Fluid Power Handbook 2016

www.fluidpowerworld.com

June 2016

2016 Fluid Power

HANDBOOK

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Fluid Power

HANDBOOK

FLUIDPOWERWORLD.COM

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FLUID POWER WELCOME

to the fifth edition of the Fluid Power Handbook. This year, we have again changed the look and feel of this issue, and we welcome your feedback on these changes. 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 the the impact of efficiency and green engineering on fluid power technology.

In addition to the print magazine, Fluid Power World has been growing online. You can search our unique, high-value editorial content online at www.fluidpowerworld.com for the latest and greatest news, application stories, technical features and more. But we’ve also expanded our Tips sites to cover niche areas that are important to you: PneumaticTips.com, MobileHydraulicTips. com, HoseAssemblyTips.com, and coming soon, SealingandContaminationTips.com.

Fluid power is, quite simply, the use of a liquid or a gas to move a load and accomplish work. The technology is used in countless industrial as well as everyday environments, from automobile assembly lines to passenger airliners and from packaging equipment to huge earth-moving pieces of equipment. Fluid power can be used for extremely tough tasks, such as boring new underground mining tunnels—or for very gentle operations, such as lifting and stacking large sheets of glass. Through various components and actuators, fluid power systems can push, pull, lift, rotate or grip almost any load.

While fluid power has been around for centuries, its heyday was undoubtedly sparked by the end of World War II, when countless soldiers returning to their home countries brought back their mechanical expertise and real-world experiences. The founding of many famed domestic fluid power component manufacturing companies—many still around today—can be traced to this era. In the years thereafter, some of the associations that have so shaped the industry were begun, most notably the National Fluid Power Association (1953) and the International Fluid Power Society (1960).

Fluid power systems are comprised of a myriad of components, including pumps, cylinders, valves, hose, fittings, gauges, sensors, filters, seals and

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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 will 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 2012 study by the Department of Energy discovered that between 2.0% and 2.9% 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 averages 22%.

While that percentage is low, it also illustrates 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 a lot in store for the technology. We continue to see hydraulic and pneumatic components become more Internetfriendly, wireless and capable of being monitored and controlled from distant locations. We’re eagerly watching to see what the coming years will bring to the world of fluid power.

PAUL HENEY EDITORIAL DIRECTOR

www.fluidpowerworld.com

6/16/16 2:49 PM

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INSIDE

pneumatictips.com • mobilehydraulictips.com • hoseassemblytips.com • sealingandcontaminationtips.com

02 Fluid Power Overview 06 Fluid Power Fights Back 16 Gauges 19 Fluid Power Safety 23 Shock Absorbers and Linear Dampers 27 Retaining Rings

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28 Pneumatics Overview 30 Air Brakes and Clutches 32 Air Dryers 35 Air Springs 36 Compact Cylinders - Actuators 38 Cylinders - Pneumatic Actuators 42 Cylinder Slides/Rodless Cylinders - Pneumatic 44 FRLs 47 Grippers - Pneumatic 51 Miniature Pneumatics 54 Pneumatic Tubing 58 Vacuum Components 61 Valves - Pneumatic 66 Hydraulics Overview 68 Accumulators 71 Bar Stock 74 Clutches and Brakes 75 Cylinders - Hydraulic 81 Filters - Hydraulic 84 Filtration Systems 86 Fittings and Flanges - Hydraulic 90 Fluids - Hydraulic 93 Heat Exchangers 96 Hose - Hydraulic 99 Hose Couplings - Hydraulic 103 Manifolds - Hydraulic 106 Motors - Hydraulic 110 Pressure Transducers 113 PTOs and Gearboxes 116 Pumps - Hydraulic 119 Replacement/Rebuilds 123 Rotary Actuators - Hydraulic 126 Seals 130 Valves - Hydraulic www.fluidpowerworld.com

6/17/16 4:58 PM

JUNE 2016 • vol 3 no 5 • www.fluidpowerworld.com

EDITORIAL Editorial Director Paul J. Heney pheney@wtwhmedia.com @dw_editor Managing Editor Mary Gannon mgannon@wtwhmedia.com @dw_marygannon Associate Editor Mike Santora msantora@wtwhmedia.com @dw_mikesantora Assistant Editor Michelle DiFrangia mdifrangia@wtwhmedia.com @wtwh_michelle Contributing Editor Josh Cosford @FluidPowerTips Contributing Editor Ken Korane kkorane@wtwhmedia.com @fpw_kenkorane Contributing Editor Carl Dyke @carlindustry

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SALES VP, Publisher Michael Ference 408.769.1188 mference@wtwhmedia.com Tom Lazar 408.701.7944 tlazar@wtwhmedia.com @wtwh_Tom Jim Powers 312.925.7793 jpowers@wtwhmedia.com @jpowers_media Neel Gleason 312.882.9867 ngleason@wtwhmedia.com @wtwh_ngleason Courtney Seel 440.523.1685 cseel@wtwhmedia.com @wtwh_CSeel Mary Ann Cooke 781.710.4659 mcooke@wtwhmedia.com Michelle Flando 440.670.4772 mflando@wtwhmedia.com @mflando

With over 3 billion parts installed and failure rates less than 1PPM, SFC KOENIG is recognized for excellence in sealing and flow control. The original inventor of the pre-assembled metal-to-metal seal solution, we offer: • a variety of sizes, material types and pressure ratings • engineering support throughout the design process • the easiest method of sealing drilled holes, with faster installation and fewer errors • proven effectiveness in a diverse variety of hydraulic, automotive and precision high performance applications.

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FLUID POWER WORLD does not pass judgment on subjects of controversy nor enter into disputes 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 publication. Every effort is made to provide accurate information.

However, the 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.

FLUID POWER WORLD does not endorse any products, programs, or services of advertisers or editorial contributors. Copyright© 2016 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 systems, without written permission from the publisher.

Contact us to review your application and request samples.

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SUBSCRIBER SERVICES: To order a subscription or change your address, please visit our web site at www.fluidpowerworld.com

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FLUID POWER WORLD is published by WTWH Media, LLC, 6555 Carnegie Avenue, Suite 300, Cleveland, OH 44103.

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FLUIDD POWERR FIGHTS BACK Mike Santora • Associate Editor

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The demand for green engineering is running through all technologies as of late, fluid power included. And yet, hydraulics and pneumatics are still battling their reputation as a 19th century anachronism. So what’s helping drive fluid power to its greener future? Naturally, the answer is more monetary than Mother Nature.

www.fluidpowerworld.com

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IN MANY

BY DYNAMICALLY CONTROLLING the pump speed to match the duty cycle, the Sytronix system from Bosch reduces the energy required to supply hydraulic power to the press, thereby reducing energy consumption. The system also reduces noise and waste heat generated by the hydraulic system.

www.fluidpowerworld.com

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ways, green fluid power technology is just starting to hit its stride. Twenty years ago, a green, efficiency-minded design was little more than a feather in the cap of a fluid power system. If the fluid power industry wasn’t actively fueling its dirty technology reputation, it wasn’t working too hard to fix it either.

That’s no longer the case as of the last few years. Those considering fluid power design solutions won’t have it any other way.

For a technology that treads with such a heavy industrial and environmental footprint, it’s easy to assume high-level oversight would be the norm. But it isn’t. The choice to design smaller, more efficient fluid power systems has largely been left to the will of manufacturers. For the moment at least, even Tier IV and Tier V emissions standards are having little impact on design concerns. The legislative push that nudged other industries to design green has not been nearly as strong in fluid power. It’s as if industry outsiders view fluid power as so technologically crude already, improvements would not make much of a difference. Fluid power professionals know that’s not true, but the idea is fairly common. Kevin Gingerich, manager, Communications and eBusiness at Bosch Rexroth, said that hydraulic power is far from the power source relic that it’s often viewed as.

“People seem to think of hydraulic technology as a technology from the past: Not intelligent, not precise, capable of brute force only, but today’s hydraulic drive systems are precise, compact and intelligent,” he said.

Gingerich also explained that hydraulic drive systems can position axes to a few micrometers. They can also be situated in smaller spaces because they introduce less heat into a system. “Really, the more you know about modern hydraulics, the more you realize just how modern, capable and relevant this amazing technology is,” he said.

While regulatory bodies may be silent on the need to increase fluid power’s friendliness to the environment, fluid power end users are not. Frank Langro, director of marketing and product management for Festo, said consumers began sending consistent feedback for smaller, more efficient designs about five years ago.

“The bigger, well-known companies were in the lead on this; dollars and cents drove the demand,” Langro said.

“All companies want to be consumer friendly. They want to take steps to create more sustainability and protect the environment as well.”

Langro added an example, stating that if you save three cents on every widget you produce, and you’re producing millions of widgets, well, that’s quite a bit of money.

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Gingerich also echoed this idea of increased consumer expectations. “Twenty years ago, nobody was asking for increases in efficiency at the level that customers are asking now.” He also said the competition for design improvements has gone global.

“Green energy needs have created an innovation revolution that is seeing new technologies being developed all the time,” Gingerich said. “It has also created a much higher level of expectation among customers, especially in Europe where energy costs are much higher.”

Out of the West, China is also heavily involved with research and development. Back from a recent Asia trip, Dr. Monika Ivantysynova, director of the Maha Fluid Power Research Center at Purdue University, said students overseas are looking to innovate and make a big impact on the industry.

“You know what is going to help green engineering in fluid power? Competition. I just returned from China and I gave talks there in front of graduate students,” Ivantysynova said. “I think the global competition will help make faster progress. We need new engineering. We need a new set of architectures for all kinds of fluid power systems that are different, but not too much different. I personally think fluid power has a bright future but we can do much better.”

EFFICIENCY Langro spoke about the NFPA’s decision a few years ago to take a hard look at the challenges fluid power would be facing in the future. The NFPA’s road mapping process lead to a predictable destination: efficiency.

