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Fluid Power overview
06 07 08 10 16 18 21 24 28 32 34 38 40 43 47 50 53 56
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
59 60 66 70 71 74 76 80
Pneumatics overview Pneumatic actuators Air compressors Air springs FRLS Pneumatic hose & tubing Vacuum components Pneumatic valves
84 86 90 93
Gauges Miniature fluid power controls Safety Shock absorbers
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FLUID POWER OVERVIEW FRLS
KEEPING
YOU INFORMED WELCOME
to the ninth edition of the Fluid Power Handbook. Each year, our editorial staff works hard to add to the detailed information we’ve already accumulated on hydraulic and pneumatic components and systems. Once again this year, we’ve incorporated several new frequently asked questions into many of the Handbook sections, dealing with sizing, selection and specific component types. Additionally, you will see new briefs in a few categories — compact pneumatic cylinders, hydraulic hose, HPUs, and fluids — where we worked with distributors and system integrators to highlight how fluid power technologies improved machine and system designs. We plan to expand these case histories in 2021 — please let us know about any stories that you may like to see!
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. This year, as we continue to deal with the Coronavirus pandemic, we understand more than ever the importance of being a source of knowledge for the industry. This is why we continue to publish basics info in this Handbook as well as in all of our publications. It is also why we try to find new avenues to help current and future users of fluid power systems understand these technologies. We have pivoted with our Fluid Power Technology Conference and for the time being, have turned it into a virtual event. Every other Tuesday at 2 p.m., you can register to attend a session by industry experts and manufacturers themselves to learn about trends in fluid power, technologies basics and more. Additionally, we are continuing our partnership with LunchBox Sessions’ Carl Dyke, for his every-other-Tuesday YouTube live series, in which he uses his on-site trainer and popular Live Schematics to bring specific technologies to life for viewers. While some say that fluid power is a static, mature technology, there’s still much in store for the technology. We have discovered with the shutdown that occurred this year just how essential fluid power really is and we’re happy to report on the innovations that continue in our industry. We may be down, but we’re certainly not out.
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 Caterpillar equipment building our roads and cities to the John Deere equipment servicing our farmland and the Komatsu equipment harvesting the raw materials from our mines. That’s not to say that hydraulics can’t be precise, however. You will find the technology on passenger airliners and military jets, as well as on machine tools and material handling equipment. Hydraulics differs from pneumatics in that the medium being used to transmit power is a liquid as opposed to a gas. The liquid is generally hydraulic f luid, which is based on a mineral oil base stock. In some cases, water can be used —but this requires the use of very specialized components and is not altogether common. Hydraulic fluid has low compressibility (or a high bulk modulus) and generally a good thermal capacity. Naysayers may argue that hydraulics is a dirty, loud and even an environmentally unfriendly technology. However, that lazy argument doesn’t ring true for fluid power engineers. Those claims merely indicate that the systems being described are improperly designed, installed or maintained. Understanding the operation of and parameters for the application is critical, as is a good working knowledge of sealing and how to deal with contamination. Even something as basic as adding a new component to a sealed hydraulic system can introduce contamination — something that leads to eventual systemic breakdown. The bottom line is that, as in any industrial system, smart engineering design and regular maintenance will avoid problems in the future.
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HYDRAULIC ACCUMULATORS IMAGE COURTESY OF GPM CONTROLS
HYDRAULIC
ACCUMULATORS
HYDRAULIC
accumulators are devices that store and discharge energy in the form of pressurized fluid. They’re analogous to rechargeable batteries or capacitors in electrical systems, and they are often used to improve hydraulic-system efficiency. These pressure vessels hold hydraulic fluid and a compressible gas, typically nitrogen. The housing or shell is made of materials like steel, stainless steel, aluminum, titanium and 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 common types are bladder, piston and diaphragm hydraulic accumulators. Bladder accumulators, as the name implies, use a flexible closed
bladder inside the shell to separate the gas and fluid. They typically have large ports that permit rapid fluid discharge and help ensure that the device is relatively insensitive to dirt and contamination. Bladder-type accumulators are usually designed to have a 4:1 pressure ratio (maximum pressure to gas-charged pressure) to protect the bladder from excessive distortion and material strain. Experts tend to view bladder accumulators as the best general-purpose units. They come in a wide range of standard sizes, and good response characteristics make them well suited for shock applications. Depending on the design, a bladder can be easily replaced in the event of failure or damage. Piston accumulators are much like hydraulic cylinders without the rod. Similar to other accumulators, a typical piston accumulator consists of a fluid section and gas section, with the movable piston separating the two. Less common are piston accumulators that replace high-pressure gas with a spring or heavy weight to apply force to the piston. Piston accumulators are generally recommended for large stored volumes — to 100 gallons or more — and can have high flow rates. Pressure ratio is limited only by the design, but they’re usually not recommended for shock applications. They are often built for rugged, heavy-duty operations. However, they are more sensitive to contamination that can damage the seals — although most piston accumulators are readily repaired by replacing the piston seals. Diaphragm accumulators operate much like bladder accumulators. The difference is that instead of a rubber bladder, this version uses an elastic diaphragm to separate the oil and gas volumes. Diaphragm accumulators are economical, compact and lightweight devices that offer relatively small flow and volume typically to around one gallon. A diaphragm accumulator can handle higher compression ratios of up to 8 to 10:1 because the rubber barrier does not distort to the same degree as a bladder. They also enjoy wide mounting flexibility, are insensitive to contamination and quickly respond to changes in pressures, making them suited for shock applications.
