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FLUID POWER
OVERVIEW
Welcome to the seventh edition of the Fluid Power Handbook. Every year, our editorial
staff works hard to add to the detailed information we’ve already accumulated on hydraulic and pneumatic components and systems. Once again this year, we’ve incorporated some new frequently asked questions into many of the Handbook sections. We had very positive feedback to this concept last year, and I hope you’ll find the new topics useful in your daily work. As always, we have updated the graphical look of the issue, too. You will find updated sections on many components and other areas of fluid power interest, some all-new concepts, and a fascinating trends piece on how biomimicry has become a popular design trend among component and system manufacturers. Fluid power systems are comprised of components that include pumps, cylinders, valves, hose, fittings, gauges, sensors, filters, seals, and reservoirs. Some components are considered absolute necessities, while others are optional and used to refine the system for more precise operation or to increase the lifespan of the system or its individual parts. Throughout this handbook, we detail many of the more common and widely used components, explaining their operation, their place in the system, and how an engineer should correctly specify them. While fluid power can be used in most any industry or application, it is commonly seen in markets that include packaging, off-highway, mining, offshore/marine, medical, material handling, construction, aerospace, automation, robotics, and entertainment. And fluid power is an important technology. A study by the Department of Energy several years ago showed that between 2.1% and 3.0% of the United States’ energy is transmitted through fluid power equipment. While that is an impressive statistic, the study also found that the efficiency of fluid power systems ranges from 8% to 40%, averaging a mere 22%. These numbers illustrate how much improvement there is to make. The study concludes that a 5% improvement in efficiency over 5 years is possible by instituting best practices in industry. Or, even better, a 15% improvement over 15 years is possible with a strategic R&D program. Doing this would save the U.S. an amazing $37 billion. While some say that fluid power is a static, mature technology, there’s still much in store for the technology. We continue to see hydraulic and pneumatic components become more Internetfriendly, wireless and capable of being monitored and controlled from distant locations, taking preventative maintenance to new levels. One last important note: I’m happy to announce that Mary Gannon, who has been with the publication since its inception, has been promoted to Editor of Fluid Power World. Mary has a keen knowledge of the industry, the technology and the people; I’m sure many of you have interacted with her over the years. Starting with next month’s issue of the magazine, watch for her face in the front of the magazine with the lead editorial opinion. Congratulations, Mary, this is very well deserved!
PAU L J. HEN EY V IC E PRESIDEN T , EDITORIAL DIREC TOR
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INSIDE THE 2018 FLUID POWER HANDBOOK
60
02 06
Fluid Power Overview Fluid Power and Biomimicry: The nature of robotics’ future
14 Hydraulics Overview 15 Accumulators 16 Bar Stock 19 Cylinders 24 Filters 30 Filtration Systems 32 Fittings and Flanges 36 Fluids 42 Hose 46 Hose Couplings 50 Hydraulic Power Units 56 Manifolds 60 Motors 64 Pumps 68 Replacement/Rebuilds 72 Seals 77 Sensing Technologies 81 Valves
86 Pneumatics Overview 87 Compact Cylinders - Actuators 88 Cylinders - Pneumatic Actuators 90 Cylinder Slides/Rodless Cylinders 94 FRLs 96 Grippers 98 Miniature Pneumatics 100 Pneumatic Tubing 103 Vacuum Components
106 Gauges 108 Retaining Rings 110 Fluid Power Safety 114 Shock Absorbers 120 Ad Index
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In the last decade, engineers have begun to seriously reexamine the world’s first and best engineer: nature. Interest in the efficiency and elegance of design in nature has exploded and fluid power is helping propel new designs into the future.
Fluid power
and biomimicry: The nature of robotics’ future
Mike Santora • Associate Editor
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B I O M I M I C R Y
With companies like Festo and Boston Dynamics now creating machines lifelike enough to capture the pop culture imagination, we’re as likely to see the word biomimicry in our Facebook feeds as we are at a tech conference. One of the technologies helping to drive this thrust in biomimicry-influenced robot design is fluid power. To get a better understanding of how fluid power is evolving in nature-inspired robot design, we took a look at some of the cutting-edge projects these two companies have been working on. We asked Festo’s Head of Bionic Learning Network, Dr. Heinrich Frontzek, what he thinks is going to happen with fluid power technology in biomimicry for the future. He said that it’s Festo’s belief that bionics is one of the enablers of fluidics, and that it can enrich fluidics. This relationship is how they find new possibilities and movement patterns while remaining energy-efficient.
Boston Dynamics’ Atlas humanoid robot has been through a recent redesign. This newer Atlas model is about 1.5 m tall and weighs 80 kg. It has an increased strength density to near human levels, is completely power autonomous (running between 30-60 min, depending on what it is doing) and has 28 degrees of freedom. www.fluidpowerworld.com
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The BionicCobot is based on the human arm not only in terms of its anatomical construction but like its biological role model, the pneumatic lightweight robot solves many of its tasks with the help of flexible and sensitive movements.
“Fluid power is useful for collaborative robotics. For example, the BionicCobot is based on the human arm not only regarding its anatomical construction. Like its biological role model, the pneumatic, lightweight robot solves many of its tasks with the help of flexible and sensitive movements. Due to this flexibility, it can work directly and safely together with humans,” Frontzek said. Another application for fluid power technology in Festo’s future concepts is the FlexShapeGripper. The FlexShapeGripper uses this principle of the chameleon’s tongue to grip the widest range of objects in a form-fitting manner. Using its elastic silicone cap, it can even pick up several objects in a single gripping process and put them down together, without the need for a manual conversion. The gripper consists of a double-acting cylinder, of which one chamber is filled with compressed air while the second one is permanently filled with water. This second chamber is fitted with elastic silicone molding, which equates to
“Bionics provides new approaches to product development. Applied science, which derives findings from nature and transfers this knowledge to technology, is a field for the future.” 8
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the chameleon’s tongue. The volume of the two chambers is designed so that the deformation of the silicone part is compensated. The piston, which closely separates the two chambers from each other, is fastened with a thin rod on the inside of the silicone cap. Adding to the hype is Boston Dynamics, which was founded in 1992 but has managed to create all the buzz of being the new kid on the block. Clips of some of the company’s earlier projects like the DARPA-funded BigDog led to viral video fame and it isn’t hard to see why. BigDog was the first robot with four legs articulated like a real animal. Running at up to 10 kmh, and climbing 35° slopes, videos of BigDog in action both exhilarated and frightened engineers and sci-fi geeks alike. The internet couldn’t get enough. www.fluidpowerworld.com
5/21/18 3:32 PM
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B I O M I M I C R Y
Since BigDog’s inception in 2005 the company has come a long way. Boston Dynamics took the lesson’s learned from BigDog and other early projects and created Spot, a new robot with better stability and mobility in rough terrain. There are also several new robots each with an all new bag of tricks. If you’re reading this article, you’ve likely seen the video of their humanoid robot, Atlas handle a series of box jumps before sticking the landing on a back flip. At the time of this writing the video has more than 14 million views since it was published in mid-November of 2017. What’s next? According to Boston Dynamics, the company is looking to use its past innovations to help evolve into more focused niches within robotics. “We began as a spin-off from the Massachusetts Institute of Technology, where we developed the first robots that ran and
maneuvered like animals. Now we are taking the next step, combining the principles of dynamic control and balance with sophisticated mechanical designs, cutting-edge electronics, and software for perception, navigation, and intelligence. Boston Dynamics has an extraordinary technical team of engineers and scientists who seamlessly combine advanced analytical thinking with
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bold engineering and boots-in-the-mud practicality. “We pride ourselves in building machines that both break boundaries and work in the real world.” Creating the power necessary for lifelike movement is not without it challenges. Festo specifically dealt with flexibility issues early on. Frontzek explained: “One important feature of pneumatics— and of the pneumatic robot arm BionicCobot too— is its flexibility inherent to the system. It comes about because the air in the drive chambers can be compressed. On the BionicCobot, this leads to a critical benefit over robots that are moved with geared motors, for example. In the event of a collision with an obstacle, the ability of the air to compress immediately takes effect. Therefore, you need complex
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B I O M I M I C R Y
regulation and control technology which are considerable challenges.” As each drive chamber of the seven axes is operated separately with a valve on the BionicCobot, the pressure level of each axis can be individually adjusted. The torques of each separate joint can thus be controlled depending on the application. In this way, the robot can also fulfill tasks where it has to be powerful. For example, if it is supposed to hold an object in a certain position or apply a counteracting force. “To handle these issues, we installed pneumatic drives in the BionicCobot’s joints, which move the robot. Sensors are also fitted in the axis modules for measuring the joint angles and the pressure for recording the chamber pressures,” Frontzek said. “Although the necessary regulation and control technology is complex, it enables a so-called path follower control, which is not possible with standard pneumatics: in robotics, a required movement is usually planned in so-called gripper coordinates and converted into joint angles. The individual movements of the seven joints on the BionicCobot are thus coordinated so that the whole robot arm performs the required movement. In this way, the BionicCobot can help with many assistance activities—as a third hand.” Frontzek also said his impression is that perhaps the interest in biomimicry has been increasing because people easily get in touch with technology inspired by nature. It always feels a bit familiar for them. Automation technology has the potential to carry out everyday tasks in spaces like factories for gripping, moving and positioning goods as well as controlling processes. “Nature performs all of these tasks instinctively and efficiently. What could be more logical than to examine these natural phenomena and learn from them? That’s why in 2006 Festo set up the Bionic Learning Network, a research network linking Festo to well-known universities, institutes, development companies and private inventors.” Interest in the types of research engineers are doing at the Bionic Learning 12
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Network is wide reaching. Frontzek said engineers from many industries want to learn more about how fluid power and biomimicry can work together to solve application challenges. “We don’t see that there is one industry that is more interested than others. For example, AUDI is interested in the FlexShapeGripper. Our FinRayGripper has become a product (new name as a product DHAS). It is, for example, used in the food industry to handle hot bread coming out of the oven or fruits and vegetables,” Frontzek said. “Bionics provides new approaches to product development. Applied science, which derives findings from nature and transfers this knowledge to technology, is a field for the future. Bionics made by Festo is concerned with understanding the workings of nature to transfer them to technical applications. “Our main business is automation technology. Our future concepts aim to make automated movement patterns and all our automation products, in general, more energy-efficient and offering our customers new features using bionics, e.g. an adaptive gripper that can handle fragile goods carefully and easily.” FPW
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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 psi, 5,000 psi, or even much higher) and do it in a reasonable amount of space. That’s why we see hydraulics at work in some of today’s most demanding applications, from the Caterpillar equipment building our roads and cities to the John Deere equipment servicing our farmland and the Komatsu equipment harvesting the raw materials from our mines. That’s not to say that hydraulics can’t be precise, however. You will find the technology on passenger airliners and military jets, as well as on machine tools and material handling equipment. Hydraulics differs from pneumatics in that the medium being used to transmit power is a liquid as opposed to a gas. The liquid is generally hydraulic fluid, which is based on a mineral oil base stock. In some cases, water can be used—but this requires the use of very specialized components and is not altogether common. Hydraulic fluid has low compressibility (or a high bulk modulus) and generally a good thermal capacity. Naysayers may argue that hydraulics is a dirty, loud and even an environmentally unfriendly technology. However, that lazy argument doesn’t ring true for fluid power engineers. Those claims merely indicate that the systems being described are improperly designed, installed or maintained. Understanding the operation of and parameters for the application is critical, as is a good working knowledge of sealing and how to deal with contamination. Even something as basic as adding a new component to a sealed hydraulic system can introduce contamination—something that leads to eventual systemic breakdown. The bottom line is that, as in any industrial system, smart engineering design and regular maintenance will avoid problems in the future.
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HYDRAULIC
ACCUMULATORS
HYDRAULIC ACCUMULATORS are devices that store and discharge energy in the form of pressurized fluid.
They’re analogous to rechargeable batteries or capacitors in electrical systems, and they are often used to improve hydraulic-system efficiency. These pressure vessels hold hydraulic fluid and a compressible gas, typically nitrogen. The housing or shell is made of materials like steel, stainless steel, aluminum, titanium and fiberreinforced composites. Inside, a moveable or flexible barrier—usually a piston or rubber bladder—separates oil from the gas. In these hydropneumatic units, hydraulic fluids only compress slightly under pressure. In contrast, gases can be compressed into smaller volumes under high pressures. The re-expansion of the gas is what supplies energy back into the system. Potential energy is stored in the compressed gas and released on demand to force oil from the accumulator and into a circuit.
IMAGES COURTESY OF FREUDENBERG SEALING TECHNOLOGIES
To use the device, the gas volume is first precharged—generally to around 80 to 90% of the minimum system working pressure. This expands the gas volume to fill most of the accumulator with only a small amount of oil remaining inside. In operation, the hydraulic pump raises system pressure and forces fluid to enter the accumulator. The piston or bladder moves and compresses the gas volume because fluid pressure exceeds the precharge pressure. When a downstream action such as actuator movement creates system demand, hydraulic system pressure falls and the accumulator releases the stored, pressurized fluid to the circuit. Then the charging cycle begins again. Three common types are bladder, piston and diaphragm hydraulic accumulators: Bladder accumulators, as the name implies, use a flexible closed bladder inside the shell to separate the gas and fluid. They typically have large ports that permit rapid fluid discharge and help ensure that the device is relatively insensitive to dirt and contamination. Bladder-type accumulators are usually designed to have a 4:1 pressure ratio (maximum pressure to gas-charged pressure) to protect the bladder from excessive distortion and material strain. Experts tend to view bladder accumulators as the best general-purpose units. They come in a wide range of standard sizes, and good response characteristics make them well suited for shock applications. Depending on the design, a bladder can be easily replaced in the event of failure or damage.
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Piston accumulators are much like hydraulic cylinders without the rod. Similar to other accumulators, a typical piston accumulator consists of a fluid section and gas section, with the movable piston separating the two. Less common are piston accumulators that replace high-pressure gas with a spring or heavy weight to apply force to the piston. Piston accumulators are generally recommended for large stored volumes—to 100 gallons or more—and can have high flow rates. Pressure ratio is limited only by the design, but they’re usually not recommended for shock applications. They are often built for rugged, heavyduty operations. However, they are more sensitive to contamination that can damage the seals— although most piston accumulators are readily repaired by replacing the piston seals. Diaphragm accumulators operate much like bladder accumulators. The difference is that instead of a rubber bladder, this version uses an elastic diaphragm to separate the oil and gas volumes. Diaphragm accumulators are economical, compact and lightweight devices that offer relatively small flow and volume—typically to around one gallon. A diaphragm accumulator can handle higher compression ratios of up to 8 to 10:1 because the rubber barrier does not distort to the same degree as a bladder. They also enjoy wide mounting flexibility, are insensitive to contamination and quickly respond to changes in pressures, making them suited for shock applications. 5 • 2018
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BAR
STOCK
BAR STOCK IS, QUITE SIMPLY, A BILLET, SLUG OR BLANK FORM OF RAW METAL that is purified to manufacture strong, metal components. Although it is available in a variety of shapes, it is most commonly found as round cylinders or long rectangles in fluid power use. Depending on what use it is for, bar stock can mean one of two things in hydraulics. Piston rod bar stock is used to make the strong metal rods used in cylinders, while manifolds, subplate mounts and plumbing will rely on different types of stock.
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. According to Adam Hart, plant manager at Higginson Equipment in Burlington, Ont., 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.” 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
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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. 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 www.fluidpowerworld.com
5/21/18 2:55 PM
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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. 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.
Lower Cost Per Part Look to Dura-Bar to perform better than castings and many lowmedium carbon steels for hundreds of applications. Industries using Dura-Bar for their parts include machine tool, fluid power, oil and gas, industrial, defense, mining, agriculture, automotive and more. Dura-Bar’s dense grain structure, lack of porosity and shrinkage make for a highly consistent materials with superior machinability, thereby reducing production costs. With continuously cast Dura-Bar, no costly patterns are necessary and bars are available as quickly as the next day from an extensive network of distributors throughout North America and in China. There are many ways to save with Dura-Bar Continuous Cast Iron. Customers who have made the switch profit most from significant total part cost reduction due to: • Faster Machining Speeds & Feeds • Zero Defect Guarantee • 10% Lighter Weight • Superior Vibration Damping • Pressure Rated to 6,500 psi Contact us to discuss your application at sales@dura-bar.com 800-BAR-MILL (227-6455) or visit www.dura-bar.com
Bar 9-17_FPW_bleed_REVISED.indd stock - HYDRAULICS - FPW HB 5-18_Vs4 MG.indd 18 Durabar 1
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HYDRAULIC
CYLINDERS FEW THINGS REPRESENT A FLUID POWER SYSTEM the way a hydraulic cylinder does. These true workhorses operate in industrial and mobile applications. 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. Selecting the right cylinder for an application is critical to attaining maximum performance and reliability, which means taking into consideration several design and performance parameters. Fortunately, an assortment of cylinder types, mounting methods and “rules of thumb” are available to help select the appropriate cylinder.