“It was pretty much unanimous that you saw efficiency as the leader. Compact and intelligent products that are easy to use, intelligent, plug-and-play applicable and more diagnostic, were needed,” he said.

One of the initial hurdles though, was that end user demands had not reached the fever pitch needed to spark action.

“OEMs weren’t always on board,” Langro said.

“Producing the lowest cost machine and adding new developments that could create a more efficient machine just wasn’t their priority.”

Some companies were thinking long term regardless of demand. Nic Copley, VP, Technology and Innovation for Parker Hannifin, said, “The general awareness of the need to be more conscious of green issues has been going on for quite a few years. I first noticed it back in the ‘90s when there was a big push to start moving away from systems that had oil lubrication in them. A lot of work around development materials was done just to make sure that you could develop pneumatic systems that would run oil-free

THE ENERGY EFFICIENCY module is a completely new, intelligent combination of service units, sensors and fieldbus technology from Festo.

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or lubrication-free for the lifetime of the product. I think it's been a long, incremental, step-by-step pathway, but probably the last five to eight years has been much more focused.”

Copley went on to remind us that people have been forecasting the demise of fluid power’s relevance for years. But we haven’t seen that happen. The technologies that were supposed to put fluid power out to pasture are now working with it. Copley said he expects hybrid technology to be influential in solving future fluid power efficiency issues.

“I expect we will see a lot more combining of technology. I think that in the fluid power world, we'll probably see a lot more merging between the electro-pneumatic and the hydraulic worlds,” he said.

“We’ll see the benefits of all the technologies converging to make sure we have far more effective and efficient systems in the future.”

For pneumatics in particular, the efficiency of electrical input power to compressed air outlet power continues to keep compressed air costs up. Vicki Gonzalez of Nexmatix noted that the pneumatics industry has been around for more than a century, but many of the fundamental products, like directional control valves, were designed in a time when companies did not think about the cost of energy to compress air.

“A great example is the directional control, which is designed to exhaust air during every stroke,” she said. “The DOE estimates that manufacturers today spend $8.8 billion on electricity to compress air and these pneumatic systems have efficiencies of less than 20%. That’s over $6 billion dollars we could save if we started designing for efficiency.”

As for hydraulics, leakage remains public enemy number one.

Leakage poses environmental, safety and efficiency threats. The

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IN THIS IMAGE we see Festo’s Valve Terminal MPA – Serial Communication for Comprehensive Diagnostics Concepts.

primary solution is preventative maintenance—creating smarter, more efficiency-minded products. At Rexroth, for example, it has even become a company-wide initiative.

“We call it 4EE, or Rexroth for Energy Efficiency. It serves as the basis for our energy advice for industrial companies and for the development of energy-efficient solutions in optimizing our own operations. It’s a systematic initiative to create energy efficiency across all drive and control technologies, with four levers that cover the spectrum of opportunities in efficiency: Energy System Design, Efficient Components, Energy Recovery and Energy on Demand,” said Gingerich.

Overall, in industrial production, companies are optimizing machines and processes by taking advantage of a diversity of technologies. The focus of which is mostly the reduction of power consumption and the increase of operating efficiencies. The focus of how this is done varies from company to company but most of the talk revolves around a few concepts. The creation of lightweight components, for example, is popular for creating cleaner and more efficient operation and a transition toward hybrid technologies.

Mike Tuohey, sales and marketing communications manager at Piab, also said this hybridization is crucial in most companies’ green engineering strategy. “For example, as a design focus, now electronics and pneumatics are both improving operating efficiencies. This addresses many of the concepts of green engineering.”

Additionally, companies are designing electronic controls that are better than ever at monitoring a system and allocating energy on an as-needed basis. Integrated sensor technology is the focal point of predictive maintenance. Langro said that this type of technology allows for the monitoring of pressure flow and hazardous system abnormalities.

“We released a SHOWN HERE is Piab’s piCLASSIC vacuum product that is part of pump. These vacuum our MS series of air supply pumps are 40% more units and it is an energy energy efficient compared to a rotary efficiency module. It's an style pump. intelligent on-off valve with

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a flow sensor and pressure sensor.

If the unit notices that the machine is in an idle state, the on-off valve adjusts accordingly,” Langro said.

“This way there is no unnecessary air consumption through leaks or any other losses in the machine.”

LEADING THE WAY There are several new concepts leading the way in green fluid power. For example, Rexroth’s Sytronix system is, again, another development focusing on the “energy on demand” concept.

It provides power to a hydraulic pump only during active cycles— as opposed to constant pressure, which requires a constant energy supply. With conventional hydraulic systems found on many industrial machines, an electric motor typically runs at a constant speed to power the hydraulic unit, regardless of whether pressure is needed at that moment. Therefore, the motor is always running and consuming energy even if the machine is idle.

The Sytronix pump drive regulates the speed and consequently, the energy consumption of the electric motor according to machine demands. Depending on a machine’s cycle characteristics and power rating, the pump drives achieve a 30 to 80% energy savings.

Nexmatix’s valve technology is helping engineers in pneumatic applications. The valves use designs that act as plug-and-play replacements for standard off-theshelf valves. These plug-and-play alternatives typically recycle 30% of the compressed air used in pneumatic actuator applications without compromising actuator

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function. The reduced compressed air usage directly correlates to less energy cost. This valve technology can also be applied to the majority of 5-port/3-position and 5-port/2-position valve lines with little cost difference.

As one might expect, Industry 4.0 advances are also on the horizon in fluid power. Festo plans to build on its valve manifold system by integrating OPC UA into the platform.

Piab has noticed the Big Data trend in fluid power as well. “Adding data measuring capabilities at the component level is important now.

Now there is the need for tapping into the Internet of Things and Big Data revolution. We’re seeing the need to provide the ability to monitor components and develop data to support improved production and efficiency,” said Tuohey.

Companies are continuing to zoom-in on fluid power efficiency at the component level. Now, that focus has translated into a global shift in how we think about fluid power designs. Langro noted, “The saying is if you don't measure it, nobody will pay any attention.” The green energy trend has clearly gained the attention of fluid power component manufacturers. Not only are they paying attention, they are working harder than ever to bring noticeable change to the industry. Designing for efficiency is good for both the green of eco-friendly technology and the green of collective company coffers. Regardless of motive, fluid power reaps the benefits.

“People seem to think of hydraulic technology as a technology from the past: Not intelligent, not precise, capable of brute force only, but today’s hydraulic drive systems are precise, compact and intelligent.”

- Kevin Gingerich

MANAGER, COMMUNICATIONS & eBUSINESS BOSCH REXROTH

www.fluidpowerworld.com

6/16/16 3:00 PM

Sytronix: Put the best of hydraulics and electronics to work for you.

Your move?

We’re in.

Rexroth’s Sytronix Variable-Speed Pump Drives combine the reliability of powerful hydraulics with the efficiency of electronics for energy efficiency that pays off quickly. Along with energy savings up to 80%, the many benefits of these drives include lower noise emission (as much as 20dB), a more compact design and enhanced control functionality and accuracy. With Rexroth, you also get the support and expertise of a truly global company. If you’re ready for a powerfully efficient solution, make your move to Sytronix.

www.boschrexroth-us.com/sytronix For expert advice call 1-800-REXROTH (739-7684) or email us at info@boschrexroth-us.com

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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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New twist on pneumatic muscles p.32

From the basic to advanced: selecting directional control valves p.38

Industrial variable speed drives p.48

February 2016

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More efficient

mobile hydraulic

troubleshooting PAGE 42

Electrohydraulic controls

improve ship stability

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p.40

Understanding proper

air preparation p.48

Spotlight on offline filtration

p.56

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April 2016

Hydraulics define

R

Ag machinery relies on the versatility

of hydraulics p.26

Twelve tips for better cylinder selection

p.34

the evolution of construction machinery

The hydraulics of injection molders

p. 48

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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

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gauge designs continue to evolve. The evolution of pressure gauges for fluid power applications has, generally, been 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 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 zero-referenced 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 zero-referenced against ambient pressure and is used in 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

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

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.

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.

The pressure range at which the gauge will be working is a primary selection factor for the type of material

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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.

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, it’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 A LOOK at the internal gauge failure. functions of a bourdon tube pressure guage.

PHOTO COURTESY OF NOSHOK

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FLUID POWER SAFETY

FLUID POWER

safety

SAFETY

is a critical aspect to any fluid power system, not just from the basic level of keeping components plumbed properly, but also in overall levels of machine safeguarding. It is critical to evaluate the entire system, 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 13849-1:2006, 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 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

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SAFETY CONTROL SYSTEM includes poppet and proportional valves. Image courtesy of Ross Controls

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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.

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When fluid is well controlled through some sort of metering device, such as a flow control, counterbalance or proportional valve, the actuators and loads are prevented from running away. For example, when a cylinder is installed rod down, and loads are typically in tension, it is often installed with a meter-out configuration to prevent the load from running away with the cylinder. Although it is a safe method to prevent a runaway load, rod-side pressure intensification can be a risk. If intensification blows out piston seals, then the load can drop regardless, which is counterproductive to the meter-out philosophy.

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Indexing and positioning elements

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Leveling elements and supports

Safe hydraulics involves the consideration of a safe machine, first and foremost, and this is regardless of the form of motivation.

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.

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Operating elements

HYDRAULIC SAFETY

Control elements

Hinges and connections

Rotary controls

To avoid the pitfalls of metering out, a counterbalance valve can be used instead. The counterbalance valve is considered a pressure valve, but in reality, it controls the speed of an actuator. These valves ensure a cylinder will only move as rapidly as pump flow is dictating, regardless of load-induced pressure or intensification.

If a pilot signal is not seen from the opposite work port, the valve remains shut, preventing the load from dropping. Often mounted directly to the cylinder work port, the counterbalance valve also prevents dropping loads when hose or tube failures occur.