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BAR
STOCK IMAGE COURTESY OF DURA-BAR
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 8
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Equipment in Burlington, Ont., piston rod stock is nearly as varied as what is produced from the steel industry, but some are more common than others. “The most common bar stock material by far, is 75 kpsi 0.0005-in. (1⁄2 thou) chrome plated steel bar,” said Hart. “There are also many other options. With a steel piston rod, you can increase the tensile strength … up to 100 kpsi, and the chrome can be increased to 0.001.” He is describing the tensile strength and the chrome plate thickness of the bar stock, which is important because most cylinders spend half their time pulling. Additionally, thicker chrome results in superior corrosion resistance. Other techniques are employed to strengthen the rod stock. “Most large diameter piston rods are induction hardened, which helps improve impact resistance,” said Hart. “If an end user keeps breaking male rod threads, sometimes this stronger material can help improve the longevity of the cylinder.” Regarding cylinder finish treatments, in extreme conditions, such as corrosive or salinated fluid exposure, rod stock can be further upgraded to stainless steel. “Some end users require corrosion resistance for their process, which is where stainless steel steps in,” said Hart. “Most grades of stainless steel can have a chrome finish.” However, stainless steel is not the only finish available. “Aside from chrome, the only other common finish treatment for piston rods is nitride. This is an extremely durable finish. It is a chemical process that hardens and darkens the material, which provides wear and corrosion resistance.”
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BAR STOCK
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. 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
CA N I BU Y D I FFE R E N T TYP ES O F BA R STO C K A L LOYS FO R HY D R AU L I C M A N I FO L D S ? YES. Aluminum bar stock is often sold as either 6061 “wrought aluminum” for pressures up to 3,000 psi (210 bar) but is sometimes available as 7075 aircraft aluminum, which withstands up to 5,800 psi (400 bar). 7075 is extremely strong and very expensive and is used only in applications where lightweight and high-pressure combine. Grade 65-45-12 ductile iron is an economical, high-strength option used for high-pressure applications up to 5,000 psi (350 bar). It is readily available and easy to machine. Steel in the grade of 1018 or 1045 is also sometimes used for manifolds up to 5,000 psi. Either grade is readily available and inexpensive, but some prefer the machineability of ductile iron. For the ultimate high-pressure capacity, 4041 high alloy steel provides a pressure capacity of up to 10,150 psi (700 bar). Also known as chromoly steel, it offers excellent machineability for a fair price. most common system of circuit construction, and they all require a manifold to interface with. A manifold for a D03 valve, for example, is around 3 in. tall and 3 in. deep, but can be as long as needed to mount any number of valve stacks. The manifold most often has pressure and tank drillings running its length. Each “station” of the manifold, where the valve mounts with four bolts, has four drillings mating up with the pressure and tank passages, as well as mating up with the work ports, which are drilled on the side of the manifold in a vertical arrangement. Bar stock manifolds can be drilled as either parallel or series circuits, depending on the application. Bar stock can be cut into smaller slices and drilled in similar arrangements to bar manifolds to create subplate mounts. The subplates allow one valve to mount atop, with four ports on each of the four sides. Bottom-ported subplates are also available, but are rarely used, because of their tricky mounting, and ports all on one surface, making plumbing difficult. Bar manifolds have plenty of material to enable the addition of a relief valve cavity, but subplates have no such luxury of real estate.
Both manifolds and subplates are available in sizes from D02 to D08, and many manifold accessories are available to help complete the hydraulic circuit, such as tapping plates, cover plates and gauge blocks. Bar stock can also be used to clean up plumbing on machines by reducing the need for adapters and fittings. By drilling ports into a bar, a header or manifold can provide a junction to common feed or return lines, so that each tube or hose plumbs neatly into the same source. Manifolds and headers can reduce leak points, but also add a look of professionalism compared to a mess of tees and adapters. Bar stock is great for mounting components, such as test points, transducers or pressure switches. The bar material can also be anodized any color, or even just treated for corrosion resistance by clear anodizing for aluminum or nickel plating for ductile iron. Lastly, because bar stock is so commonly used in various applications, it is readily available through every fluid power distributor in North America.
IMAGE COURTESY OF DURA-BAR
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HYDRAULIC
CYLINDERS IMAGE COURTESY OF RAM INDUSTRIES
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 10
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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 singleacting 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.
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
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move as the cylinder is stroked, resulting in a possible bent rod or excessive wear.
Common types of mounting include: Flange mounts — Strong and rigid, but have little tolerance for misalignment. It is recommended to use cap end mounts for thrust loads and rod end mounts for loads under tension.
Side-mounted cylinders — Easy to install and service, but the mounts can sometimes create a bending moment as the cylinder applies force to a load, increasing wear and tear. To avoid this, specify a stroke at least as long as the bore size for side mount cylinders (heavy loading tends to make short stroke, large bore cylinders unstable). Side mounts, such as side lugs, need to be well aligned and the load supported and guided. Centerline lug mounts — Absorb forces on the centerline, but require dowel pins to secure the lugs to prevent movement at higher pressures or as a result of shock loads. Pivot mounts — Absorb force on the cylinder centerline and let the cylinder change alignment in one plane. Common types include clevises, trunnion mounts and spherical bearings. Because these mounts allow a cylinder to pivot, they should be used with
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WHEN SHOULD YOU USE STAINLESS STEEL CYLINDERS? STAINLESS STEEL air and hydraulic cylinders are used in applications where corrosion resistance is the utmost priority. Standard cylinders are made from combinations of alloy steel, such as 1018, 1045 and 4140, which are all susceptible to oxidation and rust in humid or wet applications. Even when epoxy painted, carbon steel cylinders exposed to surface abrasion, dents or harsh chemicals can wear away any paint, leaving exposed steel to corrode. Marine environments often require the usage of stainless steel cylinders, both onshore and offshore. Saltwater is especially damaging to standard alloy steel, which will rust rapidly when exposed to saline. Maritime cylinders used for cranes, boat lifts, davits or other machinery do well when made from stainless steel alloys. Offshore oil rigs are especially prone to aqueous corrosion, and although special construction and coatings can mitigate some oxidation, the nature of pneumatic and hydraulic cylinders makes them more prone to corrosion. Manufacturing cylinders in 316-grade stainless steel produces an especially resistant raw material for offshore applications. Stationary marine applications are only half of the equation when it pertains to the effectiveness of stainless steel. Shipbuilders use stainless actuators in locations most prone to corrosion, such as trim actuators, steering components or life craft hoists. An onshore crane failure will just be annoying and time-consuming to repair, but the failure of a steering cylinder could be deadly, so all precautions must be taken to encourage reliability. Stainless actuators are often the best answer. Environments containing caustic or corrosive chemicals will make do with stainless steel cylinders as well. Although poor for marine applications, 304 stainless steel works well in the metals industry where chemicals like sodium hydroxide are used in the cleaning process. Another tough environment requiring the use of stainless cylinders are pulp and paper mills. The black liquor by-product is extremely corrosive to steel, so 304 stainless comes to the rescue once again, extending the life of linear actuators in demanding applications. That being said, even stainless cylinders will eventually corrode when exposed to black liquor, showing you how demanding the environment is. Sometimes a cylinder must not only resist the damaging effects of its ambient environment, but it must also aid in the protection and safety of that environment as well. Food grade applications often require stainless steel air and hydraulic cylinders. Indeed, they excel here because food applications are often damp or altogether