CYLINDER TYPES The three most common types of cylinders are tie-rod, welded and ram, the latter of which is single acting, meaning it is powered in one direction only. Tie-rod cylinders can be single acting, although they are most often powered in both directions. They have machined, square caps and heads being forced together against the barrel by high-tensile steel tie rods www.fluidpowerworld.com
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fastened by nuts, making them easy to disassemble and repair in the field. Welded cylinders employ a steel barrel with a cap welded to the bottom and the end treatment subsequently welded to the cap. The rod and piston assembly then has to be assembled around the head—which uses a buttress thread for strength—and is tightened into the barrel. Finally, the single-acting ram is typically just a rod inside a barrel with a single port and requires either a spring or mass to retract. For all 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 5 • 2018
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PURPOSE-BUILT
SMART CYLINDER SOLUTIONS When integrating hydraulic linear actuators in to your closed loop system, make sure you are working with the right cylinder manufacturer. Aggressive Hydraulics has been designing and manufacturing custom hydraulic cylinders incorporating linear displacement transducers for years. We are not tied to any one type of transducer technology or manufacturer. Instead our team of Engineers and Technicians will work with you to determine the best type of transducer to fit your application, operating conditions and parameters. • We use every major manufacturer of transducers. • Our Smart Cylinders are working in every major industry including sub-sea.
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H Y D R AU L I C C Y L I N D E R S
WHAT ARE APPLICATIONS FOR HYDRAULIC CYLINDERS?
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However, knowing the operating requirements is only half of the challenge. Cylinders must be able to withstand extreme high or low temperatures for some designs. They may be required to handle issues like humidity or salt water for marine hydraulic systems. Wherever
Hydraulic cylinders are used in countless applications. They are often seen at work in both industrial applications (including hydraulic presses, cranes, forges, and packing machines), and mobile applications (such as agricultural machines, construction vehicles, and marine equipment). They’re essential to the operation of excavators, loaders, balers, telehandlers, man-lifts, drill-rigs, and dump trucks—not to mention operating booms, arms, lifts, platforms, and buckets. Hydraulic cylinders are the most effective and efficient method of pushing, pulling, lifting and lowering. And, when compared with pneumatic, mechanical or electric systems, hydraulics can be simpler, more durable, and offer greater power density. For example, a hydraulic pump has about ten times the power density of an electric motor 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. That means taking into consideration several parameters. Fortunately, an assortment of cylinder types, mounting techniques and rules of thumb are available to help. When you’re designing an application that involves hydraulic cylinders, it’s important to consider operating conditions. Cylinders must match a specific application in terms of the amount of pressure, the force exerted, and space requirements imposed by the machinery’s design.
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temperatures typically rise to more than 300° F, standard Buna-N nitrile rubber seals may fail—so you’ll have to choose cylinders with Viton synthetic rubber seals instead. When in doubt, assume operating conditions will be more rugged than they appear at first glance. The type of metal used for cylinder head, base and bearing can make a significant difference. As noted in the main body of this article, most cylinders use SAE 660 bronze for rod bearings and medium-grade carbon steel for heads and bases. But users should also consider extreme temperatures. Typical carbon steels used in cylinder components are generally suited for around –5 to 200° F. In arctic conditions well below 0° F, for example, standard steel can become brittle and may require alternative materials. In addition, to prevent an extended rod from bending under its own weight, use additional support, such as a stop tube or a larger diameter rod, which increases strength. Some questions to consider include: What is the maximum pressure range for the application? Do I need the cylinder to push or pull—or both? What push or pull tonnage is required? What stroke length will be required? What mounting method is being used? How much support will the piston and cylinder require?
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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. CYLINDER MOUNTING METHODS Mounting methods also play an important role in a cylinder’s performance. Generally, fixed mounts on the centerline of the cylinder are best for straight line force transfer, ideal column loading and avoiding excessive wear. Pivoting mounts, such as clevis or trunnion, require care in application, because of their capacity to move as the cylinder is stroked, resulting in a possible bent rod or excessive wear. Common types of mounting include: Flange mounts—Strong and rigid, but have little tolerance for misalignment. It is recommended to use cap end mounts for thrust loads and rod end mounts for loads under tension. Side-mounted cylinders—Easy to install and service, but the mounts can sometimes create a bending moment as the cylinder applies force to a load, increasing wear and tear. To avoid this, specify a stroke at least as long as the bore size for side mount cylinders (heavy loading tends to make short stroke, large bore cylinders unstable). Side mounts, such as side lugs, need to be well aligned and the load supported and guided.
Centerline lug mounts—Absorb forces on the centerline, but require dowel pins to secure the lugs to prevent movement at higher pressures or as a result of shock loads. Pivot mounts—Absorb force on the cylinder centerline and let the cylinder change alignment in one plane. Common types include clevises, trunnion mounts and spherical bearings. Because these mounts allow a cylinder to pivot, they should be used with rod-end attachments that also pivot. Pivoting mounts are required for many applications, such as booms and buckets, but are also most prone to rod buckling, especially as the rod reaches end of stroke. 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 snap-ring cylinders rated for 2,000 psi should be avoided. Cylinders are designed with safety factors of 2:1 to 4:1, so sometimes running slightly overlimit might be acceptable, but not double. WHAT STROKE LENGTH WILL BE REQUIRED? Ensure that the machine has appropriate clearance, because the longer retracted length of the cylinder should be factored. Also, if stroke is too long, additional support will be required, such as a guided load or stop tube. WHAT MOUNTING METHOD IS BEING USED? Flange mounting is often best because the load is transferred along the centerline of the cylinder. Noncenterline mounting calls for additional support to avoid misalignment, but these are required when the mechanism must pivot through an arc, so load calculations must be factored accurately. PUSH OR PULL OR BOTH? Any cylinder can be used as single acting, which is powered in one direction only, but it can only push or pull. 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 piston’s load-bearing effect. 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. A double acting cylinder is powered in both directions to push and pull.
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H Y D R AU L I C C Y L I N D E R S
WHAT PUSH OR PULL TONNAGE IS REQUIRED? Always assume peak loads will require additional strength. The rule of thumb is to choose a cylinder with a tonnage rating of 20% more than required for the load; however, this is always application-specific, so it’s best to consult a hydraulic professional before you make tonnage assumptions. Cylinder force (lb) is equal to the area of the piston (in.3) times pressure (psi), or F=AxP. 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 stainless-steel 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 applied seals can reduce friction and wear, lengthening service life, but incorrect types can lead to downtime and maintenance headaches as a result of failures. Every manufacturer likes to use a different 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 big difference in performance and reliability. Most cylinders use bronze for rod bearings and medium-grade carbon steel for heads and bases. 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 corrosion resistance. 5 • 2018
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HYDRAULIC FILTERS
INLINE CARTRIDGE FILTER ASSEMBLY. IMAGE COURTESY OF HYDAC
MANY FLUID POWER SYSTEMS fail simply because there is too much particle contamination in the fluid medium. In fact, some estimate that 75% of all fluid power failures can be attributed to contamination-related issues. Thus you, as an engineer, technician or end-user, who ignores filtration does so at the peril of your hydraulic system. There are multiple reasons why your hydraulic fluid becomes contaminated. Every hydraulic machine is first manufactured with builtin contamination during machining, cutting, welding and grinding of the reservoir and fixed plumbing. Additionally, contamination ingression also occurs from either new oil (which is dirtier than you imagine) or external sources such as fallout, grime, mud and dust. Finally, the components in your system generate their own particles when friction components such as bearings, pistons, spools and swashplates rub together. Removing all forms of particle contamination is your highest priority to ensure a long, reliable life for your hydraulic machine. Filters are your first line of defense to reduce the number of particles in your fluid. Filters also prevent excessive internally-generated contamination, considering particles exacerbate the rate of internally generated contamination, acting like liquid sandpaper. There are several types of filters for you to choose, the most popular of which are inline cartridge and spin-on filter assemblies. The inline cartridge filter assembly is popular and is available for pressure and return lines. These assemblies have a drop filter cartridge (as shown in the cutaway on this page) that can be removed and replaced when
WHERE SHOULD YOU LOCATE HYDRAULIC FILTERS IN YOUR SYSTEM? Is there an ideal location for placement of hydraulic filters in an operating system, and if so what is the correct type of filter to use? Is one filter sufficient, or should multiple filters be used? Can different types of filtration be combined? Most hydraulic systems use a single return line filter, most often either an in-tank unit or an inline spin-on filter assembly. Return filters are typically the most economical and practical, if only one filter must be used. They clean all fluid returning from the system, which removes
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any internally generated particles. You should be careful when selecting return line filters, however, because spikes in both pressure and flow can occur in return lines. Pressure filters are common for use after the pump, effectively safeguarding downstream components from particles either ingested into the reservoir or generated by the pump itself. You can even purchase pressure filters without a bypass valve, which normally allows flow to circumvent the filter when it begins to clog.
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OFFLINE FILTER SYSTEM. IMAGE COURTESY HYDAC
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they become clogged. Spin-on filter assemblies are also used for inline applications, although their location is typically limited to return lines. Some manufacturers make heavy-duty assemblies, rated for upwards of 500 psi, which make them ideal for you to use in return lines experiencing pressure spikes, like the examples from MP Filtri, pictured. Other filter options exist, such as bag filters and suction strainers. Bag filters are used when large volumes of fluid are being processed, such as is required in steel mills. They are less common in a “live” system where they’re exposed to primary pump flow but are instead more popular in high volume kidney loop “offline” systems (see sidebar Where should I location my hydraulic filter?). Also fairly common are suction strainers installed in the reservoir’s pump outlet port. They’re often made from woven steel fibers and are designed to remove larger chunks of contamination that could harm a pump. Care must be taken to ensure your suction strainer doesn’t impact pump inlet conditions, as excessive flow resistance increases the likelihood of pump cavitation and resulting damage. Filter construction is also important when you choose your assembly. Construction dictates not only where your filter can and
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H Y D R AU L I C F I LT E R S
WHERE SHOULD YOU LOCATE HYDRAULIC FILTERS IN YOUR SYSTEM? (CONTINUED) When ultimate filtration is required, an offline system is most efficient. Offline filtration requires a dedicated pump and motor to circulate tank flow through an often very fine filter with high dirt holding capacity. Often called a “kidney loop” filter, these systems run even with the machine does not, and are not exposed to unstable operating conditions related to the primary circuit. The downside is the added expense of an additional pump, motor and filter assembly. Regardless of which location you install your filter (which is, of course, all three), ensure it operates using a bypass indicator. The bypass indicator comes in either a pop-up device, a pressure gauge or lighted switch. When backpressure measured in the filter reaches a preset point—typically between 60-80% of bypass valve cracking pressure—the indicator warns you the element must be soon changed.
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HEAVY-DUTY FILTERS FROM HYDAC
should be located, but also the flow and pressure rating of the assembly. Material construction ranges from plastic or aluminum for low-pressure (500 psi or less) inline or return line assemblies. For medium pressure locations (1,000-3,000 psi), aluminum or steel housings are required. High-pressure filter assemblies (those rated higher than 3,000 psi) require steel construction for both their filter head and bowl, and are often installed with elements constructed for higher collapse pressure, such the heavyduty filters from HYDAC, pictured. The construction design of a filter assembly varies specifically with its installed location, and differs based on where in the circuit you locate the filter. Any filter installed in a working pressure line requires the capacity to survive that very pressure, and then some for safe measure. Return line filters are generally only required to handle backpressure related to flow, which increases due to both flow intensification and also to pressure differential created from the clogged element itself. Filters are sized appropriately to handle the maximum flow possible with reasonably low backpressure. Filter assemblies are installed with bypass valves that open when backpressure reaches a predetermined level. The backpressure is created as the element becomes clogged with particles. As the bypass valve opens,
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FLUID POWER HANDBOOK
fluid sidesteps the element itself, flowing around it to avoid excessive and damaging backpressure, especially in return lines. As a bonus, larger filter assemblies have higher dirt holding capacity, which itself is a critical design consideration. Once you arrive at a filter assembly suitable to your application’s installation requirements, have selected the appropriate pressure rating, and then sized it appropriately to reduce backpressure, you can continue by considering how finely you want to filter your fluid. Every filter manufacturer of reasonable reputation tests and then publishes their filtration ratings, expressing the lowest micron size the filter will efficiently remove, and what that efficiency rating is. You hear filters referred to as their micron rating, such as 5 micron. Anyone can throw a rating at a filter and call it a day, but how you qualify that number dictates how effective the filter is at removing particles of the rated size. Manufacturers must express the beta ratio measured at the given particle rating size for the rating to mean anything. The beta ratio expresses the difference between particles measured before a filter and then after the filter. The higher the ratio, the higher number of particles were trapped in just one pass through filter on a dedicated test rig with special test dust. For example, a beta ratio of 200 represents that for every 200 particles entering upstream of the filter only one particle makes it through. Just as you must specify beta ratio with micron rating, so too must you specify micron rating with beta ratio; they are arbitrary without each other. Written properly, it may read β5 ≥ 200, which means the filter is rated for 5 microns, greater than or equal to beta 200. Once the beta ratio is known, some simple math converts that number to an efficiency rating. Simply take the beta ratio, subtract one, and then divide by the beta ratio. For example, (200-1)/200 = 0.995, or 99.5% efficiency. The previous example tells us our hydraulic filter removes particles of 5 microns and larger at an efficiency of at least 99.5% in a single pass. Always look for the highest beta ratio you can find.
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DID YOU KNOW?
Flow intensification occurs when a differential cylinder retracts; the flow entering the rod side results in higher velocity from the same flow entering a smaller volume.
HEAVY-DUTY FILTER ASSEMBLIES. IMAGE COURTESY OF MP FILTRI
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ADVANCEMENT IN FLUID CONDITIONING
To ensure equipment up time, reduced maintenance costs, and protect both the engine and the drive system from damage, optimum fluid conditioning is critically important.
• Provides Local Visibility to the Fluid Condition of Critical Systems. • Integrated micro VSD, (Variable Speed Drive), pump/motor provides optimal flow for accurate sensor readings in variable conditions. • The HY-TRAX Manually Controlled Fluid Sampling System allows a user to retrieve ISO cleanliness levels from a reservoir tank or a low-pressure line (<50 psi max). ®
• Unique HELIOS pleat geometry – optimizes media area open to flow to calm the flow in areas between pleats reducing ∆P. • Outer wrap perforations insure optimized flow onto the filter pleats and help to minimize pressure losses.
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HYDRAULIC
FILTRATION SYSTEMS FILTRATION is always on a hydraulic maintenance technician’s mind. But for some machinery or in some plant setups, inline hydraulic filters are not enough to keep a hydraulic system functioning properly. Most hydraulic systems are installed with at least a return filter, which semi-purifies fluid before it is once again welcomed into the reservoir. However, what if a single return filter is not enough? What if your cleanliness codes are not achieved, even if you’ve upgraded to a finer filter media? A pressure filter is an option, which will keep the components downstream of the element one step cleaner. But what if using a pressure filter is impossible, due to plumbing difficulties or pressure drop considerations? A solid option to increase filtration effectiveness is with an offline filter system, often called kidney loop filters. Offline filtration uses a dedicated lower pressure pump (still often a hydraulic pump), which draws fluid from the reservoir and then flows that fluid through a dedicated filter assembly—usually of a highquality 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
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allows for live selection of either filter so the other can be replaced. Most filter manufacturers offer a filter system dedicated to the offline filtration market that is highly efficient and offers high dirt holding capacity. Offline filtration is typically the highest quality in a manufacturer’s product line, which is reflected in the cost of these products. To help justify the purchase of such a system, they are often sold as portable units, small units that can be carried by a handle, or large units requiring a wheeled cart to manage their bulk. These units can be wheeled from machine to machine, where a suction tube is placed into a port of the reservoir and then passes through its own filters before being injected back into the tank. Depending on the size of the tank, the filter system’s flow rate and filter quality, one might leave the filter system running on the machine for hours or perhaps days. Permanently mounted offline systems are now more commonly used as well. They are often mounted to a panel, either near the reservoir or directly attached to it. Eliminating intermittent filtration of the portable type ensures that fluid is clean from storage to service. Some of these filter systems are installed with auxiliary electronics, such as particle counters. A particle counter will give you a live reading of the ISO Code of the oil passing through the unit, so you can leave the unit running until the desired code is achieved. If this type of system seems out of your reach, note that some hydraulic distributors will rent these machines out for a reasonable cost. www.fluidpowerworld.com
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H Y D R AU L I C F I LT R AT I O N S YS T E M S
WHEN SHOULD YOU FLUSH A NEW OR REBUILT HYDRAULIC SYSTEM? Particulate contaminants circulating in fluid power systems cause surface degradation through general mechanical wear, such as abrasion, erosion, and surface fatigue. This wear causes increasing numbers of particles to be formed, the result being that wear increases if this chain reaction of wear is not properly contained (by reducing contamination). Gaps within components grow larger, leakage oil flows increase in size and operating efficiency (e.g. of pumps, cylinders) decreases. To avoid this, you should always remove particulate contamination in new systems, before startup, and later when the system is in use. The overall contamination level of a hydraulic system being built in an assembly plant is generally high. The total contamination (number of particulates) consists of contamination that already exists in the new fluid, contamination that is already on/in supplied components and contamination introduced during the build process from the surroundings. The sum of all the contamination can be considered the Initial Contamination Level. The investment in time and equipment to reduce the Initial Contamination Level is worthwhile because of a reduction of costly warranty claims and for quality control and tracing purposes of products. If the fluid was not maintained to recommended cleanliness standards during use, a warranty claim could be rejected. Fluid in the reservoir should be flushed with a filter cart or a kidney loop system. It is recommended to flush the system and all the sub-functions by activating them to allow fluid to circulate and to flush particulates back in to the tank where they can be captured, either by the system filters (e.g. return filters) or by external, offline filter systems (filter carts, kidney loop systems). A general rule is to achieve a cleanliness level of the hydraulic fluid in the reservoir that is 1 to 2 ISO Codes below the recommended Target Fluid Cleanliness level for the system.
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HYDRAULIC
FITTINGS AND FLANGES HYDRAULIC FITTINGS connect conductors such as hoses, pipes and tubes to the components in a hydraulic system. They allow the pressurized fluid to move through the system without leakage. Available in a variety of styles to change and direct, change or split flow, most fittings have a male and female component that join to form a connection. They can be manufactured as unions, plugs, crosses and elbows. It is critical to carefully identify threads on the fitting connections, as these threads can look almost identical from one standard to another. However, because they are not mates, they will not properly engage. In addition to reviewing documents from SAE, NFPA and ISO to help identify each standard to confirm diameter and thread type, keeping thread identification kits on hand can help with this concern. See the included sidebar for more on thread ID kits. 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 32
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More is more! 10,000 new products We have added over 10,000 new fittings and accessories to our product line including an expanded selection of DIN fittings, DOT and brass products and hose tails. Contact Brennan today and see why More is More!
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WHY SHOULD YOU USE THREAD IDENTIFICATION KITS? Thread Identification kits are available for countless styles of hydraulic hose fittings, particularly for common styles and sizes, for example, NPT, SAE, metric and BSPP fittings. These kits are designed to reduce confusion between different styles of fittings, which have fairly similar threads. They’re also used to differentiate between male and female fittings. Although fittings may look the same, they will not mate together properly — which can be a serious problem. If you try to mate parallel threads of two different styles together, you’ll quickly see leakage and eventually damage to the threads. Individuals in maintenance or repair of hydraulic equipment should have thread identification kits on hand when they need to replace fittings, and especially for components manufactured for or in the European market. Most kits come with the most common sizes of both male and female thread fittings and they are usually color-coded with a matching guide on the lid to make it easy to tell the different styles apart. The size is often imprinted right on the fitting. These kits comes in a tough, locking case that be can easily used throughout a plant or job site. Using the kit’s included fittings along with your ID/OD calipers to measure thread inner and outer diameters and thread pitch gauges for thread pitch and size will help prevent costly mistakes.
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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
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very few applications require anything other than Buna Nitrile or Viton. When selecting a fitting, several considerations are important. Most non-flanged fittings have a gender— called male and female—that are joined together to form a union. Most fittings are sized based on the size of
the conductor (size of hose, pipe or tube), and overall dimensions can vary greatly based on fitting type, even for the same size conductors. Additionally, most fitting types are available in a multitude of materials, including plastic, brass, steel, stainless or specialty metals like Monel. Each are applied in applications based on the fluid medium and ambient conditions, and each has different performance characteristics that allow customization within a fitting type. Often the first choice is to match the fitting to a similar material of the conductor or component that it is connecting to: plastic to plastic, steel to steel and stainless to stainless. Geometry is also an important consideration, and geometry is typically identified by alphabet letters the fittings resemble. Fittings are available inline to change the direction of flow in various increments (45˚ or 90˚ elbows [L]), or a swivel to allow two joined sections to rotate. They can also split or combine flows with run and branch www.fluidpowerworld.com
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FITTINGS AND FLANGES
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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 cross-sectional area. O-ring face seal, SAE straight thread and ISO 6149 fittings have a seal, normally Buna N, contained within a groove to seal the fluid. It is important for the seal to be compatible with the fluid and the operating temperature range. An elastomeric seal greatly reduces the possibility of leakage caused by vibration, thermal cycling and pressure cycling. SAE J518 split flange fittings are used on larger line sizes, starting at 1/2 in. (-8) but coming into predominance at 2 in. (-32) and above. A flange head with an O-ring groove on its face is attached to a conductor (hose, tube or pipe) and is secured to the port, which could be a flat-face fitting or a pad on a pump, valve or cylinder, by a clamp with four bolt holes. The clamp can be whole, but is often split so that a quarter of the diameter of the flange head is on either side of the centerline of the 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 market is 37°, and it is popular enough that the 24° and 45° versions are rarely used. The fittings can be designed to clamp onto a tube by means of a sleeve or ferrule, and care needs to be taken so that the correct size is used because inch and metric tubing sometimes have sizes that are close to overlapping. The quick disconnect allows multiple reconnections of the assembly without causing excess wear or concern for thread damage. Some fittings allow www.fluidpowerworld.com
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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 contamination, which was addressed with the advent of flat-face couplers, which have no recess to collect contamination. Staple and band fittings are low-pressure fittings. Band fittings are attached to the hose by a barbed or beaded end being inserted into a hose and a band clamp securing the connection. This method is only for extremely low pressures. Staple fittings have a cylinder with an O-ring and a bead further up on it that slides into a socket. The connection is secured by a staple that goes through both sides of the connection behind the bead, although it is still typically used for low pressure or suction lines.
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HYDRAULIC
FLUIDS against metal surfaces. Without a method to carry thermal energy away from these surfaces, overheating can result with damage to seals, valve plates and other components. As oil returns to the reservoir, it often passes through a cooler to help maintain an optimal temperature range before being pumped out to the system once again. Hydraulic fluid can also carry heat from a warmed tank or, via a special warming circuit, into a cold system to help minimize the possibility of damage during cold starts. Closed-loop hydraulic systems require a special bleed-off circuit connected to tank, to ensure that some oil is always being cooled. A charge or replenishing pump brings the cooled oil back into the circuit to help maintain a suitable overall system temperature. Contaminant removal. Hydraulic fluid can suspend and carry solid particles and water away from sensitive components. Filters and other conditioning devices perform the critical job of stripping and trapping the contaminants, and letting clean fluid return to the circuit.
Image: istockphoto.com
HYDRAULIC FLUID is the medium used to provide consistent and powerful transmission of energy throughout a hydraulic system. That, in turn, allows actuators and drives to generate linear or rotary motions and perform work. This simple energy-transfer function is only achieved by a fluid that does not easily trap gasses. Trapped gas and foaming problems would bring a higher level of compressibility to a fluid that is usually relied upon to support a very stiff, fast-reacting system that functions safely, repeatedly, reliably and efficiently. Where transmitting energy is the core function of hydraulic fluid, it is also useful in four secondary functionsâ&#x20AC;&#x201D;heat transfer, contamination removal, sealing and lubrication. Heat transfer. Hydraulic machines produce a lot of excess heat in normal operation, often caused by inefficiencies within the components themselves. Pumps and motors allow fluid to pass through the fine clearances between internal parts when system pressures are high. The heating in this situation is caused by large volumes of fluid molecules rubbing
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H Y D R AU L I C F L U I D S
Knowing that solid contaminants are suspended in a fast-moving stream of fluid is not a substitute for monitoring cylinder wear or for a suitable filtration program. When a flow valve opens only slightly, and the pressure differential across the valve is high, the same suspended particles may now cause erosive damage to a valve as though it had been shaped with a file or a grindstone. Sealing. While seals and O-rings close the major clearance between some parts, the hydraulic oil finishes the job in the fine clearances where the solid material of an undersized shaft seal might cause damage. Another example is the spool valve which has a seal at each end to prevent oil from escaping to atmosphere. Inside the valve each notch and undercut on the spool is sealed from the next only by the tight tolerance of the spool and valve bore, together with the oil’s surface tension and resistance to shearing. Critical to this sealing function is the viscosity of the fluid. Viscosity index (VI) which is the change in viscosity over a swing of temperature changes, is also a key factor. A fluid with a high VI number is able to resist changes in viscosity as it heats up, allowing the fluid to maintain a consistent seal. Lubrication. Lubrication is required in most hydraulic components to protect internal parts from frictional wear. Oil provides fullfilm lubrication between moving parts, such as the slippers and valve plate of a piston pump. Without the lubricating properties of oil, hydraulic
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HYDRAULIC FLUID TEST MATCHES REAL-WORLD CONDITIONS In recent years, operating pressure and power density in many newer hydraulic systems have increased significantly. Not too long ago, pressures in circuits topped out at around 3,000 psi. Now, systems running at 5,000 to 7,000 psi or more aren’t uncommon. On one hand, that can mean better performance, smaller and lighter components and, possibly, sizeable cost savings. On the other, it can also increase the susceptibility to wear and premature failures in hydraulic systems. New-generation pumps and motors are designed to handle higher loads and offer long life. However, one can’t always say the same for the fluids they use. That’s because the type and quality of oil has a significant impact on the wear characteristics of hydraulic components and, with it, the reliability and longevity of fluid-power systems. Unfortunately, as hydraulic components and systems have rapidly evolved to meet more-rigorous operating conditions, the standards for rating fluids have struggled to keep pace. According to engineers at Bosch Rexroth, the problem is that while pressures, temperatures and oil-circulation rates have risen sharply, the minimum requirements of relevant standards like ISO 15380 and ISO 12922 do not reflect the elevated loads and stresses that today’s hydraulic fluids face.
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systems would be unreliable with a very short life for many components. The majority of hydraulic machines use refined mineral oil base stock or a synthetic oil. These oils are formulated and manufactured to specific industrial test standards for important properties such as viscosity, viscosity index and pour point. These three properties along with ambient and operating temperatures are often carefully considered when choosing a fluid. If ambient machine temperatures are low, one would choose an oil with lower rated viscosity and pour point. Pour point is simply the temperature at which oil will still pour. If a machine sees varying temperatures, as happens to an all-weather mobile machine, a high viscosity index is crucial. It is important to consider the viscosity requirement as specified by component manufacturers. A piston pump, for example, may require a viscosity between 16 and 40
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centistokes. Centistokes describe the kinematic (measured while flowing) viscosity of a fluid, regardless of temperature. This data helps the user select a final ISO viscosity that will conform to required kinematic viscosity, at the final operating temperature. Hydraulic oil has a package of chemical additives designed to improve the performance of both the oil and the components in the hydraulic system. These additives can improve the foaming resistance of the oil, or help to quickly release at the tank any trapped air brought into a hydraulic system via bad cylinder or motor seals, or through poorly sealed hose connections. Rust and oxidation inhibitors are powerful chemicals that can ultimately reduce the internal production of particle contaminants as they trap water and keep it away from ferrous metal surfaces. Vane pumps are among the most efficient from a volumetric standpoint. There is little if any clearance between the knife-edge of the vane and the cam ring.
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HYDRAULIC FLUID TEST MATCHES REAL-WORLD CONDITIONS (CONTINUED) One result is that a high-pressure circuit may run trouble-free with one fluid but quickly fail with another—even though both fluids meet the same standard. Until now, machine manufacturers could only rely on arduous and expensive trialand-error tests to find suitable fluids that meet their application requirements. To address that, Bosch Rexroth recently developed a new evaluation method for rating hydraulic fluids that realistically reflects higher performance demands. In the test, described in Rexroth data sheet RDE 90235, technicians assess the behavior of fluids and their interactions with components like axial-piston pumps and motors in hydrostatic drives, under real-life conditions. The test reportedly goes far beyond current DIN and ISO requirements. Rating criteria are based on component weight and dimensional changes, material compatibility issues, and the degree of fluid breakdown during the tests. From this, technicians report conclusions on the long-term running behavior of the components based on how well the test fluid stands up. Currently worldwide, products from a number of oil and lubricant manufacturers meet all the requirements and make the list. These include: Atak Madeni, Fuchs, Gazpromneft Lubricants, Hermann Bantleon, Hindustan Petroleum, Liqui Moly, Panolin, Petrol Ofisi, Shell International, SRS Schmierstoff, Petronas Lubricant, TongYi Petroleum, Total Lubricants, and Zeller + Gmelin. According to fluid suppliers, these approved high-performance fluids offer a host of benefits for machine builders and users. Not only can equipment run with higher loads and temperatures, components see less wear and last longer, reservoirs and lines can be smaller, and fluid change intervals are longer—all which leads to less downtime and lower costs, versus machines running on conventional hydraulic oils.
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To help provide a lubricating boost for these pumps, a hydraulic fluid with an antiwear or extreme pressure additive is required. These additives react with metal surfaces creating a thin, sacrificial lubricant film. The overall additive package often separates highquality fluids from economy priced hydraulic oil, where a poor additive mix can actually become corrosive to the yellow metals (brass and bronze) used in hydraulic components. Hydraulic fluid is the life-blood of many mobile and stationary machines. It is difficult to overemphasize the care that should be taken to maintain this precious medium. It is equally important to remember that while a quality fluid has been engineered and designed to perform challenging tasks, it cannot compensate for a system with an undersized reservoir or a motor with an excessive shaft load. If the hydraulic components are properly specified and the overall system is well designed, a good quality hydraulic fluid will serve the critical function that ties the pump and the actuator together, along with all components in between.
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HYDRAULIC
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HOSE
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HYDRAULIC HOSE is a common and important element in countless industrial and mobile machines. It serves as the plumbing that routes hydraulic fluid between tanks, pumps, valves, cylinders and other fluid-power components. Plus, hose is generally straightforward to route and install, and it absorbs vibration and dampens noise. Hose assemblies—hose with couplings attached to the ends—are relatively simple to make. And if specified properly and not overly abused, hose can work trouble-free for hundreds of thousands of pressure cycles. Hydraulic hoses often consist of an inner tube, one or more layers of reinforcement, and an outer cover. Each constituent should be selected with the intended application in mind. Typical operating and performance parameters include the size, temperature, fluid type, pressure-holding capacity and environment, to name a few. Reinforced hose is constructed with some structural element— styles include spiral wire, textile braid, wire braid, wire helix and other designs in many plies or layered configurations. The inner tube contains the fluid and keeps it from leaking to the outside. The cover protects the reinforcement layer. Other construction options for hydraulic hose include coiled, corrugated or convoluted. 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
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integral end connections, anti-static, lay flat, crush-proof and flame-resistance characteristics. In addition, material considerations include the type of fluid being conveyed and its concentration, as well as substances that may attack the hose cover. Hose selection must ensure compatibility if it is to convey special oils or chemicals. The same holds for hose exposed to harsh environments. Substances such as UV light, ozone, saltwater, chemicals and pollutants can cause degradation and premature failure. For in-depth fluid compatibility data, consult the manufacturer. While hydraulic hose is usually constructed of multiple materials, the most commonly used primary materials include elastomers, fluoropolymers and silicone, thermoplastics, metal, and composite or laminated structures. Elastomeric or rubber hydraulic hose are often selected for their flexibility. Fluoropolymer hose offer good flex life, superior chemical and corrosion resistance and can handle high www.fluidpowerworld.com
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HANDBOOK
HOW DO YOU PROTECT HYDRAULIC HOSE? Hydraulic hose assemblies rarely operate in controlled, protected environments. Rather, rough and demanding conditions are the norm. Thus, hose is often exposed to flying debris in mobile equipment and the likes of weld spatter and hot metal chips—or even the errant lift truck—in industrial settings. Add to that, hoses subjected to continuous movement and machine vibration can rub against each other and adjacent equipment. That can erode hose covers, expose the reinforcement to dirt and moisture, and lead to quick failure. Thus, it’s essential to protect hydraulic hose against abrasion, physical impact, crushing loads and high temperatures to ensure long, reliable operation. Here are some common ways to safeguard hose. Shielding. Perhaps the best way to keep hose safe is to create a physical barrier against harm. Something as simple as a sheet-metal guard can protect vulnerable sections against impacts, sparks, weld spatter and intense heat near furnaces and foundry equipment. When space limitations or convenience means shielding isn’t suitable for an application, there are several other options to consider. Spring guards. Spring guards, as the name implies, are wound steel springs typically zinc-plated for corrosion resistance. They come in various diameters slightly larger than the hose OD. They install by simply sliding over the hose and are available in standard lengths or in bulk coils that can be cut to fit. Steel-spring hose guards support hydraulic hoses and help protect against abrasion, cuts, deep gouges, impact, and accidental compression. A related product is the open/closed coil hose guard. It fits over the hose OD but only extends about 12 in. from the coupling. Closed spring coils (each wire coil abuts the next) run about half the length—for firm support and attachment near the fitting. The remaining open, spaced coils typical of conventional spring guards provide strain relief. The device helps prevent the hose from excessive bending and kinking adjacent to the coupling. Protective sleeving. Protective sleeves are textile tubes that slide over a hose. They’re made of materials like woven nylon or polyester and provide excellent resistance to external abrasion and UV radiation. Some products are rated for MSHA flame resistance. Others, made of materials like polyamide, protect against hydraulic hose bursts and pinhole leaks. Sleeves are also an effective way to bundle together similarly rated high- or low-pressure hoses that bend and flex in the same direction.
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Protective wrap. This product is much like a protective sleeve, but is designed to affix after assemblies are already installed. The abrasion-resistant fabric material comes flat and includes a Velcro-type closure to easily wrap around and fasten over the hose or bundle several hoses together. Plastic spiral guards. Flat, spiral-shaped plastic guards wrap around a hose and, depending on the application, can be secured without removing an assembly from a machine. They’re made of materials such as heavy-duty polyethylene and have a flat profile with rounded edges to prevent damage to the hose. Plastic spiral guard offers a light and economical way to protect hoses from abrasion, cuts, impact and crushing loads, as well as UV attack. The plastic guard is relatively easy to install and it can bundle together several hose lines. Fire sleeve. A number of companies offer heat and flame-resistant hose protection products, often termed “fire sleeve.” This tough, flexible insulating cover slides over a hose and clamps to the fittings. Typically made of woven fiberglass or material like high-temperature silicone, it protects hydraulic hoses and pneumatic tubing against extreme radiant heat, sparks, weld spatter, molten metal and glass splash, as well as open flames. Point-of-contact sleeve. Rather than covering an entire assembly, sometimes only short lengths see contact with other surfaces and face abrasive wear. These slipon protectors minimize harm to hoses and fluid lines, as well as wire harnesses and battery cables. They’re made of inexpensive but durable materials like PVC and attach with cable ties or band clamps.
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H Y D R AU L I C H O S E
QUALITY HYDRAULIC HOSE & ACCESSORIES NEW
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temperatures. Thermoplastic hydraulic hose offers tight minimum bend radii and excellent kink resistance. Metal hoses can handle high temperature flow materials and often can handle higher pressures. They can be either stiff or flexible. Flexible hoses are easier to route and install, compared with rigid tubing and pipe. They lessen vibration and noise, dampen pressure surges and permit movement between parts. In addition, increasing demands for higher productivity, efficiency and environmental compatibility are forcing hose manufacturers to improve product integrity—hoses now withstand higher pressures, extreme heat and cold and accommodate a range of fluids including today’s “green” variants. Most hoses are manufactured to SAE J517, European Norm (EN) or ISO Standards. These standards predominate in the Americas, Europe and Australia, and are also used throughout Asia.
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HYDRAULIC
HOSE COUPLINGS
IMAGE COURTESY OF KURT HYDRAULICS
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WHILE SELECTING the proper component for any aspect of a fluid power system is important, choosing the right hoses, tubing, fittings and assemblies is a true safety issue. Selecting the incorrect style can cause failures, property damage, or even personnel injury. Understanding the hose assembly and following the installation instructions provided by manufacturers will reduce these risks. For engineers looking to specify hose couplings, they need to consider a few things besides correct size. Will the couplings be reused or permanent? Will they need a locking mechanism to prevent involuntary disconnection? Is one-handed operation required? Couplings can be two types: permanent and fieldattachable (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 pre-assembled (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. 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.
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HOW CAN QUICK COUPLINGS IMPROVE HOSE SAFETY? Hydraulic and pneumatic hose safety can be ensured by using quick-connect fittings or quick couplings that make a repeated connection and disconnection between fluid lines and the equipment they are attached to. Quick couplings are used in both hydraulic and pneumatic applications and are designed for easy hand operation. These safety devices feature a male end—or plug—that is inserted into a female end—or socket—to make a secure, leak-tight seal. In extremely high-pressure hydraulic applications, a leak or accidental disconnection can cause serious personal injury or damage to machinery. In pneumatic applications, compressed air presents great dangers for hose whip. The designs usually feature a one-way sleeve to allow for break-away with a tool when a coupling is clamp mounted. Two-way sleeves allow for one-hand disconnection. In two-way designs, twisting and pulling the two ends breaks the connection.
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One of the most common designs is the flat face design, which is available as push-to-connect, threaded or screw-in. They eliminate any cavities where fluid or air can rest, thus removing the chance for trapped pressure and leakage. Flat face couplings provide high flow and low pressure drop and their sleeve-locking feature reduces the chance of accidental connection, removing leakage and spillage risks.
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IMAGE COURTESY OF KURT HYDRAULICS
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.
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H Y D R AU L I C H O S E C O U P L I N G S
HOW CAN QUICK COUPLINGS IMPROVE HOSE SAFETY? (CONTINUED) Ball or bearing types feature springloaded balls lodged in cavities to make the connection. They can be disconnected with just one hand. For aggressive media, one should use non-spill designs. Nonlatching couplings are heavily used in medical and test applications where frequent change-outs are common. Bayonet couplings feature two plugs that are engaged and locked by completing a quarter-turn to push the male end into the female socket. Disengaging and separation is the same, but in the opposite direction. When selecting a hose coupling assembly, the lowest rated component is the actual safety factor. So, if you have a 5,000 psi-rated hose but a 4,000 psi coupling, the safety factor of the assembly is 4,000 psi. When working with aggressive media, using color-coded or key locks helps avoid cross connection. Ensure sealing materials and all internal materials are compatible with the fluids being used. And if available, check alert rings to ensure a system is properly connected; for example, some
Hydraulic hoses that claim to meet SAE J517 standards (for example, SAE 100R1 and SAE 100R2) need to be designed for, and certified to, the criteria defined by SAE. That criteria includes stringent dimensional tolerances (inside, outside and braid diameters), compound and reinforcement types, length changes, cold flexibility and ozone and heat resistance. There are also burst pressure and impulse requirements in J517. Those requirements are for coupled assemblies, and SAE states that “the general and dimensional standards for hydraulic hose fittings are obtained in SAE J526.” Hydraulic hose fittings that meet SAE J516 standards are similarly well defined by SAE as to material type, dimensions, finish and so on. The SAE manual also specifically states that J516 fittings are intended to be used “in conjunction with
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manufacturers use a red alert ring that disappears when a system is properly connected. The dangers of oil injection injuries are real with high-pressure hydraulics. This is why hydraulic high-pressure quick couplings require more than just a Teflon tape seal or thread sealant. The higher pressure you are using, the stronger the seal required—ranging from rubber metal seals and copper seals to fully metal seals. Ensuring a safe end of life with quick couplings is also critical. When a coupling fails, you don’t want it to explode and have locking balls flying out. The design should fail with predetermined leakage paths, so fluid comes out slowly. To avoid unplanned failure, conduct regular inspections and maintenance to check for wear and damage. Ensure there is no brinelling occurring where the locking ball is. Also, a hose could fail before the coupling so a visual inspection for abrasion and cuts is necessary. In pneumatic applications, safety couplings are designed to vent compressed air when the sleeve is pulled back while holding the nipple and hose assembly in place. After all the air is vented out of the system, only then does it release the hose, which prevents hose whip, popping noises and even moisture from escaping the system.
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
POWER UNITS
T-SHAPED RESERVOIR AND HPU IMAGE COURTESY OF HENGLI AMERICA
NEARLY EVERY SINGLE HYDRAULIC SYSTEM shares one thing in common — they’re run by a hydraulic power unit. Although some units are multitaskers, like the engine on a tractor, most often they’re purpose built for the single task of converting mechanical energy into hydraulic energy. The scope of a power unit ranges from fractional horsepower electric units to monstrous constructions in the hundreds of horsepower. The power unit exists to supply the machine with hydraulic energy in the form of pressure and flow, without which you have idle components. You must first calculate the pressure and flow required by the actuators in the systems. You may calculate this step more than once as you balance performance with economy, as very few machinery OEMs have no limits, financial or otherwise. After you arrive at your pressure and flow requirements, you specify the pump type and size. Pump cost and complexity are vast but this dictates the level of performance you can expect to achieve with your actuators. The type of pump used correlates with the direction you must take with reservoir design, filtration and complexity of pumping. A gear pump, for example, requires only suction and pressure lines. A load sensing piston pump, conversely, will add to that a case drain line and one or more hookups for the load sense network.
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The pump now defined, the rest of the power unit can be built around it. You must now choose the size of your reservoir. Although opinions vary, you can’t go wrong with the advice to size it as large as possible. Limitations will exist for cost and footprint, but on average, expect to need at least three times pump flow at minimum, to ideally five times if it can be achieved. Every multiple of pump flow provides a precious extra minute of fluid dwell time. Reservoir size is critical for many reasons. A large volume of hydraulic fluid relative to pump size gives time for fluid to cool before being drawn back into the circuit where heat soaks in once again. Large tank volume means large tank surface area, and in addition to the first point, this large surface www.fluidpowerworld.com
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WHICH TANK STYLE TO CHOOSE
IMAGES COURTESY OF LDI INDUSTRIES
IMAGE COURTESY OF HALDEX
The reservoir style you choose will be based on price and available space. You should choose the largest reservoir money and space will allow, and if possible, the tank should permit flooded suction. A flooded suction occurs when the pump inlet port is below oil level, either inside or outside the reservoir. L-shaped, T-shaped and elevated reservoirs provide the best combination of attributes, but are large and costly. I’ll describe your primary options, and their advantages and disadvantages. The JIC Style reservoir, top right, is an industry standard welded-top tank that comes with two removable end covers on opposing ends. The removable end covers allow you to access internal plumbing and components for assembly and repair, but also allows for easy cleanout when the reservoir is empty. Components are typically welded or mounted to the top of JIC reservoirs, such as pump/motor groups, filters, heat exchangers, etc. You can also mount a JIC reservoir on an elevated platform, which provides the below-mounted components with flooded suction and ease of access. Similar to JIC are DIN reservoirs, but with different standard dimensions. JIC reservoir advantages: industry standard, many sizes available, flexible mounting, cleanout panels JIC reservoir disadvantages: large size, fixed top plate, requires fabrication Vertical style reservoirs, left, are small and versatile, providing many top mount options with just a few volume options. “Vertical” refers to the vertical mounting of the pump/motor group with a bellhousing dripping down into the reservoir and the motor sticking straight up into the air. There are many lids which provide various combinations of cutouts for the pump/ motor, filler cap and open space for other components to mount to. The vertical mount reservoir is a compact, economical and versatile option available from 5 to 50 gallons or so. Standard components can be bolted on, avoiding the requirement for welding. Vertical reservoir advantages: compact, economical, versatile, easy to fabricate Vertical reservoir disadvantages: limited space, difficult to service, limited size/options The “L” Shaped reservoirs, bottom right, provide the highest function and serviceability, giving the user flooded suction, space for many components, and a flat bottom for lifting. Pump suction lines are separated with locking ball valves, allowing the pump to be serviced without theatrics. The pump/motor mounting surface is best included with the deck riser and drain, allowing all spilled oil to be caught and disposed of properly without mess. As well, L-Shaped reservoirs can be had with removable lids, cleanout panels or both. The T-Shaped reservoir is also available for mounting dual, opposed pump/motor groups. L-shaped reservoir advantages: generous space, ease of service, cleanout panels, many sizes L-Shaped reservoir disadvantages: cost, footprint
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H Y D R AU L I C P O W E R U N I TS
provides a radiation layer to improve cooling. Additionally, with more fluid, particles settle more effectively than if they immediately re-enter the circuit, as with smaller tanks. Opposite to the settling of particles, air bubbles are given more time to rise, reducing the potential for cavitation-related damage from aeration. After you calculate tank volume, you must now consider the reservoir construction type. Reservoir style (see sidebar on Which tank style to choose) plays an important role in ensuring the pump inlet conditions are ideal, preventing conditions favorable to cavitation. Economics are also primary here, ranging from the vertical type at the low end, to the L-shaped at the upper end. The former is compact but difficult to service, while the latter is highly serviceable but large and expensive. Highly complex hydraulic systems consist of many components—some related to the
function of the circuits, like manifolds, directional valves and pressure valves— and other components required for fluid conditioning and monitoring. Filters, heat exchangers and pressure gauges are components added to ensure safe and reliable power unit operation. Because of ease and convenience, as many components as possible should mount to the reservoir. As such, these components command much real estate, and reservoirs are oversized to accommodate. Use your hydraulic schematic as the first step in the power unit creation process. A hand-drawn schematic helps you choose actuators and major components, but a detailed drawing helps with the visualization of component layout. A circuit drawing should be modified to include every component that will exist on the power unit, not only for the assembly technician to understand
how to install and plumb all the components, but for future troubleshooting and repair. Experienced hydraulic designers know what a power unit needs, but seeing the circuit helps spot gaps where less obvious components should be drawn, and then subsequently added to the bill of materials. Test points, ball valves, bellhousings, drive couplers etc., are all important and should be included. Once a schematic is complete, a bill of materials (BOM) can be created from it. The most important set of components is the pump/motor assembly. It includes the chosen pump, a motor of adequate power capacity, a pump-motor mount and drive coupler set. The pump/motor mount — often called the bellhousing — rigidly fixes the C-Face electric motor to the pump and provides a gap to install the couplers. A coupler slides on the motor shaft, its mate slides on the pump shaft and then a synthetic rubber insert is placed between them before the couplers are pushed
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together and fixed in place with set screws. Be sure to select a coupler set rated for the required horsepower and pressure spike potential. Once together, the pump/motor assembly is mounted to the reservoir, preferably with isolation mounts. These mounts are either welded or bolted to the reservoir and consist of two metal plates galvanized onto either side of a chunk of rubber. This isolator prevents excessive pump/motor vibration from resonating through the steel plates of the reservoir; an important requirement because hydraulic power units are already prone to nasty harmonic noise pollution. In succession, the remaining “fixed” components are mounted to the reservoir. Accumulators mount off the side, valve banks to the top, filtration inside or on top, level/ temperature indicator located to the side, ball valves (very important!) hard plumbed
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to the suction port, and any other component either welded or bolted to the tank are now installed. With all fixed components now complete, you can fabricate the power unit’s plumbing. Most designers prefer tube but it is timelier to fabricate. It is semi-permanent and is more reliable in the long run. Hose can be used for plumbing as well, although hose invariably fails. Hose has its place, however, especially if noise and vibration is a concern. Tube transmits vibrations more readily, but hose can often dampen it. Most frequently, you plumb a power unit using a combination of hose and tube. At the end of fabrication, the unpainted components are removed, and the power unit is cleaned and prepped for paint. Epoxy paint is best because it resists oil and provides a
durable finish. When the paint dries, the technician re-assembles the unit, and the final stages of power unit fabrication begin. The electrician now wires the electrical components, and this includes adding any electrical enclosures, transducers, switches etc. Wiring of the motor itself will often occur on sight at commissioning, but the electrician wires the valves and control boxes when the power unit is manufactured, to allow for testing. Once electrical is complete, the technician fills the tank with oil and jogs the motor from the electrical panel to ensure the motor is turning the correct direction. The technician turns on the pump, checks for leaks and tests the HPU at pressure. The final step, after successful testing, includes draining the tank, installing on a skid and wrapping up for shipping.
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IMAGES COURTESY OF DAMAN PRODUCTS CO.
HANDBOOK
HYDRAULIC
MANIFOLDS
IN SIMPLEST TERMS, a manifold is a component from which you attach other things. A slightly less elementary explanation is that it cleans up plumbing—and this is why you should care about this unassuming block of metal that ultimately makes for smoother system design. A hydraulic manifold is a housing for surface and/or cartridge valves that regulates fluid flow between pumps, actuators and other components in a hydraulic system. It can be compared to a home’s electrical panel. Just as raw electrical power comes to the panel and is distributed to various household circuits to do work (provide light, power the dishwasher, operate the garage door), hydraulic oil under pressure is routed to the manifold by a pump where it is diverted to various circuits within the manifold to do work. The role of a manifold is to bring the hydraulic circuits to life through the creation of a block machined in a manner consistent with the original circuit design. All valves have a series of orifices to which drilled holes in the manifold must communicate. The configuration of these drilled holes in the manifold is the representation of the defined circuit. The manifold is the central muscle control of the hydraulic system receiving inputs from switches, manual operations (levers) or electronic feedback systems. These inputs energize 56
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various valves mounted on or in the manifold, while specific oil pathways allow oil to flow through hydraulic lines to the appropriate actuator to perform work. The complex matrix of variables can make manifold design and component selection a challenging and rewarding art form, as size, weight, function, performance and operating environment are always part of the design consideration. In addition to providing a neat and logical layout, consolidating components into a manifold reduces space and pressure drop. This results in fewer fittings, more efficient assembly times and reduced leak points. Manifolds are sometimes viewed as black boxes, as they can be highly complex with upward of 500 holes communicating with each other www.fluidpowerworld.com
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FLUID POWER HANDBOOK
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 operationâ&#x20AC;&#x2122;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
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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. Laminar manifolds are always custom-designed. Valves and other connections can be located where appropriate for a specific application. But because of the permanently shaped flow passages and brazed construction, this type of manifold cannot be modified easily if future circuit changes become necessary. Because there are so many configurations available for manifold design, there are several software packages available to help the engineer design a system. With advances of these design software packages and CNC technology, the installed cost for custom solid-block manifolds, even small runs, is highly competitive to systems using modular blocks or discrete components.
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H Y D R AU L I C M A N I FO L D S
WHERE ARE MANIFOLDS USED? Manifolds are used in many fluid power applications, depending upon the application. Manifolds can be used to mount valves or to consolidate plumbing. When used for mounting valves, they are the interface between the valves and the ports to be plumbed into.
No Kinks No Hose Twisting
With industrial style hydraulic valves, such as ISO valves mounted on D03 or D05 patterns, for example, the valves terminate with plain ports surrounded by O-rings, and cannot be plumbed directly into a hydraulic system. The manifold is a block, or series of adjoining blocks, which has an interface for the valve(s) to mount to, ports for the fluid to travel, and then ports to plumb the manifold to the rest of the circuit. The pressure passage can be parallel to the valve pressure ports, or in series, joining the tank to pressure port of subsequent valves.
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The advantage of manifold mounted valve systems is in their modularity; standard valves of various, easily exchangeable iterations can be mounted to a manifold to customize the circuit and its number of actuators. A standard valve series can be mounted to manifolds employing any type of port, such as NPT, Metric or ORB, rather than producing every valve with every version of port, saving manufacturing and inventory costs.
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Manifolds can be used outside of valving, as well. A manifold can simply be a chamber with two or more ports joined in series to reduce plumbing. For example, a return line manifold with six smaller ports joining to one large tank port will save the need for a series of expensive tee’d together fittings, reducing both cost and the chance of leakage. A manifold can be used less commonly in pressure lines, cleaning up plumbing and improving appearance.
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HYDRAULIC
MOTORS
ORBITAL MOTOR. IMAGE COURTESY OF DANFOSS POWER SOLUTIONS
60
Hydraulic motors are used in any application requiring rotational force, also known as torque. A hydraulic motor converts hydrostatic energy into mechanical energy by pushing vanes, gears or pistons attached to a crankshaft. The power capacity of a hydraulic motor is dictated by its displacement, and the speed in which it rotates. They are used in industrial applications such as augers, conveyors, and mixers, as well as rolling mills, where their robust nature and resistance to heat make them a perfect solution. Hydraulic motors can be used in any application an electric motor can, but require no electricity, and can withstand more environmental extremes, such as submersion and extreme heat. Motors can be sized small enough for high-speed drills, or large enough for injection molding machines. Hydraulic motors are especially well suited to mobile machinery, where they are the primary drive for most off-highway equipment. Hydrostatic drive systems act to transmit engine power to the drive wheels, with exceptional versatility and reliability. Hydraulic wheel motors are used on wheeled loaders, skid-steer loaders, articulating loaders and backhoe loaders, where the motor’s quick and smooth reversibility make them perfect for the application. Motors are also used in tracked vehicles, such as excavators of all sizes, bulldozers, and specialty machines, such as bridgelayers or drill rigs. The high power density of hydraulic motors allow them to achieve earth-moving torque in a relatively small package. Hydraulic motors can also be used in hybrid hydraulic vehicle drive systems, where the motor can also absorb energy from the drive wheels to pump fluid into an accumulator, which can subsequently send flow back to that motor for a burst of acceleration from low speed. Hydraulic motors can be either fixed- or variable-displacement and operate either bidirectionally or uni-directionally. Fixed-displacement motors drive a load at a constant speed while a constant
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RADIAL PISTON MOTOR. IMAGE COURTESY OF BOSCH REXROTH
input flow is provided. Variabledisplacement 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, www.fluidpowerworld.com
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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. 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. 17-1864_Kaw_Horz_9x5.25.pdf
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WHAT TO KNOW WHEN SPECIFYING MOTORS There are several important things to consider when selecting a hydraulic motor. You must know the maximum operating pressure, speed and torque the motor will need to accommodate. Knowing its displacement and flow requirements within a system is equally important.
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BENT AXIS HYDRAULIC MOTOR. IMAGE COURTESY OF LINDE HYDRAULICS
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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 costeffectively 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.
LSHT MOTOR. IMAGE COURTESY OF MUNCIE POWER PRODUCTS
WHAT ARE THE DIFFERENT TYPES OF HYDRAULIC MOTORS? Keep in mind that the three different types of motors have different characteristics. Gear motors work best at medium pressures and flows, and are usually the lowest cost. Vane motors, on the other hand, offer medium pressure ratings and high flows, with a mid-range cost. At the most expensive, piston motors offer the highest flow, pressure and efficiency ratings. Gear motors feature two gears, one being the driven gear—which is attached to the output shaft—and the idler gear. Their function is simple: high-pressure oil is ported into one side of the gears, where it flows around the gears and housing, to the outlet port and compressed out of the motor. Meshing of the gears is a bi-product of highpressure inlet flow acting on the gear teeth. What actually prevents fluid from leaking from the low pressure (outlet) side to the high pressure (inlet) side is the pressure differential. With gear motors, you must be concerned with leakage from the inlet to outlet, which reduces motor efficiency and creates heat as well. In addition to their low cost, gear motors do not fail as quickly or as easily as other styles, because the gears wear down the housing and bushings before a catastrophic failure can occur. At the medium-pressure and cost range, vane motors feature a housing with an eccentric bore. Vane rotors slide in and out, run by the eccentric bore. The movement of the pressurized fluid causes an unbalanced force, which in turn forces the rotor to turn in one direction. These designs are available in a variety of different styles, including radial, axial and other less common
designs. Radial-piston motors feature pistons arranged perpendicularly to the crankshaft’s axis. As the crankshaft rotates, the pistons are moved linearly by the fluid pressure. Axial-piston designs feature a number of pistons arranged in a circular pattern inside a housing (cylinder block, rotor or barrel). This housing rotates about its axis by a shaft that is aligned with the pumping pistons. Two designs of axial piston motors exist— swashplate and bent axis types. Swashplate designs feature the pistons and drive shaft in a parallel arrangement. In the bent axis version, the pistons are arranged at an angle to the main drive shaft. Of the lesser used two designs, roller star motors offer lower friction, higher mechanical efficiency and higher start-up torque than gerotor designs. In addition, they provide smooth, lowspeed operation and offer longer life with less wear on the rollers. Gerotors provide continuous fluid-tight sealing throughout their smooth operation. Pressure (outlet) side to the high pressure (inlet) side is the pressure differential. With gear motors, you must be concerned with leakage from the inlet to outlet, which reduces motor efficiency and creates heat as well. In addition to their low cost, gear motors do not fail as quickly or as easily as other styles, because the gears wear down the housing and bushings before a catastrophic failure can occur.
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HYDRAULIC
PUMPS
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Hydraulic pumps are used in literally every single hydraulic power transmission system. A hydraulic pump is the device that converts mechanical energy into hydraulic energy, which is a combination of pressure and flow. A hydraulic pump can be any device that you can input force into to create pressure, which in turn creates flow. Most hydraulic pumps have a mechanical input from an internal combustion engine or electric motor. These prime movers input their mechanical power to the hydraulic pump in a rotational fashion. The input shaft of the pump will be connected to gears, vanes or pistons of the hydraulic pump, where they will rotate or reciprocate to transfer pressure (force) to the hydraulic fluid. As long as the force (pressure) created by the pump is high enough, flow will occur at a rate dictated by the displacement volume of the pump and the speed at which it rotates. These pumps, also called positive displacement pumps, have a small clearance between rotating and stationary parts. A specific amount of fluid is delivered to the system for each revolution. Positive-displacement pumps can be further divided into two categories: fixed- and variable-displacement. Fixed-displacement pumps provide a single, specific volume displacement per revolution. In variable-displacement pumps, displacement per cycle can vary from zero to maximum volumetric capacity. Some of the more widely used types of positive-displacement pumps are gear, piston and vane. Gear pumps can be either internal or external styles. External gear pumps are one of the most popular types used in modern hydraulic systems. Gear pumps produce flow by using the teeth of two meshing gears to move the fluid. Their simple construction ensures limited purchase costs and servicing. They feature decent mechanical and volumetric efficiency, compact dimensions and low weight/power ratio. Of the three common types of positive displacement pumps, gear pumps are the least efficient; their appeal is low cost and simple design.
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External gear pumps can be equipped with straight spur (the most common type), helical or herringbone gears. In operation, the drive gear and driven gear rotate, creating a partial vacuum at the pump inlet (where gear teeth unmesh) that draws fluid into gear teeth. Gear teeth mesh at the outlet, forcing fluid out of the pump. Internal gear pumps contain one internal and one external gear. They pump fluid in the same manner as external spur gear pumps. In the basic design, the internal gear, which drives the outer gear, has one tooth less than the outer gear. As they mesh, the teeth create sliding seal points. 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. www.fluidpowerworld.com
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Efficient hydraulic fluid supply. Variable displacement axial piston pumps operate using several pistons arranged in a circular pattern in a cylinder barrel. As the barrel and pistons rotate, a swashplate adjusts the depth to which the pistons are inserted into the barrel, effectively varying the fluid output volume to the loads.
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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 IM AG operation. EC OU Piston pumps supply high flows at high speed. RT ES Two types of piston pumps—axial and radial—are YO FE manufactured in both fixed- and variable-displacement AT ON 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, variabledisplacement 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.
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IMAGE COURTESY OF PARKER
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 variabledisplacement pump is obtained. Radial-piston pumps are characterized by a radial piston arrangement within a cylinder block. As pistons reciprocate, they convert rotary shaft motion into radial motion. One version has cylindrical pistons, while another uses ball-shaped pistons. Another classification refers to porting: Checkvalve radial-piston pumps use a rotating cam to reciprocate pistons; pintle-valve 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).
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REPAIR, REBUILD
& REMANUFACTURING WHEN FLUID-POWER COMPONENTS like pumps, motors, valves and cylinders fail, the overriding goal is to get a system up and running again as soon as possible. One option is simply to purchase and install a new replacement part, but a repaired or remanufactured component is often the more economical option. Depending on the unit, almost every constituent part can be replaced or repaired, provided a suitable replacement is available. As a general rule, a repair makes sense if the cost doesn’t exceed 60 to 70% of the cost of a new component. Beyond that, the user is typically better off with a new unit. However, if a new product is not readily available and a critical or expensive machine or production line is down, repair is still practical at a higher cost. Technicians first make a full inspection and diagnose the problem. Sometimes components wear out. But premature failures typically result from contamination, cavitation, overpressurization, and excessive heat. Thus, installing a robust filtration system, keeping components cool, and following a disciplined maintenance program are critical to extending component and machine life. Finally, some components break due to incorrect application, installation or commissioning—failures that are often preventable. Technically savvy repair shops tear a unit down and try to bring it back to “as new” condition. All critical dimensions and surfaces are inspected and measured. Seals and low-cost consumable parts like springs, washers and shims tend to be replaced. Likewise, bearings will be inspected and possibly replaced. IM AG In more-serious cases, E CO say internal wear due to UR TE contamination damage in SY a piston pump, lapping FL UI
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BEFORE, ABOVE, AND AFTER, BELOW, REBUILT PUMPS. IMAGES COURTESY OF ENGINEERING TECHNOLOGY SERVICES
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the surfaces might be suitable if still within acceptable tolerances. Otherwise, the technician may need to remanufacture or replace rotary barrels, pistons, and other internal parts. That can ultimately extend to replacing other major components like housings, covers, relief valves, controllers and charge pumps. In the worst case, a completely new unit can be built from parts, although that is not the norm. Another issue is whether to repair/ remanufacture a unit with parts sourced from the original hydraulic OEM or with aftermarket replacement parts. The hydraulic repair industry broadly falls into three business sectors. Hydraulic component manufacturers often tend to serve large machine builders directly, with their own parts. But they often leave user service and support to distributors and large, sophisticated repair houses. Some use OEM parts exclusively, some do not. Further removed are smaller shops that can find OEM replacement parts costly and not readily available. After completing the repair, testing and calibration are also critical to ensure repairs are done right the first time. This is necessary simply because hydraulics has gotten more complex. Newer components routinely have integrated electronics, digital controls and sophisticated software. Unfortunately, the complexity of todayâ&#x20AC;&#x2122;s hydraulics means pure mechanical aptitude is no longer sufficient to fix many components. Components with higher operating pressures and electronics controls also tax the capabilities of test equipment at many repair facilities. Small shops will make repairs and perhaps run basic tests, but they are not capable of performing fullfunction tests. Larger repair shops have made significant investments in state-of-the-art test stands for qualifying dynamic open and closedloop systems. Not surprisingly, the price of a repaired or rebuilt component can vary widely depending on the www.fluidpowerworld.com
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expertise behind it. Experience and specialized skill is required to correctly diagnose, rebuild and test hydraulically powered equipment. Costs include the expertise for diagnosing the failure, recommending the proper repair, whether to rework or replace a part, and whether to use genuine or aftermarket parts. This ultimately speaks to the overall competency of the repair company and the capabilities of the staff and testing equipment. The quality of the repair shouldn’t be driven solely by price, either. Companies that shop by price alone are usually disappointed by the outcome, as the repair may not hold up. Consider the time and labor needed to remove a failed pump or cylinder from a machine and install a rebuilt one, plus the cost of the repair itself, as well as the cost of machine downtime and lost productivity, and it quickly adds up. Cutting corners on parts or testing and finding that the repair quickly fails, and forcing the user to start over at square one, gets expensive. That’s why it’s essential to get repairs right the first time.
FOUR TIPS FOR SELECTING THE RIGHT MOBILE HYDRAULIC REPAIR A repair or rebuild performed by an authorized service center ensures equipment will have longer life and better performance because they are performed by factory trained technicians with genuine service parts and completed wtih a renewed factory warranty. Consider these four criteria when seeking a repair or rebuild of hydraulic components. 1.
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R E PA I R , R E B U I L D & R E M A N U FAC T U R I N G
Factory trained technicians — Service technicians should be factory trained and certified to perform quality repairs and rebuilds just like the manufacturer. Genuine service parts — Select a repair shop that only sells, services, and inventories genuine manufacturer’s service parts while following factory approved rework specifications. Factory warranty — Most reputable repair and rebuild companies offer a warranty in keeping with the manufacturers they carry. Don’t settle for less. Timing — Finally, repair and rebuild should compete with new unit lead-time. The repair shop should stock the most commonly used service parts, allowing factoryquality component repair and rebuild to be performed in as little as two weeks.
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IMAGE COURTESY OF TRELLEBORG SEALING SOLUTIONS
HYDRAULIC
SEALS
ONE OF THE ARGUMENTS often used against hydraulic systems is that dreaded word, leakage. But with proper sealing, leakage won’t be a problem with your system. Sealing is a vital factor in the proper function of hydraulic applications. Leakage from the cylinder or across the piston, along with the ingress of unwanted contaminants, can not only decrease the lifespan of the components, but it will affect the efficiency of the entire application.
MATERIAL OPTIONS Proper material choices will be determined by the seal’s environment. Different types of chemicals react differently to different fluids, while some materials have higher pressure and temperature limits. They also must be able to withstand extrusion, so materials are very applicationspecific. Polyurethane Polyurethane is an organic material whose chemical composition is characterized by a large number of urethane groups. Urethanes belong to the thermoplastic elastomers (TPE) family and close the gap between thermoplastic and elastomeric materials regarding hardness, deforming behavior and consistency. Within certain temperature limits, polyurethane possesses the elastic characteristics of rubber combined with the advantages of a rigid plastic. The composition of the material is determined by three components: polyol, diisocyanate and a chain extender. The type and amount of these materials used, and the reaction conditions, are 72
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decisive in determining the properties of the resulting polyurethane material. In general, polyurethanes possess the following properties: yy high mechanical, tensile strength yy good abrasion resistance yy modulus of elasticity is variable yy wide range of hardness values, while retaining good elasticity yy good resistance to ozone and oxygen yy outstanding resistance to abrasion and tear Temperature range for use: –30 to 80° C; high performance types (compounds) up to 110° C in mineral oils (long-term exposure temperature).
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Acrylonitrile-Butadiene-Rubber (NBR) NBR is a polymer of butadiene and acrylonitrile. The acrylonitrile (ACN) component affects the following properties of the NBR: yy yy yy yy yy
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: yy good resistance to swelling in aliphatic hydrocarbons; greases; fire retardant hydraulic fluids of Groups HFA, HFB and HFC yy 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 yy high swell in aromatic hydrocarbons, chlorinated hydrocarbons, flame retardant hydraulic fluids of the Group HFD, esters 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.
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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: yy tough resistance to high heat yy excellent resistance to oil, hydraulic fluid and hydrocarbon solvents yy good flame retardance yy low permeability to gases yy 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.
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H Y D R AU L I C S E A L S
WHY IS HYDRAULIC FLUID COMATABILITY IMPORTANT FOR SEALS? In a fluid power system, fluid compatibility is often discussed with respect to seals. In other words, does the chemical composition jibe with the fluid composition? Chemical composition is the composition of the seal itself. What type of rubber, essentially, is it made from? Generations ago, fluid power seals were made of things like leather and natural rubber, which had poor composition—completely across the board. Also, seal compatibility must be considered with respect to whether the seal performs properly across the required temperature ranges, which could include extremes. These are things that the system designer and operator need to be mindful of. If you have machinery that’s grooming ski slopes, that needs to have a different type of seal set than with something that only works in high temperature applications like a steel mill, where there’s tons and tons of heat. If you look at the difference between, say, Buna Nitrile and polyurethane, you will see that the temperature range tends to be different. The polyurethane one is usually good for colder temperatures. Some of them are better in higher temperatures. Some urethanes, like polyurethane compounds, are not so good with some of the HE fluids. EPDM rubber is generally suitable for water but not compatible with mineral-based oils. It’s rated for low temperatures. Fleurocarbons, which are Viton—can be used effectively in oil but not for water … but these materials are also good for high temperatures. Buna Nitrile, again, is a good all-around material. It’s the most common seal material for fluid power applications. It’s compatible with both oil and water but not recommended with synthetic and fire-resistant fluids. It has a medium temperature range. Polyurethanes are high durometer, meaning they have a higher hardness. They’re also low friction. A lot of polyurethane U-type seals or cup seals are good for low friction applications like flight simulators. They also have a medium temperature raise. Plastics have good chemical compatibility—they’re really hard and resistant to a lot of things, so they are good for wide temperature ranges but they’re typically very hard. This can reduce their sealing effectiveness. The softer rubbers are better at sealing because they deform easy. Teflons feature good wear and chemical resistance and a very wide temperature range. PTFE materials also can result in poor sealing because they’re very tough and very rigid.
Polytetrafluoroethylene (PTFE) PTFE is a polymer of tetrafluoroethylene. This non-elastic material is characterized by: yy slippery surface that repels most media yy non-toxic at working temperatures up to 200° C yy low coefficient of friction against most opposing surfaces made of other materials; stiction and friction are almost the same yy excellent electrical insulating properties (almost independent of frequency, temperature and weathering effects) yy chemical resistance that exceeds that of all other thermoplastics and elastomers
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liquid alkali metals and a few fluorine compounds attack PTFE at higher temperatures
The temperature tolerance is between –200° and 260° C; PTFE has some elasticity even at extremely low temperatures; therefore it is used in many extreme cold temperature applications. Most hydraulic applications require the use of a spring or elastomeric component to energize a lip seal configuration because of the low elasticity and tendency to cold flow over time. SIX COMMON SEAL DESIGNS Following is a list of some of the most common seal designs used in fluid power applications.
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Piston Seals yy provide sealing of the piston and barrel, critical to the function of the cylinder yy most often a lip-seal design, but can also be O-rings, T-seals, and so on yy must provide efficient sealing, but also reasonably low friction yy made from various seal materials, depending on application yy require system pressure to effectively activate the lip seal
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Wipers yy provide aggressive wiping force yy prevent mud, water, dirt and other contamination rom entering the system yy allow lubricating oil film to return to system on inward stroke yy protect main sealing elements, thus increasing life of seals yy often made from polyurethane, which offers high abrasion resistance yy often used as a linkage pin grease seal
yy yy yy yy yy
Rod Seals prevent system fluid from escaping to atmosphere must provide sealing function at low and high pressure require excellent extrusion and wear resistance should provide good pump-back capability for lubricating oil film often must withstand up to 6,000 psi
Buffer Seals yy must withstand high pressure exposure yy protect the rod seal against pressure spikes yy feature a pressure-relieving capability that prevents pressure build-up between seals yy increase rod seal life yy allow for wider extrusion gaps yy require high wear resistance Wear Bands yy prevent contact between metal parts in the cylinder yy center rod and piston from housing elements yy increase seal life O-rings yy most commonly used in static applications and radial or axial deformation to maintain sealing contact force yy double-acting, so seal on both sides of a component yy can be used as energizing elements or as primary seals yy self-acting, so do not require additional system pressure or speed to create the seal
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POSITION
SENSING
Several technologies exist to provide position feedback or data of a cylinder’s position. The sensor converts the position into a proportional analog or digital signal. Hall-Effect Transducers use a magnet that communicates with the Hall chips, which then give an output to the internally built microprocessor. The output from the microprocessor is converted to a signal required by the user interface such as voltage, current, PWM or digital output. Using Hall-effect technology on linear position sensors allows manufacturers to make small, compact designs that can be mounted internally or externally to a cylinder. Hall-effect technology is well suited to mobile applications as it is highly resistive to shock and vibration. It is used commonly on steering and depth applications. LVDTs or Linear Variable-Distance Transducers are durable and resist shock and vibration while offering high repeatability. These absolute linear position/displacement transducers convert a linear displacement into an analog electrical signal. Their design includes transformer coils wound around non-magnetic coils.
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LVITs—Linear Variable Inductive Transducers—are contactless position sensing devices, with sensing ranges up to 30 inches or more. Most designs feature an inductive probe surrounded by a conductive tube. This is attached to the moving object to make the reading. These contactless position sensing devices use eddy currents developed by an inductor in the surface of a conductive movable element to vary the resonant frequency of an L-C tank circuit. Modern electronics using microprocessors and small component size makes high performance possible, achieving linearity errors of less than ±0.1% and temperature coefficients of 50 ppm/°F, along with either analog or digital outputs.
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Pressure 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. Some pressure transducers output current rather than voltage, and are then often referred to as transmitters. These values typically fall within tens of thousandths of amps. When choosing the output of a pressure transducer, it is important to keep in mind the input requirements of the device that will be accepting the signal, the distance the signal must travel and possible interference that can be found in the environment around the system. Important performance criteria to consider are the pressure transducer’s operating pressure range, maximum rated pressure, accuracy and operating temperature range. The operating pressure range demarcates the intended pressure bounds at which the transducer has been designed to perform optimally. The maximum rated pressure is the highest allowable pressure that the pressure transducer
is rated to withstand. The accuracy of the transducer is usually represented by suppliers in terms of ASME B40.1 grades: 4A (0.1%), 3A (0.25%), 2A (0.5%), A (1%), B (2%), C (3%) and D (4%) deviance from the true pressure value. A good pressure transducer is designed to operate independently of temperature; however, the operating temperature specifies a “safe” range; operating outside of this temperature may significantly affect the accuracy of pressure sensing. For typical industrial applications, select a 0.5% accuracy class. This should be sufficient for most closed-loop systems. Higher accuracy will quickly increase the price. Before making that investment, determine if the rest of your system requires this higher accuracy. Accuracy is a constant value found on the data sheet. Unfortunately, most hydraulic systems start cold and get hot, so your actual pressure accuracy will depend on temperature change. The overall accuracy is accuracy class plus error due to temperature change.
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Servo Kinetics (SKI) has been in business for 40 years, before many of the competitor’s even existed. Our capacity for servo and proportional valve repair far exceeds any of our competition. SKI serviced the NASA Houston Shuttle Simulator for 25 years, through three major upgrades of the Electro-Hydraulic, closed loop motion system.
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INITIAL RUN-IN PERFORMANCE/FAILURE ANALYSIS THAT IS DOCUMENTED AND INCLUDED ON FINAL TEST REPORT SENT BACK WITH YOUR VALVE.
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Encoders are used heavily on mobile machinery. Available with capacitive, optical or magnetic sensing and with incremental or absolute position sensing, these devices accurately determine the stroke of the cylinder. Rotary and linear devices are available, but rotary styles are most common. These devices measure position and resolution in pulses per revolution. Some rotary encoder designs use a wire-actuated sensor mounted directly inside the cylinder. Others are mounted externally. Others are based on Hall-effect, magnetostrictive, or inductive technologies. Linear encoders are available with resistive, capacitive, optical or magnetic sensing with incremental or absolute position sensing to accurately determine the stroke of the cylinder.
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Magnetostrictive Transducers measure the distance between a position magnet attached to the component in motion and the head-end of a sensing rod that is attached to the axis to be measured. The magnet does not touch the sensing rod, so no parts can wear out. The sensing rod mounts along the motion axis to be measured and the position magnet attaches to the member that moves. An electronics module sends an analog or digital position reading to a controller or other receiving device. Also within the electronics housing is the electrical connection interface, either an integral connector or cable and visual diagnostic LEDs to ensure proper wiring, power, and magnet positioning.
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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 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, as is 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 is 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
MONOBLOCK VALVE. IMAGE COURTESY OF PRINCE MANUFACTURING
example, one would describe a double-acting single monoblock valve as “4-way, 3-position,” or simply a “4/3 valve.” (See sidebar for more.) 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.
HOW DO YOU SELECT DIRECTIONAL CONTROL VALVES? A hydraulic system will either have a fixed-displacement pump—which requires an open center valve—or a variabledisplacement pressure-compensated pump that limits pressure. In the latter, you need a closed-center valve. Typically, with a mobile valve an open center can be converted to closed center by adding a closed-center plug in the outlet section. This will block off the open-center core when the spools are in neutral. The flow rate of your source is necessary in determining how small or large of a valve is needed. If you size a valve too large, you risk trouble shifting it. If you size it too small, you risk energy loss from excessive backpressure, resulting in heat. Excessive heat causes lost lubricity and varnishing of internals. Valves are shifted mechanically with a lever, cam or pilot, but also with an electric solenoid. If the application is cost-sensitive, manual control is best, as no other electric control package is required. The electric control option requires a power source and switch at minimum, but provides advanced operation for automation or sophisticated proportional control applications. In addition, when choosing a directional valve, consider the following questions:
Is it a parallel or tandem circuit (from variable or fixed pumps)? A parallel circuit is one where all valves’ pressure ports are plumbed together simultaneously, which requires a variable displacement pump or unloading circuit. Valves in a (continued on page 84)
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H Y D R AU L I C VA L V E S
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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 valveâ&#x20AC;&#x2122;s simple spring-loaded ball or poppet. Sequence, counterbalance and brake valves are all forms of relief valves with added utility or functionality, such as reverse flow checks or pilot operation built in. The 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 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 provide controlled, fixed flow to one part of the circuit (sometimes at the sacrifice of another part) and only if 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 loadinduced pressure variances. Pressure compensators are a type of flow control valve available as a single component, often added to other valves in a circuit to provide flow rate accuracy independent of load, such as with an electronic proportional valve. Proportional valves are considered both flow and directional valves, and not only meter flow, but also control the direction flow is metered in. Proportional valves use pulse-width modulation to maintain voltage while controlling current. Varying the
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FLUID POWER HANDBOOK (continued from page 81)
parallel system require a blocked P-port in neutral. A tandem circuit is one where the tank port of the first valve flows into the pressure port of the next valve, and again to the next valve. A fixed displacement pump is typically used, which forces fluid to all downstream valves. Do you need an open or closed center for oil flow? Open and closed center refers to the pattern of flow in the through the valve in its neutral state. An open center valve will flow through all ports freely in neutral, while a closed center valve has all ports blocked in neutral. As you would suspect, a closed center valve requires a variable displacement pump, while an open center valve typically works best with fixed displacement pumps. What is the volume of flow from the source? The size of the valve is dictated by the flow required by the actuators and produced by the pump. Higher flow requires larger valves, and the cost of that size increases price exponentially. When sizing valves, consider the different spools and how they play a role on pressure drop. How many directions do you want the oil to flow expressed as 3-way, 4-way)? The “ways” a valve flow are bidirectional, and essentially refers to the number of ports the valve has. A valve with 4-ways, for example, has a pressure port, tank port and two work ports (A & B). How many spool positions are needed? Spool positions are defined by the shape of the valve body and the machining of the grooves in the spool. As a
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spool moves within the body, it opens a channel to allow flow between the work ports. Valves typically contain two or three positions, including neutral, unactuated position. Few valves have four positions. What is the spool action, such as spring center to neutral or various detent selections? The spool is located in the body and held in place by springs or detents. The spring will hold a two position valve off to one side in its unactuated state, and two springs will hold a three position valve centered in its unactuated state. Detented valves have no springs, but use a mechanism to hold a valve in place in its positions, whether two, three or four positions.
current modifies the force of the magnetic field and subsequently 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. An electronic valve controller can provide adjustable minimum and maximum settings. A minimum current value is needed to move the spool past its “dead zone” 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 stick the spool within the body. 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.
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PNEUMATICS
OVERVIEW
PNEUMATICS IS THE TECHNOLOGY OF COMPRESSED AIR, although many manufacturers today refer to it as a type of automation control. Pressurized gas— generally air that may be either dry or lubricated—is used to actuate an end effector and accomplish work. End effectors can range from the traditional cylinder design to more 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 very specific applications where hazards are critical—for example, in a mine or on an offshore oil platform—where a single stray spark could mean total disaster and lost lives. By its nature, air is easily compressible, and so pneumatic systems tend to absorb excessive shock, a feature that can be useful in some applications. Most pneumatic systems operate at a pressure of about 100 psi, a small fraction of the 3,000 psi that many hydraulic systems experience. As such, pneumatics is generally used when much smaller loads are involved. A pneumatic system generally uses an air compressor to reduce the volume of the air, thereby increasing the pressure of the gas. The pressurized gas travels through pneumatic hoses and is controlled by valves on the way to the actuator. The air supply itself must be filtered and monitored constantly to keep the system operating efficiently and the various components working properly. This also helps to ensure long system life. In recent years, the control available within pneumatic systems (thanks to advanced electronics and componentry) has increased greatly. Where once pneumatic systems could not compete with many comparable electronic automation systems, the technology today is seeing a renaissance of sorts. More and more, pneumatics is being used in interesting ways that would have been unthinkable a decade or two ago. Creative applications from soft robotics to pneumatic muscles are consistently making the news, showing not only the creativity of the engineering community, but also the inherent flexibility and adaptability of this important technology.
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COMPACT
ENGINEERS ARE SOMETIMES CONFRONTED with situations where space and weight are limited, but high force is still a requirement. In these types of circumstances, compact cylinders may be best suited in for the job. Common applications include medical devices, robotics, packaging, and semiconductor, among others. These low-profile components have been shortened relative to standard pneumatic cylinders. They may be up to 50-60% shorter than the normal cylinder, but still maintain the capacity to exert the same force as their larger counterparts. Important parameters for the proper selection of a compact cylinder can be broken up into general, dimensional, performance, material and features. Originally called the “Pancake cylinder,” these miniature cylinders were first invented in 1958 by Al Schmidt, to fill a need for force in a tight, enclosed space. The basic intent was to get the most stroke in a short overall length using common machined parts and seals. Over the years, this design has been further developed, with many additional features and options to satisfy a variety of customer applications. This round body cylinder has a smooth, clean outside diameter for ease of machinery cleaning. Even though initially used for strokes less than 1 in., manufacturing methods have allowed increased strokes to as much as 4 in. Non-metallic rod bushings and piston bearings can accommodate extreme or unforeseen loads for long-term durability. Other compact or mini cylinders vary quite a bit. They can be rectangular or square shaped, offer numerous mounting features and can be placed with adjacent cylinders at a close 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 non-rotating styles. They are available in single-acting and double-acting versions. www.fluidpowerworld.com
compact cylinders - PNEUMATICS - FPW HB 5-18_Vs3 MG.indd 87
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CYLINDERS
HOW DO YOU SELECT A COMPACT CYLINDER? When sizing a compact cylinder, it is important to know the followingapplicationdatapointstochoosethebestcomponent for your application:
Operating psi, force required (Force = Pressure x Piston Area) Stroke Preferred mounting, foot print Spring return or double acting
In addition, you should know information about the application where the cylinder will operate to ensure it meets its lifecycle requirements:
Ambient temperature Media temperature Environment Excessive loads other than required axial force Load guiding (non-rotating) requirement
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PNEUMATIC ACTUATORS
CYLINDERS
PNEUMATIC ACTUATORS are simple and cost-effective mechanical devices that use compressed air acting on a piston inside a cylinder to move a load along a linear or rotational path. That motion can be in any form, such as blocking, clamping or ejecting. Unlike their hydraulic alternatives, the operating fluid in a pneumatic actuator is simply air, so leakage doesn’t drip and contaminate surrounding areas. There are many styles of pneumatic actuators including diaphragm cylinders, rodless cylinders, telescoping cylinders and through-rod cylinders. The most popular style of pneumatic actuator consists of a piston and rod moving inside a closed cylinder. This actuator style can be sub-divided into two types based on the operating principle: single-acting and doubleacting. Single-acting cylinders use one air port to allow compressed air to enter the cylinder to move the piston to the desired position, as well as an internal spring to return the piston to the “home” position when the air pressure is removed. Double-acting cylinders have an air port at each end and move the piston forward and back by alternating the port that receives the highpressure 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.
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They all look alike... they cost the same ...
P N E U M AT I C AC T U ATO R S — C Y L I N D E R S
HOW DO I CHOOSE?
An air cylinder will generally leak the most at the shaft, at the point where the rod moves in and out—the location of the rod seal. Some technicians recommend putting a small amount of soapy solution (bubbles) in this area to better see if this is the source of the leak. Other areas include welded seams or at the air connection points—where the air lines enter the cylinder body. Air cylinders generally leak because their seals have worn out, sometimes exacerbated by internal rusting of the metal components. If a piston or rod seal is the culprit, these can be replaced, and seal kits are widely available. Some cylinders are the non-repairable type, and if this is what you are dealing with, the entire cylinder will have to be replaced.
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. WHY DO AIR CYLINDERS LEAK? While a leaking pneumatic cylinder does not represent the environmental and safety catastrophe that a leaking hydraulic cylinder does, it’s still a serious situation that you should pay attention to. In addition to the fact that air leakage means wasted energy, it’s also a sign that either the system was designed improperly (say, with high side loads that are damaging the cylinder upon extension), or more likely, that the cylinder is nearing the end of its useful life and has to be replaced.
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PNEUMATIC RODLESS
& CYLINDER SLIDES
WHEN AN APPLICATION calls for power and linear motion while also supporting side loads, pneumatic rodless and cylinder slides (also known as guided cylinders) are up to the task. Unlike standard pneumatic cylinders, which are unable to hold the position of the piston rod, pneumatic slides can stabilize and hold a load because there is no rotating rod to cause side loads. Because they feature a non-rotating platform to mount other actuators and tooling, they are ideal for automation applications where there is repeated pick-andplace of parts. Rodless cylinder slides are different from standard pneumatic cylinders because they have no piston rod that extends outside of the cylinder body. This makes them ideal for long-stroke applications or where space is limited. The rodless design eliminates common problems from side loads, such as rod bending and overhang, among others. Instead of the rod, a magnetic or mechanical coupling system connects an internal piston to an external carriage. They are also popular choices when longer distances of travel are required, or when the overall length must be minimized due to space constraints. Typical uses for these slides include conveyor stops; part ejection and positioning; opening and closing safety doors, gates or curtains; and multi-axis configurations. In many of these applications, the need for side load capacity and non-rotating capability is critical. There are several considerations you should be aware of when selecting the best type of pneumatic cylinder slides. These include:
Load capacity required. The total payload must be calculated to start the selection process. Life required from the slide. The bearing system selected will have an impact on the expected life of the unit along with the required speed and payload. Speed required. The slide speed is a critical component including the ability of the slide to handle the kinetic energy as the load stops at the end of travel. Cylinder shock pads, cylinder cushions or shock absorbers may be required based on the load and speed of the slide. Deflection needed. The amount of deflection will vary based on the bearing system and the payload being carried. This deflection will affect the positional accuracy of the slide.
Many manufacturers of pneumatic cylinder slides provide specification and sizing software to allow the proper selection of the slide required for various applications.
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P N E U M AT I C R O D L E S S & C Y L I N D E R S L I D E S
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.
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Gantry slides can handle heavier loads while traveling longer distances with a higher thrust than most other designs. Similar to thruster slides, they use a moving or reciprocating carriage between two fixed bars for their motion. 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
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travels with less deflection based on the bearing system being supported on each end. Like the thruster style slide, the saddle carries the load versus the cylinder’s piston rod. Another type of slide is a rodless slide. In this case, the bearing system is attached to the rodless cylinder directly on one or both sides of the cylinder. The cylinder’s piston is linked to a carriage mounted upon the bearing system, offering load carrying capability as well as resistance to side loads. Rodless slides offer the most space savings as the cylinder’s travel is contained within its own overall length. 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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COMPRESSORS GENERATE PRESSURIZED AIR, but that exiting air typically contains dirt and water. Before it can travel downstream to valves and actuators, it must be filtered, regulated and sometimes lubricated. Otherwise, left untreated, it can damage products, cause premature component wear, attack seals and cause them to leak, and permit rust and corrosion in tools and piping—all leading to faster breakdowns and higher maintenance and operating costs. An air line filter traps particle and liquid contamination in compressed air. It captures solid particles (dust, dirt, rust), and also separates liquids (like water and oil) entrained in the compressed air. Filters are installed in the line upstream of regulators, lubricators, directional control valves and air-driven devices such as cylinders and motors. 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 allow the excess downstream load- or head-induced pressure to be exhausted. Non-relieving regulators, when similarly adjusted, will not allow the downstream pressure to escape. The trapped air will need to be released by some other means; for example, by operating a downstream valve.
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FRLS
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.
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As aready mentioned, air must be filtered before it can be used. It goes without saying that it is important to use the correct filter for an application. Here are two important considerations when sizing and selecting filters for compressed-air systems. Micron rating. Particle-removal filter elements are rated by the size of particles they trap. The smaller the micron rating, the smaller the particles it will capture. Thus, a 40-micron element, for example, is designed to remove particles 40 microns and larger. In industrial applications, standard general-purpose filters are generally rated to handle particulates from 40 to 5 microns, depending on the size. Higher-efficiency filters are also available to remove even smaller particles—1 micron or less. The downstream components and equipment determine the micron rating a filter element requires. Most industrial equipment and tools like air wrenches typically only need 40-micron filtration, while more-sensitive components and instruments typically demand 5-micron elements. Food, beverage and pharmaceutical applications routinely demand even finer filtration. It’s a good idea to consult with the equipment manufacturer to determine exact filtration requirements. But because smaller micron-rated elements have a higher pressure drop across the filter and shorter element life, don’t overspecify the filter. Opting for a finer-than-necessary filter will actually increase energy consumption, change-out intervals, and overall operating and maintenance costs. Flow capacity. In addition to contamination-removal efficiency, also consider the airflow demands of the application. Filters are generally available in many sizes to handle flows as high as 1,000 scfm. After engineers understand the required flow for a pneumatic application, select the proper-size filter by determining the maximum allowable pressure drop across the filter. To help with that task, most manufacturers supply flow-versus-pressure drop calculators or curves for a given element. To use such a chart, first determine the application’s inlet pressure to obtain the suitable pressure and flow performance. Next, determine the acceptable pressure drop across the filter. Generally, for good performance experts, recommend a filter with a 2 to 5 psig pressure drop. Then find the flow. If flow is too low, select a larger port or body size to give the required flow. Likewise, if flow is higher than necessary, select a smaller unit. Manufacturers may offer several different filters with a particular port size, but all may have different flow capacities. Typically, specify the product with the lowest pressure drop at the required flow rate. Also note that the curves are based on a dry element; a wet element will have lower flow and higher pressuredrop ratings. Other considerations. Other practical considerations when specifying air filters are pressure and temperature operating ratings, and the types of bowls, drains and differential-pressure gages available. Finally, don’t overlook quality and efficiency. A well-designed filter removes water and particles efficiently and has a low pressure drop at a given flow, which conserves energy. It also has a better ability to trap and hold contaminants without restricting flow, and requires lessfrequent replacement, versus lesser-quality units.
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PNEUMATIC
GRIPPERS
GRIPPERS are used in many pneumatic systems, as a means of grasping or holding parts; grippers may move, insert, stack or orient the parts to achieve a desired step in a manufacturing process. Grippers, which use compressed air to actuate the jaws, come in multiple styles, including parallel and angular, and may use a rack and piston arrangement, a lateral direct coupling or opposing jaws. They may have two, three or four jaws, depending on the type and shape of part that they will be interacting with. These components can be used in many applications, including: yy yy yy yy yy
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Oversize loads Heavy loads Part ejection Part seating Part handling
Clamping and fixturing Pick and place Material transfer Camming and indexing
Industries served include robotics, medical device manufacturing, semiconductor manufacturing, laboratory processing, sensor manufacturing, plastic molding/blow and injection, vehicular lighting and trim, pharmaceutical and biotech, and even bearing manufacturing. Pneumatically operated industrial grippers have been commercially available for only a few decades, and prior to that, most gripping mechanisms were designed and fabricated by machine builders employing a combination of pneumatic cylinders and external tooling for each individual application. Their most common purpose is to grasp or enclose parts for transfer, insertion or assembly in automated manufacturing and processing systems. Additional uses may include operating in environments that are hazardous for human presence. Pneumatic grippers provide their motion and actuation by introducing compressed air into a chamber of the 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 gripperâ&#x20AC;&#x2122;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 gripperâ&#x20AC;&#x2122;s jaws. A directly opposing linkage is attached to another jaw. These two opposing jaws then make up the simple angular gripping device. With the appropriately designed and attached tooling, this type of gripper can provide a fairly large force in an angular fashion. The force is generally directly proportional to both the pressure applied to the piston and the length of the linkage or toggle. The length of the tooling attached to the gripper also affects the force capacity, as the gripping point must be within the specified gripping distance/ range. One of the limitations of this angular style of gripper jaw motion is that lacking sophisticated tooling, it can only efficiently grip a limited size range of parts. A variation of this angular gripper could include a third, or even a fourth jaw, defining a center gripping axis or region. The parallel gripper design employs one of several different types of mechanisms that convert the piston or its rod motion to a parallel jaw travel, including:
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a lateral direct coupling of a pair of pistons to guided opposing jaws moving opposite each other, such as with a wide opening gripper
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P N E U M AT I C G R I P P E R S
yy a design where opposing jaws are driven by a component of the piston rod riding on a cam surface portion of the jaws yy a rack, powered by pistons, driving a pinion that in turn employs a scotch yoke cam device to drive opposing jaws yy a rather complicated mechanism where the piston or one of its members drives a scroll mechanism similar to a machine lathe-chucking device 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.
The Right Gripper for Your Part
- Robotic end effector solutions - Angular & parallel, many sizes and options available - Unique solutions available - For handling various sized parts - Superior design & delivery
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MINIATURE
IMAGE COURTESY OF CLIPPARD INSTRUMENT LABORATORY
PNEUMATICS
THE AREA OF MINIATURE PNEUMATICS is a specialized niche that sees a lot of use in applications such as medical/dental instruments, test equipment, analytics, pharmaceuticals, entertainment/ animatronics, semiconductor, HVAC systems, aerospace, down-hole oil tools, machine tools, ink-jet printing and process control systems. Sometimes referred to as precision pneumatics, miniature pneumatics consists of a range of components that have been miniaturized for use in light- and medium-duty applications with lowto-medium pressure ranges. Size and weight constraints matter in these systems and the need for precision is high. System pressures of 20 psi are not uncommon in miniature pneumatic applications. Miniature pneumatic products encompass a range of scaled down parts, including valves, cylinders, fittings, manifolds and tubing. Specialized components, such as nozzles and screens, are also seen on occasion. For example, the evolution of medical equipment has expanded beyond the hospital environment and toward the home care and ambulatory environments. This has made portable battery-operated variants of traditional stationary equipment more attractive. In todayâ&#x20AC;&#x2122;s culture, the persistent demand for reliable and innovative products compels companies to integrate new and more advanced technology into smaller packages. The medical market is the major sector for growth in miniature pneumatics right now. Industry experts 98
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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 metal-to-metal contact of the moving armature and stationary core. Quieter operational design found in some miniature pneumatics uses so-called whisper technology to greatly reduce sound levels. These valves are used for medical applications flowing gas or air, such as dialysis machines, patient monitors, ventilators/respirators and other bedside medical devices. www.fluidpowerworld.com
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Industrial Microhydraulics The Lee Company can now supply a full range of microhydraulic components such as check valves, relief valves, shuttle valves, flow controls, restrictors, screens and Betaplugs for high volume, low cost industrial and automotive applications.
The Lee Company 2 Pettipaug Road, Westbrook CT 06498 Tel: 860-399-6281 800-533-7584
www.leeimh.com Westbrook • London • Paris • Frankfurt • Milan • Stockholm
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PNEUMATIC
SYSTEM DESIGNERS use pneumatic hose and tubing to convey pressurized air to actuators, valves, tools and other devices. There are a great number of types and sizes of tubing and hose on the market, so a design engineer should consider a number of important factors to select the right one for a given task. Perhaps the best place to start is with the tubing’s construction. Tubing manufactured for pneumatic applications may be extruded of a single material or reinforced internally, typically with textile fibers, for higher strength. Air hose generally consists of an inner tube, one or more layers of reinforcing braided or spiral-wound fiber, and an outer protective cover. In broad terms, hose is more rugged than tubing—but it tends to cost 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 100
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HOSE & TUBING
spikes or otherwise operate above maximum working pressure. Also note that some products handle vacuum to approximately 28 in.-Hg without collapse. Thermoplastic tubing is made from several common materials. But manufacturers offer countless variations of polymer formulations to suit specific needs. Typical tubing materials used in pneumatic applications include:
Polyurethane tubing is strong, flexible, kink and abrasion resistant, and it withstands contact with fuels and oils. www.fluidpowerworld.com
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P N E U M AT I C H O S E & T U B I N G
It’s commonly used in pneumatic actuation and logic systems, robotics and vacuum equipment, and in a variety of semiconductor manufacturing, medical and laboratory applications. Nylon tubing is tough, light and dimensionally stable. It can be formulated for higher-pressure pneumatics, flexibility for routing in tight spaces, high flexural-fatigue resistance and low water absorption. Polyethylene tubing is often used in low-pressure pneumatics and pneumatic controls. It has wide resistance to chemicals and solvents, good flexibility and relatively low cost. HDPE tubing comes in semi-rigid versions that resist cuts and physical damage and has a higher burst pressure than polyethylene tubing. Polyvinyl chloride (PVC) tubing is light and generally more flexible than nylon and polyethylene, offers good chemical resistance and can be repeatedly sterilized. It is suitable for 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.
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 www.fluidpowerworld.com
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limits. In particular, be aware that elevated temperatures will impact working pressure, especially as tubing materials reach their upper temperature limit. For example, one version of nylon tubing with a 0.25-in. ID has a rated burst pressure of 1,250 psi at 75° F, but only 600 psi at 200° F. Many fittings are available to build pneumatic tubing and hose assemblies for connection to mating components. These include barbed, push-to-connect, swaged, crimped and reusable versions; and they come in plastic, brass, steel, stainless steel and other materials. First and foremost, designers should ensure the fitting and tubing or hose are compatible with each other, as not every fitting design is suitable for every type of tubing or hose. Other important considerations for fittings include pressure-holding capacity, ease of installation, size and weight, corrosion resistance and, of course, cost. Tubing is extruded in straight lengths and stored on reels, but it can also be molded into spring-like coils. This lets the tubing extend considerably as 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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WHAT ARE THE BENEFITS OF LOOPING MY COMPRESSED AIR PIPING? Most compressed air piping systems are radial feeds, with one-way piping branches feeding out from the compressor room in all directions. Often, the piping starts out in larger size and, if the design engineer wanted to save costs, the diameter of the piping might be reduced near the end of the lines. Some compressed air savings can often be gained by installing a looped compressed air system rather than radial feed, with the savings in pressure drop having much to do with diversity. It is fairly common to find the radial compressed air lines all sized to be the same—however, the flow in each branch may not be equal. Therefore, the pressure loss in one section of the piping may be very different than the other. The pressure loss in the section with the heaviest flow becomes the line with the lowest pressure at the end. With compressed air systems, having proper pressure is important. A critical tool or machine receiving low pressure due to excessive pressure loss in compressed air piping will cause problems, and a common way to solve this is to boost the compressor discharge pressure to compensate. Boosting the pressure raises the pressure at the critical end use, but also all the other unregulated uses in the plant. Increasing the compressor discharge pressure causes the compressor to consume more power, about 1% per 2 psi increase. It also causes higher flow in any unregulated uses by about 1% per every 1 psi increase. This also increases the compressor power. Looping the piping has two effects, it equalizes the pressure drop because now the critical end use has at least two more diverse parallel paths to receive compressed air. And having a loop typically means the piping is the same size throughout the loop, rather than reducing the size near the end. Having a parallel path is very effective in reducing the pressure loss. Because the pressure loss in piping varies with the square of the internal velocity, best case would have one half the flow in each line, which would result in one quarter the pressure loss. Further to this, if pipe size is increased, there is a significant increase in internal area of the pipe, which also reduces internal velocity. For example, use of 3-in. pipe rather than 2-in. would reduce the pressure loss by a factor of eight. Similar pipe size comparisons exist for other size upgrades. For this reason, in terms of energy efficiency, where practical, it is always good to consider looping your piping— and using multiple loops of well sized piping is the best of all.
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VACUUM
COMPONENTS
VACUUM IS PRESSURE that is lower than atmospheric—14.7 psia at sea level. In a vacuum system, the difference between atmospheric and vacuum pressure creates the ability to lift, hold, move and generally perform work. There are two types of vacuum applications: closed, or non-porous; and open, or porous. In a closed system, removing air progressively decreases the air density within the sealed, confined space and creates a vacuum. In an open system, a vacuum unit must remove more gas molecules than are able to leak back into the system. Vacuum is typically divided into three areas of application, depending on the level of vacuum required. Low-level vacuum applications are typically those requiring high flows and low force. These systems are primarily serviced by blowers. Screen printing on cloth is one application that falls into this range. The majority of industrial vacuum falls within the range of 6 to 29.5 in.Hg. Application examples include pick-and-place and thermoforming. Scientific or process applications encompass the deepest levels— approaching a near-perfect 29.92 in.-Hg. Flow in this range is minimal. Examples of applications are ion implantation and space simulation. The vacuum generators that evacuate air and create the required low pressure come in an extensive array of types, sizes, designs and efficiencies to suit widely ranging applications. Two basic types are electric-motor-driven vacuum pumps and vacuum ejectors. Vacuum pumps. Mechanical vacuum pumps generally fall into one of two different types: positivedisplacement and dynamic/kinetic. Displacement vacuum pumps essentially operate as compressors
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with the intake below atmospheric pressure and the output at atmospheric pressure. They draw in a fairly constant volume of air, which is mechanically shut off, expanded, and then ejected. The main feature of vacuum pumps of this type is that they can achieve a high vacuum with low flow rates. Types include reciprocating piston, rotary vane, diaphragm and rotary screw. They are often suited for precision industrial applications. Kinetic vacuum pumps cause gas particles to flow in the delivery direction by applying additional force during evacuation. Rotary blowers, for example, operate according to the impulse principle: a rotating impeller transfers kinetic energy by impacting air molecules. In operation, air is drawn in and compressed on the suction side by the impeller blades. These vacuum pumps generate a relatively low vacuum, but at high flow rates (high suction capacity). They are usually suited for handling extremely porous materials, such as clamping cardboard boxes. Among the advantages, typical positive-displacement industrial pumps generate up to about 98% vacuum— beyond the capability of ejectors. And blowers can offer high suction rates well beyond 1,000 m3/hr. However, electromechanical vacuum pumps tend to operate continuously with vacuum requirements regulated by valves. And compared to ejectors, they are larger, heavier, and usually cost more.
COMPARING THE REAL COSTS OF VACUUM GENERATORS In some cases, the preferred type of vacuum generator for an application is fairly obvious—such as a vacuum blower when the task requires low vacuum and high flow. But in many settings, the choice is not clear-cut. While maximizing performance for a given task is important when using a vacuum system, users should also look at overall costs before specifying essential components. Given that energy is a valuable and, at times, expensive resource, the cost of running a suitable vacuum system should play an important role in any design. With electric-motor-driven vacuum pumps, users can measure and assess energy costs based on prevailing electricity prices and electrical power consumption. For circuits based on vacuum ejectors, one should investigate the amount of energy necessary to operate a pneumatic vacuum ejector with compressed air. Let’s take a closer look at these two basic types of vacuum generators, and how they affect the bottom line.
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VAC U U M C O M P O N E N TS
COMPARING THE REAL COSTS OF VACUUM GENERATORS (CONTINUED) Start with some straightforward considerations to help compare the economics of vacuum generators of similar performance. For example, let’s compare annual costs for owning and operating a vacuum ejector (with or without an air-saving function, or both) and for an electrically driven vacuum pump. First determine factors like the purchase prices of a vacuum pump and an ejector, the maintenance costs (typically minimal for an ejector), electricity price and the cost to generate compressed air.
pick-up, transport, vacuum discharge, unloading and return to the start position to begin the next cycle. The amount of time required for each step depends on the vacuum generator. An ejector with an air-saving function consumes air (energy) only for perhaps a fraction of a second while picking up the workpiece. An ejector without an air-saving function consumes compressed air during pick-up and transport of the workpiece. This might take a few seconds. A vacuum pump consumes energy for the entire operation cycle, as the pump does not normally switch off.
In a typical vacuum-handling operation, for instance, then determine how many hours the equipment runs each year; and the operating cycle where work steps include evacuation, workpiece
From this information, one can calculate the actual annual run time for the devices. Knowing the operating pressure and air consumption for the ejectors, and the cost of compressed
Vacuum ejectors. Vacuum ejectors basically generate vacuum using pneumatically driven nozzles without moving parts. They produce high vacuum at relatively low flow rates. A classic ejector consists of a jet nozzle (also called a Laval or venturi nozzle) and, depending on the design, at least one receiver nozzle. Compressed air enters the ejector and a narrowing of the jet nozzle accelerates the flowing air to up to five times the speed of sound. The ejector has a short gap between the jet-nozzle exit and the entry to the receiver nozzle. Here, expanded compressed air from the jet nozzle creates a suction effect at the gap which, in turn, creates a vacuum at the output vacuum port. Vacuum ejectors come in two basic versions, single and multi-stage. A single-stage ejector includes a jet nozzle and one receiver nozzle; a multi-stage ejector has a jet nozzle and several nozzle stages, each of which has a larger diameter in proportion to the falling air pressure. Air
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air, lets engineers determine the annual operating cost of the devices. Vacuum pumps driven by electric motors require similar attention. Run time per year times the cost of energy to drive the motor equals the yearly operating cost. Given all that, one can compare overall costs for the vacuum ejectors and vacuum pump. Don’t forget that a vacuum system includes initial, maintenance and energy costs. Often, vacuum pumps have a lower operating cost but higher initial and maintenance cost. And ejectors with air-saving features can lessen air consumption and narrow the operating-cost gap between the two. While ejectors may use more energy, their simple design keeps initial costs and maintenance costs to a minimum. In many applications, ejectors more than justify their existence.
drawn in from the first chamber, combined with compressed air from the jet nozzle, is thus used as a propulsion jet for the other chambers. In both versions, air exiting the receiver nozzle generally discharges via a silencer or directly to the atmosphere. Among their benefits, vacuum ejectors are compact, lightweight, and relatively inexpensive and they respond quickly, with fast start and stop times. They resist wear, can mount in any position, experience no heat build-up in operation, and consume energy only as needed—as they switch off when no vacuum is needed. On the downside, vacuum ejectors only generate pressures to about 85% vacuum, and do not produce extremely high suction rates.
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HANDBOOK
GAUGES
PRESSURE GAUGES MEASURE A FLUID’S INSTENSITY. They ensure reliable operation and reduce the risks of pressure spikes or changes that could cause damage to the system. In addition, they prevent leaks by alerting personnel of unusual changes in system pressure. Hydraulic pressure gauges are available to measure up to 10,000 psi, though typical hydraulic systems operate 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 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. Pressure is measured in many locations throughout pneumatic and compressed air systems. It is measured at receiver(s), as well as every system FRL or stand-alone regulator and sometimes at pneumatic actuators. These gauges can be rated up to 300 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 pressure range at which a hydraulic gauge will be working is a primary selection factor for the type of material used to make the gauge. Gauges operating at higher pressures generally tend to be made of materials such as steel; when operating at lower pressures, they tend to be made of bronze.
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Most pressure gauges in North America come with a 1/4-in. NPT male, but SAE thread is gaining popularity. The use of testpoint adapters at various locations on the hydraulic system allows for measurement during troubleshooting with just one gauge. 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 throughout the system. Most gauges are 2 1/2 in. in diameter, and can be top-mount or panel-mount styles. Common threats to gauge reliability are vibration, pulsation and pressure spikes. Therefore, it’s best to look for gauges designed specifically for hydraulic applications to reduce costly downtime. A forged brass case prevents resonant frequencies from destroying internal components; a liquid-filled case protects
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G AU G E S
A PRIMER ON GAUGE FUNCTION
the gauge from vibration and extreme pressure cycles; and a restrictor prevents damage from pressure spikes. Although the liquid used in the gauge varies from application to application, glycerin offers higher the viscosity so it dampens the vibrations mor. When choosing between a dry, water- or glycerin-filled gauge, it is also important to consider temperature range, needle response time required, changes in pressure and expected vibration. Gauge accessories, such as specialized restrictors, piston snubbers or diaphragm seals, may be help prevent premature gauge failure.
The most common gauges are Bourdon tubes and bellow gauges. Bourdon tubes take pressure and convert it into mechanical energy. This energy moves a dial in the gauge, displaying the pressure in the system. Bourdon tube gauges 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. Bellow gauges function similarly to Bourdon tubes, but they use a spring to judge the amount of energy to push the dial. The spring is expanded and compressed by the pressure in the tubes and the energy created by that movement is transferred into gears that move the pressure dial.
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RETAINING
RINGS
RETAINING RINGS fit into a machined groove either on the inside of a bore or on the outside of a shaft and are used to reduce vibration and withstand axial loading while holding 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 be honed to the shaft. SPECIFYING RINGS FOR FLUID POWER SYSTEMS The most important detail to know when selecting a retaining ring 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. Also, if the ring will be subjected to axial thrust loads, the ring specifications need to be checked to determine whether a light-, medium- or heavy-duty ring is required. Finally, the material needs to be selected based on what type of corrosive media the ring will be subjected to.
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FLUID POWER
SAFETY
UNDERSTANDING where stored energy exists in any fluid power system is critical to safe machines and personnel. It is important to keep components plumbed properly, but also have the correct levels of machine safeguarding in place—from properly labeled lockout/tagout systems to safety valves to ensure redundancy and safe shutdown. It is critical to evaluate the entire system and its complete schematics, including the electrical portion, to minimize exposure to unnecessary risk. Systems are rated based on the weakest link in the control chain. Several standards (including ISO 13849-1:2006, ANSI/ASSE Z244.12003 [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: yy must be functionally redundant yy 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 yy must return to a safe position in the event of a loss of pressure or other such event yy must be able to inhibit further operation upon detection of a fault condition until such condition is corrected yy should have a dedicated, specific function- reset input and should prohibit the ability to perform a reset by simply removing or re-applying pneumatic or hydraulic power, and must not automatically reset. Providing control reliability with fluid power is not quite the same as with electrical controls, however. For instance, plain redundancy in a safety circuit requires the equivalent function of four valve elements, not just two. Two of the four valve elements handle the inlet function while the other two elements handle the stop function (energy release). Many self-designed systems risk having hidden, potential flaws, which can lead to unsafe conditions because they are unseen, unexpected and, therefore, excluded
ELWOOD’S ISO-LOCK VALVE WAS DEVELOPED TO LOWER THE AMOUNT OF DISRUPTION CAUSED BY FULL SYSTEM SHUTDOWN IN ORDER TO REPAIR VALVE STACKS. THE UNIQUE SANDWICH PLATE ISOLATION DESIGN ALLOWS MAINTENANCE TEAMS THE ABILITY TO LOCKOUT ALL PORTS WITH ONE SIMPLE QUARTER TURN AND THEN LOCKOUT VIA A HASP. IMAGE COURTESY OF ELWOOD CORP. 110
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HOW CAN YOU ENSURE HYDRAULIC SAFETY? The two most critical considerations in hydraulic safety are with control of flow and pressure. Safe control of loads being manipulated by hydraulics is often primary. However, the safe containment of pressurized fluid often means loads are also stable, as a catastrophic leak of fluid could cause a failure of the system to control the load. Safe control of a hydraulic load involves ensuring fluid is accurately metered. The potential for pressure and/ or load-induced pressure to impart energy into oil is high. As such, the oil wants to give up that energy to the ambient surroundings, but is prevented from doing so by only the integrity of the components and conduits of the hydraulic system. The pressure either wants to escape past flow control or directional valves, or it attempts to exit to atmosphere through seals or plumbing failure points. 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 SAFETY VALVES FROM BUCHER HYDRAULICS with a meter-out configuration to prevent the load from runHELP PROTECT PERSONNEL AND MACHINES IN ning away with the cylinder. Although it is a safe method to MOBILE AND INDUSTRIAL SETTINGS. 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. 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. A leak in the cylinder hose or tube can also cause catastrophic failure and personal harm. When fluid is free to exit the actuator at a broken conduit, it can no longer hold a load up. The counterbalance valve prevents the load from dropping should there be a conduit failure. A pilot-operated check valve will provide the same safety function of a counterbalance valve, but is designed for static load-holding. It will hold a load indefinitely, but is not as smooth at controlling load-induced movement as the latter valve is. Pressure is the very essence of a hydraulic system, and it is required to provide the force density that makes a hydraulic system so effective. But pressure can easily rise through intensification, load spikes, “water-hammer,” and thermal expansion, to name a few. If pressure isn’t controlled and limited, components can fail, seals can give way and either example can render a machine unsafe. For this reason, hydraulics uses many types of pressure control valves. Relief valves are used to limit pressure, preventing damage and limiting power requirements. A relief valve is often used 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.
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SAFETY
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 electrical-control fault, such as when a relay might be stuck in the open or closed position. The second abnormal condition is when a valve develops diminished performance, such as when it becomes sticky or sluggish. In such cases, the valve reaches the proper position, but slower shifting affects safe stopping distances or precise timing. The ANSI B11.19-2010 standard mandates a monitoring system that detects these conditions for critical applications and the ANSI/PMMI B155.1-2011 standard requires diminished performance monitoring if stopping time can be affected. An easy solution is to use a self-monitoring, Category-3 or -4 valve, designed to detect both conditions. The use of double valves remained relatively unheard of for many years, except in a few select industries, such as stamping presses, which first initiated control reliability requirements. Double valves provide dual internal functions (redundancy) so that an abnormal function of one side of the valve does not interfere with the overall normal operation. At the same time, the double valves sense abnormal operation on either side of the valve and then inhibit further operation until the problem has been corrected and the valve deliberately reset. This sensing and inhibiting function is commonly referred to as monitoring. Two standard air valves, whether in parallel or in series, cannot perform the same safeguarding function as a double valve providing this critical function. By simply incorporating two standard air valves into the circuit, no provision is made to sense the abnormal operation of one side of the valve or, even more preferable, diminished performance such as slow shifting. In addition, there is no provision for inhibiting further operation of the circuit until the valve is repaired. If one valve actuates abnormally, the second one continues to function and
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redundancy is lost. The circuit doesnâ&#x20AC;&#x2122;t recognize lost redundancy, nor would it halt operations as a warning that redundancy has been compromised. Then, if the second valve also actuates abnormally, there is no back up, and control integrity no longer exists. Double valves are appropriate for pneumatic and hydraulic equipment anytime reliability is an issue. Typical applications include e-stop, two-handcontrol, 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.
DOUBLE VALVES, SUCH AS THIS DM2 SERIES C DOUBLE VALVE, RETAIN A LOCKOUT CONDITION TO PREVENT UNINTENTIONAL RESET WITH REMOVAL OF AIR OR ELECTRICITY. IMAGE COURTESY OF ROSS CONTROLS.
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INDUSTRIAL
SHOCK ABSORBERS
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MACHINE BUILDERS are always on the lookout for ways to run equipment faster and increase throughput and productivity. However, components moving at high speeds often must decelerate and stop without damaging the equipment or payload. Otherwise, the consequences are excessive loads, vibration and noise that can compromise safety and machine reliability. Engineers can sometimes dampen motion with products like inexpensive elastomeric bumpers, simple air cushions or gas-spring linear dampers. But these typically have a limited ability to absorb energy and decelerate objects. Shock absorbers, in contrast, provide controlled deceleration by converting kinetic energy to thermal energy. In action, motion applied to a hydraulic shock absorber’s piston forces pressurized fluid through specially designed internal orifices. That restricts flow and generates heat which, in turn, transfers to the metal body and dissipates to the environment. After impact, a spring typically
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returns the piston rod to the starting location. Shocks are used in a wide range of applications, from automotive manufacturing and lumber processing to robots, cranes and packaging equipment. When choosing a shock absorber, one must specify the stroke length, compressed length, extended length, cylinder (body) diameter and rod diameter. The stroke length is the distance between the compressed and extended length. The cylinder diameter is an important factor in determining whether the cylinder will fit into the desired location, and how the shock absorber will be affixed to the adjacent structure. HOW DO YOU SIZE A SHOCK ABSORBER? Sizing a shock absorber is relatively straightforward. Several reputable manufacturers offer online calculators, but here are a few guidelines to quickly come up with suitable products for a given task. Manufacturers’ web sites and data sheets typically list products by parameters like stroke, usable velocity range, maximum amount of energy that can be absorbed per cycle, maximum force capacity, and the maximum propelling force it can handle, as well as dimensions and other relevant details. Before sizing a shock absorber, however, users first need to determine the
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relevant operating conditions, including the weight and velocity of the moving mass and how frequently the shock is loaded. For simplicity, let’s look at a linear-motion application and use Imperial units for the calculations.
size and recalculate the work energy. The next step is to calculate the total energy, Et (lbin.) per cycle, shown as: Et = Ek + Ew
Determine kinetic energy in the system from: Ek = W/(722)(V2) where Ek = kinetic energy, lb-in.; W = weight of moving mass, lb; and V = velocity of moving mass, in./sec. This equation represents the amount of kinetic energy that the shock absorber will convert to thermal energy on each impact. Next calculate the work energy in the application, defined as the amount of energy an external device generates to move the load: Ew = Fd(S) where Ew = work or drive energy, lb- in.; Fd = drive force, lb; and S = stroke of the shock absorber, in. Note that Fd should not exceed the unit’s maximum rated propelling force. If it does, select a larger
Again, if this exceeds the model’s energy-absorbing capacity, select a larger unit and recalculate the work energy. Otherwise, the shock’s temperature may rise beyond rated limits and critical internal components like hydraulic seals could fail. If the application uses more than one shock absorber, divide the total energy Et by the number of shocks to determine the total energy per shock. Then determine the total energy a unit must convert in one hour. That’s because even though a shock might absorb an acceptable amount of energy in a single impact, it might not be able to dissipate the generated heat if the cycle rate is too fast. Here, multiply Et by C, the total number of cycles per hour: Etc = Et(C)
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INDUSTRIAL SHOCK ABSORBERS
The device’s hourly capacity must exceed this calculated amount. If not, choose a larger absorber (and recalculate Ew if the stroke changes) or, possibly, add an external oil tank or a cooling device to help dissipate the heat. Finally, consider the shock force, Fp (lb) in the application. Shock force, in essence, is the resistive force required by the shock absorber to stop the moving load: Fp = Et/(Sη) where S = the stroke of the shock absorber and η is the unit’s damping efficiency. While the efficiency can vary with the type and model, 85% efficiency is a good baseline for typical industrial shocks. This is important when selecting a suitable shock absorber because the machine structure and mounting must have the necessary strength and rigidity to withstand the transmitted force. The efficiency of various units is measured by evaluating how much of the shock’s stroke is used for actual damping of the motion. Shock absorber efficiency increases as more energy dissipates over the stroke, and more-efficient products typically yield the lowest shock forces for a given stroke. Considerations such as the machine’s structural integrity and the payload’s ability to withstand forces without damage are also key to successful damping configurations. And some applications or payloads may have specified g-load rating limits. For example, an operator housed in a large overhead crane must be protected from excessive g-forces. Calculate this g-load from: g = (Fp – Fd)/W The above calculations help ensure that a given shock absorber meets all operating parameters. Again, make certain that the selected model matches or exceeds requirements for energy absorbed per cycle and per hour, as well as the shock force. Otherwise, it will likely cause damage or fail prematurely. Shock absorbers may be made from aluminum, steel and stainless steel, or thermoplastic. Steel is used when high strength is required. The other materials provide varying balance between strength, weight, corrosion resistance and cost. Additionally, the rods can be treated with chrome to provide corrosion resistance and increase surface hardness. Nitride will increase the hardness by introducing nitrogen into the outer surface of the rod. There are also a number of important shock absorber features to consider. Adjustable shock absorbers allow the stiffness of the response to be monitored and fine-tuned. This is usually accomplished by adding or removing hydro/pneumatic medium from the shock absorber by way of a valve. Locking capability allows the position of the rod to be fixed at a given position.
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