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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.

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Fluid Power

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.

Relief valves are used to limit pressure, preventing damage and limiting power requirements. A relief valve is often used

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.

Two general abnormal conditions can affect valve safety.

The first is similar to an electricalcontrol 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, 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 B155.1-2011 standard requires diminished performance 22

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to control main system pressure or isolated sub-circuits. Some circumstances require sub-circuits to operate at different pressures than others, and the pressure-reducing valve can achieve this. It limits pressure downstream of itself, and in some cases, can be a reducing-relieving valve as well. This type of valve can control downstream pressure spikes or thermal expansion, where heated fluid increases in pressure. Some hydraulic systems can use a combination of various pressure valves to ensure circuit pressure is safely limited at every part of the machine.

So by controlling both pressure and flow in a hydraulic system, best practices of safety can be observed. A machine failure is unsafe in the best scenarios, so ensuring a hydraulic circuit is designed for safety will prevent injury to operators.

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 actuates abnormally, the second one continues to function and redundancy is lost. The circuit doesn’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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SHOCK ABSORBERS & LINEAR DAMPERS

SHOCK ABSORBERS &

linear dampers Shock absorbers and dampers work to provide smooth deceleration of a given payload. A pneumatic or hydraulic shock absorber will use fluid or gas power for deceleration, and a spring to return the piston to its initial position. However, linear dampers typically do not. They facilitate the smooth closing of hinged doors, such as garage doors.

Important parameters for the proper selection of a linear damper can be broken up into general, dimensional, performance, material and features.

IMAGES COURTESY OF ACE CONTROLS

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WHAT ARE THE SELECTION CRITERIA? The general criteria when selecting a shock absorber are the damping method and damping direction. The damping method can be elastomeric, pneumatic or hydraulic. The damping direction can be either in compression or extension.

When choosing a shock absorber, one must specify the stroke length, compressed length, extended length, cylinder 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 housing, or what housing to use. The rod diameter is used to determine how the shock absorber will be affixed to a given component.

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Deceleration & Vibration

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Fluid Power

Motion Control Custom control of hand forces

Safety Products Protection for all machine designs under any condition

Vibration Control Isolate unwanted vibrations

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Technology:

The performance of the shock absorber can be determined from the maximum force, energy per cycle and maximum cycles per minute. The maximum force, also called the P1 force, is the greatest rated force that the shock absorber can supply. The energy per cycle is the amount of energy that the component can absorb, and will also depend on the stroke length. The maximum cycles per minute is a measure of how quickly the shock absorber can return to its unstretched length.

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

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.

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RETAINING RINGS

RETAINING

rings

RETAINING

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 thin-walled 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.

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.

PHOTO COURTESY OF SMALLEY STEEL RING

AXIAL PISTON PUMPS use Spirolox retaining rings to hold the bearing and spindle in the housing. Photo Courtesy of Smalley Steel Ring

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pneumatics OVERVIEW

PNEUMATICS

is the technology of compressed air, but in some circles today, manufacturers choose to 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 application-specific 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 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.

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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 a great deal. 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 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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AIR BRAKES

& clutches AIR

IMAGE COURTESY OF EATON

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brakes serve as the main brake to provide parking and E-stop braking functions in a variety of applications, including draglines, grinding mills and shovels in mining, drawworks for drilling rigs, and marine winches.

They provide the quick actuation and stopping times that are necessary in emergency situations or to stop large industrial machines for routine maintenance. They are actuated with compressed air rather than hydraulic fluid or oil.

Air clutches help transfer torque between the engine and transmission, rather than providing stopping power. They rely on compressed air for engagement and disengagement. By controlling the air pressure applied to the clutch, you can achieve smooth controlled deceleration and even rapid stops. In addition, they offer quick and safe disengagement.

Air brakes are rotors or discs attached to a driveline shaft that, when the brake is applied, bring the rotating machinery to a stop. When the brake is applied, a piston is pneumatically actuated, forcing the friction pads to clamp the rotor discs. Several factors go into sizing an air brake; most notably, the required horsepower, stopping time and total energy are musts for selecting the right brake for the application.

Drum-style constricting clutches are pneumatically actuated and are used to connect and disconnect to rotating shafts to transmit power and motion.

Air-actuated brakes and clutches offer a number of advantages, such as fast response rates and safe operation. Since they are pneumatically actuated versus hydraulically actuated, the risk of spills, leaks and fires is eliminated. Leaked or spilled oil can create an incident that can take hours or days of downtime to clean up, creates a hazardous working environment and puts end users at risk of expensive environmental fines.

Unlike electrical options such as variable frequency drives and other AC drive motors, air brakes are capable of not only stopping motion, but also manual operation in the case of a power failure. Electrical options are usually more complex as a result, and may require some type of a fail-safe brake to slow or stop motion in case of power outage.

To meet the needs of more complex machinery, newer brakes and clutches can include sensing and monitoring technology to make the equipment more responsive and able to provide feedback or warn users of problems. Solutions on the market today include brake and clutch products with embedded temperature sensors or slip detection.

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AIR

Fluid Power

dryers

IN PNEUMATIC

systems, compressors draw in ambient air that always contains some amount of water vapor. The process of compressing air to around 100 psi raises the air’s temperature, but also dew point and water-holding capacity.

Any subsequent cooling downstream will likely cause some water vapor to condense. Liquid water in pneumatic systems creates problems such as corrosion and bacteria growth, which, in turn, leads to sluggish controls or component breakdowns. Thus, dryers are often needed to mitigate water issues.

In a typical system, air flows from the compressor to an aftercooler that lowers the temperature, causes condensation and removes the majority of water. Sometimes coalescing filters are installed to remove additional water.

But if a circuit requires further treatment, air dryers are a must. They remove most or all of the water in compressed air before it reaches critical components or processes. Here’s a look at the major types.

Refrigerated dryers remove water by cooling the compressed-air temperature and causing condensation. An internal moisture separator collects the liquid water and sends it to a drain. Refrigerated dryers typically generate air with pressure dew points between 35 and 40° F. They tend to be used in general plant operations. They may not be suitable for more-critical processes that demand extremely dry air, and they aren’t designed for circuits that see subfreezing temperatures.

Refrigerated dryers are considered fairly economical to purchase and operate. They fall into two categories, cycling and non-cycling. As the names imply, one type runs intermittently and the other runs continuously. Users should consider cycling dryers, which only power up to meet demand—and thus reduce electricity consumption and energy costs.

INGERSOLL RAND’S heat-ofcompression desiccant dryers are highly energy efficient.

INGERSOLL RAND’S heated desiccant dryers have an external heat source.

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Desiccant dryers work on a different principle—they adsorb moisture from the air stream and onto a desiccant material in a reversible process. They produce low dew points, so they are a good choice in subfreezing conditions or when processes require extremely dry air. Two types are heatless and heated.

Heatless desiccant dryers house a desiccant material in two adjacent tanks, called the drying tower and regenerating tower. Moisture-laden compressed air flows into the drying tower, where it passes over and binds to the porous desiccant. Extremely dry air, with pressure dew points of –40 to –100° F, exits the dryer.

This adsorption process also generates heat, typically raising the air temperature by as much as 20° F. To remove

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water from the desiccant, about 15% of the dry air from the first tower is ported to the second, regenerating tower. There, the dry compressed air expands to atmospheric pressure as it travels through the desiccant and pulls water off the desiccant, aided by the higher air temperature due to the heat of adsorption.

Moist air is then discharged from the system. The dryers cycle between drying and regenerating operations at regular intervals, so one tower always dries the incoming air.

Heated desiccant dryers, as the name implies, have a heater in the circuit. Much like the heatless version, desiccant in one tower removes moisture from the flowing air. Resulting pressure dew points again can range from –40 to –100° F. A second tower regenerates the spent desiccant. Valves divert approximately 8% of the air exiting the drying tower and pass it through a heater. This hot, dry air then passes over the desiccant in the regenerating tower, freeing the previously captured moisture. Moist air is then discharged, usually through a muffler, to the outside.

When weighing the virtues of heatless versus heated desiccant dryers, keep in mind economics. The cost of generating compressed air can be sizeable, and heated dryers use about 50% less compressed air for regeneration. On the other hand, heaters can require a lot of electricity. Thus, users should review the specifics of an application to determine which type costs less to operate.

A related type of desiccant dryer is the heat-of-compression dryer. These are special versions of desiccant dryers that reuse heat generated by oil-free air compressors and are highly energy efficient.

In HOC dryers, hot (often above 300° F) high-pressure air from a compressor first SECOTEC SERIES refrigeration dryers from Kaeser travels to the regeneration Compressors are rated for flows to 14.3 m 3 /min.

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tower and frees moisture from the desiccant. Flow then exits and passes through an aftercooler, reducing air temperature to around 100° F and removing some water.

Cool air then runs through the drying tower and exits with dew points of approximately –40° F. HOC dryers coupled with oil-free compressors generate high-quality compressed air free of water and oil, and are suitable for food, beverage and pharmaceuticalmanufacturing applications. HOC dryers cost more up front, compared to refrigerated and conventional desiccant dryers.

However, because they’re powered by waste heat from the compressor, operational costs of HOC dryers are minimal.

Membrane dryers rely on the selective permeability of specially engineered membrane materials, and on pressure differences inside the dryer. Small water molecules in the air can pass through microscopic pores in the membrane; larger nitrogen and oxygen molecules cannot.

A dryer consists of a cylindrical vessel filled with a bundle of hollow, membrane tubes. In action, untreated compressed air enters the dryer and flows through the tubes. Also, the volume outside the tubes but inside the vessel itself is at atmospheric pressure, creating a pressure differential across the tube walls. Inlet air passes through the tubes and the pressure differential lets only water molecules pass through the membrane, and dry air exits the unit.