IMAGE COURTESY OF HIGGINSON EQUIPMENT
wet. A bottling plant, for example, uses stainless steel extensively, not just for extended life of the machinery, but to prevent corroding or rusting machinery from contaminating the product. Because stainless steel is less likely to rust, its surface finish remains true for longer periods of humid exposure. A clean, smooth surface prevents the adhesion and accumulation of food particles and bacteria, which is clearly the top concern for food and beverage production. Food grade cylinders are manufactured to different standards compared to “off-the-shelf” cylinders, with features added to reduce the accumulation of bacteria. For example, a quality stainless air cylinder used in food and beverage might have a polished surfaced to prevent bacterial adhesion, rounded corners to reduce burrs and welded or threaded construction to eliminate cavities or pockets. Typical tie-rod cylinders, for example, provide hiding spaces under the tie rod locations, making washdown less effective. Food grade applications can be so extreme as to eliminate externally adjustable cushions, which provide a pocket for food and bacteria to reside. Cushions may still exist but will be the nonadjustable type without any exterior cross drillings. Even the rod wiper may be manufactured from food grade polymers to prevent cross-contamination. Although stainless steel cylinders are more costly than standard steel alloys, some applications absolutely require their superior corrosion resistance properties.
What is the maximum pressure for the application?
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.
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rod-end attachments that also pivot. Pivoting mounts are required for many applications, such as booms and buckets, but are also most prone to rod buckling, especially as the rod reaches end of stroke.
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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
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HYDRAULIC CYLINDERS FRLS
load is transferred along the centerline of the cylinder. Non-centerline mounting calls for additional support to avoid misalignment, but these are required when the mechanism must pivot through an arc, so load calculations must be factored accurately.
Push or pull or both?
Any cylinder can be used as single acting, which is powered in one direction only, but it can only push or pull. When a cylinder pushes, protection against rod buckling and bending must be ensured, which can be achieved through oversized rod material or with a stop tube to prevent full extension, taking advantage of the piston’s load-bearing effect. When a cylinder pulls, there is little concern for buckling, but you should ensure your force calculations factored 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.
What push or pull tonnage is required?
Always assume peak loads will require additional strength. The rule of thumb is to choose a cylinder with a tonnage rating of 20% more than required for the load; however, this is always application-specific, so it’s best to consult a hydraulic professional before you make tonnage assumptions. Cylinder force (lb) is equal to the area of the piston (in.3) times pressure (psi), or F=AxP.
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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
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on the safe side and choose a cylinder design capable of more of than you will ask of it. Fluid type — Most hydraulic systems use a form of mineral oil, but applications using toxic synthetic fluids — such as phosphate esters — require Viton seals, which will not break down or swell in the fluid. Once again, Buna-N seals may not be adequate to handle some synthetic hydraulic fluid, although the gentler synthetics, such as PAO-based stock, will be fine. Hydraulic systems using high water-based fluids may require stainless-steel construction, as well as PTFE (Teflon) seals, especially if no glycol is used in the fluid. Seals — Seals are the most vulnerable component of a hydraulic system. Properly applied seals can reduce friction and wear, lengthening service life, but incorrect types can lead to downtime and maintenance headaches as a result of failures. Every manufacturer likes to use a different style, so it is important to replace them with a similar type and material when rebuilding. Cylinder materials — The type of metal used for cylinder head, cap and bearing can make a big difference in performance and reliability. Most cylinders use bronze for rod bearings and medium-grade carbon steel for heads and bases. But stronger materials, such as 65-45-12 ductile iron for rod bearings, can provide a sizable performance advantage for tough industrial tasks. The type of piston rod material can be important in wet or high-humidity environments (like marine hydraulics) where stainless steel may be more durable than the standard case-hardened carbon steel with chrome plating used for most piston rods. A new option for rod surface treatment is nitriding, which is an oxidation process to increase the surface hardness of metals and corrosion resistance.
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• ASSEMBL
IN ED •
Hydro Leduc N.A. Inc is open for business!
Beginning in July, our outside sales team will be calling on customers and distributors that are ready to get back to work and turn this economy around! As you can see, Hydro Leduc N.A. Inc. has a new assembly plant here in the USA to assemble your bent axis piston motors. We are located in Katy, Texas to ensure the shortest lead time possible for our Distributors and OEM customers. We are proud to say “we’re assembled in America.” If you’re ready to get back to normal, we hear you loud and clear!
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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 enduser, who ignores filtration does so at the peril of your hydraulic system.
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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
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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 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 (1,000-3,000 psi), aluminum or steel housings are required. High-pressure filter assemblies (those rated higher than 3,000 psi) require steel construction for both their filter head and bowl, and are often installed with elements constructed for higher collapse pressure.
HYDRAULIC FILTERS
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 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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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 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.
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FILTRATION SYSTEMS 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 semipurifies 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?
