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2021 FLUID POWER HANDBOOK
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P N E U M AT I C T I PS .CO M • M O B I L E H Y D RAU L I C T I PS .CO M • H OS E A SS E M B LY T I PS .CO M • S E A L I N GA N D CO N TA M I N AT I O N T I PS .CO M
CONTENTS
05 Fluid Power overview
06 07 09 THE 2021 FLUID POWER HANDBOOK 11 16 20 22 25 29 32 36 38 41 44 50 52 55 58
84
44
11
77 88
32 41
62 63 70 73 74 77 80 84
Hydraulics overview Accumulators Bar stock Cylinders Filters Filtration systems Fittings & flanges Fluids Hose Hose couplings Hydraulic power units Manifolds Motors Pumps Repair, rebuild & remanufacturing Seals Sensing technologies Hydraulic valves Pneumatics overview Pneumatic actuators Air compressors Air springs FRLS Pneumatic hose & tubing Vacuum components Pneumatic valves
88 Gauges 90 Safety 93 Shock absorbers 96 Ad index
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EDITORIAL
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FLUID POWER WORLD does not pass judgment on subjects of controversy nor enter into dispute with or between any individuals or organizations. FLUID POWER WORLD is also an independent forum for the expression of opinions relevant to industry issues. Letters to the editor and by-lined articles express the views of the author and not necessarily of the publisher or the publication. Every effort is made to provide accurate information; however, publisher assumes no responsibility for accuracy of submitted advertising and editorial information. Non-commissioned articles and news releases cannot be acknowledged. Unsolicited materials cannot be returned nor will this organization assume responsibility for their care. FLUID POWER WORLD does not endorse any products, programs or services of advertisers or editorial contributors. Copyright© 2021 by WTWH Media, LLC. No part of this publication may be reproduced in any form or by any means, electronic or mechanical, or by recording, or by any information storage or retrieval system, without written permission from the publisher. SUBSCRIPTION RATES: Free and controlled circulation to qualified subscribers. Non-qualified persons may subscribe at the following rates: U.S. and possessions: 1 year: $125; 2 years: $200; 3 years: $275; Canadian and foreign, 1 year: $195; only US funds are accepted. Single copies $15 each. Subscriptions are prepaid, and check or money orders only.
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SUBSCRIBER SERVICES: To order a subscription please visit our web site at www.fluidpowerworld.com FLUID POWER WORLD (ISSN 2375-3641) is published seven times a year: in February, April, June, July, August, October, and December by WTWH Media, LLC; 1111 Superior Ave., Suite 2600, Cleveland, Ohio 44114. Periodicals postage paid at Cleveland, OH & additional mailing offices. POSTMASTER: Send address changes to: Fluid Power World, 1111 Superior Ave., Suite 2600, Cleveland, OH 44114
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FLUID POWER OVERVIEW
10 YEARS OF FLUID POWER KNOWLEDGE WELCOME TO THE TENTH
edition of the Fluid Power Handbook. It’s exciting to be part of this milestone, as we continue to update our information on hydraulic and pneumatic components and systems. This year, we’ve incorporated more technical sidebars with frequently asked questions, sizing and specification tips and looks at a specific components within each category. 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 almost 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. Although the world is continuing to open up after the Coronavirus pandemic, we know that the way engineers consume their information changes on a daily basis. The pandemic helped to speed this up, so while we will keep publishing the print and online basics in this Handbook as well as in all of our publications and websites, we still work to find new avenues to help current and future users of fluid power systems understand these technologies. Last year, we turned our in-person event, the Fluid Power Technology Conference, into a series of virtual events, and those webinars have continued throughout this year; we have several slated in the coming months. Visit fluidpowertechconference.com for details on past and upcoming webinars. Additionally, we are continuing our partnership with LunchBox Sessions’ Carl Dyke, for his YouTube live series, in which he uses his on-site trainer and popular Live Schematics to bring specific technologies to life for viewers. We plan to resume in-person events in 2022 and are currently in the process of developing more fluid power basics videos, kicking off with our symbology series, with more to come. And our Women in Fluid Power podcast is making a return in the near future, so visit fluidpowerworld.com often to see all that is new. The fluid power industry has carried on as an essential business even during the tightest lockdowns. While our members are dealing with increased orders and supply chain issues, they persevere. That’s what fluid power does. It is strong, powerful and reliable.
Mary C. Gannon, Editor mgannon@wtwhmedia.com @DW_marygannon
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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 equipment building our roads and cities to the machines servicing our farmland and massive 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. And combining electric technologies with hydraulics helps to increase efficiency and more. 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 breakdown. The bottom line is that smart engineering design and regular maintenance will avoid problems in the future.
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HYDRAULIC ACCUMULATORS
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.
BLADDER ACCUMULATORS | COURTESY OF ROTH HYDRAULICS
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 fiber-reinforced 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. 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 types exist: bladder, piston and diaphragm accumulators. Bladder accumulators use a flexible closed bladder inside the shell to separate the gas and fluid. They usually 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 generalpurpose units. They come many standard sizes, and good response characteristics make them suitable for shock applications. A bladder can be easily replaced in the event of failure or damage. 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 highpressure 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, heavy-duty operations. However, they are more sensitive to
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PISTON ACCUMULATORS | COURTESY OF LIEBHERR-COMPONENTS AG
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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.
HOW DO YOU SAFELY PRECHARGE AN ACCUMULATOR? WHEN HIGH-PRESSURE HYDRAULICS FAIL, the result is often a whimper as a seal blows out and pressure decays to useless levels. The harm to the surrounding environment and spectators is often little or none, which is why high-pressure hydraulic systems are so popular. However, when high-pressure pneumatic systems fail, you don’t want to be on the same city block, let alone next to the machine. A hydropneumatic accumulator charged to 3,000 psi is essentially a tube of nitrogen stuffed with over 200 times the volume of air compared to its surrounding atmosphere. That 3,000 psi wants badly to become 14.7 psi again, and when given the opportunity, will disperse its energy as rapidly as possible to equalize. Accumulator safety is a lesson just as important as fluid injection or loading holding concepts. Any opportunity to handle an accumulator is an opportunity to tip, drop or break one as well, and more often than not, the handling occurs during the precharge operation. Accumulators are pressure vessels, and as such need to be respected for their potential to return to atmospheric pressure. It goes without saying only approved gases, such as nitrogen, should be used for the charging of accumulators. Your cutting torch’s oxygen tank, thankfully, won’t even connect to the plumbing of your charge kit.
ACCUMULATOR CHARGE KIT. | COURTESY OF GPM CONTROLS
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Nitrogen cylinders are to be stored upright and chained or strapped to a cart or wall, which reduces the chance of accidental tipping. When not in use, nitrogen tanks are best left with their valves closed, fittings disconnected, and the protective cap secured. Leaving the charge hose attached to the tank is an invitation for wasted gas, especially if the valve isn’t completely tight, so remember to tighten the valve after removing the hose. The habit of replacing the protective cap between uses is an opportunity to check the valve is once again tight. Before installing the charge kit, set the protective cap aside safely to ensure it doesn’t fall or get damaged, preventing later installation back onto the tank. It’s important to install the charge kit’s nitrogen fitting into the tank port before loosening the tank valve. Once the fitting is tightened with a wrench, take the opportunity to double-check all other hose connections, fittings and valves on the charge assembly. Most have bleed valves that must be tight before charging as well. The final connection will be the gas chock to the accumulator valve, which itself should be tight but not overly so. Once all lines and fittings are secure, open the gas chock valve if so equipped. Should precharge still exist in the accumulator, the pressure reading will display on the charge head’s gauge. You may now open the nitrogen valve slowly, and as long as tank pressure is higher than accumulator pressure, precharge will rise. When designated pressure is reached, close the nitrogen and gas chock valves. Note that gauge pressure often reads higher while the nitrogen tank valve is open. To confirm precharge accuracy, re-open the gas chock valve to display precharge pressure, which may be lower than initially observed. Follow the previous procedure to fill the accumulator slightly higher to account for hysteresis, and be aware that just measuring precharge pressure in small accumulators will lower the precharge pressure. Once precharge is set and confirmed, double-check all valves are closed, and then remove the accumulator from the charge hose. You may now remove the charging hose from the tank and confirm once again the valve is shut before replacing the protective cap.
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BAR STOCK MOLTEN IRON IS POURED INTO THE CONTINUOUS CASTING PROCESS TO FORM BAR STOCK. | COURTESY OF DURA-BAR
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 0.0005in. (1⁄2 thou) chrome plated steel bar,” said Hart. “There are also many other options. With a steel piston rod, you can increase the tensile strength … up to 100 kpsi, and the chrome can be increased to 0.001.” He is describing the tensile strength and the chrome plate thickness of the bar stock, which is important because most cylinders spend half their time pulling. Additionally, thicker chrome results in superior corrosion resistance.
Other techniques are employed to strengthen the rod stock. “Most large diameter piston rods are induction hardened, which helps improve impact resistance,” said Hart. “If an end user keeps breaking male rod threads, sometimes this stronger material can help improve the longevity of the cylinder.” Regarding cylinder finish treatments, in extreme conditions, such as corrosive or salinated fluid exposure, rod stock can be further upgraded to stainless steel. “Some end users require corrosion resistance for their process, which is where stainless steel steps in,” said Hart. “Most grades of stainless steel can have a chrome finish.” However, stainless steel is not the only finish available. “Aside from chrome, the only other common finish treatment for piston rods is nitride. This is an extremely durable finish. It is a chemical process that hardens and darkens the material, which provides wear and corrosion resistance.” 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 for 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.
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DUCTILE IRON BAR STOCK. | COURTESY OF DURA-BAR
Aluminum is a popular choice for bar stock material when system pressure is 3,000 psi or less. It is easier to work with than ductile iron, and is also lower in physical mass and overall cost. However, when working pressure is higher than 3,000 psi, iron is required to withstand the additional stress. Ductile iron, such as Dura-Bar, is a continuous cast (iron) that is less brittle than standard cast iron and is pressure rated to 6,500 psi. Ductile iron is a compound with a highly controlled microstructure, improving strength and machinability. Although forged steel is another option for bar stock, it is rarely used on less than the most extreme applications. Whatever name you know them by best — ISO, cetop, NG6, D03 — the industry standard modular stackable valves are the most common system of circuit construction, and they all require a manifold to interface with. A manifold for a D03 valve, for example, is around 3 in. tall and 3 in. deep, but can be as long as needed to mount any number of valve stacks. The manifold most often 10
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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 www.fluidpowerworld.com
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.
HYDRAULIC CYLINDERS
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 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 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.
CUSTOM HEAVY-DUTY CYLINDER WITH SPHERICAL BEARING. | COURTESY OF RAM INDUSTRIES
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CYLINDER MOUNTING METHODS
Mounting methods also play an important role in a cylinder’s performance. Generally, fixed mounts on the centerline of the cylinder are best for straight line force transfer, ideal column loading and avoiding excessive wear. Pivoting mounts, such as clevis or trunnion, require care in application, because of their capacity to move as the cylinder is stroked, resulting in a possible bent rod or excessive wear.
TYROD CLEVIS TYPE HYDRAULIC CYLINDERS ARE AVAILABLE IN BOTH 2,500 PSI AND 3,000 PSI. | COURTESY OF RAM INDUSTRIES
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.
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.
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 over-limit 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. Non-centerline mounting calls for additional support to avoid misalignment, but these are required when the mechanism must pivot through an arc, so load calculations must be factored accurately.
Push or pull or both?
HEAVY-DUTY HYDRAULIC CYLINDERS. | COURTESY OF HIGGINSON EQUIPMENT
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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 loadbearing effect. When a cylinder pulls, there is little concern for buckling, but you should ensure your force calculations factored in 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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WHAT ARE THE MOST COMMON PISTON ROD PROBLEMS ON HYDRAULIC CYLINDERS? IT WOULD HELP IF YOU never underestimate the importance of your cylinder rod. The rod is not just the object transmitting the force from the cylinder’s piston but also the interface between that cylinder and your machine. Before you can correctly select and then design the piston rod, you must understand the potential areas of failure. The following are the top five most common piston rod problems on hydraulic cylinders. 1. The rod is improperly sized for the application. This can manifest in two ways, and believe it or not, a rod can be too large. Undersized piston rods have low column strength, especially in long stroke applications. A long, thin rod will bend or break when pushing loads heavier than they’re rated. Low column strength can remedy this with a larger diameter rod or a stop tube to provide added support. Conversely, if a cylinder operates in tension, an oversized rod will actually reduce retraction force. This problem occurs at the design stage, so be sure you accurately calculate retraction force with an understanding of differential cylinders. The rod takes up area on the piston, reducing the cylinder’s force in retraction, causing the cylinder to retract with less force than it extends. 2. Rod thread breakage. A thread may break for any of many reasons. From an application perspective, choosing the incorrect thread may be the first mistake. Thread diameter plays a part in its strength, so choosing a standard male thread when a full-size thread would be more robust is a big mistake. The quality of the thread plays a part in reliability, as well. If the radius of the thread turn-down (the gap between the rod shoulder
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and its thread) is not correctly machined, it leaves the window open for breakage. 3. Rod scratches leading to seal damage. Rod scratches rarely affect the strength of the cylinder, especially if they’re minor. However, dents and scratches in the rod rub against the rod wiper and seal, damaging it over time. Using only rod material made from induction hardened steel will prevent dings on the surface. Also, be sure to select chrome at least a thousandth of an inch thick. A thick chrome surface further hardens the steel’s outer shell, which provides critical corrosion resistance. Corrosion on the rod also creates pits that may damage the rod seals. 4. Corrosion of the rod thread. When you machine off the entire crust of the rod thread, you no longer have chrome. Because chrome protects from corrosion, the rod thread is now left to the elements. If your cylinder operates in humid environments or around harmful chemicals, corrosion or rust may weaken the rod end over time. Choose the appropriate alloy for your application. Rod material is readily available in various alloys of stainless steel. Stainless steel won’t rust, and if you order it with chrome, it provides the same surface protection as described earlier. 5. Mechanical damage. Although industrial hydraulic cylinders rarely run into objects or other machinery, mobile machinery is susceptible to such accidental damage. Even the massive rods used on excavators are no match for the power of which the boom, arm and bucket cylinders are capable. Running the cylinders into a bridge upright will ding or dent a rod, no matter how thick the chrome or case hardening.
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 applicationspecific, 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
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HYDRAULIC CYLINDERS
DOUBLE ACTING TELESCOPIC CYLINDER USED IN THE RAIL MAINTENANCE INDUSTRY. | COURTESY OF AGGRESSIVE HYDRAULICS
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.
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HYDRAULIC FILTERS 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 built-in 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 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
HYDRAULIC AND LUBE FILTERS | COURTESY OF SCHROEDER INDUSTRIES
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For over 18 months the world has been changing and we at Zinga want to let you know that we are still here to help like we have been for over forty years. Through a pandemic, rising material costs, and labor shortages, we are still providing unmatched quality, first in class customer service, and expert level solution assistance because that’s how we operate. Our products are still trusted by top brands and our competitors alike, so if you haven’t taken the time to look at Zinga lately, maybe it’s time to take another look. Zinga, we are here to help.