A percentage of dry “purge” air (usually 10 to 20%) recirculates outside the tubes and carries the water vapor away. Depending on the design, pressure dew points can range from around 40° to as low as –40° F. Membrane dryers are compact, lightweight, require no power and have no moving parts, so they cost little to operate and do not require routine maintenance. These units are often recommended for point-of-use applications, near electrical or explosive hazards, and in remote locations.

With any type of dryer, experts recommend mounting standard and coalescing filters upstream. That keeps particulates, oil and liquid water from entering a dryer, maintains high efficiency, and helps ensure long life.

Another recommendation is don’t over-specify a dryer.

Operating every part of a compressed air system at the lowest possible dew point is rarely necessary, is expensive and almost always wasteful. On the other hand, don’t skimp on drying and specify a dew point that‘s too high. Damage from water in a system is equally expensive. Experts say users should only supply the degree of dryness needed for each application, especially when it varies by process or machine. And, like any system, consider up-front and operating costs, flow rates and performance capabilities when specifying a dryer.

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AIR SPRINGS

IMAGE COURTESY OF FEST O

AIR

springs AIR SPRINGS

have 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

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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 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 length—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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COMPACT

Fluid Power

cylinders

COMPACT

cylinders have been shortened relative to standard pneumatic cylinders. They may be up to 50% 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.

The “Pancake cylinder” was the original compact pneumatic cylinder, invented by Al Schmidt in 1958, 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 to provide long-term durability.

SELECTING A COMPACT CYLINDER APPLICATION DATA NEEDED FOR

OTHER ITEMS TO

SIZING A COMPACT CYLINDER AND

CONSIDER:

CHOOSING THE BEST COMPONENT: • • • •

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

• • • • •

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

Other compact cylinders vary quite a bit. They can be square shaped, offer numerous mounting features and can be placed with adjacent cylinders at a close center-to-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 nonrotating styles.

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TESY

ABC OF F

O

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PNEUMATIC ACTUATORS:

cylinders

MANY

industrial applications require linear motion during their operating sequence. One of the simplest and most cost effective ways to accomplish this is with a pneumatic actuator, often referred to as an air cylinder. An actuator is a device that translates a source of static power into a useful output motion.

It can also be used to apply a force.

Actuators are typically mechanical devices that take energy and convert it into some kind of motion. That motion can be in any form, such as blocking, clamping or ejecting, but typical motions are rotational or linear in scope.

Pneumatic actuators are mechanical devices that use compressed air acting on a piston inside a cylinder to move a load along a linear path. Unlike their hydraulic alternatives, the operating fluid in a pneumatic actuator is simply air, so leakage doesn’t drip and contaminate surrounding areas.

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

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

Single-acting cylinders use one air port to 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. www.fluidpowerworld.com

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PNEUMATIC CYLINDER SELECTION

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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 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.

When selecting an air cylinder, it is important to properly match the cylinder to the application, particularly in terms of required force. The theoretical force available in the actuator is the piston surface area multiplied by the supplied air pressure. Spring force must be subtracted from this value for single-acting cylinders. The actual force applied to the load will be 3 to 20% less due to pressure losses in the system.

When the required piston surface area (A) is known, the bore diameter (d) can be found by the formula:

Stroke length is determined by the required travel of the machine element driven by the actuator. The final selection criterion is the cylinder mounting arrangement and the resulting configuration.

There are many different configurations available from various manufacturers. The more common ones include rigid nose or tail mount, trunnion mount, rear pivot mount and foot mount. Once the basic actuator size and configuration are known, other options such as end-of-stroke cushions or special seals should be considered. In some applications, position detection switches are required, typically accomplished with a magnetic piston and switches.

There are many factors such as system contamination, corrosion, minor leaks and wear that will affect the available air pressure and flow used to drive the actuator. An actuator and fluid power system should be sized correctly so as not to waste energy, with a margin added to account for minor reductions in pressure and flow due to the factors listed above.

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PNEUMATIC CYLINDER SLIDES/

rodless cylinders

PNEUMATIC

cylinder slides, and rodless cylinders (sometimes referred to as guided cylinders) are designed 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 multiaxis configurations. In many of these applications, the need for side load capacity and non-rotating capability is critical.

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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 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

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PNEUMATIC CYLINDER SLIDES/RODLESS CYLINDERS

There are several considerations 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.

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SPECIFYING CYLINDER SLIDES

IMAGE COURTESY OF FABCO

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.

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 AIR

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 airdriven devices such as cylinders and motors.

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

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

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.

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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 determines the range of pressure adjustment. General purpose regulators are available in relieving or nonrelieving 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

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allow the excess downstream load- or headinduced pressure to be exhausted. Nonrelieving 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.

Downstream equipment flow and pressure requirements must be determined to properly size the correct regulator for the application. Similar to filters, manufacturers offer flow characteristic charts for their products to help chose the correct regulator.

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.

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 to the one shown.

SELECTING FRLs

IMAGE COURTESY OF PARKER HANNIFIN

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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.

By referring to the flow charts provided by the FRL manufacturer, you can size the assembly to match the flow required. An important consideration of selecting FRL size is the flow characteristics of the regulator. 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

6 • 2016

too close to inlet pressure, the FRL will start to reduce flow, or even stop flowing altogether.

On top of flow limitations possible with the regulator, you must calculate the flow of the filter and lubricator. Flow reductions are cumulative, as every inline component can reduce flow further, which emphasizes the importance of avoiding arbitrary FRL selection based on port size alone.

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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PNEUMATIC GRIPPERS

PNEUMATIC

grippers

PNEUMATICALLY

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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 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.

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Fluid Power

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 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

SIZING GRIPPERS

IMAGE COURTESY OF BIMBA MFG.

48

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 some-

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what, 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.

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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.

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MINIATURE PNEUMATICS

MINIATURE

pneumatics IN MANY

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ways, small is still the new big in pneumatics as of late. Whether it’s for end-use application purposes or to reduce manufacturing costs and increase efficiency—the trend to go small or go home continues.

Miniature pneumatics, sometimes called precision pneumatics, consists of a range of pneumatic components that have been miniaturized for use in light- and medium-duty applications with low-to-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 pneumatics has found applications in 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.

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.

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A selection of miniature fluid power components currently available include the following: Plugs are the most successful method to seal drilled holes for critical applications. More than 100 million are in use where permanent leak-proof seals are required.

Restrictors offer small size, precision performance and long life. Single orifice and multi-orifice models cover a wide range of restriction and can be customized to meet special requirements.

Flow controls including restrictor check valves, directional flow controls and constant flow valves are available in a wide range of sizes to suit many applications.

Nozzles can produce either a hollow cone of atomized spray or an accurately targeted jet of lubrication fluid.

Relief valves provide safety relief, linear or high pressure operation. They cover a wide range of applications.

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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

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

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

Contact The Lee Company.

Innovation in Miniature 2 Pettipaug Rd, Westbrook CT 06498-0424 860-399-6281 | Fax: 860-399-2270 | 1-800-LEE PLUG | www.TheLeeCo.com

The Lee Company

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In today’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.

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, battery-powered 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 metalto-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.

MINIATURE PNEUMATICS

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Shuttle valve styles include: spring biased with the emergency port normally closed; detented, where the poppet latches to close off either the emergency port or the normal port; selective shuttle valves with a nonbiased loose ball design; and inverse shuttle valves, where the lower pressure inlet port is open to common instead of the higher pressure inlet port.

Safety screens protect hydraulic components from contamination.

Manufacturers offer intermediate- and high-pressure etched screens; they can be customized to meet special requirements.

Piloting solenoid valves are designed for use in aerospace hydraulic and fuel systems, oil tool applications and other challenging high-pressure applications.

These are critical in space and weight savings, as well as power consumption.

Pressure components include miniature restrictors for damping pressure transients, adjustable pulsation dampers and miniature passive peak pressure recorders.

THE NEXT GENERATION DELIVERY SOLUTIONS AVENTICS Quick Ship Program. Kjell Lyngstad, Director AVENTICS North American Sales, helps ensure his customers have the high-performance pneumatics they need through our Quick Ship program. It offers a wide selection of our most popular pneumatic products, ready to ship with fast, reliable delivery that meets or beats market lead times. And standardizing on these products saves time over your system’s lifecycle.

Get the current catalog: Visit www.aventics.us/QuickShip

AVENTICS Corporation Lexington, KY info.us@aventics.com

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PNEUMATIC

hose & tubing THE

basic function of pneumatic hose and tubing is to convey pressurized air to actuators, valves, tools and other devices. But there are countless types and sizes of tubing and hose on the market, so engineers should consider a number of important factors to select the right one for a given task.

Start with construction. Tubing for air applications may be extruded of a single material or reinforced internally, typically with textile fibers, for higher strength. Pneumatic 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.

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PNEUMATIC TUBING

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: •

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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 semirigid 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 low-pressure medical applications and can be formulated to meet FDA specifications for contact with food and drugs. PVC tubing is typically clear, and thus well-suited where visible indication of flow is necessary.

Polypropylene tubing can be formulated for food-contact applications, resists chemical attack and withstands UV radiation in outdoor applications. www.fluidpowerworld.com

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Engineers should weigh the features, benefits and drawbacks of each material.

One important factor is compatibility of fluids conveyed through the conduit.

For 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.

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. 6 • 2016

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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 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.

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VACUUM

components

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

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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 FLOW 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.

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FREE AIR CAPACITY 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.

VACUUM FORCE 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.

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LEVELS OF VACUUM

HANDBOOK

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

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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. The largest number of applications occur here. 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.

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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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Your Control for

COMPRESSED AIR 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, and supplied through a network of piping at a certain pressure level.

AIR SUPPLY PRESSURE 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.

EVACUATION TIME The amount of time it takes to evacuate a given volume to a desired level of vacuum.

ENERGY CONSUMPTION 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.