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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 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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Leadership in Fluid Power
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RAM Industries Inc. RAM Industries Hydraulic Cylinder Custom Solutions RAM’s expertise involves establishing a close technical rapport with every client to share ideas and gather critical information regarding cylinder fit and performance requirements. RAM has extensive experience working with purchasers, engineers, manufacturing and maintenance teams, and entrepreneurs developing new product innovations that require custom cylinders. RAM serves a diverse range of OEM clients in construction, oil & gas, mining, forestry, transportation, industrial equipment and agricultural industries across North America. The RAM team offers ongoing engineering support, R&D consultation, prototyping services, and state of the art testing capabilities. Join the growing number of companies that trust and rely on RAM! RAM Industries Inc. 1-877-799-1005 www.ramindustries.com
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RAM Industries Inc. was established in 1973 with a focus on hydraulic cylinders. Today, over 45 years and 5 factory expansions later, custom cylinder design and manufacturing remains at the core of RAM’s operations! RAM offers OEMs a wide range of standard or custom product designs, including: • Single and Double Acting Cylinders • Single and Double Acting Multi Stage Telescoping Cylinders • Re-phasing Cylinder Sets • Position Sensor (Smart) Cylinders • Heavy Duty Construction Cylinders • Stabilizing Cylinders Buying custom built hydraulic cylinders gives OEMs increased design flexibility and options compared to off-the-shelf offerings. Our sales and engineering experts work with your team to determine the exact fit and function cylinder requirements that will give your equipment a distinct advantage! RAM is well-versed in cylinder componentry, design and manufacturing methods, as well as the latest technologies in materials to ensure your cylinders function exactly as required. Customized cylinder options include: • Mounting Configurations • Leading Edge Seal Technologies • External Guards for Ports, Hoses and Oil Lines • Bolt-in Glands • Position Sensor Technology • Replaceable Bushings and Bearings • Integrated Valves such as Relief, Counterbalance, PO Check • Custom Color Paint Finishes Choosing a custom engineered hydraulic cylinder solution will give your equipment the leading edge!
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WHEN SHOULD YOU FLUSH A NEW OR REBUILT HYDRAULIC SYSTEM?
HYDRAULIC CYLINDER CUSTOM SOLUTIONS
PARTICULATE CONTAMINANTS
circulating in fluid power systems cause surface degradation through general mechanical wear, such as abrasion, erosion, and surface fatigue. This wear causes increasing numbers of particles to be formed, the result being that wear increases if this chain reaction of wear is not properly contained (by reducing contamination). Gaps within components grow larger, leakage oil flows increase in size and operating efficiency (e.g. of pumps, cylinders) decreases. To avoid this, you should always remove particulate contamination in new systems, before startup, and later when the system is in use. The overall contamination level of a hydraulic system being built in an assembly plant is generally high. The total contamination (number of particulates) consists of contamination that already exists in the new fluid, contamination that is already on/in supplied components and contamination introduced during the build process from the surroundings. The sum of all the contamination can be considered the Initial Contamination Level. The investment in time and equipment to reduce the Initial Contamination Level is worthwhile because of a reduction of costly warranty claims and for quality control and tracing purposes of products. If the fluid was not maintained to recommended cleanliness standards during use, a warranty claim could be rejected. Fluid in the reservoir should be flushed with a filter cart or a kidney loop system. It is recommended to flush the system and all the sub-functions by activating them to allow fluid to circulate and to flush particulates back in to the tank where they can be captured, either by the system filters (e.g. return filters) or by external, off-line filter systems (filter carts, kidney loop systems). A general rule is to achieve a cleanliness level of the hydraulic fluid in the reservoir that is 1 to 2 ISO Codes below the recommended Target Fluid Cleanliness level for the system.
RAM has over 45 years experience serving OEMs in custom designed hydraulic cylinders. RAM will design and build hydraulic cylinders that meet your exact equipment fit and performance specifications. RAM offers: 3D Modeling Prototyping Reverse Engineering Testing to SAE Standards Technical Support Position Sensor Technologies Mate Specialty Materials Integrated Valves Custom Paint & Packaging Mixed Delivery Sizes
Contact the RAM team of experts today! RAM Industries Inc www.ramindustries.com T: 1.877-799.1005
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HYDRAULIC FITTINGS & FLANGES
HYDRAULIC IMAGE COURTESY OF MAIN MANUFACTURING
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. Fitting connection types include: welded (socket weld, butt weld, slip on); threaded (NPTF, BSPT [both not recommended but used], SAE straight thread, ISO 6149; BSPP); flanged; barbed; quick-disconnect; push-to-connect; 37˚ flare; 24˚ cone; and inverted flare, among others. When selecting a type of fitting, some important considerations are working pressure, vibration, type of fitting, desired attachment, size of piping, flow, material of the conductor or component, and price. The fluid power industry is trying to transition to fittings with an elastomeric seal — generally O-rings — to prevent leakage. These include, but are not limited to, the SAE straight thread, face seal, ISO 6149, and SAE J518 (Code 61 and Code 62) flanges. Seal construction
must be compatible with the type of fluid being used in the system, although very few applications require anything other than Buna Nitrile or Viton. When selecting a fitting, several considerations are important. Most nonflanged 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. 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 IMAGE COURTESY OF ANCHOR FLUID POWER this seat is forced against a mating www.fluidpowerworld.com
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W H AT ’S T HE D I FFE R E N C E BE T W E EN F I T T I N G S A N D A DA P T E R S ? Adapters are components manufactured to join two or more thread forms or sizes to one another. In some cases, adaptors may also be fittings. For example, if the ORB female cylinder port I spoke of earlier needed to attach to a female JIC hose end, the adapter fitting would be ORB male by JIC male. I should note, ORB adapters are almost always fittings, since their very nature defines a “boss” as a female port in a component. Outside of a fitting, adapters allow the interchange of competing component standards, such as SAE, JIC, Metric, JIC and NPT, to name a few. We’re not so lucky to enjoy one standard, so sometimes you need to join a metric hose end to a BSPP fitting, requiring an intermediate adapter to join the two. Adapters could also be used to join or split plumbing, such as a run or branch tee, which itself may have other adapters installed into it to accept multiple standards of hose ends for a single connection. IMAGE COURTESY OF TOMPKINS Adapters allow the attachment of unequal size components as well. Consider the rare occasion when optional components are HYDRAULIC FITTINGS AND ADAPTERS are often assumed installed into a system yet they just happen to have the same port to be the same. Any given technician installing components during sizes. Going from one diameter to the next requires an adapter. The plumbing will tend to use the verbiage familiar to him or her and will adapter may have two female, two male or one of each type, but use the words interchangeably. Of course, they are not the same, or will always exist with different sizes on each end. Some adapters are they wouldn’t be segregated as two words in the English language! jump sizes, going up two or more standard sizes, such as from ¼ in. Fittings install into other major components to allow plumbing NPT right up to ½ in. NPT. of hoses and tubes that wouldn’t otherwise be possible. You install Adapters provide creative solutions, as well. A hydraulic hose fittings into the work ports of pumps, motors, cylinders and valves to may have 1 in. JIC connections, so a run tee adapter with one end 1 facilitate connection of tubes and hoses to these major components. in. JIC female, one end 1 in. JIC male and the third with ¼ in. JIC male For example, a cylinder may have O-ring boss female ports will allow a test point or pressure machined into its head and cap, and although some hydraulic hose gauge to be added inline to that ends are available with ORB male ends, they are rare. More often section of plumbing. than not, the ORB port has a fitting installed with either JIC or O-ring Although used face male, while the hose is manufactured with the corresponding interchangeably, fittings and JIC female or ORF female end. This configuration allows for easy adapters vary by name only installation and removal, and in the off chance there is damage to the based on where and how threads, it’s easier to change a fitting rather than an entire hose. they’re used. When in doubt, Fittings for hydraulic application are often made from forged and consider fittings to be what goes machined steel, which are extremely strong and rigid. Occasionally, into the components you’re fittings are machined from steel billet, although this is rare. The plumbing, and adapters to fitting will include a thread corresponding to the component’s port, facilitate plumbing the rest of but on the other end will include a thread corresponding to the hose the system together. end thread form. IMAGE COURTESY OF ADAPTALL seat, generally by torquing a swivel nut on one fitting, engaging with a threaded portion of the mating fitting. The angle of the seat and face for most JIC fittings in the North American market is 37°, and it is popular enough that the 24° and 45° versions are rarely used. The fittings can be designed to clamp onto a tube by means of a sleeve or ferrule, and care needs to be taken so that the correct size is used because inch and metric tubing sometimes have sizes that are close to overlapping. The quick disconnect allows multiple reconnections of the assembly without causing excess wear or concern for thread
damage. Some fittings allow 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 flatface couplers, which have no recess to collect contamination. Staple and band fittings are low-pressure
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fittings. Band fittings are attached to the hose by a barbed or beaded end being inserted into a hose and a band clamp securing the connection. This method is only for extremely low pressures. Staple fittings have a cylinder with an O-ring and a bead further up on it that slides into a socket. The connection is secured by a staple that goes through both sides of the connection behind the bead, although it is still typically used for low pressure or suction lines.
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HYDRAULIC
FLUIDS
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.
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As oil returns to the reservoir, it often passes through a cooler to help maintain an optimal temperature range before being pumped out to the system once again. Hydraulic fluid can also carry heat from a warmed tank or, via a special warming circuit, into a cold system to help minimize the possibility of damage during cold starts. Closed-loop hydraulic systems require a special bleed-off circuit connected to tank, to ensure that some oil is always being cooled. A charge or replenishing pump brings the cooled oil back into the circuit to help maintain a suitable overall system temperature. Contaminant removal. Hydraulic fluid can suspend and carry solid particles and water away from sensitive components. Filters and other conditioning devices perform the critical job of stripping and trapping the contaminants, and 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
HYDRAULIC FLUIDS H YDRAUL I C SYST E M S TA K E T HE HEAT MINING MACHINES and equipment face tough and unforgiving operating conditions, and the hydraulic systems they rely on have to stand up to the abuse and, in some cases, extreme heat. Facing a unique combination of challenges, a mineral-processing OEM approached GS Global Resources, Mukwonago, Wisc., for engineering assistance while designing a massive 3,300-hp rock crusher, reportedly the largest open-pit cone crusher in the world. Hydraulic cone crushers feed raw material between two cone-shaped structures, one nested inside the other. Hydraulic actuators apply pressure to the two opposing cones, which rotate and stroke to efficiently pulverize rock and ensure high production rates. Aside from its considerable size, the cone crusher would also be located in Zambia, posing multiple environmental issues including high ambient temperatures and an extremely caustic water supply. Due to the hot climate, large air coolers traditionally used in this application could
not effectively remove heat from the oil. That’s a problem for several reasons. Excessive heat reduces oil viscosity, which lowers the fluid’s ability to lubricate components and, in turn, induces surface wear and speeds failure. Heat also accelerates oxidation and breaks down additives which protect the entire system, and leads to the formation of sludge, varnish and other contaminants. High fluid system temperatures can also prematurely break down seals and other plastic components. As an alternative method to reduce temperatures, this OEM would normally turn to shell-and-tube water coolers. However, to be fully effective, this application would require twelve 10-in. shell-and-tube heat exchangers, which would have led to an incredibly large footprint. GSGR was tasked with developing a different, more-efficient cooling method that could handle this unique set of environmental concerns. The GSGR engineering team proposed installing flat-plate heat exchangers, a cooling technology that had yet to be used in the mining industry. Because this type of unit offers more flexibility than shell-and-tube systems in terms of the amount of heat that can be extracted, it could be
adjusted to work in extremely hot climates. One issue, however, was the pH of the local water supply. As the water used in this application was so caustic that it would eat away at the stainless steel plates typically used in this type of heat exchanger, titanium plates were substituted, which were durable enough for the local water supply. To test the new design, GSGR engineers developed a complex procedure to recreate the environmental constraints and operational issues. They simulated the heat load of the rock crusher and ran oil to the cooler. The flat-plate heat exchanger was supplied with water heated to 93° F, which was 8° higher than the hottest estimated temperatures in the machine’s environment. This elaborate test set-up let technicians monitor water and oil temperature changes across the cooler and guarantee that this cooling technology would perform well on site. As a result, the OEM was able to launch the system in the field with confidence that it could work efficiently even in temperatures beyond what was required. The manufacturer embraced the newer, more durable and compact technology, and has since incorporated it into four additional machines. GS Global Resources gsglobalresources.com
ENGINEERS AT GS GLOBAL RESOURCES DEVELOPED A UNIQUE FLAT-PLATE HEAT EXCHANGER TO COOL HYDRAULICS IN A MINING APPLICATION.
(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 all-weather mobile machine, a high viscosity index is crucial. It is important to consider the viscosity requirement as specified by component manufacturers. A piston pump, for example, may require a viscosity between 16 and 40 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.
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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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1-800-248-DMIC
HBS - Hose Barb To Split Flg SBE - Split Barb Elbow 90 Deg ESB - Split Barb Elbow 45 Deg
CSN - Downline Coupling CSN (P) - Downline Coupling Piloted HCSN - High Pressure Coupling HCSN (P) - High Pressure Coupling Piloted
HBL - Hose Barb To Male Thread BSE - Hose Barb To Male Thread 90 Deg EBS - Hose Barb To Male Thread 45 Deg
BV3D Actuator
SNS - Male Thread To Female Thread SNE - Male Thread To Female Thread 90 Deg ESN - Male Thread To Female Thread 45 Deg FL4SU - Flange
BVALP - Low Profile Suction Ball Valve
BVAL - Suction Ball Valve
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SWBARB - Hose Barb To Weld
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BVH - High Pressure Ball Valve
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SSW - Swivel Socket Weld FSW - Fixed Socket Weld
BVLS - Stainless Steel Ball Valve
COMPANION FLANGES FMC - C.61To Male Thread SFC - C.61 To Male Thread 90 Deg ESC - C.61 To Male Thread 45 Deg
SWEL90 - Weld Elbow 90 Deg SWEL45 - Weld Elbow 45 Deg SWEL90SR - Short Radius Weld Elbow
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HYDRAULIC
HOSE
HYDRAULIC
hose is a common and important element in countless industrial and mobile machines. It serves as the plumbing that routes hydraulic fluid between tanks, pumps, valves, cylinders and other fluid-power components. Plus, hose is generally straightforward to route and install, and it absorbs vibration and dampens noise. Hose assemblies — hose with couplings attached to the ends — are relatively simple to make. And if specified properly and not overly abused, hose can work trouble-free for hundreds of thousands of pressure cycles. Hydraulic hoses often consist of an inner tube, one or more layers of reinforcement, and an outer cover. Each constituent should be selected with the intended application in mind. Typical operating and performance parameters include the size, temperature, fluid type, pressure-holding capacity and environment, to name a few. Reinforced hose is constructed with some structural element — styles include spiral wire, textile braid, wire braid, wire helix and other designs in many plies or layered configurations. The inner tube contains the fluid and keeps it from leaking to the outside. The cover protects the reinforcement layer. Other construction options for hydraulic hose include coiled, corrugated or convoluted. Coiled hose is designed for flexibility and elasticity. This feature often makes it expandable and easy to store. Corrugated hose contains corrugations, pleats or spiral convolutions to increase flexibility and capacity for compression and elongation. Multi-element hydraulic hoses are constructed of more than one hose formed or adhered together in a flat, ribbon or bundled configuration. Additional features to consider include whether the hose requires integral end connections, anti-static, lay flat, crush-proof and flame-resistance characteristics. In addition, material considerations include the type of fluid being conveyed and its concentration, as well as substances that may attack the hose cover. Hose selection must ensure compatibility if it is to convey special oils or chemicals. The same holds for hose exposed to harsh environments. Substances such as UV light, ozone, saltwater, chemicals and pollutants can cause degradation and premature failure. For in-depth fluid compatibility data, consult the manufacturer.
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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.
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FLUID POWER HANDBOOK HY D R AU L I C H O S E E X P ERTIS E HE L PS A C U STO M E R FUL FIL L HI S G R E E N - L I V I N G D R EAM POWER DRIVES INC., a manufacturer and distributor specializing in fluid transference and filtration, was faced with a unique challenge when a customer came into one of their Express Hose Centers with the intention of fully restoring a 1960 Case 430 backhoe loader.
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Simplified hose configuration, less hose and adapters combine to reduce repairs and downtime cutting the overall cost of hydraulics.
The customer was looking to bring the tractor back to its former shape and condition — in order to use it to start and cultivate a greenhouse operation. He started researching options, to determine what was required for the project, and experienced difficulty finding such information for a restoration of this scope, as the original Case Corp. no longer exists. The tractor clearly needed a lot of work, and he pored over what information he could find; it was immediately clear that he had to completely rebuild the frame and engine, as well as replace all of the hydraulic hoses. The damaged hydraulic hoses were brought to the PDI team at their Tonawanda, N.Y. Express Hose Center, where they identified each hose and fitting, taking into account the pressure ratings and internal structures, in order to replicate 18 new hydraulic hose assemblies. The expertise of the Eaton Aeroquip-certified technicians allowed for a quick turnaround on this order. Additionally, the customer took care to ask for added hose protectors, to help preserve his new investment. After completing the restoration and installing all of the new hoses, the 1960 tractor now runs like it is brand new. Not only is the tractor a sight to behold once again, but it is helping this loyal PDI customer fulfill his dream of creating and maintaining a selfsustaining greenhouse, which will produce both heat and electricity. Now he has a reliable machine for decades to come. Restoring agricultural machinery — especially machinery that is no longer produced — can sometimes feel like a heavy burden. In this case, Power Drives was able to quickly identify and rebuild the entire hydraulic hose system on this Case tractor. It’s important to not try to take on a project of such magnitude alone; any mistake in hydraulics can be a dangerous one. When you don’t quite know where to start, it’s always best to consult the experts.
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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 they need a locking mechanism to prevent involuntary disconnection? Is one-handed operation required? Couplings can be two types: permanent and fieldattachable (reusable). Permanent couplings are generally more reliable, easier and quicker to attach than fieldattachable couplings, which makes them widely used in industry. Crimping or swaging equipment (sometimes both) is needed to put a permanent coupling on a hose. Permanent couplings can be pre-assembled (one piece), with a ferrule permanently attached to the stem. Higher-pressure hoses use field-attachable couplings, as well as permanent couplings. Field-attachable couplings fit right on the hose using only a wrench and a vise. No special equipment is required. While handy, they do cost more than permanent couplings and take more time to attach. There are three common types of coupling interfaces used in hydraulics today: thread interface, mated angle and O-ring. Threaded couplings have two types of threads: male (outside threads) and female (inside threads). The National Pipe Tapered for Fuel (NPTF) has, as the name implies, a tapered thread. When the male and female components are threaded together, the tapered threads deform, applying pressure on one another, and thus making a tight seal. Mated angle couplings form a seal when the male and female threads are screwed together. Two types of mated angle seals are SAE 45° and JIC 37°, but there are others. The NPSM seal is a mated angle. Couplings with angle seats for sealing have straight or 32
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IMAGE COURTESY OF RYCO
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 www.fluidpowerworld.com
HYDRAULIC HOSE COUPLINGS
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.
HOW CAN QUICK COUPLINGS IMPROVE HOSE SAFETY? HYDRAULIC AND PNEUMATIC
hose safety can be ensured by using quick-connect fittings or quick couplings that make a repeated connection and disconnection between fluid lines and the equipment they are attached to. Quick couplings are used in both hydraulic and pneumatic applications and are designed for easy hand operation. These safety devices feature a male end — or plug — that is inserted into a female end —or socket — to make a secure, leak-tight seal. In extremely high-pressure hydraulic applications, a leak or accidental disconnection can cause serious personal injury or damage to machinery. In pneumatic applications, compressed air presents great dangers for hose whip.
IMAGE COURTESY OF BESWICK
The designs usually feature a one-way sleeve to allow for break-away with a tool when a coupling is clamp mounted. Two-way sleeves allow for one-hand disconnection. In two-way designs, twisting and pulling the two ends breaks the connection. One of the most common designs is the flat face design, which is available as push-to-connect, threaded or screw-in. They eliminate any cavities where fluid or air can rest, thus removing the chance for trapped pressure and leakage. Flat face couplings provide high flow and low pressure drop and their sleeve-locking feature reduces the chance of accidental connection, removing leakage and spillage risks.
IMAGE COURTESY OF CEJN
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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. 34
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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. 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.
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HYDRAULIC POWER UNITS H Y DRAU L I C S POW E R S U LT R A- HI G H P RE S S U R E FI R E FI G H T I N G HMA FIRE, based in Fall River, Wisc., develops fire-fighting equipment and systems for a range of on- and off-road vehicles — from custom mini-pumpers and skid units for ATVs to add-on packages for full-size fire vehicles. Its goal is to make fighting fires safer and more efficient by using ultra-high pressure (UHP) technology. The company recently teamed up with Kraft Fluid Systems, Strongsville, Ohio, to develop a compact unit that’s hydraulically powered. Ultra-high pressure is defined by the National Fire Protection Agency as pump pressures above 1,100 psi. Spraying water at these pressures breaks down conventional water droplets into scores of smaller particles which, in turn, increases the total droplet surface area ten-fold. Greater surface area means more contact with the fire and more efficient heat absorption. When a water droplet absorbs heat it converts to steam which, in turn, displaces oxygen, cools the superheated environment and extinguishes the fire. Tests show that UHP can put out fires in about half the time and using only one-fifth the water, compared to conventional systems that supply water at around 125 psi. This makes vehicles equipped with HMA technology viable, effective alternatives to traditional firefighter pumper trucks. Recently, the engineering team at Kraft was asked to help HMA design a hydraulic system for a Ford F550 truck that would power two 20 gpm water hand lines while not overloading the PTO (pilot take off) under any conditions. And it had to fit within the very limited space available on the F550 platform. In addition to the UHP drive system, Kraft also had to design a second circuit to supply low-pressure water to the inlet of the UHP pump. To meet hydraulic power requirements, the Kraft experts selected a compact Danfoss H1P 53 cc pump. The variabledisplacement, axial-piston pump is noted for high-power density and excellent efficiency, along with a
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
KRAFT FLUID SYSTEMS DEVELOPED A COMPACT HYDRAULIC DRIVE TO POWER AN ULTRA-HIGH PRESSURE FIREFIGHTING SYSTEM FOR A FORD F550 TRUCK.
durable, robust design that’s suited for demanding working conditions. The pump uses electric displacement control along with a built-in speed sensor to ensure precise output. It’s rated for working pressures up to 380 bar (5,511 psi). A fixed-displacement Danfoss series 90, 75 cc axial-piston motor was selected to drive the UHP’s triplex water pump. Information from a transducer that reads the water pressure on the UHP system, along with data from the speed sensor on the Danfoss pump, is transmitted to a Danfoss mobile electronic controller that monitors and regulates the H1P pump’s displacement. As a result, the system maintains maximum torque on the H1P drive pump. Whether one hose line is used, or both, the closed-loop control will back off the pump displacement or increase it in order to maintain the programmed maximum system torque, thanks to the compact and efficient hydraulic system that powers the UHP firefighting equipment. “We know ultra-high pressure,” said Greg Filut, HMA Fire’s production manager. “Kraft Fluid Systems knows fluid power hydraulics and control. The experts at Kraft were instrumental in helping us design from the concept stage all the way through run-off of this very important vehicle for our company.” Kraft Fluid Systems | kraftfluid.com
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 www.fluidpowerworld.com
is highly serviceable but large and expensive. Highly complex hydraulic systems consist of many components — some related to the function of the circuits, like manifolds, directional valves and pressure valves — and other components required for fluid conditioning and monitoring. Filters, heat exchangers and pressure gauges are components added to ensure safe and reliable power unit operation. Because of ease and convenience, as many components as possible should mount to the 7 • 2020
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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
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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 pumpmotor 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
HYDRAULIC POWER UNITS
Quick Release Couplings
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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 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 www.fluidpowerworld.com
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Kuriyama Hydraulic Connections Quick Release Couplings catalog features the Argus line of products from Alfagomma®.
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.
Argus couplings are made from high strength carbon steel with TOP COAT chrome-III zinc plating that provides superior resistance to oxidation. Available parts with sizes from ¼" to 1 ½" conform to ISO-A and ISO 72411 B standards. Flat Faced couplings, sizes from ¼" to 1", prevent contamination in construction equipment. Screw-type quick couplings are also available which provide connections with poppet or flat face valves.
Phone: (847) 755-0360 | Fax: (847) 885-0996 360 E. State Parkway | Schaumburg, IL 60173 sales@kuriyama.com | www.kuriyama.com 7 • 2020
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HYDRAULIC IN
MANIFOLDS
simplest terms, a manifold is a component from which you attach other things. A slightly less elementary explanation is that it cleans up plumbing — and this is why you should care about this unassuming block of metal that ultimately makes for smoother system design. A hydraulic manifold is a housing for surface and/or cartridge valves that regulates fluid flow between pumps, actuators and other components in a hydraulic system. It can be compared to a home’s electrical panel. Just as raw electrical power comes to the panel and is distributed to various household circuits to do work (provide light, power the dishwasher, operate the garage door), hydraulic oil under pressure is routed to the manifold by a pump where it is diverted to various circuits within the manifold to do work. The role of a manifold is to bring the hydraulic circuits to life through the creation of a block machined in a manner consistent with the original circuit design. All valves have a series of orifices to which drilled holes in the manifold must communicate. The configuration of these drilled holes in the manifold is the representation of the defined circuit. The manifold is the central muscle control of the hydraulic system receiving inputs from switches, manual operations (levers) or electronic feedback systems. These inputs energize various valves mounted on or in the manifold, while specific oil pathways allow oil to flow through hydraulic lines to the appropriate actuator to perform work. The complex matrix of variables can make manifold design and component selection a challenging and rewarding art form, as size, weight, function, performance and operating environment are always part of the design consideration. In addition to providing a neat and logical layout, consolidating components into a manifold reduces space and pressure drop. This results in 38
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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 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 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 especially suited to mobile machinery, where they 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 motor’s speed control and smooth reversibility make them perfect for use on backhoes, skid-steers 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 several parameters, including torque capacity, speed range, pressure limitations, efficiency and displacement. Displacement is the amount of fluid needed to turn the output shaft one revolution, and it is usually 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, fixed40
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IMAGE COURTESY OF BOSCH REXROTH
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 There are several types of hydraulic motor, including 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 www.fluidpowerworld.com
HYDRAULIC MOTORS
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 mediumpressure 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).
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W H AT ARE P TO T H R O U G H - D R I V E M OTO R S ? IF YOU’VE SPENT any reasonable amount of time working on or around mobile machinery, you’re very familiar with PTO-driven machinery, including the hydraulic pump. The PTO shaft exiting the rear of a farm tractor is used to power mechanical implements through a drive shaft, such as bailers or mowers, themselves sometimes attached to the tractor itself via a 3-point hitch. Pumps are also mounted to the PTO shaft, providing an easy source of hydraulic energy in addition to the tractor’s secondary hydraulic outputs. For on-road machineries, such as stone slingers, dump trucks or fire engines, no such external drive shaft is used. Instead, the transmission of the vehicle has a removable panel where a power take-off is mounted to provide a drive shaft or pad mount for pumps
or other purposes. A hydraulic pump, for example, mounts either directly or closely to the PTO unit, supplying hydraulic energy for various machine functions. What both tractor and on-road PTO options require is a drive shaft terminating at the pump, eliminating any other purpose or potential for driving mechanical functions. Although options like a split shaft take-off or PTOs with multiple take-offs do exist, they’re expensive and bulky. As well, the location of the PTO drive is previously limited to under the truck or behind the tractor. A great new concept in the world of machine drives is the PTO through-drive motor. A PTO through-drive motor allows the unique ability to mount the drive system anywhere your creativity conceives, so long as it’s run via a hydraulic pump elsewhere. Imagine mounting a drive shaft for a pond pump mounted at the end of an excavator’s arm? Or being able to mount a PTO driven implement to the back of your skid steer loader, powered by auxiliary hydraulics? Linde offers such an animal, if you were doubting its existence, available as their HMV/R-02 series PTO motor. Essentially, it’s a through-drive motor with splined
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joints at either end allowing the attachment of various sizes of PTO shafts. Provided are two opposed shafts allowing it to fit in the driveline of existing equipment, so hydraulics can now power machinery previously only brought to life mechanically. Just as with the standard HMV-02 series of motors from Linde, displacements are available all the way up to 331 cc/rev, and with up to 500 bar (7,250 psi), peak torque is well over 2,200 Nm (1,628 lb-ft), twisting with enough force to power most PTO driven machinery on the market. The Linde system works with five standard shaft sizes, each running ANSI B92.1 splined outputs.
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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 fixeddisplacement 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 torque that the motor will need to accommodate. Knowing its displacement and flow requirements within a system is equally 42
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important. The type of 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 rating, as this will factor in whether it runs cost-effectively or not. In addition, a component that is easy to repair and maintain or is easily changed out with other brands will reduce overall system costs in the end. Finally, consider the motor’s size and weight, as this will impact the size and weight of the system or machine with which it is being used.
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HYDRAULIC PUMPS
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.
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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. Fixeddisplacement pumps provide a single, specific volume displacement per revolution. In variable-displacement pumps, displacement
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per cycle can vary from zero to maximum volumetric capacity. Some of the more widely used types of positive-displacement pumps are gear, piston and vane. Gear pumps can be either internal or external styles. External gear pumps are one of the most popular types used in modern hydraulic systems. Gear pumps produce flow by using the teeth of two meshing gears to move the fluid. Their simple construction ensures limited purchase costs and servicing. They feature decent mechanical and volumetric efficiency, compact dimensions and low weight/power ratio. Of the three common types of positive displacement pumps, gear pumps are the least efficient; their appeal is low cost and simple design. External gear pumps can be equipped with straight spur (the most common type), helical or herringbone gears. In operation, the drive gear and driven gear rotate, creating a partial vacuum at the pump inlet (where gear teeth unmesh) that draws fluid into gear teeth. Gear teeth mesh at the outlet, forcing fluid out of the pump. Internal gear pumps contain one internal and one external gear. They pump fluid in the same manner as external spur gear pumps. In the basic design, the internal gear, which drives the outer gear, has one tooth less than the outer gear. As they mesh, the teeth create sliding seal points. Because their transition zone from low to high pressure (the area over the crescent) is relatively long, internal gear pumps can offer lower noise levels than some other types of pumps. Gears are made of special steel and are
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often case hardened and quench hardened. