Lee Company..................................................... 103 Tompkins Industries, Inc..........................................3, 40 Trelleborg Sealing Solutions ...................................... 75 Veljan Hydrair Inc. ....................................................... 63 World Wide Fittings ................................................... 13 Yates Industries, Inc................................................... IFC Zero-Max ..................................................................... 39

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