High Flow... up to 4000 scfm

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LIFTING FORCE The lifting capacity of a suction cup, determined by multiplying pressure times the area.

VOLUME 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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valves

COMPRESSED

air pneumatic systems require methods of safe and precise control of the actuators unique to their accoutrement.

Although the medium is fluid, just as hydraulic or process water 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

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HANDBOOK

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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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

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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 doubleacting or spring-return 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 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.

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PNEUMATIC VALVES

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 springoffset 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 deenergizing 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, rubber-to-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. They are limited to two- or threeway 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.

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Spool valves use a notched metal cylinder that slides within a precisely machined body, drilled with three to five ports, or even 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 not only efficient, but also extremely resistant to contamination, making them great for dirty environments.

MOUNTING CONSIDERATIONS Pneumatic directional valves come in both standard and non-standard mounting configurations. The non-standard valve is constructed at the whim of the manufacturer, with port layout, operator style and mounting options unique to their 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 field bus connection, making the operation of a dozen valves as easy as one connector.

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hydraulics OVERVIEW

HYDRAULIC

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 and 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 farmlands 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 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.

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HYDRAULIC

Fluid Power

accumulators ACCUMULATORS

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are pressure vessels that store hydraulic energy and deliver that energy back to the system on demand. This is analogous to the way a car battery or electronic capacitor stores energy. In hydro-pneumatic accumulators, compressible gas (nitrogen) is used to keep stored fluid pressurized, and the re-expansion of the gas is what supplies the energy back into the system. Hydro-pneumatic accumulators are used extensively in the global fluid power industry.

In industrial plant operations, accumulators suit multiple purposes: pulsation dampeners, emergency power source, thermal expansion, transfer barrier for fluid separation, as prefill for large volume press applications, noise attenuation, leakage compensators, dispensers for pressurized lubricants, auxiliary power and others.

There are three primary designs of accumulators, each employing a different method to separate gas from liquid. Bladder accumulators use a flexible closed bladder inside a forged steel shell. Diaphragm accumulators use a flexible open diaphragm (membrane) separating two steel clam-shells either welded or threaded together. Piston accumulators consist of a moveable hydraulic piston within sealed tubes and soft seals on the piston to separate liquid from air.

For high-pressure bladder accumulators, seamless chromemoly steel has been used extensively

for more than 40 years. This material has a high tensile strength, which must withstand the massive load exerted by hydraulic pressure. The material lends itself well to forming and machining and has a burst pressure more than 350% of design pressure, also considered 3.5:1 safety factor conforming to ASME Code.

Various coatings can enhance resistance to rust, corrosion and abrasion to the accumulator finish. Most notable is the internal coating of phenolic resins.

This material provides excellent resistance to many common process fluids and has shown excellent chemical resistance.

It is easily applied, and for bladder accumulators, aids in applications where lubrication from the working fluid is poor.

Because of the smooth surface, it helps prevent the bladder from adhering to the internal wall surface during operation.

Electro-less nickel plating is also very popular, which is a coating applied approximately 2-mm thick, internally and externally. Electro-less nickel has proven to be a very durable material, cost effective, and also has been shown to be a viable alternative, in many applications, to the significantly more costly stainless steels, while maintaining the strength integrity of carbon steel. The nickel plating variation, as with the phenolic coated cousin, can be combined with 316 stainless steel connections. These connections include the liquid and gas ends of the accumulator, along with all internal component parts.

A bonus quality of nickel plating is that it protects the accumulator on the outside, for those applications where water washdown or corrosive vapors may be present. Electrolytic action between the stainless steel and electro-less nickel is seldom seen.

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There are several accumulator manufacturers who produce accumulator housings using 316 stainless steel, however, because 316 stainless steel does not have the tensile strength of high carbon steels, the wall thickness must be increased to improve pressure capacity, especially for accumulator designs seeing over 500 psi maximum allowable working pressure. Stainless-steel hydraulic accumulators are usually special order, both in the piston and bladder configurations, and therefore may have extended delivery times.

The most common and most widely used of all accumulators are for the fluid power market. Hydro-pneumatic accumulators are sometimes designed to operate up to 6,000 psi, although 3,000 psi or less are more common. Both piston and bladder accumulator manufacturers provide the fluid power industry with variations from 1.5 in.3

of volume up to 120 gal capacity. These are the most common accumulators selected by OEM manufacturers for injection molding machines, rubber processing machines, die casting machines, machine tools, presses and a variety of automated machines and process lines with extensive use in the landbased and offshore oil industry for safety valves and production applications. Another classification of accumulator is of the 500-psi bladder design, which offers large-capacity—up to 140gal and larger. These bladder accumulators are most commonly found in process areas where large piping networks are distributed through one or several buildings. These networks carry a variety of fluids, such as water, fuel oil, lubricants and chemicals. These accumulators are sized and placed in specific locations to primarily eliminate shock. Secondary applications of these large capacity accumulators are for thermal expansion/ contraction, pump start up and shut down, and for fire protection.

Smaller accumulators, known as the diaphragm type, are mostly used for pulsation dampening where flow rates are relatively low. These accumulators are usually non-repairable and must be disposed of when wear or failure occurs; however, repairable diaphragm accumulators are now more common from some manufacturers. The diaphragm accumulator is a relatively low cost device, rated up to 10,000 psi, but can be used on low-pressure applications as well. They are very reliable products, and with proper precharge and maintenance, their reliability will be further extended. Diaphragm accumulators may be phenolic coated internally or coated internally and externally with electro-less nickel plating, just as with other accumulators. They also have optional elastomers for specific fluid compatibility. Some manufacturers offer these in 316 stainless steel housings at reduced operating pressures.

The design is simple—they feature the anti-extrusion plug built into the bladder and with the repairable design, bladders can be replaced in minutes.

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HYDRAULIC BAR STOCK

HYDRAULIC

bar stock THE TERM

bar stock is often used in fluid power settings, but 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 will examine all of these uses.

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 0.0005-in.

(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.”

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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.”

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.

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

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HYDRAULIC BAR STOCK

DURA-BAR CONTINUOUS Cast Iron Bar Stock

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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clutches & brakes HYDRAULIC CLUTCHES A clutch is a device used to transmit mechanical power from a prime mover (electric motor or internal combustion engine) to a transmission or driven device. As opposed to a mechanical clutch, a hydraulic clutch uses a fluid coupling device to transmit rotational power at variable operating speeds and without problems of shock loads. The fluid coupling device contains a driving turbine, or torus, also known as the pump, which creates flow within the chambers of the fluid coupling enclosure. This flow drives the output turbine, which is connected to the driven member of the transmission system.

Clutches have found use in industrial, automotive, railway, airline and agricultural settings. When selecting a hydraulic clutch, important criteria to consider include what applications, performance specifications and compatibility are desired.

The performance of a hydraulic clutch can be broadly specified according to its torque rating, power, rotational speed and maximum pressure. The maximum torque is the greatest rated torque the clutch drive can bear, and is typically the limited factor in most applications. The maximum power capacity refers to the operating power of the load the clutch is intended to operate under, which is a factor of torque and speed. Rotary speed is the intended operating speed that the clutch has been designed to rotate at, while the maximum pressure is the greatest pressure that the clutch can withstand.

When considering compatibility of the clutch with driving and driven members, the clutch dimensions, clutch shaft geometry and drive connection type are important considerations. Significant clutch shaft dimensions are bore diameter, clutch diameter, length and weight. Clutch shaft geometry can be in-line, parallel, right angle or another arrangement. The connection type of the clutch shaft can be in-line, through shaft or flanged at the ends.

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HYDRAULIC BRAKES Hydraulic brakes are used to stop or slow rotating mechanical systems by converting kinetic energy into heat energy. When triggered, they transfer pressure through the use of a fluid and this pressure triggers the braking mechanism by pushing on a piston in a brake caliper or slave cylinder. These braking mechanisms consist of brake types such as drum brakes, disk brakes, band brakes and cone brakes. Each of these braking styles have different braking properties, but they are all powered, by way of hydraulic fluid, from the pressure of a master cylinder or pump. The main specifications considered with brakes are speed, power, torque and maximum pressure.

When selecting a brake, the style of braking is the first choice an engineer must decide on. Drum brakes work by applying hydraulic pressure, pushing a braking pad or shoe against the rotating surface, creating friction and slowing the spinning surface down. Disk brakes function by squeezing the rotor with calipers to slow it down. Band brakes work by tightening the band material around the rotating drum, similar to older stationary bikes. A cone brake works by forcing two cone-shaped devices toward each other, one of which is the rotor and the other is the friction material.

COMBINATION PACKAGES Because hydraulic brake and clutch components both use fluid power, they can be integrated and manufactured for ease of compatibility and installment into the application. When both clutch and brake components are present, braking can be facilitated by the disengagement of the clutch mechanism, which transmits the power needed to apply the brakes.

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HYDRAULIC CYLINDERS

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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.

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What is the maximum pressure for the application?

The hydraulic cylinder must be rated to work within the 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 snapring 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.

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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

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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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MS2 — Foot Side Lugs

MS7 — End Lugs

MF2 — Cap Rectangular Flange

MF6 — Cap Square Flange

MT4 — Intermediate Trunnion

MS3 — Centerline Lugs

MP1 — Cap Detachable Clevis

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.

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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.

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What mounting method is being used?

MF1 — Head Rectangular Flange

MT2 — Cap Trunnion

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

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.

MF5 — Head Square Flange

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Push or pull or both?

What push or pull tonnage is required?

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 loadbearing 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 direction to push and pull.

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.

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.

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KEY SPECIFICATIONS 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.

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 stainlesssteel construction, as well as PTFE (Teflon) seals, especially if no glycol is used in the fluid.

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.

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 65-45-12 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.

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HYDRAULIC FILTERS

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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 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.

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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.

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HYDRAULIC FILTERS

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.

SELECTION CRITERIA •

•

•

Flow describes how much fluid flow is permissible through the filter, the maximum flow rate.

Pressure describes the maximum operating pressure that the filter can withstand.

Port size is the size of the inlet and outlet ports that need to connect to the hydraulic fluid system.

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SOMETIMES,

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.

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, separated by a three-way ball-valve. This design allows

FILTER CARTS can include particle counters to provide live ISO Code readings, as seen on the back of this design from MP Filtri.

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FILTRATION SYSTEMS

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.

FILTER CARTS can range from very simple systems that simply filter fluids to complex, such as this unit from Schroeder Industries, which features contamination and water saturation sensors, multiple filters, touchscreen panels and more.

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HYDRAULIC

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fittings & flanges HYDRAULIC

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.

SPLIT FLANGE adapters and split flange clamps.

Image Courtesy of Anchor Fluid Power

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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.

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 a “dash” 32 size is a 2-in. (32/16) thread. A Y-flange may split a 2-in. flow into two reduced 11⁄2-in., instead of creating three 2-in. connections to more closely match the "Serving Industry for 55 years" cross-sectional area.

O-ring face seal, SAE straight * SAE 4-BOLT * SAE, JIS, DIN thread and ISO 6149 fittings have a seal, normally Buna N, contained within * FLANGE a groove to seal the fluid. It is impor* METRIC PORTS ADAPTERS tant for the seal to be compatible with the fluid and the operating tempera* STAINLESS * SPECIALS ture range. An elastomeric seal greatly reduces the possibility of leakage caused by vibration, thermal cycling and pressure cycling.

SAE J518 split flange fittings are Experience - 55 years of hydraulic experience used on larger line sizes, starting at 1⁄2 Availability - Standards & Specials in stock in. (-8) but coming into predominance at 2 in. (-32) and above. A flange head Assistance - MAIN's engineers are part of the with an O-ring groove on its face is SAE, NFPA, and ISO committees attached to a conductor (hose, tube or pipe) and is secured to the port, which Request MAIN's catalog WWW.MAINMFG.COM/flg could be a flat-face fitting or a pad on A US Manufacturer a pump, valve or cylinder, by a clamp Info@MainMfg.com with four bolt holes. The clamp can be PH: 800-521-7918 whole, but is often split so that a quar3181 Tri-Park Drive ter of the diameter of the flange head is on either side of the centerline of the manufacturing products, inc Grand Blanc, MI 48439

HYDRAULIC FLANGES AND COMPONENTS

' M A I N ' A D VA N TA G E S

MAIN

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bolt holes to help minimize torque on the clamp.

The screws used are tightened to a high torque value to avoid problems with fatigue. In many cases, using pipe or tubing, the flange connections have operated within their specified working pressure for decades.

Flare fittings, such as the JIC 37˚, are fittings with a conical end face and the seal is formed when this seat is forced against a mating seat, generally by torquing a swivel nut on one fitting, engaging with a threaded portion of the mating fitting. The angle of the seat and face for most JIC fittings in the North American marSPLIT FLANGE adapters and split flange ket is 37°, and is popular enough that the 24° and 45° versions clamps. Image Courtesy of Anchor Fluid Power 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 conMade for maximum portability and tamination, which was addressed with the immediate use, Tompkins Test Point Kits are advent of flat-face couplers, which have portable, compact, and durable. Since the no recess to collect contamination. fittings are designed to be connected under Staple and band fittings are low-prespressure, you can test quickly and at multiple sure fittings. Band fittings are attached to points while feeling confident that they the hose by a barbed or beaded end bewon't leak under vibration or peak pressure. ing inserted into a hose and a band clamp securing the connection. This method is You can create your own custom kit online or only for extremely low pressures. Staple select from either the JIC or Flat Face Kits. 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 through both sides of the connection behind the bead, although it 800.255.1008 | tompkinsind.com is still typically used for low pressure or suction lines.

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HYDRAULIC FLUIDS

HYDRAULIC

fluids HYDRAULIC

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.

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

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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 6 • 2016

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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 waterretention 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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HEAT EXHANGERS

WAT E R -C O O LED sh Im a g e e ll c a n A P I o u r te s y o f T -a n d -t u b e heat e h Heat T x r a n s fe e r m a l T r a n s fe r P r o c h a n g e r s r Co. d u c ts ,

HEAT

exchangers ONE

of the critical conditioning requirements of hydraulic fluid is that it is maintained at an optimal operating temperature. As 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.

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

Hydraulic systems use heat exchangers 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 selfexplanatory. It will use a fluid such as water or air to transfer heat into or away from hydraulic

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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 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.

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AIR-COOLED fluid conditioning system Image courtesy of Thermal Transfer Products, an API Heat Transfer Co.

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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 increases 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 can remove over 300 hp worth of heat, they can block a commercial doorway because of their unwieldy size. Liquid-to-liquid 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. www.fluidpowerworld.com

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FEP & PFA

HEAT EXHANGERS

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 tubing, transferring heat from the hydraulic fluid to the 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 hydraulic systems.

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

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hoses

A 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 searching for 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 reinforcement—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, antistatic, 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 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

IMAGE COURTESY OF KURIYAMA OF AMERICA

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Simplified hose configuration, less hose and adapters combine to reduce repairs and downtime cutting the overall cost of hydraulics.

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 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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S = SIZE What is the inside diameter required? Is the outside diameter a factor? How long does it need to be?

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T = TEMPERATURE What is the temperature of the media? What is the surrounding ambient temperature?

A = APPLICATION Where is this hose being used?

M = MATERIAL TO BE CONVEYED What is the media being transferred? Is the tube of the hose compatible?

P = PRESSURE What is the maximum system pressure? Is there any external force or pressure on the hose?

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E = ENDS OR COUPLINGS What are the terminal end connections required to mate with the ports?

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D = DELIVERY When are the hoses required? How many?

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HYDRAULIC HOSE COUPLINGS

HYDRAULIC

hose couplings IMPROPER

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?

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A COLLECTION OF quick connect couplings, including flat face, screwto-connect, threadless connection and pressure diagnostic quick disconnects. Image courtesy of CEJN

Couplings can be two types: permanent and field-attachable (reusable). Permanent couplings are generally more reliable, easier and quicker to attach than field-attachable 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 preassembled (one piece), with a ferrule permanently attached to the stem.

Higher-pressure hoses use field-attachable couplings, as well as permanent couplings. Field-attachable 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.

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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.

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 high-pressure 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.

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.”

FLAT FACE COUPLERS Image courtesy of Holmbury Inc.

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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

DECOMPRESSION BLOCK COUPLER Image courtesy of Holmbury Inc.

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.

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HYDRAULIC MANIFOLDS

HYDRAULIC

manifolds 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 typical work performed by hydraulics is to rotate, clamp, press, extrude, lift, shear, extend and position.

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For example, in the operation of a backhoe loader, the operator manipulates control valve levers, which, in turn, actuate valves mounted in or on a manifold.

Connected to the manifold are various hoses that terminate where hydraulic power is transformed into mechanical action. This transformation occurs when a hydraulic cylinder or motor responds to oil under pressure, and either extends, retracts, turns or brakes. Depending on which levers are being controlled, the operator can extend and swing the articulating arm of the backhoe loader or scoop gravel with the bucket, thus converting hydraulic power into 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.

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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 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 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.

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HYDRAULIC MANIFOLDS offer a variety of paths for fluid to flow through a hydraulic system. These acrylic models demonstrate complexities in their design.

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HYDRAULIC MANIFOLDS

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 solid-block manifolds, even small runs, is highly competitive to systems using modular blocks or discrete components.

CONSIDERATIONS WHEN SELECTING A HYDRAULIC MANIFOLD

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Fluid type Seal materials Material and finish Environmental conditions and temperatures Pressure (maximum and working) Duty cycle Flow conditions (pump, accumulator and return) Port sizes, types and locations Number and type of valves Electrical voltage and connection Mounting

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HYDRAULIC

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Three Common Designs 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 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 EXTERNAL GEAR MOTOR 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

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motors are rotary actuators that convert hydraulic (fluid) energy into mechanical power. They work in tandem with a hydraulic pump, which converts mechanical power into fluid (hydraulic) power. Hydraulic motors provide the force and supply the motion to move an external load.

Three common types of hydraulic motors are used most often today— gear, vane and piston—with a variety of styles available among them. In addition, several other varieties exist that are less commonly used, including gerotor or gerolor (orbital or roller star) motors.

Hydraulic motors can be either fixed- or variable-displacement and operate either bi-directionally or uni-directionally. 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 “breakaway” 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. www.fluidpowerworld.com

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C E L E B R A T I N G

PERMCO IS A LEADING MANUFACTURER OF HIGH-PRESSURE HYDRAULIC GEAR / VANE PUMPS AND MOTORS, FLOW DIVIDERS, INTENSIFIERS, AND ACCESSORIES. AVAILABLE IN A WIDE VARIETY OF SIZES AND CONFIGURATIONS TO SUIT YOUR APPLICATION NEEDS. www.permco.com Global manufacturer of process control and factory automation solutions PERMCO, INC., 1500 Frost Road, P.O. Box 2068, Streetsboro, OH 44241 Toll Free: 800.626.2801 | Local: 330.626.2801 | Fax: 330.626.2805

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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 gauging with a flowmeter. You can also manually measure by rotating the motor’s shaft one turn and collecting the fluid manually.

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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.

Vane Motors 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.

VANE MOTOR

Piston-type motors SPECIFYING HYDRAULIC 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 fire-resistant 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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VARIABLE, AXIAL PISTON MOTOR, with the bent-axis design.

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 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 DUAL-DISPLACEMENT MOTOR, main drive shaft. with swashplate design.

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 fluidRADIAL PISTON MOTOR 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.

FIXED, AXIAL PISTION MOTOR, with the bent-axis design.