International Tim Cooney
tim.cooney@filtrationgroup.com +1.608.524.4200
Eastern Region Sales Dave Holcomb
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Western Region Sales Stan Askeland
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upwards of 500 psi, which make them ideal for you to use in return lines experiencing pressure spikes. 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 Hydraulic Filtration Systems article to follow to learn more). 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 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 (1,0003,000 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. 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, fluid sidesteps the element
WHAT IS THE DIFFERENCE BETWEEN CELLULOSE AND GLASS FILTER MEDIA MANUFACTURERS MAKE cellulose fibers from long chains of plant material processed into a pulp and then drawn out into strands. Glass media comes from a more complicated process of first drawing molten silica fibers and then combining them into a strand using a resin binder. The product of either process looks surprisingly similar, but their performance resembles each other very little. Both cellulose and glass media has a place in filtering hydraulic fluid from damaging particles. However, their effectiveness may best be compared to how both an electric scooter and luxury sedan both serve their purpose – the context is essential. Cellulose filter media offers a thick layer of fiber cloth, pleated and then wrapped around a perforated core. Cellulose may sometimes go by “paper” fiber, which correctly implies a sheet-like fiber pattern across the cloth. Defined as surface media, particles get trapped closer to the paper media’s leading-edge, where particles accumulate and create a resistance to flow. Some cellulose media also inherently absorb water. This property may be a benefit or disadvantage, depending on your filter’s purpose. Some hydraulic systems are exposed to water contamination, and a cellulose element is excellent at absorbing free water (just not dissolved water). Please note that as water content increases, so does pressure drop as the fibers swell in size. As far as raw filtering ability goes, the cellulose media merely passes the exam with a minimum grade. Even the best cellulose filter elements are likely less than 75% efficient, which describes a filter that may or may not filter passing particles of a given size. Such a low efficiency gives the media a “nominal” rating, meaning the media’s 18
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given micron rating should be taken with a grain of salt. So if cellulose medium’s benefit list is shorter than its downside list, why use it? It is inexpensive to manufacture, which makes it the scooter in the previous vehicle analogy. A replacement spin-on filter element may often cost little more than your venti blonde Americano. With that in mind, you must also consider what a cellulose element is protecting. A log splitter employs commodity-level hydraulic pumps, valves, and cylinders, where economy takes priority over quality. A paper filter media protects essential hydraulic components just fine, and every 20-year-old log splitter is proof of this. The filter manufacturer uses glass media yarn to create rolls of cloth — the foundation for synthetic glass filter elements. The consistent nature of the fiber layers combined with multiple layers provides glass media superior dirt holding capacity. This “depth media” traps particles through their entire thickness and not just on the surface. Glass media offers exceptional filter quality, capable in some cases of filtering 3-micron particles with 99.95% efficiency. This effectiveness qualifies the glass media as an “absolute” filtration rating, where lab tests confirm their rating. Immune from water absorption, any free water within your hydraulic oil will not increase pressure drop. However, if you require such absorption, you’ll have to install supplemental cellulose filter assemblies. Indeed, the glass media filter offers superior dirt holding capacity, finer and more efficient filtration and immunity from water. In many ways, it is superior to cellulose paper filters, so what’s the downside? Cost is easily the downside. A high-quality filter comes at the relative price of a luxury sedan, and such quality filtration is a must to protect expensive hydraulic machinery. Which should you choose? The best you can afford.
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HYDRAULIC FILTERS
IN-LINE MOUNTED FILTERS FOR MOBILE AND INDUSTRIAL USE | COURTESY OF EATON
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 its 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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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?
loop filter neither affects nor is affected by the main hydraulic system, it is a consistent and stable way to keep the oil clean. The pressure drop of often low-micron filter media will never be additive to system pressure drop, especially those related to flow surges in the tank lines of machines with rapid cycle times of cylinders. It is not uncommon to see 5- or even 3-μm offline filters with high beta ratios. Offline filtration also enables changing of filter elements while the machine is running, as shutting down the kidney loop has no association with machine operation. Some filtration systems employ duplex filters, which are two filter assemblies installed in parallel, separated by a three-way ball-valve. This design allows 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
A solid option to increase filtration effectiveness is with an offline filter system, often called kidney loop filters. Offline filtration uses a dedicated lower pressure pump (still often a hydraulic pump), which draws fluid from the reservoir and then flows that fluid through a dedicated filter assembly — usually of a high-quality medium — and then right back into the tank. Sometimes hydraulic power units have dedicated offline filter systems, whose only jobs are to circulate fluid from their reservoirs and filter it as they do so. Because a kidney 20
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SELF-CONTAINED MOBILE FILTRATION UNIT FOR LOWER VISCOSITY HYDRAULIC FLUIDS ENABLES BULK OIL HANDLING, FLUID TRANSFER AND RESERVOIR OR GEARBOX CONDITIONING. | COURTESY OF HY-PRO FILTRATION
might leave the filter system running on the machine for hours or perhaps days. Permanently mounted offline systems are now more commonly used as well. They are often mounted to a panel, either near the reservoir or directly attached to it. Eliminating intermittent filtration of the portable type ensures that fluid is clean from storage to service. Some of these filter systems are installed with auxiliary electronics, such as particle counters. A particle counter will give you a live reading of the ISO Code of the oil passing through the unit, so you can leave the unit running until the desired code is achieved. If this type of system seems out of your reach, note that some hydraulic distributors will rent these machines out for a reasonable cost.
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HYDRAULIC FITTINGS & 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.
JIC FLANGE ADAPTERS. | COURTESY OF TOMPKINS INDUSTRIES
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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; quickdisconnect; push-to-connect; 37˚ flare; 24˚ cone; and inverted flare, among others. When selecting a type of fitting, some important considerations are working pressure, vibration, type of fitting, desired attachment, size of piping, flow, material of the conductor or component, and price. The fluid power industry is trying to transition to fittings with an elastomeric seal — generally O-rings — to prevent leakage. These include, but are not limited to, the SAE straight thread, face seal, ISO 6149, and SAE J518 (Code 61 and Code 62) flanges. Seal construction must be compatible with the type of fluid being used in the system, although very few applications require anything other than Buna Nitrile or Viton. When selecting a fitting, several considerations are important. Most non-flanged fittings have a gender — called male and female — that are joined together to form a union. Most fittings are sized based on the size of the conductor (size of hose, pipe or tube), and overall dimensions can vary greatly based on fitting type, even for the same size conductors. Additionally, most fitting types are available in a multitude of materials, including plastic, brass, steel, stainless or specialty metals like Monel. Each are applied in applications based on the fluid medium and ambient conditions, and each has different performance characteristics that allow customization within a fitting type. Often the first choice is to match the fitting to a similar material of the conductor or component that it is connecting to: plastic to plastic, steel to steel and stainless to stainless. Geometry is also an important consideration, and geometry is typically identified by alphabet letters the fittings resemble. Fittings are available inline to change the direction of flow in various increments (45˚ or 90˚ elbows [L]), or a swivel to allow two joined sections to rotate. They can also split or combine flows with run and branch tees [T], “wahys” [Y] and crosses [+]. Fittings, particularly elbows, are offered in a variety of drop lengths, which is the distance from the centerline of one opening—called a port—to the end of the other port.
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HYDRAULIC FITTINGS & FLANGES Hydraulic Live Swivels Inline & 90°
BSPP AND ISO FLANGE ADAPTERS AND COUPLINGS | COURTESY OF MAIN MANUFACTURING
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
Available In BSPP
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 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 lowpressure 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. 7 • 2021
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What’s less popular but no less effective, especially for large diameter conduits, is weldless, threadless plumbing. Typically manufactured using proprietary connection systems, they use specialized equipment to permanently crimp together sections of tubing quickly and easily. These systems are offered in two standard styles. The crimped technology uses a long connector with soft sealing technology located within the ID. Two tube/pipe sections are butted together, and the connector is slid over the two sections and swaged (crimped) against the tubes to create a positive seal. The flange technology uses flared joints which are interfaced with flanges. A flare slides over the tube before a cone is inserted into the end of each pipe. Long bolts connect the two opposed flanges and are torqued tight to force the flared ends deep into the pipe, expanding it into the tube for a positive seal. Pipe diameter is manufactured up to 10 in. The more common sizes are 2 in. or less because the pressure capacity is reduced for extreme bore pipe. For the crimped style plumbing, smaller sizes are more common because the crimping tool is not manufactured for large bore piping. For flared joint systems, the internal cones and external flanges are offered in sizes up to 10 in., although the pressure capacity for pipe this large is reduced. Just like other plumbing systems, the pressure rating is determined by tube diameter, tube wall thickness and the nature of the connection. Systems are available up to 6,000 psi or higher, especially with smaller bore plumbing. Advantages include cleanliness (less need to purge or flush the system), installation ease (it’s often easier than making a hose assembly), installation time and reduced safety/inspection requirements.
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HYDRAULIC FLUIDS
HYDRAULIC FLUIDS 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 functions — 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 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.
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WHAT IS THE VARNISH POTENTIAL RATING? PROBLEMS THAT CROP UP IN hydraulic systems are often caused by contaminants like dirt and water in the oil that lead to corrosion, wear and premature failure. But many performance problems are due to another contaminant: varnish that coats control valves, restricts filters, and fouls reservoirs and other internal parts. Most hydraulics users are aware of hard, insoluble contaminants like dirt, debris and wear particles. Varnish, in contrast, is considered a soft, insoluble contaminant. Varnish primarily results from excessive heat and oxidation of the oil as well as degradation of additives. The degradation process accelerates as the fluid undergoes continued exposure to air, water, and high temperature. Varnish particles are less than 1 micron in size and are not measured by traditional particle-count testing methods or removed by conventional filtration. Varnish forms a thin-orange, brown or black film deposit — a tough, hard contaminant that coats internal machine surfaces. The reaction products tend to precipitate onto cooler surfaces, like reservoir walls, and onto valves and other working parts, especially when equipment shuts down overnight or on weekends. Varnish deposits on hydraulic system components leads to trouble such as: •
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Sluggish operation, as increased friction and stiction in mechanical parts such as spools and sleeves cause valves to stick or seize. Restricted oil flow and clogging of small orifices, valves, strainers, and filters. Reduced bearing clearances that limit lubrication. The ability to capture hard, abrasive contaminants that accelerate wear. Acting as an insulator, restricting heat transfer from reservoirs and heat exchangers and raising operating temperatures. Serving as a catalyst, speeding fluid deterioration and forming more varnish.
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All of these issues can affect performance and efficiency. It can increase wear in pumps, valves, gears and bearings. Increased friction and restricted flow raises energy consumption. Accelerated corrosion hastens component failure. In the end, the cycle time to manufacture a part or move an implement increases, lowering productivity. Valves and filters must be replaced more often. Maintenance costs rise and equipment life suffers. In extreme cases, sensitive hydraulic valves can abruptly seize and fail if varnish goes undetected, costing thousands of dollars in downtime, damaged components and ruined products. Historically, visual inspections were performed to identify varnish buildup. But traditional oil analysis testing, which is important for maintaining oil and machine health, has not been a reliable, cost-effective procedure to identify the presence of varnishing conditions. Fortunately, several methods have been developed to test for varnish in a fluid and system. Membrane patch colorimetry (MPC) varnish potential testing (ASTM D7843) is an analytical test to determine the propensity for a lubricant to form varnish deposits. MPC involves filtering a defined fluid sample through a membrane, and color of the contaminants are measured against a control membrane patch. The ultra-centrifuge method spins an oil sample to separate out varnish precursors. The sediment is checked against a visual rating scale. Remaining useful life tests measure active antioxidants in the lubricant. And Acid Number (ASTM D974) measures the total acid number in the fluid. Acid depletes antioxidants, leading to varnish formation. Bureau Veritas, with U.S. headquarters in New York City, has developed an innovative program called Quantitative Spectrophotometric Analysis (QSA) that can accurately determine the likelihood of a used lubricant producing harmful sludge and varnish. The QSA program is based on a series of new procedures that determine a specific
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rating for the used fluid. Based on the lubricant type and the application, this rating lets a trained diagnostician determine the presence or likelihood of internal sludge and varnish build-up. QSA is fundamentally based on spectrophotometry. This test method measures how a chemical substance absorbs light by detecting the intensity of a light beam that passes through a solution. Specific compounds absorb or transmit light over a certain range of wavelengths. This measurement can also determine the amount of a known chemical substance. Spectrophotometry is widely used for quantitative analysis in chemistry, physics, material and chemical engineering and medical applications. Quantitative Spectrophotometric Analysis is a technique for purposely isolating and measuring the specific lubrication degradation by-products responsible for varnish formations. It is not a measure of varnish already formed; it is a determination of the lubricants propensity to form varnish and can be applied to preventing varnish formations and build-up within the system. Results are represented as the Varnish Potential Rating (VPR) with a scale of 1 (no varnish) to 100 (severe fouling). QSA is an integral part of a comprehensive test and analysis procedure Bureau Veritas calls QSA Complete. It includes spectrochemical testing (ASTM D6595), viscosity D445, acid number D974, water content D1744 , ISO particle count ISO 11171, in addition to Quantitative Spectrophotometric Analysis. Samples that show a high VPR may indicate the need to change out the oil. Some fluid-additive manufacturers have developed system cleaners that remove varnish, without manual scrubbing or hazardous solvents; and hydraulic-fluid additive packages that prevent new varnish from depositing on valves and reservoir walls.
HYDRAULIC FLUIDS 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 returning clean fluid to the circuit. 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 full-film lubrication between moving parts, such as the slippers and valve plate of a piston pump. Without the lubricating properties of oil, hydraulic 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 allweather 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 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. 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 high-quality 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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making a difference
FOR YOUR BUSINESS Our cutting-edge line of hydraulic hose and fittings is not just a product division. It’s a promise of superior service, quality, technical support and availability. We’ve built our company on impeccable customer service. Let us know how we can make a difference for your business. call 800.231.7116 or email sales@texcelrubber.com
HYDRAULIC HOSE
HYDRAULIC HOSE ALFAGOMMA HYDRAULIC HOSES WITH STAINLESS STEEL FITTINGS. | COURTESY OF KURIYAMA OF AMERICA
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 www.fluidpowerworld.com
flexibility and elasticity. This feature often makes it expandable and easy to store. Corrugated hose contains corrugations, pleats or spiral convolutions to increase flexibility and capacity for compression and elongation. Multi-element hydraulic hoses are constructed of more than one hose formed or adhered together in a flat, ribbon or bundled configuration. Additional features to consider include whether the hose requires integral end connections, anti-static, lay flat, crush-proof and flameresistance 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 7 • 2021
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FLUID POWER HANDBOOK hose exposed to harsh environments. Substances such as UV light, ozone, saltwater, chemicals and pollutants can cause degradation and premature failure. For in-depth fluid compatibility data, consult the manufacturer. While hydraulic hose is usually constructed of multiple materials, the most commonly used primary materials include elastomers, fluoropolymers and silicone, thermoplastics, metal, and composite or laminated structures. Elastomeric or rubber hydraulic hose are often selected for their flexibility. Fluoropolymer hose offer good flex life, superior chemical and corrosion resistance and can handle high temperatures. Thermoplastic hydraulic hose 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.
MEGASYS MXG 4K HYDRAULIC HOSE IS QUALIFIED TO ONE MILLION IMPULSE CYCLES. IT IS LIGHTWEIGHT, COMPACT, AND SERVES AS A HIGHLY FLEXIBLE DROP-IN REPLACEMENT FOR CONVENTIONAL SPIRAL HOSES. | COURTESY OF GATES CORP.
HYDRAULIC HOSE & FITTINGS
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HYDRAULIC HOSE
HOW DO YOU OVERCOME HOSE MAINTENANCE CHALLENGES? MOST CONSIDER A hydraulic hose to be maintenance-free. They’re often installed and forgotten until they need replacement, hopefully not under conditions of catastrophic failure. Contrary to popular belief, hydraulic hose does not enjoy infinite service life. Like any component manufactured from metal and rubber, you may undoubtedly expect failure in the hydraulic hose’s lifetime. When and how you remove a hose is up to you, whether from failure or planned replacement. It goes without saying that hydraulic hose must be installed in such a way as to prevent any physical strain, such as kinking or torquing. The hose should lay as naturally as possible, with no abnormal stress upon the connection or reinforcement. Hose maintenance means little if installation errors result in failure long before servicing is required. The correct hose length prevents excess length from sagging or rubbing against areas causing wear. You must eliminate the potential for accidental causes of hose failure, of which
the majority occur from physical damage or wear of the hose cover. Rigidly fixing the hose in place using clamps typically prevents the hose from rubbing against anything, but sometimes the machine requires bending as part of the machine function. Any hose with movement HOSE SLEEVES PROTECT HOSES FROM WEAR AND potential must be ABRASION AND ALSO FROM HARMFUL DEGRADATION protected with hose CAUSED BY UV RAYS AND HARSH CHEMICALS. wraps, which may | COURTESY OF PYTHON PROTECTIVE be flat plastic coil or SLEEVE + COVERS abrasion-resistant sleeves. Hose sleeves offer the secondary benefit inspect the outer cover of the hose beneath. of chemical protection. A dirty environment If you find any apparent cracks, leaks, or with exposure to industrial fallout increases abrasion, replace the hose immediately. the breakdown rate of rubber. A sleeve is It helps to keep track of your hydraulic clamped the entire length of the hose from hose service life. If you know a machine fitting to fitting, providing a sacrificial surface seems to experience only three or four years that prevents hose wear from both abrasion before hoses start to fail, perhaps because and chemicals. Inspect the hose sleeve or of demanding conditions, then consider wrap regularly, especially in areas known replacing all hoses within that window. to rub against the machine or For example, if after four years, three of other hoses, and replace the the twelves hoses on your machine have wraps if any damage fails to prematurely failed, consider that the others protect the hose itself. are not likely to last much longer. Increase Although a well-maintained your inspection frequency to be sure other hose assembly will last many hoses are not near failure, and if your years, even the most perfectly machine is critical to production, consider installed and maintained replacing hoses early. hose requires replacement Hydraulic hose assemblies should not eventually. Just like hydraulic be neglected because they’re a fraction pumps, filters and oil require of the replacement cost compared to periodic replacement, so too pumps or actuators. A hose failure results does a hose. One downside of in machine downtime just the same as a protective sleeves is that they pump failure – proper hose maintenance hide the hose itself, preventing is preventive, not reactive. easy inspection. Part of your preventive maintenance program should involve removing the hose wrap to
USING HOSE CLAMPS GIVES HYDRAULIC HOSES CLEAN ROUTES, HELPING TO AVOID ABRASION, WEAR AND HOSE BENDING, WHIPPING AND OTHER UNNATURAL MOVEMENTS. | COURTESY OF VERMATIC INC.
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FLUID POWER HANDBOOK
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HIGH TEMPERATURE HOSE AND COUPLINGS | COURTESY OF KURT HYDRAULICS
HYDRAULIC HOSE COUPLINGS 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 32
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they need a locking mechanism to prevent involuntary disconnection? Is one-handed operation required? Couplings can be two types: permanent and field-attachable (reusable). Permanent couplings are generally more reliable, easier and quicker to attach than fieldattachable couplings, which makes them widely used in industry. Crimping or swaging equipment (sometimes both) is needed to put a permanent coupling on a hose. Permanent couplings can be pre-assembled (one piece), with a ferrule permanently attached to the stem. Higher-pressure hoses use fieldattachable couplings, as well as permanent couplings. Field-attachable couplings fit right www.fluidpowerworld.com
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
HYDRAULIC HOSE COUPLINGS seals are SAE 45° and JIC 37°, but there are others. The NPSM seal is a mated angle. Couplings with angle seats for sealing have straight or parallel threads. The threads themselves do not seal fluids as with tapered threads. Instead, the threads function to mechanically bring the two mating angle seats together. National Pipe Straight Thread Mechanical Joint (NPSM) brings two 30° tapered seats together to make the seal. SAE 45° flare couplings are used on lower pressure applications, such as fuel lines, hot oil lines or refrigerant lines. JIC 37° angle seats are used on medium- and high-pressure lines on heavy equipment to join hydraulic hose assemblies to hydraulic system components. There are three types of O-ring seal designs: O-ring boss, flat-face O-ring seal and O-ring flange. In the boss design, straight threads make the connection while a rubber O-ring makes the seal. Threads pull the O-ring against the port, which has a machined groove for the O-ring, flattening it and making a seal that is excellent for high-pressure applications. In a flat-face O-ring seal, the O-ring sits in a groove on the male’s face. The seal is made when the O-ring of the male meets the flat face of the female. The solid male O-ring face seal fitting will mate only with a swivel female O-ring face seal fitting. O-ring flanges make high-pressure, large-diameter connections. A port is bored with a center outlet, surrounded by a smooth flat face, which has four tapped holes and four mounting bolts that tighten down onto flange clamps. There are no threads on this coupling. The flange itself has the groove for the O-ring. There are several SAE and ISO standards that cover the performance requirements of hydraulic hose assemblies. Included are the J517, J516 and J343 standards. The three most common ISO standards are ISO A, ISO B and ISO 16028. The standard for performance testing is ISO 7241-2. Hydraulic hoses that claim to meet SAE J517 standards (for example, SAE 100R1 and SAE 100R2) need to be designed for, and certified to, the criteria defined by SAE. That criteria includes stringent dimensional tolerances (inside, outside and braid diameters), compound and reinforcement types, length changes, cold flexibility and ozone and heat resistance. There are also burst pressure and impulse requirements in J517. Those requirements are for coupled assemblies, and SAE states that “the general and dimensional standards for hydraulic hose fittings are obtained in SAE J526.” Hydraulic hose fittings that meet SAE J516 standards are similarly well defined by SAE as to material type, dimensions, finish and so on. The SAE manual also specifically states that J516 fittings are intended to be used “in conjunction with hydraulic hoses specified in SAE J517 and used in hydraulic systems on mobile and stationary equipment.” SAE J343 is the standard that establishes “uniform methods of the testing and performance evaluation of the SAE 100R series of hydraulic hose and hose assemblies.” Coupled assemblies are expected to meet or exceed SAE performance if the SAE criteria described above are met. The integrity of any hose assembly depends upon the components, fittings and hose meeting the rigorous SAE requirements, and then the components being assembled by skilled personnel. This is true regardless of where the components are manufactured. 7 • 2021
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X-SERIES HYDRAULIC FLAT FACE COUPLINGS | COURTESY OF CEJN
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WHAT SHOULD YOU KNOW WHEN SPECIFYING QUICK COUPLINGS? HYDRAULIC QUICK COUPLINGS allow for rapid connection and disconnection of hydraulic hoses to machines or sections of plumbing. Also called quick-connects or disconnects, these fittings allow fluid to flow when the male and female sides are locked together. Conversely, when disconnected, the quick couplings prevent fluid leakage using check valves to block exiting fluid.
ARGUS QUICK DISCONNECT FITTINGS | COURTESY OF KURIYAMA
Q: ARE ALL QUICK COUPLERS THE SAME? A: No, there are two primary construction styles for hydraulic quick
couplings. The check valve style system uses either balls or poppets to block flow from exiting the fitting. When connected, the balls or poppets push against each other, lifting both off their seats to allow flow through the connection. Flat face couplers employ a more sophisticated interface that, when joined, will push face sections into each other, opening a large flow path through them both. Thread-to-connect couplers employ male threads on the nipple portion, and when inserted and tightened into the female coupler, squeeze together to open both check valves. Thread-toconnect couplers offer a very high-pressure rating while offering very little flow in return.
Q: WHAT SIZE SHOULD I SELECT FOR MY QUICK COUPLINGS? A: Quick couplings should be sized based on their flow capacity.
It’s a mistake to size couplings relative to the thread size on the hose or fitting to which the coupling attaches. For example, 3/8 in. quick couplings may only flow 4 gpm at 20 psi of backpressure, while the hose itself may flow double that amount. It’s okay to use bushings to reduce the larger thread on the coupler down to the size of your plumbing. If your hose thread is larger than your coupling thread, your coupling is likely undersized. HYDRAULIC QUICK COUPLERS | COURTESY OF HOLMBURY
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Q: DO ALL COUPLINGS OF THE SAME SIZE HAVE THE SAME FLOW RATING? A: No. Flat face couplers offer a more significant flow path with
reduced pressure drop. Their interlocking sleeves open an extensive cross-section than poppet-style couplings. The poppets or ball-checks of the latter interfere with the flow path, creating pressure drop.
Q: WHY DOES THE PRICE VARY SO WIDELY WITH QUICK COUPLINGS? A: Just like many (but not all) hydraulic components, you get what
you pay for. Ball-check couplings from a local tractor parts store will offer little in common with premium flat face couplers. Inexpensive couplings will corrode more quickly, which is a problem with mobile applications. As previously mentioned, the flow rate will suffer using ball-check couplings, forcing you to oversize the set and pay more regardless. Cheap couplings will also leak more readily, all the while offering little in the way of smooth connection and disconnection. Finally, the poppet style loses oil every time you disconnect it and is also prone to collecting contamination in the female end’s open face. Premium flat face couplings offer corrosion-resistant parts that are precision-machined to offer the smoothest, most reliable connection. Their large flow path offers lower pressure drop, so you’re benefited from using a more appropriately sized set. Finally, most flat face couplings offer a twist lock that prevents accidental disconnection should an object hit the sleeve.
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HYDRAULIC POWER UNITS 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. 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. 36
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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 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 cavitationrelated damage from aeration. After you calculate tank volume, you must now consider the reservoir construction type. Reservoir style 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
HYDRAULIC POWER UNIT ILLUSTRATION | COURTESY OF ADOBE STOCK
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HPUs
CUSTOM HPU DESIGN | COURTESY OF HYDROTECH INC.
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 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 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 www.fluidpowerworld.com
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. 7 • 2021
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FLUID POWER HANDBOOK
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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
FULL VALVE BANK OF SLV20 LOAD-SENSING PROPORTIONAL DIRECTIONAL VALVES OFFERS PRIORITY-FLOW-SHARING FOR MINI AND MICRO MACHINERY. | COURTESY OF EATON
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operations (levers) or electronic feedback systems. These inputs energize various valves mounted on or in the manifold, while specific oil pathways allow oil to flow through hydraulic lines to the appropriate actuator to perform work. The complex matrix of variables can make manifold design and component selection a challenging and rewarding art form, as size, weight, function, performance and operating environment are always part of the design consideration. In addition to providing a neat and logical layout, consolidating components into a manifold reduces space and pressure drop. This results in fewer fittings, more efficient assembly times and reduced leak points. Manifolds are sometimes viewed as black boxes, as they can be highly complex with upward of 500 holes communicating with each other and 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 are provided in key locations, finding a problem becomes much quicker and simpler with a manifolded system. If transducers and other data collection devices are connected to these test points, the data may be linked into the machine controller and operation’s terminal displays. Manifolds generally operate within 500 to 6,000 psi operating pressures. With additional design considerations, 10,000 psi can be achieved within the scope of steel and stainless-steel manifold designs. Although not typical in hydraulic application, 50,000 psi can be achieved with special materials and design nuances. Manifolds come in three basic types. Most common is a solid-block design that contains all drilled passages and valves for an entire system. Typical materials for solid-block manifolds are aluminum, steel and ductile iron. Block weight can reach 100,000 lb. Modular-block, or stackable design, is a subset of the drilled block. Each modular block usually supports only one or two valves and contains interconnecting passages for these valves as well as flow-through provisions. It normally is
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HYDRAULIC MANIFOLDS connected to a series of similar modular blocks to make up a system. This system is known for its flexibility within a limited range of circuit complexity. Modular block designs are generally held together with tie rods or a system of tapped holes that allows for machine screw connections. Lastly, laminar manifolds complete the manifold category. Laminar manifolds are usually made of steel, with passages milled or machined through several plates of metal. These plates are stacked or sandwiched with the various fluid paths determined by the shape of the machined passages. Solid-metal 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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HYDRAULIC MOTORS
HYDRAULIC MOTORS
LOW-SPEED, HIGH-TORQUE HYDRAULIC MOTOR FOR LIGHT- AND MEDIUM-DUTY APPLICATIONS. | COURTESY OF GRH
HYDRAULIC MOTORS
convert fluid power into mechanical energy. High-pressure fluid flow in a circuit is used to push vanes, gears or pistons attached to an output shaft, with power capacity of a hydraulic motor dictated by its design, size and speed, among other factors.
Much like electric motors, hydraulic motors generate rotational motion and torque. However, hydraulic motors require no electricity and can withstand dusty and dirty environments, extreme heat, and even submersion. Perhaps most significantly, hydraulic motors have exceptional power-to-weight ratios. In terms of power capacity, an electric motor can weigh 20 times more than an equivalent-rated hydraulic motor. Some hydraulic motors offer high speed capabilities, such as in fan drives. Others, for instance winches, move heavy loads at low speeds, sometimes less than one rpm. They are used in industrial applications such as augers, conveyors and mixers, as well as in rolling mills, where they are preferred thanks to their robust nature and resistance to heat. Likewise, hydraulic motors are often the primary drive in off-highway equipment. Hydrostatic drive systems transmit engine power to the drive wheels with exceptional versatility and reliability. Hydraulic wheel motors’ speed control and smooth reversibility are perfect for use on backhoes, skidsteers and wheeled loaders. Motors are also used in tracked vehicles such as excavators and bulldozers, where the high power density of hydraulic motors let them achieve substantial torque in a relatively small package. Hydraulic motors are rated according to parameters such as torque capacity, speed range, pressure limitations, efficiency and displacement. Displacement is the amount of fluid needed to turn the output shaft one revolution; it is rated in terms of cc/rev or cu.in./rev. The units can be either fixed- or variable-displacement and operate either bidirectionally or unidirectionally. With input flow and operating pressure constant, fixed-displacement designs provide constant torque and speed. In contrast, under constant flow and pressure conditions, a variable motor can vary torque and speed. Thus, variable motors have a wider speed range capacity. In general, valves control direction and speed of a hydraulic motor. With proper relief-valve settings, motors can be stalled without damage. And some can be used for dynamic braking.
MOTOR DESIGNS
Motor types include gear, vane and piston units. They are usually similar in construction to the analogous hydraulic pumps. Gear motors are probably the most popular designs, and they come in several versions. External gear motors feature a matched pair of spur or helical gears enclosed in a housing. One is the driven gear
— which is attached to the output shaft — and the other an idler gear. Their function is simple: high-pressure oil is ported into one side of the meshing gears and forces them to rotate. Oil flows around the gears and housing to the outlet port. It is a constant-displacement motor. A second type of gear motor is the gerotor, or internal-gear, motor. The internal gear has one less tooth than the outer gear, and it rotates and seals against the outer component to minimize bypass leakage. The inner gear connects to the output shaft, and speeds and power density of the unit can be quite high. Another variation is the roller-gerotor motor, where rollers replace the lobes of the outer gear to minimize friction. They tend to provide smooth, low-speed operation and have higher efficiencies and longer lives. One concern with gear motors is leakage from the inlet to outlet, which reduces motor efficiency and generates heat. In addition to their low cost, gear motors do not fail as quickly or as easily as other styles, because the gears wear down the housing and bushings before a catastrophic failure can occur. Vane motors operate in the medium-pressure and cost range. Torque develops by pressure acting on exposed surfaces of vanes that slide in and out of slots in the rotor, which connects to the output shaft. As the rotor turns, vanes follow the surface of a cam ring and carry fluid from inlet to outlet. Vane motors are fixed-displacement types. Piston motors are also available in a variety of styles, including radial, axial and other less common designs. Radial-piston motors feature pistons arranged perpendicularly to the crankshaft’s axis in barrels that radiate out from the drive shaft. Fluid pressure moves the pistons linearly and causes the crankshaft to rotate. This reciprocating action against a lobed cam ring can produce extremely high torques with very low to moderate speeds. 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. There are two designs of axial-piston motors. The first is the swashplate design where the pistons and drive shaft are parallel. The second is the bent-axis design, where the pistons are arranged at some angle to the main drive shaft. In this design, the up-and-down motion of the pistons is converted to rotary motion through a ball joint. Axial-piston motors are noted for high volumetric efficiency as well as good low-and high-speed performance. These motors can be fixed-displacement or variable-displacement, depending on the design. For instance, the piston stroke can be varied in the latter type by changing the angle at which the swash plate is inclined.
SPECIFYING MOTORS
There are several important factors to consider when selecting a hydraulic motor. You must know the maximum operating pressure, speed and
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FLUID POWER HANDBOOK torque that the motor will need to accommodate. Knowing its displacement and flow requirements within a system is equally important. Operating fluid and tolerance for contamination are other considerations. In broad terms, gear motors tend to be suited for medium flows and pressures, and are the most economical. Vane motors 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. Cost is clearly a major factor in any component selection, but initial cost and expected life are just one part of the equation. Users must also know the motor’s efficiency
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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. LOW-SPEED, HIGH-TORQUE MOTORS ARE A DIRECT INTERCHANGE WITH EATON/ CHARLYNN AND OTHER BRANDS. | COURTESY OF FLUIDYNE
WHEN SHOULD YOU USE ROTARY ACTUATORS?