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PRESSURE

transducers PRESSURE

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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,

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PRESSURE TRANSDUCERS

TIPS ON SPECIFYING PRESSURE TRANSDUCERS •

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Pick a robust pressure transducer. Hydraulic applications can be remarkably difficult for pressure sensors. A proper transducer design must survive mechanical stress at the installation point and hydraulic pressure spikes.

Think total cost. Troubleshooting, downtime and replacements can dwarf the original purchase cost.

If the data sheet states nothing about mechanical shock and vibration testing, ask for details or keep looking.

Consider potential pressure spikes. A rapid pressure increase that greatly exceeds normal working pressure is a pressure spike. Severe pressure spikes can exceed working pressure, offset calibration and damage electronics inside the device. Think of a pendulum swinging, where the raised ball represents this pressure peak. It is about to strike your thin metal disc, which has a circuit device glued to the opposite side. Some of the worst pressure spikes occur in manual operation during a system startup.

the distance the signal must travel and possible interference that can be found in the environment around the system.

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.

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Remember location. Mount the pressure sensor close to the main line, actuator or volume of interest, but keep location serviceable. If you think the sensor is exposed to severe mechanical shock, high vibration or physical damage, move it. Add a short length of hose and find a more stable location. Minimize the risk of early transducer failure by selecting the right device and putting it in the correct location.

Keep it short. Avoid using test or measurement hose longer than 1 or 2 ft, unless you only plan to measure static pressures. Never use long measurement hose if the transducer provides feedback to an active closed-loop system, as it slows pressure feedback, which can make your PID system unstable.

If possible, install the hydraulic port facing up or sideways. This prevents trapped air inside the transducer, which causes a delay in reading the actual hydraulic pressure. If you commission a system with maximum pressure of 1,400 psi or lower pressure, you must manually bleed air from all high points. Above this pressure, the remaining air becomes entrained in the hydraulic fluid, where it eventually escapes in the tank. (Entrained air … it sounds rather pleasant, except for the violent implosion that occurs as these air pockets collapse at high pressures. This is another kind of pressure spike that you might wish to avoid inside your pressure sensor.)

Avoid using transducers with built-in orifices. Or add a gauge snubber. These are used to improve transducer life or to hide poor pressure regulation. However, adding a hydraulic delay leads to pressure instability in a closed-loop control system.

demanding applications

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Consider two transducers, identical except different temperature ratings. First one is 0.2%/10° K; second one is 0.2%/10° K. Your hydraulic system changes from 20° to 50° C. Both transducers are 0.5% accuracy class. #1 Overall accuracy (0.5% + 0.6% ) or 1.1% #2 Overall accuracy (0.5% + 0.3% ) or 0.8% IMAGE COURTESY OF VIATRAN

For typical industrial applications, select a 0.5% accuracy class. This should be sufficient for most closedloop systems. Higher accuracy will quickly increase price.

Before making that investment, determine if the rest of your system requires this higher accuracy.

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, too. 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.

Custom Features: • Integrated linear position sensors

• Special materials for high strength / weight savings

• Integrated valves for load-holding & counterbalance

Cylinder Manufacturing Capabilities.....

Cylinder Manufacturing Capabilities: • Bore sizes ranging from 1” to 20” (even larger bores are possible)

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

• Internal Cushioning options for deceleration of motion

• Multi-stage telescopic

Visit us at: www.PrinceHYD.com

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PTOs & GEARBOXES

PTOs &

gearboxes PTOS —

The Power Take-Off, most commonly referred to by its acronym, PTO, is a common form of mechanical power delivery in the mobile machine market. The PTO is a method of transferring high power and torque from the engine (usually through the transmission) of trucks and tractors.

In combination with gearboxes and pump mounts, nearly any type of mechanical power transmission is possible.

There are three common power take-off methods in the mobile machine market: tractor style, truck transmission style and engine crankshaft-driven, although the latter is not commonly referred to as a PTO. The crankshaft-driven method of power transmission is often used for hydraulic pumps mounted to the front of an on-highway truck, such as a plow/spreader or cement mixer.

In this drive configuration, a small shaft with U-joints attaches to a yoke coupler to turn the pump.

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The tractor PTO goes back pretty much as far as tractors do. Most early PTOs were driven from the transmission, which was located at the back of the tractor, to allow for easy location of an output shaft. The transmission type of PTO is only engaged when the transmission clutch is also engaged, and is coupled directly to the transmission, so that when the clutch is depressed, the PTO isn’t driven.

If the transmission is driving the wheels, then the transmission PTO is turning. This also means the implement can backward-power the transmission as well as when the clutch is depressed, such as down a hill or if the attachment has a mechanism with high rotational inertia, resulting in surging of the drive wheels. This can be avoided by adding a dedicated overrunning clutch for the PTO, which prevents torque from being applied in the opposite direction.

A live PTO often uses a transmission clutch with two stages. The first stage of the clutch operates the driven portion

DOUBLE GEAR type power take-off Image courtesy of Muncie Power Products Inc.

of the transmission, and the second stage of the clutch controls the engagement of the PTO. This method allows independent control of the transmission, so that the PTO maintains operation regardless of transmission clutch activity, including stopping of the tractor itself. For a tractor with a mower attachment, for example, this is a minimum requirement; you can’t have the mower turn off when you feather the clutch up a hill and around a tree.

The independent PTO has an entirely separate engagement clutch and works altogether exclusively of the transmission clutch. To engage the PTO, a button or lever is pushed; it’s that simple. Usually, a lever operated clutch is required to be activated before a separate PTO switch. Not only is the system easy, the independent PTO can be activated while the tractor is stopped or moving. Independent PTOs are available in both mechanical and hydrostatic 6 • 2016

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configurations, and as you can imagine, the hydraulic type is highly popular with the proliferation of hydrostatic drives.

The PTO shaft extending from back of the tractor is a male-splined structure. There are three types of PTO shafts, and their usage is loosely defined by the power output of the tractor. Each of the shafts runs at one of two standard speeds: 540 and 1,000 rpm. Also, each of the shafts comes in one of two diameters; 13⁄8 or 13⁄4 inches.

Finally, none of the three shafts share a number of splines: 540 rpm—6 spline—13⁄8-in. shaft 1,000 rpm—21 spline—13⁄8-in. shaft 1,000 rpm—20 spline—13⁄4-in. shaft The two 1,000-rpm shafts are known as the small (13⁄8-in.) and large (13⁄4-in.) versions. Each shaft is designed for an appropriate range

of torque and horsepower transmission, and the differing dimensions ensure they are not used interchangeably. Some tractors can operate more than one speed and power range of shafts, but it is important to remember that the implements they drive are often limited to a design speed. So if a grain auger is designed to run at 540 rpm, it might not be a good idea to try to crank it up to 1,000 rpm.

Safety is serious business with PTO shafts, because unfortunately, they are infamous for causing both injury and death. Implements should only be operated with the PTO safety covers in place, and if there is a reason they don’t exist, extra caution needs to be practiced.

All modern heavy-duty trucks come with the capability to run a PTO from the transmission. The truck’s transmission will have a cover plate (or two), which can be removed, exposing a spur gear able to drive a PTO. The truck-mounted PTO is versatile, although unlike tractor PTOs, they are not quite as universal—mostly because of the countless transmissions on the market.

Regardless, few transmissions have no PTO available. The simplest form of PTO is with a single gear to mesh with the transmission’s output gear. These types are somewhat restricted in both their speed and horsepower range, but are more economical due to their simplicity.

PTOs with two or three gears are more common, and are also available for heavy-duty applications requiring high horsepower, torque and speed. Multiple gears allow for a wider range of speed or torque multiplication.

SELECTING A PTO When selecting a PTO, you will need to provide your supplier with various parameters, as these units are not universal. You will need to know your transmission model number, output speed, torque and horsepower required and the shifter style desired. The shifter engages and disengages the PTO, and is available with air shift, electric shift and mechanical (cable) shift. Some newer PTOs are operated by a clutch instead of sliding gears, but the application information changes little otherwise.

You will also need to know what pad mount is necessary for

the pump you’re running off the PTO, such as SAE 2- or 4-bolt flange, and the series, such as SAE B or SAE C 4-bolt flange. Special consideration should be paid when mounting a pump to the PTO, as port location could interfere with chassis components or the transmission itself. These issues are often resolved by installing an intermediate shaft, allowing the pump to be mounted in a clear location a couple feet behind the transmission.

Even with the high torque that can be transmitted by PTOs, there are occasions when even more torque is demanded than a tractor can transmit (or slower speed than a standard 540

or 1,000 rpm PTO). Although implements often employ pulleys and chain drives, which can be used for torque multiplication, the heaviest of demands require a gearbox for reliable and efficient operation. Using gearboxes is not limited to farm applications. They are most often seen in wheel drives and other massive-torque, low-speed applications.

WORM AND HELICAL gearboxes Image courtesy of Bonfiglioli USA

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PTOs & GEARBOXES

PACKAGING WATER TREATMENT

MINING

MEDICAL FOOD & BEVERAGE

There are a few general types of gearboxes, including planetary, spur/helical gear and worm gear. Each has advantages and applications for which they are best suited. The most common type for mobile and off-highway machinery is the planetary drive. These consist of a center “sun” gear with three or more circumferential “planet” gears, which in turn are surrounded by a ring gear. Planetary gearboxes are robust and capable of holding high radial loads, especially when the load is correctly centered.

This is why they’re often found in wheel drive applications.

Spur- or helical-gear style gearboxes use two or more gears meshed side by side, and are often used in industrial applications, such as a rolling mill.

Their size and possible gear ratios vary widely, with the upper end of torque capacity only limited by the size of gears able to be manufactured. These types of gearboxes can have input and output shafts on either the same or opposing sides.

Also quite popular are worm gear style gearboxes. They employ a helical axial input shaft, which directs force across a perpendicular gear ring that is attached to the output shaft. Operation is always with input and output 90° from each other.

Worm gear units are generally light to medium duty.

Gearboxes are applied to either increase torque or increase speed, and it should be clear that increasing one decreases the other. When used to increase torque, they can achieve this at ratios anywhere from 2:1 relative to input torque, to 10,000:1 or more … which of course would slow the output speed drastically. When used to increase speed, sometimes planetary or helical/ spur gearboxes can be used in reverse. A common use for overdriven gearboxes is for pump drives. A 1,000-rpm PTO speed doesn’t take advantage of a hydraulic pump capable of 3,000 rpm, so a speedincreasing gearbox/pump mount would allow for the use of a smaller pump. Please note, worm gear reducers do not allow input energy at their output shaft.

MATERIAL HANDLING

GEARBOXES —

We are committed to satisfying our customers’ requirements by supplying high quality products and providing excellent service on an increasingly wide scale. Our customers’ need for ongoing technical and technological improvement demands that our Group always keeps the future in mind.

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MOBILEHYDRAULICTIPS.COM P exro PUM ch R ON of Bos T S PI tesy AL r AXI e cou g a Im

HYDRAULIC

pumps POSITIVE

VARIABLE-SPEED PUMP Image courtesy of Bosch Rexroth

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th

displacement pumps are used in fluid power motion control applications. They have a small clearance between rotating and stationary parts. These pumps deliver a specific amount of fluid to the system for each revolution. Positivedisplacement pumps can be further divided into two categories: fixed- and variable-displacement. Fixed-displacement pumps provide a single, specific volume displacement per revolution. In variabledisplacement 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 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

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(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. Because their transition zone from low to high 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.

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

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CONSIDERATIONS WHEN SELECTING A HYDRAULIC PUMP 1.

2.

3.

4.

5.

6.

Displacement Pressure Angle Related torque Weight Mounting configuration

cylindrical pistons, while another uses ballshaped pistons. Another classification refers to porting: Check-valve 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 metalto-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).

CUTAWAY OF A GEARPUMP. Image courtesy of Bosch Rexroth

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REPLACEMENT/REBUILDS

repair, Arebuild ND REMANUFACTURING WHEN

IMAGE COURTESY OF ENGINEERING TECHNOLOGY SERVICES

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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.

While sources of failures come in every imaginable shape and form, the root cause generally falls into just a few areas.

Sometimes components just 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 lowcost consumable parts like springs, washers and shims tend to be replaced. Likewise, bearings will be inspected and 6 • 2016

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BEFORE AND AFTER shot of rebuild pumps Image courtesy of Engineering Technology Services

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. As a consequence, these firms often rely on parts from aftermarket sources—usually offshore parts manufacturers. While these reverse-engineered aftermarket parts almost always cost less than OEM parts, quality concerns are currently an important issue within the fluid-power industry.

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have rebuilding and testing capabilities that are as good as or better than the OEM’s themselves, said Brian Berry, president of Engineering Technology Services, a remanufacturer of high-end pumps and valves. That’s necessary simply because hydraulics has gotten more complex. One issue is that newer equipment operates at higher pressures, said Berry. While 3,000 psi used to be the upper limit for hydraulic pressure, newer systems in some cases operate to 7,000 psi or higher. That typically demands piston pumps and motors, and piston designs are more complex than lower-pressure gear and vane pumps.

And 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 www.fluidpowerworld.com

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components. Given the sophistication of the latest controls and software, there’s a shortage of qualified technicians who can handle all aspects of inspection, testing and calibration.

Components with higher operating pressures and electronics controls also tax the capabilities of test equipment at many repair facilities, said Berry. Small shops will make repairs and perhaps run basic tests, but they are not capable of performing fullfunction tests. Other, typically 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, stressed Steve Matthew, president of Wooster Hydrostatics.

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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  ROTARY ACTUATORS

HYDRAULIC

rotary actuators HYDRAULIC

IMA

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COU

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Y TES

OF

HEL

AC

rotary actuators are used for high torque, heavy-duty motion applications. They have high force capabilities, high power-per-unit weight and volume, good mechanical stiffness, and high dynamic response. They provide the heft for lifting, turning, indexing, clamping, mixing, bending, testing and steering applications among others.

Rotary actuators are compact and efficient, and produce high instantaneous torque in either direction.

This makes them widely used in precision control systems and in heavyduty machine tool, mobile, marine and aerospace applications.

Rotary actuators are coupled directly to a rotating load and provide good control for acceleration, operating speed, deceleration, smooth reversals and positioning. They allow flexibility in design and eliminate much of the bulk and weight of mechanical and electrical power transmissions.

Because they are fully enclosed, they withstand harsh conditions and are protected from dust, dirt and moisture.

ROTARY ACTUATOR STYLES The most common types of rotary actuators are vane, rack-and-pinion and helical designs.

Vane actuators are a good fit for applications requiring extremely high endurance, high rotational speeds and a need for constant control of movement without the need to maintain the load in a particular position.

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Their compact size makes vane actuators good for use in applications requiring positioning, harmonic motion and high-speed oscillating motion, and are used in tool machines, robotics and for handling containers.

In a single-vane actuator, the vane is rigidly attached to a central shaft in a cylindrical housing. The housing is divided into two chambers by a second vane or abutment shoe that is fixed to its internal diameter and extends to the output shaft. Hydraulic fluid flows to these chambers through connection ports closed to the fixed shoe. Pressure difference between the chambers acts on the moving vane area, producing torque directly on the output shaft. Flow in and out of these chambers rotates the vane and shaft. Geometry usually limits the rotary movement of a single-vane actuator to 280°.

Double-vane actuators produce twice the torque and less than half the rotation of single-vane actuators. Two vanes and barriers provide a balance that counteracts the tendencies of unbalanced loads.

Mechanical efficiencies range from 80 to 95% and vane actuators transmit torques to nearly 700,000 lb-in.

Rack-and-pinion actuators are ideal for the highest demands on torque. They are ideal for applications where there is restricted building room in an axial direction; however, the rack-and-pinion actuator requires considerably more space compared to the rotary vane and helical with equivalent torque output.

Rack-and-pinion actuators are particularly useful for heavyduty applications, and because of their constant torque output characteristics and resistance to drift, they are often used for precision control.

Rack-and-pinion actuators have low shock resistance, zero internal leakage and also have enclosed moving parts.

Helical actuators convert the linear motion of a piston into rotation by the use of helical gears that couple the piston to a rotating shaft. They feature high load capacity and are typically used to support the load as well as rotate it. This eliminates the need for other bearings or support mechanisms. Sometimes referred to as a “powered hinge,” the compact design of helical actuators makes them ideal when space constraints are a consideration. Their inherent load carrying ability allows simplification of many rotating load mechanism designs and, by eliminating other bearings and linkages, they offer the added benefit of increasing the reliability of the overall assembly.

Helical actuators produce the same torque in both clockwise and counterclockwise rotation. Average mechanical efficiency is typically 70%. Because the angle of rotation is determined by actuator length and helix angle, in theory, any rotation is possible. Many actuators have 90, 180 and 360° rotations as standard, but models with custom rotation are also common.

Typical applications for helical actuators include aerial work platform and jib arm rotation, fork rotation, boom positioning, head rotation, valve operation, waste cart tippers, and wheel steering in the agriculture, construction, energy, marine, material handling, military and mining industries.

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ROTARY ACTUATORS

Other types of hydraulic rotary actuators include enclosed piston-crank, scotch-yoke and bladder designs. Enclosed piston-crank designs have adjustable stroke for variable shaft rotation up to 110°. A rod connected to a crank arm drives the rotating shaft. In scotch-yoke actuators, two pistons are connected by a common rod. At the beginning and end of the stroke, torque output is twice the value produced at the stroke’s midpoint. Applications that require a high breaking torque to move the load find this type of actuator appropriate. In bladder designs, a pair of rubber bladders are alternately pressurized and exhausted to produce the driving force. When pressurized, the bladder pushes against a cupshaped lever arm that rotates the output shaft.

Zero internal leakage makes this actuator highly accurate and resistant to contamination.

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SELECTING A HYDRAULIC ROTARY ACTUATOR • • • • • • • • •

Duty (heavy, standard) Rotation (90°,180°, 270°, 360°, custom) Maximum output torque Maximum holding torque Acceptable backlash Maximum bearing loads (moment, thrust, radial) Hydraulic fluid Hydraulic fluid operating temperatures Mounting (flange, foot)

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HYDRAULIC STANDARD duty rod seal Image courtesy of System Seals

SEALING

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 application-specific.

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: • • • •

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high mechanical, tensile strength good abrasion resistance modulus of elasticity is variable wide range of hardness values, while retaining good elasticity

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• •

seals

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).

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

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HYDRAULIC SEALS

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.

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Below is a list of some of the most common seal designs used in fluid power applications.

ROD SEALS

WEAR BANDS

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

•

• •

•

BUFFER SEALS •

WIPERS • • • • • •

128

provide aggressive wiping force to prevent mud, water, dirt and other contamination from 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

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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 pumpback capability for lubricating oil film often must withstand up to 6,000 psi

• •

• • •

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

•

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

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

DOUBLE O-ring design Image courtesy of Trelleborg Sealing Solutions

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NEW CMA Advanced Mobile Valve Image courtesy of Eaton Hydraulics

HYDRAULIC

valves HYDRAULIC

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. Alternatively, directional valves can be complex, such as

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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.

PRESSURE CONTROLS 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

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HED 6-1

HYDRAULIC VALVES

added utility or functionality, such as reverse flow checks or pilot operation built in. The pressure-reducing 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 a 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 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 pulsewidth 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.

HED BRINGS

COMPLETE VEHICLE CONTROL SOLUTIONS

Hydro Electronic Devices, Inc. 2120 Constitution Avenue Hartford, WI 53027 Phone: 262-673-9450 www.hedonline.com

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