ECKART SM4 HEAVY-DUTY ROTARY ACTUATOR IS HIGHLY-CUSTOMIZABLE AND DESIGNED FOR TOUGH AND DEMANDING APPLICATIONS. | COURTESY OF IC FLUID POWER
ROTARY ACTUATORS are a hybrid of linear and rotary actuators. In fluid power, linear and rotary actuators are referred to simply as cylinders and motors. A cylinder creates linear force when pressure acts up its piston’s surface area, while a motor creates rotational force when pressure acts upon its gears, pistons or vanes. A rotary actuator may take elements from cylinders or motors to create torque. The difference between a rotary actuator and motor is the former rotates around a limited arc, while a motor may rotate infinitely. Two common types of rotary actuators exist. One uses vanes inside a housing with physical stops to prevent the armature from rotating past 360 or less. The other will use piston-based cylinders to convert linear force into torque via one of two ways. The more common system employs a single or dual cylinder setup that cycles a rack back and forth against a pinion gear, rotating that gear and the output shaft anywhere along its operating range. The more complicated system uses a piston with helical cut gears on its outside and internal diameters. With force applied to its piston,
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the counter-rotating force on the OD spins the piston, which in turn spins the output shaft. You use rotary actuators when you need rotary actuation over a limited arc of travel. If your function requires the rotary actuator to cycle diametrically between 90 and 180°, hard stops prevent the actuator from moving to 89° one way and 181° the other. Some designs traverse their entire functional arc, such as vane-type units. Sometimes their arc is adjustable, but they do well to rotate both directions continuously between stops. Rotary actuators based on cylinders easily stop and hold their load anywhere along the stroke of the cylinder(s) on which they’re based. These limitations are general; manufacturers provide options to use rotary actuators full or partial rotation. Counterbalance, brake and PO check valves may hold a load locked in place, just as with the actuators upon which they’re based. Rotary actuators are used in farm applications to rotate booms, arms or other devices over a particular range. They also allow actuation of highly flexible joints, such as an optional scrap handling grapple attachment for an excavator. A boom conveyor on a stone slinger truck benefits from a rotary actuator, preventing the operator from over-extending in either direction. They are excellent for material handling, such as bin tippers or pallet changers.
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ECKART E3 HYDRAULIC ROTARY ACTUATOR IS DESIGNED FOR MOBILE APPLICATIONS,BUT IS NOT LIMITED TO THEM. IT FUNCTIONS ACCORDING TO THE “HELICAL GEAR” PRINCIPLE. | COURTESY OF IC FLUID POWER
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HYDRAULIC PUMPS 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.
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P2100 2- AND 4-BOLT BI-ROTATIONAL HYDRAULIC GEAR PUMP | COURTESY OF PERMCO
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FLUID POWER HANDBOOK Gear pumps can be either internal or external styles. External gear pumps are one of the most popular types used in modern hydraulic systems. Gear pumps produce flow by using the teeth of two meshing gears to move the fluid. Their simple construction ensures limited purchase costs and servicing. They feature decent mechanical and volumetric efficiency, compact dimensions and low weight/ power ratio. Of the three common types of positive displacement pumps, gear pumps are the least efficient; their appeal is low cost and simple design. External gear pumps can be equipped with straight spur (the most common type), helical or herringbone gears. In operation, the drive gear and driven gear rotate, creating a partial vacuum at the pump inlet (where gear teeth unmesh) that draws fluid into gear teeth. Gear teeth mesh at the outlet, forcing fluid out of the pump.
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Internal gear pumps contain one internal and one external gear. They pump fluid in the same manner as external spur gear pumps. In the basic design, the internal gear, which drives the outer gear, has one tooth less than the outer gear. As they mesh, the teeth create sliding seal points. Because their transition zone from low to high pressure (the area over the crescent) is relatively long, internal gear pumps can offer lower noise levels than some other types of pumps. Gears are made of special steel and are often case hardened and quench hardened. Then gears are ground and fine finished. Proper tooth profile design and geometric proportions can reduce pulsation and noise levels during pump operation. Piston pumps supply high flows at high speed. Two types of piston pumps—axial and radial—are manufactured in both fixed- and
C40V MEDIUM-PRESSURE AXIAL PISTON PUMPS | COURTESY OF HAWE
TROUBLESHOOTING COMMON HYDRAULIC VANE PUMP PROBLEMS VANE PUMPS ARE QUIET, efficient and reliable, especially when they’re provided care. However, they’re not indestructible and experience problems like every other pump. Just like all hydraulic pumps, when taken care of, they may last you a lifetime. Part of improving your vane pump’s life span is understanding what can go wrong through intelligent troubleshooting. Vane pumps are unique in their construction, using spring-loaded and pressure-activated vanes that push outward onto the cam ring. They’re also straightforward to repair, with replaceable cartridges that include the entire rotating group plus the cam ring. To better understand the ins and outs of troubleshooting vane pumps, I solicited an expert’s help. Glenn O’Neill has been serving hydraulics for 35 years, starting as a heavy equipment mechanic. He is the GO Hydraulics owner in Scarborough, Ontario, where he services and repairs every walk of hydraulic pump, motor and cylinder. I asked O’Neill what the most common failures of hydraulic vane pumps are, to which he responded, “The most common failures are shaft leaks, hardened seals, burning of the pressure plates and rotor, usually caused y running dry or contamination.”
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A leaking shaft on a vane pump is an obvious point of failure, providing you with a visual clue something is wrong in the form of a liquid sheen dripping from the pump’s front end. I asked Glenn to elaborate on the potential causes of a shaft failure, and he continued, “Shaft leak could be caused by bearing failure, hardening of the seal, or external contamination embedding in the rubber of the seal and running on the sealing surface on the shaft, cutting a groove in the shaft.” Clearly, some failures are catastrophic, but others sneak up on you. The shaft’s hardening is especially concerning since its observation is impossible without tearing apart the pump for inspection. Regarding the shaft seal, O’Neill added, “Hardening of the seal is usually caused by overheating for a lengthy period of time.” It must be stressed that hydraulic systems best operate within a preferred window conducive to performance and longevity. Try to avoid extended periods running above 140°F to avoid seal hardening. Reflecting on other failures, O’Neill pointed out that “another failure is a rippling of the cam ring.” The cam ring surrounds the rotating group and provides the surface against which the vanes reciprocate. “Failure of cartridge kits is usually indicated by a loud noise while under pressure, and reduction of
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flow,” he added. The cam ring and cartridge noise should not be confused with cavitation, which itself sounds like marbles made their way inside. Troubleshooting is most helpful while the pump is still in operation. I asked him to discuss the tactics he uses when diagnosing a vane pump failure, to which he responded, “Two quick methods for determining a failure of the pump without removing it are noise volume and feel. When I say “feel,” (I mean to) hold the pressure line and feel for excessive heat and gurgling of fluid on the outlet line.” O’Neill confirmed, “a sound running pump should be smooth, quiet and (have) normal running temp of oil.” Manufacturers produce both variable displacement and fixed vane pumps, although fundamentally, their rotating groups differ little. Variable displacement vane pumps compensate by pushing the cam ring to the neutral position to reduce effective pump volume. I was curious if the two construction types differed in their methods of troubleshooting. However, O’Neill confirmed, “pressure compensated vane pumps endure the same type of failure as the fixed pumps.” Clearly, the takeaway is thought and care must be applied to the troubleshooting of any vane pump.
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FLUID POWER HANDBOOK
A10V45 VARIABLE DISPLACEMENT PUMP | COURTESY OF FLUIDYNE FLUID POWER
FLUID CONDUCTING SWIVEL JOINTS
Full 4:1 Safety Factor — Field Repairable — RoHS Compliant Hydraulics, Inc., swivels provide system developers the opportunity to select swivels having geometric relations of fluid ports that compliment the movement between a systems fluid ports. These products offer designers an opportunity to improve existing concepts and take a different approach to new equipment design.
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variable-displacement versions. Axial-piston pumps contain one or more pistons that convert rotary shaft motion into axial reciprocating motion. An angled cam (or wobble plate) rotates, causing pistons to reciprocate and take fluid in as they move toward the thin part of the plate. Fluid is expelled as pistons approach the thick end. In the bent-axis design, both pistons and shaft rotate, making a wobble plate unnecessary. Bentaxis pumps use the drive shaft to rotate pistons. With the longer sealing paths along the piston walls, piston pump efficiencies tend to be higher than other types of pumps. In addition, variable-displacement pumps can provide savings by only providing the pumping necessary for the function, saving additional energy and costs. Radial-piston pumps (fixed-displacement) are used especially for high pressure and relatively small flows. Pressures of up to 10,000 psi are common. Variable-displacement is not possible, but sometimes the pump is designed in such a way that the plungers can be switched off one by one, so that a sort of variable-displacement pump is obtained. Radial-piston pumps are characterized by a radial piston arrangement within a cylinder block. As pistons reciprocate, they convert rotary shaft motion into radial motion. One version has cylindrical pistons, while another uses ball-shaped pistons. Another classification refers to porting: Check-valve 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 metal-to-metal contact. Forcing the vane out of the rotating center and toward the pump housing is accomplished using spring-loaded vanes, or more traditionally, vanes loaded hydrodynamically (by the pressurized system fluid).
Swivels provide more design versatility, longer flex hose life, simplified plumbing, and ease of maintenance. Multiple Geometric Port Configurations
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A3HM SERIES PISTON PUMP FOR OPEN CIRCUIT PUMPS. | COURTESY OF YUKEN
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REPAIR, REBUILD & MANUFACTURING 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, over-pressurization, and excessive heat. Thus, installing a robust filtration system, keeping components cool, and following a disciplined maintenance program are critical to extending component and machine life. Finally, some components break due to incorrect application, installation or commissioning — failures that are often preventable. Technically savvy repair shops tear a unit down and try to bring it back to “as new” condition. All critical dimensions and surfaces are inspected and measured. Seals and low-cost consumable parts like springs, washers and shims tend to be replaced. Likewise, bearings will be inspected and possibly replaced. In more-serious cases, say internal wear due to contamination damage in a piston pump, lapping the surfaces might be suitable if still within acceptable tolerances. Otherwise, the technician may need to remanufacture or replace rotary barrels, pistons, and other internal parts. That can ultimately extend to replacing other major components like housings, covers, relief valves, controllers and
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HYDRAULIC CYLINDERS BEFORE AND AFTER REPAIR AND REBUILD | COURTESY OF YATES INDUSTRIES
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REPAIR, REBUILD & MANUFACTURING
WHAT IS THE BEST RAW MATERIAL FOR HYDRAULIC CYLINDER REPAIR? WHEN YOUR customer’s cylinder arrives on your dock for repair, how often do you consider raw material when you quote the overhaul? Probably very little. Most hydraulic cylinders are manufactured from a combination of 1018/1020 cold rolled steel for the “square” components such as flanges, heads and caps. Piston rods mostly likely adhere to 1045 alloy, while the typical piston uses cast or ductile iron.
HYDRAULIC CYLINDER REPAIR SERVICES. | COURTESY OF HYDRAULEX
charge pumps. In the worst case, a completely new unit can be built from parts, although that is not the norm. Another issue is whether to repair/remanufacture a unit with parts sourced from the original hydraulic OEM or with aftermarket replacement parts. The hydraulic repair industry broadly falls into three business sectors. Hydraulic component manufacturers often tend to serve large machine builders directly, with their own parts. But they often leave user service and support to distributors and large, sophisticated repair houses. Some use OEM parts exclusively, some do not. Further removed are smaller shops that can find OEM replacement parts costly and not readily available. After completing the repair, testing and calibration are also critical to ensure repairs are done right the first time. This is necessary simply because hydraulics has gotten more complex. Newer components routinely have integrated electronics, digital controls and sophisticated software. Unfortunately, the complexity of today’s hydraulics means pure mechanical aptitude is no longer sufficient to fix many components. Components with higher operating pressures and electronics controls also tax the capabilities of test equipment at many repair facilities. Small shops will make repairs and perhaps run basic tests, but they are not capable of performing full-function tests. Larger repair shops have made significant investments in state-of-the-art test stands for qualifying dynamic open and closed-loop systems. Not surprisingly, the price of a repaired or rebuilt component can vary widely depending on the expertise behind it. Experience and specialized skill is required to correctly diagnose, rebuild and test hydraulically powered equipment. Costs include the expertise for diagnosing the failure, recommending the proper repair, whether to rework or replace a part, and whether to use genuine or aftermarket parts. This ultimately speaks to the overall competency of the repair company and the capabilities of the staff and testing equipment. The quality of the repair shouldn’t be driven solely by price, either. Companies that shop by price alone are usually disappointed by the outcome, as the repair may not hold up. Consider the time and labor needed to remove a failed pump or cylinder from a machine and install a rebuilt one, plus the cost of the repair itself, as well as the cost of machine downtime and lost productivity, and it quickly adds up. Cutting corners on parts or testing and finding that the repair quickly fails, and forcing the user to start over at square one, gets expensive. That’s why it’s essential to get repairs right the first time.
Some consider these raw materials industry standard, and for a good reason. They’re inexpensive and easy to source but also provide the cylinder with more than adequate performance potential. However, what if we dig deeper to understand why the cylinder arrived in your shop? Perhaps understanding why the cylinder failed provides you with an opportunity to upgrade the material to better suit your customer’s needs. Your typical chromed piston rod uses 1045 carbon steel alloy, an easily machined and welded material with high strength and wear resistance. You can expect over 80,000 psi tensile strength, which defines the maximum force the material is expected to see before it will stretch. You may also expect perhaps 0.0005 in. (1/2 thou) thick chrome to further improve the surface hardness and reduce the potential for corrosion.
However, just because the above material is inexpensive or was originally on the incoming repair, we can expect to improve upon simply “good.” If your customer’s rod was dented, corroded or shorn, upgrading the rod material is in their best interest. That same 1045 base steel may be processed further to increase tensile strength well above 100,000 psi to help prevent shearing damage. As well, induction hardening strengthens the outmost layer, helping reduce damage from impacts. Finally, doubling the chrome thickness to 0.001 in. (1 thou) adds another layer of corrosion protection while offering a more reliable sealing surface. The 1018 or 1020 mild steel alloys used for most hydraulic cylinders components, much like 1045, comes stocked on the shelves of steel distributors everywhere. However, for high pressure or extreme duty applications requiring much machining or welding, consider 4140 annoy instead. You will find 4140 as the alloy of choice for cylinders requiring internal porting and cavities for valve systems, such as integrated counterbalance valves or other internal circuitry. Also known as chromoly steel, this material also takes well to further treatments, such as annealing, hardening and forging.
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FLUID POWER HANDBOOK
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HYTHANE TPU SEALS ARE USED IN DYNAMIC SEALING ELEMENTS, IN HYDRAULIC CYLINDERS. | COURTESY OF HALLITE
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 application-specific. 52
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Polyurethane
Polyurethane is an organic material whose chemical composition is characterized by a large number of urethane groups. Urethanes belong to the thermoplastic elastomers (TPE) family and close the gap between thermoplastic and elastomeric materials regarding hardness, deforming behavior and consistency. Within certain temperature limits, polyurethane possesses the elastic characteristics of rubber combined with the advantages of a rigid plastic. The composition of the material is determined by three components: polyol, diisocyanate and a chain extender. The type and amount of these materials used, and the reaction conditions, are decisive in determining the properties of the resulting polyurethane material. In general, polyurethanes possess the following properties: www.fluidpowerworld.com
y high mechanical, tensile strength y good abrasion resistance y modulus of elasticity is variable y wide range of hardness values, while retaining good elasticity y good resistance to ozone and oxygen y outstanding resistance to abrasion and tear Temperature range for use: –30 to 80° C; high performance types (compounds) up to 110° C in mineral oils (long-term exposure temperature).
Acrylonitrile-Butadiene-Rubber (NBR)
NBR is a polymer of butadiene and acrylonitrile. The acrylonitrile (ACN) component affects the following properties of the NBR: y elasticity y cold flexibility y gas permeability y compression set y 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: y good resistance to swelling in aliphatic hydrocarbons; greases; fire retardant hydraulic fluids of Groups HFA, HFB and HFC y 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 y 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.
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: y tough resistance to high heat y excellent resistance to oil, hydraulic fluid and hydrocarbon solvents y good flame retardance y low permeability to gases y high swell in polar solvents, ketones and fireretardant hydraulic fluids (i.e. Skydrol type) Newly developed materials (cross-lined by peroxides) have good resistance to media, which can only be tolerated to a small extent, if at all, by conventional FKM. Temperature range for use: about –20 to 200° C (for short periods to 230° C). Special grades: –50° to 200° C.
Polytetrafluoroethylene (PTFE)
PTFE is a polymer of tetrafluoroethylene. This nonelastic material is characterized by: y slippery surface that repels most media y non-toxic at working temperatures up to 200° C y low coefficient of friction against most opposing surfaces made of other materials; stiction and friction are almost the same y excellent electrical insulating properties (almost independent of frequency, temperature and weathering effects) y chemical resistance that exceeds that of all other thermoplastics and elastomers y liquid alkali metals and a few fluorine compounds attack PTFE at higher temperatures
HYDRAULIC SEALS
WHAT CAUSES O-RINGS TO FAIL? AN O-RING IS most often used as a dynamic or static seal. For light-duty pneumatic applications, an O-ring is a suitable for use in pistons, piston rods, valve spools and other locations in air systems. O-rings best suit the sealing of static surfaces in hydraulic applications, such as cylinder barrel end seals, pump covers, and manifold interfaces, to name a few. Static O-ring seals’ sole job is to prevent air or hydraulic pressure media from escaping cylinders, valves, pumps and other components. You must correctly design its cavity and choose the correct seal diameter and cross-section to suit that pocket. An O-ring must be squeezed upon installation, but not so much as to damage it. The rubber compound must be selected with fluid and temperature in mind as even correctly sized seals will fail when poorly matched to ambient conditions. Here are the top five reasons O-rings fail 1. Improperly designed seal pocket. A seal must reside slightly compressed within a cavity to prevent extrusion through the clearance between each surface. The space between the two sealing surfaces is called the extrusion gap, which cannot be too large. If the “E-gap” is too large, the pressure internal to the component may squeeze the rubber material out through that gap. This “nibbling” of the seal takes the form of a delicate ribbon of rubber trailing off the O-ring circumference. 2. Seal too small. A seal too small for its designed pocket will not crush
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effectively and simply allow fluid to pass. This type of failure tends to occur quickly as the seal cannot form effectively to the pocket to prevent fluid bypass. Even if the size is marginally smaller than required, the seal will fail sooner than the correct one. 3. Seal too large. If a seal is too large for its cavity, the component may never seal properly to begin with. For example, installing a piston rod into a cylinder head may be impossible if the O-ring is too large. A marginally larger seal may still allow installation to occur but will fail as the seal is compressed tighter than designed. An O-ring must be compressed enough to fill its cavity, but too much compression deforms the seal into a rectangular shape as it loses its elasticity. 4. Incorrect temperature compatibility. The polymer of the O-ring must be selected to work within the specific temperature range for the application. A Buna Nitrile seal used in a cold environment results in the hardening of the O-ring, and it loses the elasticity required to conform to the shape of its cavity. Conversely, when you expose a seal to hot ambient conditions, it may become too soft. A hot O-ring may turn gooey and will easily extrude through small E-gaps, especially in applications where the E-gap can change, such as with O-rings used as end-seals in hydraulic cylinder barrels. 5. Incorrect fluid compatibility. Not all seals work with all fluid media, and your seal polymer must be compatible with the fluid type. Many hydraulic applications using urethane seals and standard hydraulic oil will see no issues regarding compatibility. But if the fluid turns out to be waterglycol, for example, the urethane seals will crack or break apart. AN EXAMPLE OF THERMAL DEGRADATION O-RING FAILURE AND DAMAGE CAUSED BY EXCESSIVE HEAT. | COURTESY OF TRELLEBORG SEALING SOLUTIONS
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FLUID POWER HANDBOOK 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.
Piston Seals
… provide sealing of the piston and barrel, critical to the function of the cylinder … most often a lip-seal design, but can also be O-rings, T-seals, and so on … must provide efficient sealing, but also reasonably low friction … made from various seal materials, depending on application … require system pressure to effectively activate the lip seal
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Wipers
… provide aggressive wiping force … prevent mud, water, dirt and other contamination from entering the system … allow lubricating oil film to return to system on inward stroke … protect main sealing elements, thus increasing life of seals … often made from polyurethane, which offers high abrasion resistance … often used as a linkage pin grease seal
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
… feature a pressure-relieving capability that prevents pressure build-up between seals … increase rod seal life … allow for wider extrusion gaps … require high wear resistance
Wear Bands
… prevent contact between metal parts in the cylinder … center rod and piston from housing elements … increase seal life
O-rings
… most commonly used in static applications and radial or axial deformation to maintain sealing contact force … double-acting, so seal on both sides of a component … can be used as energizing elements or as primary seals … self-acting, so do not require additional system pressure or speed to create the seal
… must withstand high pressure exposure … protect the rod seal against pressure spikes
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SENSING TECHNOLOGIES
SENSING TECHNOLOGIES SENSORS ARE
used heavily in fluid power applications, to measure critical functions such as position, flow, pressure, temperature, and more. The variety of devices that exist to gauge these functions are many, but here, we take an in-depth look at some of the more commonly used devices — position and pressure sensors and transducers.
PRESSURE TRANSDUCERS PRESSURE IS
defined as the force per given area required to stop a fluid from 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, such as a programmable logic controller (PLC) or a programmable automation controller (PAC). 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 operating pressure range, maximum rated pressure, accuracy and operating M9100 PRESSURE TRANSDUCER CAN temperature range. The OPERATE IN EXTREME CONDITIONS operating pressure range AT PRESSURES TO 700 BAR. demarcates the intended | COURTESY OF TE CONNECTIVITY 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. The most common output for industrial transducers is 0 to 10 Vdc. Gaining popularity is 0.1 to 10 Vdc, because the control system can detect a transducer fault. If the pressure signal falls below 0.1 Vdc, either the cable has been disconnected or the transducer has failed. For longer cable runs, a 4 to 20 mA output is preferable. Pressure transmitters reject electrical noise, so the analog signal is clean. The 4 mA offset helps the control system detect sensor faults. However, 4 to 20 mA transmitters have 20% lower resolution, because the 0 to 4 mA is not usable.
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POSITIONING 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 Halleffect 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-Displacement 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. LVITs — Linear Variable Inductive Transducers — are contactless position sensing devices, with sensing ranges up to 30 in. 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
SGH10L POSITION SENSOR FOR HYDRAULIC AND TELESCOPIC CYLINDERS OFFERS ALMOST NO STROKE LOSS. | COURTESY OF SIKO
HOW DO LINEAR TRANSDUCERS WORK? TEMPERATURE, pressure, and displacement are typical transducer offerings. A linear displacement transducer measures linear position or distance, especially when high accuracy is needed. Linear transducers work using a principle called magnetostriction. A typical transducer uses three major components; the sensor assembly, a waveguide and a permanent magnet. The sensor assembly houses the control electronics and is responsible for sending and receiving the signal, then transmitting that signal to the PLC. The waveguide is the probe extending the length of the transducer, which emits and senses the magnetic field. Finally, the permanent magnet attaches to the travelling component the transducer measures. The sensor assembly emits a current pulse that travels the length of the waveguide, emitting a magnetic field around the length of the probe. As the pulse passes through the field of the permanent magnet, the interaction creates a strain pulse that travels back down the length of the waveguide until the sensor assembly senses it. The difference between the sent and
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received signal allows the sensing unit to precisely identify the distance of the permanent magnet along the length of the waveguide. Many designs are accurate to within hundredths of an inch or better. Also called a Linear Displacement Transducer, or LDT for short, common applications are inside linear actuators such as hydraulic cylinders. The output configurations include, but are not limited to, 0-10 V, 4-20 mA and CANBUS of various standards. When installed inside a hydraulic cylinder, the rod must be gun-drilled to allow space for the probe, and the magnet gets bolted to the piston’s back. A port boss must be welded or machined into the “position 5” of the cylinder cap (its back end), where the transducer may be screwed tight upon installation. Some hydraulic cylinder mounts prevent the back end installation, such as with ME6 or MX1 mounts, and any clevis or rear pivot type cylinders. Rear mount cylinders will take
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advantage of internally mounted sensor assemblies which employ a connector assembly mounted to one of the available cap side surfaces. Linear transducers may also operate outside of a linear actuator altogether. Stand-alone LDTs may be mounted beside or atop any actuator, where the rod end of the cylinder attaches to the mounting hardware at the end of the sensor. A bar or rod attaches the two, and the transducer is along for the ride to accurately measure linear displacement.
GEMCO 955S SMART BRIK IS A PROGRAMMABLE NON-CONTACT LINEAR POSITION SENSOR. | COURTESY OF AMETEK FACTORY AUTOMATION
SENSING TECHNOLOGIES
SS-7 SERIES LVIT IS DESIGNED FOR SUBSEA ENVIRONMENTS AND OIL AND GAS EXPLORATION. | COURTESY OF ALLIANCE SENSORS
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. 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 and the position magnet attaches to the moving member. An electronics module sends an analog or digital position reading to a controller or receiving device. The electrical connection interface can be either an integral connector or cable and visual diagnostic LEDs to ensure proper wiring, power, and magnet positioning. Encoders are used heavily on mobile machinery. Available with capacitive, optical or magnetic sensing and with incremental or absolute position sensing, these devices accurately determine the stroke of the cylinder. Rotary and linear devices are available, but rotary styles are most common. These devices measure position and resolution in pulses per revolution. Some rotary encoder designs use a wireactuated 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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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 58
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ports. Positions are equal FREP FLEX SERIES SOLENOID-OPERATED, 3-WAY FLOW to the number of positional CONTROL VALVE COMBINES ELECTRO-PROPORTIONAL envelopes. For example, one ADJUSTABLE ORIFICE AND PRESSURE COMPENSATOR. would describe a double-acting | COURTESY OF SUN HYDRAULICS single monoblock valve as “4-way, 3-position,” or simply a of the spring, the valve will open, bleeding off “4/3 valve.” fluid to limit pressure. Directional valves are available in A relief valve controls maximum pressure monoblock or sectional valves, common for either the entire system or a sub-circuit to the mobile-hydraulic industry, as well as of it, the lowest spring pressure of a system subplate mounted industrial type valves being the one to open up first. Most other such as ISO style D03, D05 and so on. Also pressure valves are based on the relief common to both mobile and industrial valve’s simple spring-loaded ball or poppet. markets are cartridge valves installed Sequence, counterbalance and brake valves into manifold blocks. Cartridge valve are all forms of relief valves with added utility manufacturers offer many unique products or functionality, such as reverse flow checks and allow high levels of creativity with or pilot operation built in. Pressure-reducing limitless available valve combinations. valve differs from the others because it limits pressure downstream of itself rather PRESSURE CONTROLS than upstream. It is used where sub-circuit A pressure valve is any component designed to pressures need to be lower, without sacrificing limit pressure. Most pressure valves are based any performance in the rest of the system. on a poppet being pushed against a seat with an adjustable spring, although pressure valves FLOW CONTROL VALVES can be a simple ball and spring configuration Flow control valves control or limit flow or use spools for high flow circuits. Their in one way or another. They are often just operation is simple: a spring pushes the poppet a needle valve, which is just a variable against a seat, and when pressure from the system is strong enough to counteract the force restriction, adjusted by a screw or knob much www.fluidpowerworld.com
HYDRAULIC VALVES
ECONOMICAL MONOBLOCK VALVES COME IN 1, 2 OR 3 HANDLES, OFFERING FLOWS UP TO 25 GPM, POWER BEYOND PLUG OPTION AND RELIEF ADJUSTABLE FROM 1,500 – 3,000 PSI. | COURTESY OF PRINCE MANUFACTURING
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 load-induced 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 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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WHAT ARE THE DIFFERENCES BETWEEN PROPORTIONAL AND SERVOVALVES? THE LINE THAT separates the performance of servo and proportional valves has blurred in recent years. Some may consider a servovalve to be any valve capable of accurately positioning your actuator with closed-loop control. However, in everyday hydraulic circles, many industry professionals describe a servovalve as a valve with a motor drive, where proportional valves use the traditional spool and core-tube armature setup. Older proportional valve designs employed a spool valve with metering notches and PWM-ready coils to infinitely vary the spool position. The performance level of these proportional valves left much to be desired but was the least expensive method to vary hydraulic flow and direction simultaneously. Having no way to control accuracy in the face of changes in pressure drop, the actual flow rate through a “dumb” prop valve will vary
YUKEN EL PROPORTIONAL VALVES ARE CLOSED LOOP, HIGH RESPONSE TYPE PROPORTIONAL ELECTROHYDRAULIC DIRECTIONAL AND FLOW CONTROL VALVES WITH OBE (ON BOARD ELECTRONICS). | COURTESY OF ALA INDUSTRIES
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based on changes in flow, system pressure and load pressure. Conversely, servovalves offered highly accurate and responsive control right out of the box. The standard torque motor design that employs a flapper-style pilot valve offers the user inherent feedback to ensure accurate flow regardless of upstream or downstream pressure variations. As the servovalve’s mainstage spool wanders out of position due to flow forces, the linkage attached to the spool directs the flapper to open the opposing pilot passage. Pressure against the spool increases until it centers itself with the flapper. This internal feedback system ensures the servovalve always flows true to the input signal regardless of pressure variations. For proportional valves to achieve any level of performance close to a motor-based servovalve, advanced electronic control was required. Because a standard coil-powered prop valve was still susceptible to flow forces, a feedback method was required to maintain the spool in its desired position. Linear differential transducers came to the rescue to monitor spool position down to microscopic distances. The transducer signal feeds back into the valve’s onboard electronics, and when it senses the spool outside of its desired position, the valve adjusts the PWM output to the appropriate coil to bring it back in line with the desired position. Proportional valves have evolved to be quite sophisticated. The frequency response, accuracy and hysteresis come close to flapper valve performance (and sometimes even surpasses them). In fact, a whole new breed of valves called “high response” have taken the proportional valve
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YUKEN LSVG SERIES OF HIGH-SPEED LINEAR SERVO VALVES FEATURE A COMPACT, POWERFUL LINEAR MOTOR TO PROVIDE DIRECT DRIVE FOR THE SPOOL. | COURTESY OF ALA INDUSTRIES
to near servovalve performance. Proportional and servovalves also differ by the way they use hydraulic fluid to create flow. A servovalve requires hydraulic tension of sorts and operates with very high pressure drop. Many servovalves require 1,000 psi Delta P to flow what a proportional valve may offer with only 200 psi or less to achieve their rated flow rate. Servovalves are the more expensive of the pair, not just for initial investment cost but for maintenance. The filtration requirements for servovalves are the most extreme for the entire hydraulic industry. Expect to achieve an ISO cleanliness code of 16/13/10 if you expect long performance life from your valve. A proportional valve makes do with 18/16/13, which is orders of magnitude less efficient; however, both valves are worth the investment in filtration. Neither can be considered inexpensive, but even small servovalves may set you back the cost of a used car – definitely worth the price of maintenance.
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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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PNEUMATIC ACTUATORS
PNEUMATIC ACTUATORS 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. Many styles exist, including diaphragm cylinders, rodless cylinders, telescoping cylinders and through-rod cylinders.
CYLINDERS THE MOST
popular style of pneumatic actuator consists of a piston and rod moving inside a closed cylinder. This actuator style can be sub-divided into two types based on the operating principle: single-acting and double-acting. Single-acting cylinders use one air port to allow compressed air to enter the cylinder to move the piston to the desired position, as well as an internal spring to return the piston to the “home” position when the air pressure is removed.
Double-acting cylinders have an air port at each end and move the piston forward and back by alternating the port that receives the high-pressure air. In a typical application, the actuator body is connected to a support frame, and the end of the rod is connected to a machine element that is to be moved. A directional control valve is used to provide a path of compressed air to the extend port while allowing the exhaust air to escape through the valve to the atmosphere. The difference in pressure on the two sides of the piston results in a force equal to the pressure differential multiplied by the surface area of the piston. If the load connected to the rod is less than the resultant force, the piston and rod will extend and move the machine element. Reversing the directional control valve will provide compressed air to the retract port, allowing exhaust to escape the extend port, and the cylinder will return back to its home position. Pneumatic actuators are at the working end of a fluid power system. Upstream of these units, which produce the visible work of moving a load, are compressors, filters, pressure regulators, lubricators, on-off control valves and flow controls. Connecting all of these components together is a network of piping or tubing (either rigid or flexible) and fittings. D2-SERIES NITRA PNEUMATIC DOUBLE-ACTING, NFPA HEAVY-DUTY TIE ROD AIR CYLINDERS | COURTESY OF AUTOMATIONDIRECT
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PNEUMATIC ACTUATORS
WHAT ARE SINGLE-ACTING PNEUMATIC CYLINDERS? IN SINGLE-ACTING PNEUMATIC CYLINDERS,
TYPES OF PNEUMATIC CYLINDERS | COURTESY OF DOC’S HYDRAULICPNEUMATIC TRAINING LLC
Pressure and flow requirements of the actuators in a system must be taken into account when selecting these upstream system components to ensure desired performance. Undersized upstream components can cause a pneumatic actuator to perform poorly, or even make it unable to move its load at all.
WHY DO AIR CYLINDERS LEAK?
While a leaking pneumatic cylinder does not represent the environmental and safety catastrophe that a leaking hydraulic cylinder does, it’s still a serious situation that you should pay attention to. In addition to the fact that air leakage means wasted energy, it’s also a sign that either the system was designed improperly (say, with high side loads that are damaging the cylinder upon extension), or more likely, that the cylinder is nearing the end of its useful life and has to be replaced. An air cylinder will generally leak the most at the shaft, at the point where the rod moves in and out — the location of the rod seal. Some technicians recommend putting a small amount of soapy solution (bubbles) in this area to better see if this is the source of the leak. Other areas include welded seams or at the air connection points — where the air lines enter the cylinder body. Air cylinders generally leak because their seals have worn out, sometimes exacerbated by internal rusting of the metal components. If a piston or rod seal is the culprit, these can be replaced, and seal kits are widely available. Some cylinders are the non-repairable type, and if this is what you are dealing with, the entire cylinder will have to be replaced.
air is supplied through one port to one side of the piston, causing the piston rod to extend in one direction for a task such as lifting an object. The other side vents air to the environment. Movement in the opposite direction occurs most often by means of a mechanical spring, which returns the piston rod to its original or base position. Some single-acting cylinders use gravity, a weight, mechanical motion, or an externally mounted spring to power the return stroke, although these designs are less common. In contrast, double-acting pneumatic cylinders feature two ports that supply compressed air to both extend and retract the piston rod. Doubleacting designs are far more typical throughout industry, with an estimated 95% of applications using this cylinder style. However, in certain applications, a single-acting cylinder is the most cost-effective and appropriate solution. In a single-acting cylinder, the design can be “base position minus” with spring return, or “base position plus” with spring extend. This depends on whether the compressed air is used to power the out-stroke or the in-stroke. Another way to think about these two options is push and pull. In the push design, air pressure creates a thrust, which pushes the piston. With the pull design, air pressure produces a thrust that pulls the piston. The most widely specified type is pressure-extended, which uses an internal spring to return the piston to its base position when the air exhausts. One advantage of the single-acting design is that in case of either power or pressure loss, the piston automatically returns to its base position. A disadvantage of this style is the somewhat inconsistent output force during a full stroke due to the opposing spring force. Stroke length is also limited by the space the compressed spring requires, as well as available spring lengths. For position detection, a magnet can be installed on the cylinder piston. Sensors mounted on the cylinder body
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can then gather information from the magnet and determine piston position. Hall effect sensors and reed switches are often used for this purpose. Another cylinder component to consider involves cushioning. As compressed air enters the cylinder, the piston will move rather quickly and potentially cause a hard impact against the head or end cap. To avoid stressing cylinder components and transmitting vibration to the rest of the machine, cushioning can be used at the ends of the cylinder. For smaller cylinders, which many single-acting cylinders tend to be, flexible shock absorbers made of polyurethane elastomers are often used. These bumpers are installed on the head and end caps or integrated into the piston itself. With regard to sizing, it’s important to remember that the cylinder diameter is directly proportional to the amount of force it can generate from the input air pressure. The following formula is useful: F = (P x A) – f F is the cylinder’s force (N); P is air pressure (MPa); A is piston area (mm2); and f is friction drag (N). Also keep in mind that with singleacting cylinders, some work is lost due to the opposing spring force. This force reduction must be taken into account when sizing this cylinder type. Diameter and stroke are the most important factors to consider during sizing calculations. Diameter refers to piston diameter, which defines its force relative to the air pressure. Available cylinder diameters are defined by the cylinder type and ISO or other standards. Stroke defines how many millimeters the piston and piston rod can travel. A general rule is that the larger the cylinder bore is, the greater force output. Typical cylinder bore sizes are 8 to 320 mm. A final consideration is mounting style. Depending on the manufacturer, many configurations are available. Some of the most common include foot mount, tail mount, rear pivot mount, and trunnion mount. The best option will be determined by the specific application and other system components.
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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.
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AVENTICS SERIES CCI (ISO 21287) COMPACT AIR CYLINDERS | COURTESY OF EMERSON
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 Originally called the “Pancake cylinder,” these features and can be placed with adjacent miniature cylinders were first invented in cylinders at a close center-to-center 1958 by Al Schmidt, to fill a need for force in dimension. Piston bearings, materials, hard a tight, enclosed space. The basic intent was anodized bore and chrome plated rods can to get the most stroke in a short overall length enhance cylinder capability for unexpected using common machined parts and seals. side loads and long-term durability. Up Over the years, this design has been further to 6-in. strokes can be accomplished with developed, with many additional features and extruded body material. Other features options to satisfy a variety may include metric of customer dimensions, extruded applications. sensor mounting and This round non-rotating styles. body cylinder They are available has a smooth, in single-acting PANCAKE COMPACT AIR CYLINDERS clean outside and double-acting | COURTESY OF FABCO-AIR INC. diameter for ease versions. of machinery cleaning.
With guide rod systems and NFPA or ISO pneumatic cylinders •
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PNEUMATIC ACTUATORS
PNEUMATIC RODLESS & CYLINDER SLIDES WHEN AN
application calls for power and linear
motion while also supporting side loads, pneumatic rodless and cylinder slides (also
Be aware of several considerations when selecting the best type of pneumatic cylinder slides. These include: •
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-and-place 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.
•
Load capacity required. 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.
Specification and sizing software allows users to select the proper slide required for various applications. The idea of applying a load to a linear actuator is common, and there are a number of types of cylinder slides that can be used for these applications. The first basic style of powered slide is commonly known as a “thruster” or cantilever type 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.
DGC RODLESS LINEAR ACTUATOR | COURTESY OF FESTO
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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. The pneumatic cylinder is attached to a saddle that supports the bearing system on each end of the slide’s travel. This type of powered slide can be used for longer travels with less deflection based on the bearing system being supported on each end. Like the thruster style slide, the saddle carries the load versus the cylinder’s piston rod.
Another type of slide is a rodless slide. In this case, the bearing system is attached to the rodless cylinder directly on one or both sides of the cylinder. The cylinder’s piston is linked to a carriage mounted upon the bearing system, offering load carrying capability as well as resistance to side loads. Rodless slides offer the most space savings as the cylinder’s travel is contained within its own overall length. 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.
CY1F LOW-PROFILE RODLESS CYLINDER | COURTESY OF SMC USA
HOW DO PNEUMATIC RODLESS CYLINDERS/SLIDES WORK? IN A RODLESS AIR CYLINDER — also called a rodless slide — motion is transmitted through the body length by an internal piston moving with the cylinder, driven by compressed air. The piston is attached to a shuttle or carriage, which supports the load and moves it in a linear direction with the piston. Rodless slides work well in applications with limited space, because when they are installed, the length is only slightly longer than the cylinder’s stroke. This means, for example, a 25 mm diameter rodless cylinder with a 1,000 mm stroke would take up 1,200 mm of space, opened or closed. Other reasons for choosing rodless cylinders include: • • • • • • • •
Reliable, repetitive linear movement Support high loads and moment forces Clean operation with minimal chance of lubrication contamination Higher speeds than conventional cylinders Loads can be moved over large distances in faster times No concerns about rod buckling Smaller footprint for long stroke applications compared to typical rod-type cylinders Minimal component maintenance reduces downtime
In general, rodless cylinders have the same basic components: a cylinder tube or barrel, a piston, inner and outer sealing bands, and end-cushioning end caps. A slide table assembly, also called a shuttle or carriage, is mounted directly above the piston, connected directly through a slot cut along the top of the cylinder
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body, indirectly through magnets located on both the piston and table, or by cables connected to the piston through a pulley. Compressed air from the end caps moves the piston and carriage along the length of the cylinder barrel in either direction. The sealing bands prevent external leakage of air pressure and allow the carriage to move back and forth. Adjustable end cushioning on both ends prevents any hard stops at the end caps. When choosing a rodless cylinder, it’s critical to consider how the load affects function, and to account for unwanted bending moments. Pitch, roll, and yaw moments are possible as well as the “jerking” forces from acceleration and deceleration of the carriage. Cylinders that are mounted vertically also need to consider the forces of gravity when moving and holding the load in place. However, these factors are straightforward to design for, and the ability to support higher loads and moment forces are an advantage over conventional rod-type cylinders. With conventional cylinders, the rod extends beyond the cylinder barrel to push or pull the load. This creates the tendency to bend and buckle, creating excess wear on the seals and bearing. The weight of the cylinder rod can also sag under its own weight, causing misalignment and more bending and buckling. Rod-type cylinders travel at different velocities in forward and return strokes and are designed for short stroke lengths. The most common applications for rodless cylinders are packaging, printing, automotive, cutting, material transfer, assembly, robotics, and electronics manufacturing. And they’re used across industries such as textiles and ceramics, testing and inspection, food and beverage production, and pharmaceuticals and chemicals to name a few.
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AIR COMPRESSORS AIR COMPRESSORS
supply the compressed air flow for all pneumatic equipment in a system. The compressor adds energy to the air, which is cleaned and conditioned by filters and dryers, then transmitted in piping for use. Compressed air is an energy intensive source of process power, about 7 to 8 units of energy are consumed at the compressor for each unit of mechanical energy produced by a typical compressed-air powered device — and of this, typically 50% of the compressed air is wasted due to leakage and inappropriate use. Most of the energy released by an air compressor is in the form of heat of compression. When discussing compressed air flows, there are various definitions that relate to compressor capacity: ACFM — actual cubic feet per minute (also called free air delivered, FAD, or inlet cubic feet per minute, ICFM). This is the flow of air taken in by the compressor at site conditions (local atmospheric pressure, temperature and humidity). In general, higher altitudes, temperatures and levels of humidity reduce the capacity of the compressor to produce a given mass of compressed air; therefore, if these conditions exist, a larger compressor must be purchased. CFM — cubic feet per minute. This is the flow of air at a certain point at a certain condition, which must be specified. With regard to sizing air compressors, it is important to understand the wide range of conditions at which the CFM can be stated. SCFM — standard cubic feet per minute. This is the flow of free air measured and converted to a set of standard conditions. The definition of SCFM for air compressor rating purposes (Compressed Air and Gas Institute based on 70
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ISO Standard 1217) is the flow of air at 14.5 psig atmospheric pressure, at 68° F and 0% relative humidity. ACFM and SCFM are both measured at atmospheric pressure, not at the pressure the air compressor produces. There are two types of compressors: positive displacement and dynamic (also called centrifugal or axial).
POSITIVE DISPLACEMENT AIR COMPRESSORS
Positive displacement compressors take in air and mechanically reduce the space occupied by the air to increase CNC LINE OF BOOSTER COMPRESSOR PACKAGES pressure. They can further be | COURTESY OF KAESER COMPRESSORS divided into rotary and reciprocating types. Rotary compressors are available in sizes from 5 to 600 hp. In rotary screw compressors, filtered air enters the inlet of the air end where male and female rotors unmesh. The air is trapped between the rotors and the air end housing. This space is reduced as the are typically two-stage units and are more rotors remesh on the opposite side of the air costly than lubricated types. Oil-free rotary end. Thus, the air is compressed and moved to screw air compressors are available as airthe discharge port. For lubricated compressors, cooled and water-cooled, with both load/ cooling fluid is injected into the housing, unload and variable speed control options. which mixes with the air to seal, lubricate and They also offer the same flexibility as oilremove the heat generated by compression. flooded rotaries when oil-free air is required. This fluid forms a thin film between the rotors Reciprocating air compressors use a that virtually eliminates metal-to-metal contact piston within a cylinder as the compressing and and wear. The fluid is separated from the displacing element. Single-stage and two-stage compressed air, cooled, filtered and returned to reciprocating styles are commercially available. the injection point. The compressed air passes Single-stage compressors are generally used through an after-cooler and water separator to for pressures in the range of 70 to 100 psig reduce its temperature and water content so it and two-stage compressors are generally used is ready for the air treatment equipment. for higher pressures in the range of 100 to Cooling takes place inside the compressor 250 psig. These types of units are most often package, so the rotary compressor is a used for smaller systems. Typically, these continuous duty, air-cooled or water-cooled compressors are not rated for continuous duty compressor package. These compact designs due to limited cooling methods and should be provide smooth, pulse-free air output and operated at duty cycles of 60% of full capacity high output volume. They are also easy to or lower, or equipment damage may result. maintain and operate. The reciprocating air compressor is Oil-free rotary screw air compressors single-acting when the compressing is use specially designed air ends to compress accomplished using only one side of the air without oil in the compression chamber, piston. A compressor using both sides of the yielding true oil-free air. Oil-free compressors piston is considered double-acting. www.fluidpowerworld.com
AIR COMPRESSORS Reciprocating air compressors are available either as air-cooled or watercooled in lubricated and non-lubricated configurations and provide a wide range of pressure and capacity selections.
DYNAMIC AIR COMPRESSORS
Dynamic compressors use the mechanical action of rotating impellers to transfer pressure to the air. The centrifugal air compressor is a dynamic compressor, which depends on transfer of energy from a rotating impeller to the air. Centrifugal compressors produce compressed air by converting angular momentum imparted by the rotating impeller (dynamic displacement). To do this efficiently, they rotate at higher speeds than the other types of compressors. Adjusting the flow by straight modulation or with the use of inlet guide vanes is the most common method to control capacity of a centrifugal compressor. By closing the guide vanes, volumetric flows and capacity are reduced with good turn-down efficiency. However, this adjustment is limited to
the upper range of flow, with the use of inefficient blow-off required to ensure the compressor does not go into a damaging condition called surge. Centrifugals can also operate using load/unload style control and have minimal unloaded power consumption. This can be a good energy efficiency measure. Efficient control of systems using multiple centrifugals requires coordination of the modulation controls and load/unload to ensure the compressors are kept from blowing off and that the compressors operate at their most efficient discharge pressure. The centrifugal air compressor is an oil-free compressor by design. The oillubricated running gear is separated from the air by shaft seals and atmospheric vents. Dynamic compressors are most often used on sizeable compressed air systems. These units are most efficient where large continuous flows of compressed air are required. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants, and natural gas processing plants.
3-PHASE, 15-HP ROTARY SCREW AIR COMPRESSOR WITH 120 GAL TANK | COURTESY OF INGERSOLL RAND
WHERE IS THE COMPRESSED AIR PRESSURE DROP? A RULE OF THUMB used in the compressed air industry is a compressor consumes 1% more energy for every 2 psi in extra pressure at the discharge. When reducing the energy consumption of a compressed air system, we must pay detailed attention to pressure differentials at various points in the system. These pressure drops all add up to higher pressure at the compressor discharge — and higher energy costs.
• •
•
There is 10-psi pressure drop across the air dryer and filters. Often, if the system is to be upgraded, choosing better system components can reduce this differential. The piping system shows a 5-psi pressure differential from end to end. This is not hugely significant, but best practices would allow only a maximum 2-psi differential at full system flow. This can be achieved by properly sizing pipes and looping piping runs. Pressure drop at the end use components, caused by undersized and misapplied FRLs, hoses, connectors and fittings adds up to 15 psi. This problem is very common and is often the cause of high compressor discharge pressures (jacked up to compensate for all the various pressure loss). Proper design and upgrade of these supply components can greatly reduce pressure loss and provide better supply pressure for tools and equipment, often increasing the performance.
FIGURE 1. THIS PRESSURE PROFILE CAN BE CONSTRUCTED BY MEASURING PRESSURE AT VARIOUS POINTS IN A COMPRESSED AIR SYSTEM. IT TELLS US WHERE WE MUST LOOK IN CORRECTING PRESSURE DIFFERENTIALS THAT COST MONEY DUE TO HIGHER PRESSURE AT THE COMPRESSORS. (SOURCE: COMPRESSED AIR CHALLENGE)
Consider the pressure diagram in the graphic above; itwww.fluidpowerworld.com appears the compressor is running at between 100 and 110 psi, yet the pressure at the critical load at the far end is only 70 psi. Some observations:
Once all the pressure differentials are addressed and reduced, the compressor pressure can be lowered, saving energy. Try do some measurements on your system to see where your problems are. If your compressors are running above 100 psi, there is likely a problem. If you want to know more about compressed air optimization, check out a Compressed Air Challenge Webinar coming soon: compressedairchallenge.org/ calendar
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PARTNERS. THE BEST PART OF ALL.
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AIR SPRINGS
AIR SPRINGS PROVIDE ACTUATION AND VIBRATION ISOLATION. AS ACTUATORS, THEY CAN PROVIDE LINEAR OR ANGULAR MOTION. | COURTESY OF ENIDINE
AIR SPRINGS AIR SPRINGS
have been used in heavy-duty vehicle suspension systems for nearly a century, where they have been able to provide usefulness by taking advantage of the compressed air required for vehicle braking systems. Air springs have provided a two-fold advantage over mechanical leaf or coil springs. One advantage with air suspension is the extra comfort provided by being able to vary the air pressure inside the spring, which changes the spring rate, and therefore, ride quality. Additionally, because variable control over air pressure adjusts the deck or trailer height, aligning loading docks to the level of the deck is possible when dock plates are unavailable. The usefulness of air springs or actuators didn’t go unnoticed in the industrial machine industry, and it was clear they could offer unique solutions for various applications. Air actuators have seen duty as shock absorbers, linear actuators, vibration isolators and tensioners, to name a few examples. They can be used to absorb shock in material handling applications, such as a saw mill, when logs are dropped onto processing stations. Air springs make some of the best vibration isolators on the market, such as would be used on a vibrating hopper or commercial laundry machine. In summation, air springs are a high-force, low-cost actuator that can operate in a linear fashion or at an angle. They can be stacked to provide longer strokes or greater angular rotation.
As air is directed into air springs, the bladders allow them to expand in a linear fashion. This permits them to be used as force developing actuators — like pneumatic cylinders — and as such, rod attachments are available to mimic the function of them. Most often, however, an air actuator is simply two end plates connected by a bladder, and as they are pressurized, force pushes the plates away from each other. As linear actuators, they can provide up to 35 tons of force, making them useful in various press applications, such as a forming press or small stamping press. Air actuators are also excellent for constant force applications, such as pulley tensioners or drum roller compression devices. All air springs are single-acting, unless they are coupled together so one extends while the other retracts. The two major types of air springs are the rolling lobe (sometimes called reversible sleeve) and the convoluted bellow. The rolling lobe air spring uses a single rubber bladder, which folds inward and rolls outward, depending on how far and in which direction it is moved. The rolling lobe air spring is available with high usable stroke length — but it is limited in strength because of its tendency to bulge, and therefore has limited force capacity. The convoluted bellow type air spring uses one to three shorter bellows, with the multiple units being reinforced by a girdle hoop. Convoluted air springs are capable of ten times the force of a rolling lobe version and twice the life cycle rating, but have less usable stroke to work with.
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FRLS 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 non-relieving types. Relieving regulators can be adjusted over a wide pressure range, and even when downstream flow is blocked at the reducing valve, relieving regulators will allow the excess downstream load- or head-induced pressure to be exhausted. Non-relieving regulators, when similarly adjusted, will not allow the downstream pressure to escape. The trapped air will need to be released by some other means; for example, by operating a downstream valve. 74
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MS SERIES FILTERS LOWER THE RISK OF PARTICLE CONTAMINATION WHEN COMPRESSED AIR COMES INTO DIRECT CONTACT WITH FOOD OR PACKAGING IN THE FOOD ZONE. | COURTESY OF FESTO
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(ACD) FRL UNIT FEATURES A MODULAR DESIGN WITH UNIFORM BODY STYLE, OFFERING BETTER VISIBILITY AND ENVIRONMENTAL RESISTANCE. | COURTESY OF SMC USA
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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 airdriven 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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PNEUMATIC HOSE & TUBING
PNEUMATIC HOSE & TUBING SYSTEM DESIGNERS
use pneumatic hose and tubing to convey pressurized air to actuators, valves, tools and other devices. 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 spiralwound fiber, and an outer protective cover. In broad terms, hose is more rugged than tubing — but it tends to cost more. Air supply and application set a baseline for product performance. Flow requirements help determine hose or tubing size. Tubing is generally specified by OD and wall thickness, while hose is specified by ID. Regardless, choosing too small an inner diameter “chokes” flow and results in pressure losses, inefficiency and excessive fluid velocity that can shorten
service life. Too large a diameter, on the other hand, results in higher than necessary weight, size and cost. Also ensure that products operate below the stated maximum working pressure. Manufacturers generally rate tubing by measuring the burst pressure at 75° F, and then divide it by an appropriate safety factor (typically 3:1 or 4:1) to determine the maximum working pressure. Keep in mind that published burst-pressure ratings are only for manufacturing test purposes, and in no way indicate that a product can safely handle pressure spikes or otherwise operate above maximum working pressure. Also note that some products handle vacuum to approximately 28 in.-Hg without collapse.
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POLYURETHANE TUBING | COURTESY OF SMC CORP. OF AMERICA
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Thermoplastic tubing is made from several common materials. Typical tubing materials used in pneumatic applications include: •
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Consider fluid compatibility. For instance, oil from air lubricators, as well as fumes or other substances ingested by the compressor, could affect the inner tube. Also, 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. 78
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Polyurethane tubing is strong, flexible, kink and abrasion resistant, and it withstands contact with fuels and oils. It’s commonly used in pneumatic actuation and logic systems, robotics and vacuum equipment, and 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. It 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.
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PNEUMATIC HOSE & TUBING
LIGHTWEIGHT REINFORCED POLYURETHANE PNEUMATIC AIR TOOL HOSE ASSEMBLIES | COURTESY OF KURIYAMA OF AMERICA
External mechanical influences can also hasten hose and tubing failure. Protect against excessive flexing, twisting, kinking, tensile and side loading, and vibration as well as abrasive wear, snagging or bending beyond the minimum bend radius. Replace and discard any hose or tube that is cut, worn or otherwise damaged. Don’t overlook the effect of temperature and heat on tubing materials — inside and outside the assembly. Always operate within minimum and maximum temperature limits. Tubing is extruded in straight lengths and stored on reels, but can be molded into spring-like coils. This lets the tubing extend considerably as needed, then retract to a compact configuration for 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. Tubing variations 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.
HOW DO YOU SELECT COMPRESSED AIR HOSE SIZES? TRUE STORY: a fiberglass parts plant was having air pressure problems in their plant, so much so that they had turned their air compressor up to 140 psi. This was the highest it could go, which made it consume 15% more power. But still, their most critical air tool was having performance problems. An air auditor investigated and found that the air tool in question needed about 25 cfm at 90 psi to run at peak performance. The auditor rigged up a test pressure gauge by placing it on a T connection right at the air inlet of the tool. When he pulled the trigger the air pressure fell to 44 psi. Looking back from the tool to the main compressed air distribution line, the auditor found 50 feet of ¼-in. hose and three quick connect couplers, all undersized for the tool flow. This tool was poorly connected to the system resulting in greatly decreased performance. The supply hosing was upgraded to 3/8-in. and the resulting tool pressure when running settled above 90 psi, even when the compressor discharge pressure was reduced to 110 psi to save energy. This greatly increased tool performance. The table shown in the graphic shows the huge effect going to larger size hose has on the pressure drop. Use this to help improve your system.
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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
SOFTGRIP VACUUMBASED PINCH GRIPPER FOR THE AUTOMATION OF THE FOOD/ CHOCOLATE INDUSTRY. | COURTESY OF PIAB
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SCROLLVACPLUS OIL-FREE BACKING AIR PUMP | COURTESY OF LEYBOLD
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: positive-displacement and dynamic/kinetic. Displacement vacuum pumps essentially operate as compressors 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. www.fluidpowerworld.com
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Don’t go into freefall! Keep control of load lowering and load holding with counterbalance valves, pilot-operated check valves and other types of load control valves. Carl is looking forward to a conversation with you during this YouTube Live event on July 22.
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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 multistage. 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 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.
LEMAXIO MINI-VACUUM PUMP IO-LINK | COURTESY OF COVAL
HOW DO YOU MAKE VACUUM WITH COMPRESSED AIR? IT SEEMS BIZARRE, but it is possible to
Due to the high cost of compressed air, it is wise to always assess your vacuum generators for cost. Is it a continuous requirement or intermittent? Often, continuous duty requirements can best be supplied using a mechanical vacuum pump. Consider the vacuum eductor pictured in the photograph in Figure 1. This unit consumes a continuous 50 cfm and runs on a 24-hour x 7 day per week duty cycle, feeding sand to a silo in a metal foundry. It consumes about $10,000 of electricity per year. Use of a mechanical vacuum source, or even better, a mechanical sand elevator could save substantial annual cost. On the other hand, low duty cycle applications like the vacuum pick-up devices in Figure 2 can FIGURE 1. CONTINUOUS often be more economically COMPRESSED AIR GENERATED supplied by compressed air VACUUM CAN BE EXPENSIVE COMPARED TO ELECTRIC powered generators. With DRIVEN SOURCES. proper control, vacuum pick up can be set to use minimal compressed air during pick up operation and turn off the vacuum when not required. More sophisticated vacuum venturi can also be purchased
make vacuum with compressed air. Vacuum eductors, sometimes called ejectors, or venturi use a flow of compress through a specialized nozzle to create vacuum without a mechanical vacuum pump. The process in its simplest form uses no moving parts, so it is very reliable. It is also very energy intensive; like most compressed air powered devices, a vacuum venturi consumes about 10 times the power of a mechanical vacuum pump for the same flow rating. This is not the fault of the generator, but reflects on the inefficiency of compressed air.
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FIGURE 2. SUCTION PICK-UP DEVICES CAN OFTEN BE MORE ECONOMICALLY SUPPLIED BY COMPRESSED AIR POWER.
with a built-in sensor that shuts off the flow of compressed air when the vacuum level within the pick-up cups is adequate. If this application was suppled by a dedicated mechanical vacuum pump, which would likely run continuously, the electrical cost would be higher. When generating vacuum from compressed air, it is best to consider the duty cycle of the application. With good control, the cost can be reduced substantially — even with compressed air powered generators.
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PNEUMATIC VALVES CONTROLLING
pneumatic actuators in compressed air systems requires safe and precise functionality. Although the medium is fluid, just as hydraulic systems, the execution of control is different in many ways than with a liquid. What is shared in the conduction of any fluid power medium is the need for valves to control force, velocity and direction of movement.
AIR PREPARATION
MANUALLY OPERATED VHEF PNEUMATIC VALVE | COURTESY OF FESTO
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Pressure relief valves will control pressure at their inlet port by exhausting pressure to atmosphere. Relief valves are typically used only in receivers or air storage devices, such as accumulators, as a means to prevent excessive pressurization. As such, relief valves are often called safety valves and are not typically appropriate for use anywhere but the air preparation stage. Pressure regulators in pneumatic systems limit pressure downstream of the unit by blocking pressure upstream at the inlet. Regulators are used in the air preparation stage, as well as in control of cylinders and motors. The letter R in the acronym FRL stands for regulator, which is installed downstream of the receiver tank, but before the circuit they are regulating pressure for. Sometimes multiple stages of pressure reduction are required, especially with a large centralized compressor and receiver feeding various workstations. A regulator can control pressure within the main grid of distribution plumbing, but sometimes air is piped directly www.fluidpowerworld.com
to an FRL at each workstation or machine. Pressure at this main header could be 120 psi or more, but a branch circuit could be regulated at 90 psi, for example. Most regulators are capable of relieving downstream pressure, which prevents that downstream pressure from elevating as a result of load-induced pressure or thermal expansion. Pressure regulators can be had as stand-alone units, but sometimes a filter is attached to kill two birds with one stone. Regulators are most often available as a component of a modular set, with a filter, regulator, lubricator or dryer, and can be assembled in any combination. The regulator will have an inlet port, outlet port and a port for the pressure gauge, with which they are most often included. Pressure regulators can also be used to control pressure for individual actuators, such as an inline regulator or work-port mounted regulator. These are typically quite small and included with reverse flow check valves, as would be required for double-acting function of a cylinder, for example. Further still, differential pressure regulators are offered by some manufacturers to maintain a set pressure differential between the two ports, rather than just maintaining downstream pressure. It should be noted that all pressure regulators are adjustable, most often with screws or knobs.
FLOW CONTROLS
Also common in pneumatic systems are valves to control flow. There are fewer available types of flow valves compared to pressure or directional valves, but most circuits apply them to make for easy
PNEUMATIC VALVES ASCO SERIES 256/356 SOLENOID VALVES | COURTESY OF EMERSON
adjustment to cylinder or motor velocity. Controlling velocity in pneumatic systems is more complex than in a hydraulic system because pressure differential between the work ports of a cylinder plays a larger part. Flow control valves for pneumatic systems are quite simple, usually available in two configurations used in two different ways. One configuration is merely a variable restriction, with a screw or knob adjustment to open and close a variable orifice, which is also often referred to as a needle or choke valve. The other type introduces a check valve, which allows free flow in one direction and restriction in the opposing direction. For whatever reason, this valve has hijacked the name flow control all for itself. Flow control valves are applied in two different ways: meter in or meter out. Meter in is the method of controlling the rate of airflow as it enters a motor or cylinder. When metering in, a cylinder will move rapidly with high force and efficiency, but the motion of the piston is prone to spongy and unpredictable movement. When metering out, the cylinder velocity is more stable and repeatable, but efficiency and dynamic force are lost to the energy required to push past the flow control. Regardless, most pneumatic applications operate using meter-out flow controls because the disadvantages are easy to overcome by increasing upstream pressure. A method of increasing cylinder velocity, typically for double-acting or spring-return cylinder retraction functions,
is to add a quick exhaust valve to the cap side work port. Because cylinders retract faster than they extend as a result of differential air volumes, it is harder to evacuate the cap side air volume without oversized valves or plumbing. A quick exhaust valve vents directly to air from the cap side work port and massively reduces the backpressure created upon retraction, permitting rapid piston velocity.
DIRECTIONAL CONTROL VALVES
Pneumatic directional valves are available in many sizes, styles and configurations. At the basic end of the spectrum is the simple check valve, which allows free flow in one direction and prevents flow in the reverse direction. These can be installed anywhere from right after the receiver to within a flow control valve itself. As directional valves grow in complexity, they are specified under a general naming practice related to the number of positional envelopes of the valve and the number of work ports in the valve, and specifically in the order described. For example, if it has five ports, port 1 will be for pressure inlet, ports 2 and 4 for work ports, and 3 and 5 for the exhaust ports. A valve with three positions will have a neutral condition, extend condition and retract condition. Putting it all together, this describes a five-way, www.fluidpowerworld.com
three-position valve, also referred to as a 5/3 valve. The common configurations seen in pneumatics are 5/3, 5/2, 4/2, 3/2 and sometimes 2/2 valves. Also part of the description of a directional valve is its method of both operation and positioning. The valve operator is the mechanism providing the force to shift the valve between its positions. The operator can be a manual lever, electric solenoid, air pilot or cam mechanism, to name a few. Some valves are a combination of these, such as a solenoid pilot valve, which is a tiny valve providing pilot energy to move the main-stage valve. Positioning of any valve is achieved by either a spring, such as with a 5/2 spring-offset valve, or with detents, as in 5/2 detented valves. A 5/2 spring-offset valve will return to its starting position when energy is removed from its operator, like deenergizing the coil, or removing pilot pressure. A 5/2 detented valve will stay in the position it was last activated to until the operator switches it again. Pneumatic valves are manufactured in various incarnations. Poppet valves are simple, using a spring to push a face of the poppet down on its seat. Construction can be metal-to-metal, rubber-to-metal or even with diaphragms. Poppet valves can often flow in one direction, just as a check valve, but need to be energized to flow in reverse. They are limited to two- or threeway port configurations, although they can 7 • 2021
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mimic four- or five-way valves when used in parallel. They offer typically high flow conductance for their size, and are generally resistant to contamination. Spool valves use a notched metal cylinder that slides within a precisely machined body, drilled with three to five ports, or seven ports if the valve is pilot operated. Low-end valves consist of only a spool and body, and are prone to internal leakage. Better valves use seals in the body or spool to prevent leakage between ports. High-end spool valves are constructed with precision, often requiring fine lapping procedures during manufacturing, and with their tight tolerances, often require few seals, improving reliability and longevity. Other forms of high-end valves use a sliding block of metal or ceramic, which is efficient and extremely resistant to contamination.
MOUNTING CONSIDERATIONS
JSY SERIES COMPACT 5-PORT SOLENOID VALVE | COURTESY OF SMC CORP. OF AMERICA
Pneumatic directional valves come in both standard and non-standard mounting configurations. The non-standard valve’s port layout, operator style and mounting options are unique to each manufacturer’s product. They can be inline, subplate mounted or sectional stacks mounted in a row. Because each manufacturer does mounting differently, it is best to research the product appropriate for your application. Luckily, most manufacturers have lines of standardized valves suiting one or more specification, such as ISO 5599-1, with its staggered oval ports; this means one manufacturer’s valve will fit the subplate or manifold of another manufacturer’s. Port and electrical connections are standardized with most valves as well. NPT ports are common, but many new valves come with push lock fittings on the subplate itself. Electrical connectors for standardized valves are frequently DIN, mini-DIN or with fieldbus connection, making the operation of a dozen valves as easy as one connector.
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GAUGES PRESSURE GAUGES
are more common and digital pressure gauges with customizable firmware allow process measurement of pressure-based measure a fluid’s intensity. They ensure measurement of leaks or other parameters reliable operation and reduce the risks like torque, load, force and hardness. Pressure is measured in many locations of pressure spikes or changes that could throughout pneumatic and compressed air cause damage to the system. In addition, systems. It is measured at receiver(s), as well they prevent leaks by alerting personnel of as every system FRL or stand-alone regulator unusual changes in system pressure. and sometimes at pneumatic actuators. These gauges can be rated up to 300 psi. Hydraulic pressure gauges are available to Pressure is measured in three ways measure up to 10,000 psi, though typical — absolute, gauge and vacuum. Absolute hydraulic systems operate in the 3,000 to pressure is a measure of actual pressure 5,000 psi range. Hydraulic gauges are often including ambient air, which is zeroinstalled at or near the pump’s pressure port referenced with a perfect vacuum, but for indication of system pressure, but can be can be as high as 14.7 psi at sea level. installed anywhere on the machine where Absolute pressure readings are considered in pressure needs to be monitored — especially applications interacting with ambient air, such if sub-circuits operate at a pressure rate as the compression ratio calculation for flow different from pump pressure, such as after (cfm) requirements. Gauge pressure is zeroa reducing valve. Often, pressure-reducing referenced against ambient pressure and is valves have a gauge port to tap into, allowing used in most applications operating in, but you to directly monitor its downstream not with, ambient air, such as in fluid power pressure setting. systems. Disconnected from equipment, They are now designed with hydraulic gauge pressure will read zero. Finally vacuum friendly pressure connections (such as SAE/ “pressure” is expressed in Torr, or referenced Metric straight threads) to prevent system against ambient pressure, as with “in.-Hg” leaks. Analog gauges with custom scales (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. The most common are Bourdon tubes and bellow gauges. Bourdon tubes take pressure and convert it into mechanical energy, which 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 HEAVY-DUTY, LIQUID FILLED the pressure range. BRASS GAUGE One important | COURTESY OF NOSHOK characteristic to note is the cross 88
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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. Most pressure gauges in North America come with a 1⁄4-in. NPT male, but SAE thread is gaining popularity. The use of test-point adapters at various locations on the hydraulic system allows for measurement during troubleshooting with just one gauge. The testpoint 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 21⁄2 in. in diameter, and can be top-mount or panel-mount styles. Common threats to reliability are vibration, pulsation and pressure spikes. It’s best to look for gauges designed for hydraulic applications to reduce costly downtime. A forged brass case prevents resonant frequencies from destroying internal components; a liquid-filled case protects the gauge from vibration and extreme pressure cycles; and a restrictor prevents damage from pressure spikes. 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, prevent premature gauge failure.
WHEN SHOULD YOU USE FLOW METERS? A FLOW METER IS a mechanical or digital device manufactured to measure and indicate flow. The most common flow meter is the mechanical variable area construction type, which employs a spring-loaded orifice-equipped piston. As flow passes the piston, the pressure differential moves the piston assembly against the spring, displaying the flow rate reading.
HEDLAND FLOW METER | COURTESY OF BADGER METER
A flow meter should be installed in a hydraulic circuit to provide operational information for health and performance. They may be permanently installed or simply used as troubleshooting components during periods of machine failure. Which installation option employed depends on the initial machine build (and its budget) or the machine build specifications’ depth. Flow meters are not inexpensive accoutrements, so their location must
be thoughtful, especially when large units are required. The cost of flow meters rises exponentially with size, so frequently, only the maintenance team uses them for troubleshooting. Regardless, should the budget exist, a flow meter provides valuable data in various locations. Installing a flow meter in the primary pressure line(s) of the pump(s) is the first step to understanding your system in breakdowns. I open the conversation to multiple pumps because a grouping run in parallel provides difficulty diagnosing failure when you’re unsure which, if any, of the pumps are not functioning. The first step to diagnose an unknown stoppage of machine function is first to confirm flow exists. A flow meter installed after a pump confirms pump flow, but also of pump health should the flow reading be reduced. Recording periodic pump flow provides insight. If the flow
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GAUGES
reading drops over time, it may be a sign of pump wear. Furthermore, adding a second flow meter to the pump’s case drain line provides the maintenance team with additional insight. As a pump wears, its case flow accelerates as more fluid bypasses internally. Regular checks and recordings of the case flow rate will provide clues to pump condition, enable you to change or repair the pump before it fails entirely. Lastly, flow meters work well in subcircuits with critical functions requiring predetermined flow, especially those related to cycle time performance. High volume production machinery requires 100% throughput. The installation of flow meters before a motor or cylinder circuit allows the production team to confirm that the flow supports the cycle rate. If machine performance drops, the flow meter confirms whether flow to the circuit is the culprit.
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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.
M35 SERIES DOUBLE VALVE WITH SOFT-START SUPPLIES AIR TO A ZONE OR ENTIRE MACHINE UNTIL SIGNALED TO SHUT OFF AND EXHAUST RESIDUAL DOWNSTREAM PNEUMATIC ENERGY FROM THE MACHINE. | COURTESY OF ROSS CONTROLS
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Several standards (including ISO 138491:2006, ANSI/ASSE Z244.1-2003 [R2008] and ANSI/PMMI B155.1-2011) define the control system as including not only input, sensing and interlock devices, but also output devices such as pneumatic and hydraulic valves. The function of a fluid control valve mimics that of an electrical-control relay and, therefore, is subject to the same rules for classifying safety integrity. Thus, properly specified machine safeguarding systems include provisions for pneumatic valves, including: … must be functionally redundant … must be monitored for faults (including diminished performance faults, which may create the loss of redundancy), without depending on external machine controls or safety circuitry
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… must return to a safe position in the event of a loss of pressure or other such event … must be able to inhibit further operation upon detection of a fault condition until such condition is corrected … should have a dedicated, specific functionreset input and should prohibit the ability to perform a reset by simply removing or re-applying pneumatic or hydraulic power, and must not automatically reset. Providing control reliability with fluid power is not quite the same as with electrical controls, however. For instance, plain redundancy in a safety circuit requires the equivalent function of four valve elements, not just two. Two of the four valve elements handle the inlet function while the other two elements handle the stop function (energy release). Many self-designed systems risk having hidden, potential flaws, which can lead to unsafe conditions because they are unseen, unexpected and, therefore, excluded from design and safety reviews. A good example is the spool cross-over conditions or ghost positions of a valve, which are usually not shown on schematics. 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
FLUID POWER SAFETY 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 redundancy is lost. The circuit doesn’t recognize lost redundancy, nor would it halt operations as a warning that redundancy has been compromised. Then, if the second valve also actuates abnormally, there is no back up, and control integrity no longer exists. Double valves are appropriate for pneumatic and hydraulic equipment anytime reliability is an issue. Typical applications include e-stop, two-hand-control, light curtains, safety gates, pneumatic locking devices for safety gates, hydraulic brakes, air brakes, amusement rides, hoists, elevators, pinch-point applications, or any other application where control system integrity depends on valve operation.
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SHOCK ABSORBERS
SHOCK ABSORBERS 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 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,
SEVERAL MODELS AND STYLES OF INDUSTRIAL SHOCK ABSORBERS | COURTESY OF ACE CONTROLS
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HOW ARE INDUSTRIAL SHOCK ABSORBERS DIFFERENT FROM OTHER DAMPING SYSTEMS? SHOCK LOADS ARE COMMON IN INDUSTRIAL EQUIPMENT — BOTH AS A NORMAL RESULT OF THE APPLICATION AND AS UNFORESEEN LOADS THAT OCCUR WHEN THERE’S A VARIANCE IN THE OPERATION OR PROCESS. WHETHER INCORPORATED INTO THE INITIAL DESIGN OR ADDED AFTER IMPLEMENTATION, INDUSTRIAL SHOCK ABSORBERS ARE OFTEN THE BEST CHOICE FOR MITIGATING THE FORCES THAT RESULT FROM SHOCK LOADS.
however, users first need to determine the relevant operating conditions, including the weight and velocity of the moving mass and how frequently the shock is loaded. For simplicity, let’s look at a linear-motion application and use Imperial units for the calculations.
A MOVING LOAD has kinetic energy proportional to the mass of the load and the square
Determine kinetic energy in the system from:
of its velocity: KE = ½ mv2.
Ek = W/(722)(V2)
The basic purpose of a shock absorber is to remove kinetic energy from the load being stopped, convert it to thermal energy, and dissipate it as heat, thereby preventing the kinetic energy from propagating through the equipment and surrounding structures as shock loads. Shock absorbers can be used on both linear and rotary loads, whether the load is sliding, rolling, or free-falling. Other shock and vibration mitigation devices, such as springs and rubber bumpers, are effective at slowing or stopping loads, but they mostly absorb the kinetic energy from the load and introduce it back into the system as rebounding, or “bouncing” of the load. Pneumatic cushions are similar to shock absorbers in their ability to convert kinetic energy to thermal energy. But because air is compressible, their stopping forces are nonlinear, increasing sharply at the end of the stopping stroke. Dashpots can also be used to slow and stop loads, but they also rely on air and provide non-linear resistance, with the stopping force peaking at either the beginning or the end of the stroke (depending on whether the device is used in “push” or “pull” mode). Industrial shock absorbers, on the other hand, provide a relatively linear reaction force throughout the entire stopping stroke, without rebounding or bouncing at the end of deceleration. This lack of bouncing also allows shock absorbers to provide faster stopping times than other damping systems. The result is fast, smooth, and predictable deceleration of the load. The basic design of a shock absorber includes a double-walled cylinder, a piston, and a return mechanism for the piston. The inner cylinder is filled with a noncompressible fluid, such as hydraulic fluid or oil, and has orifices located at exponential intervals (to match the exponential nature of the kinetic energy equation). When the load makes contact with the piston rod, the piston pressurizes the fluid and forces it through the orifices. As the piston moves through its stroke, the orifices are closed off one-by-one. This ensures that pressure inside the cylinder remains constant and provides a linear stopping force for the load. As the fluid is pressurized and forced through the orifices, its temperature increases rapidly, and this heat (thermal energy) is then expended to the outer body of the shock absorber and the surrounding environment. COMPARISON OF VARIOUS DAMPING TECHNOLOGIES
Stopping force (N)
Hydraulic dashpot Pneumatic cylinder cushions Industrial shock absorbers
Springs or rubber buffers
Stopping stroke
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MOST DAMPING DEVICES EXHIBIT NON-LINEAR STOPPING FORCE, WHICH CAUSES SHOCKS AT THE BEGINNING OR END OF THE STOPPING STROKE. SHOCK ABSORBERS, HOWEVER, PROVIDE NEARLY CONSTANT STOPPING FORCE THROUGHOUT THE ENTIRE STROKE.
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where Ek = kinetic energy, lb-in.; W = weight of moving mass, lb; and V = velocity of moving mass, in./sec. This equation represents the amount of kinetic energy that the shock absorber will convert to thermal energy on each impact. Next calculate the work energy in the application, defined as the amount of energy an external device generates to move the load: Ew = Fd(S) where Ew = work or drive energy, lb-in.; Fd = drive force, lb; and S = stroke of the shock absorber, in. Note that Fd should not exceed the unit’s maximum rated propelling force. If it does, select a larger size and recalculate the work energy. The next step is to calculate the total energy, Et (lb-in.) per cycle, shown as: Et = Ek + Ew Again, if this exceeds the model’s energyabsorbing 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)
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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AD INDEX AAH Fluid Power............................. 61
GRH Power...................................... 49
Prince Manufacturing...................... 59
Ace Controls.................................... 91
HAWE Hydraulic.............................. 45
Proportion-Air.................................. 76
Adaptall........................................... 89
HED (Hydro Electronic Devices)... 19, 54
RAM Industries Inc........................... 15
Aggressive Hydraulics..................... 13
Hydraulics Inc.................................. 48
SIKO.................................................. 6
ALA Industries................................. 64
J.W. Winco, Inc................................ 27
SMC Corporation of America.......... 87
AutomationDirect.............................. 1
Kuriyama of America, Inc................. 30
Spartan Scientific............................. 86
Clippard.......................................... BC
LunchBox Sessions.......................... 81
Super Swivels................................... 23
Coxreels........................................... 33
Main Mfg......................................... 24
Texcel Rubber.................................. 28
Delta Computer Systems................. 57
Milwaukee Cylinder......................... 14
The Lee Company........................... 69
DMIC............................................... 92
Motion............................................. 72
Tompkins Industries..................IFC, 24
Emerson...................................75, IBC
MP Filtri USA................................... 21
Ultra Clean Technologies................... 3
Fairview USA Inc.............................. 78
Nexen Group................................... 66
Veljan Hydrair.................................. 40
Festo................................................ 83
Next Level Manufacturing............... 39
Vermatic Inc..................................... 35
FluiDyne Fluid Power...................... 43
Permco, Inc...................................... 47
Zinga - Filtration Group................... 17
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Profitability depends on maximizing production availability. I need to prevent unexpected machinery downtime caused by valve and cylinder failures.
YOU CAN DO THAT Emerson analytics solutions for pneumatic systems provide actionable insights that support your predictive maintenance strategies. Scalable, non-invasive, continuous monitoring provides performance and lifecycle management information for valves and cylinders. This creates visibility into their health and remaining life, helping to reduce unplanned downtime, ensure production quality and improve throughput. To discover how our smart pneumatics solutions can increase your overall equipment efficiency, visit: Emerson.com/Digital-Transformation-Pneumatics
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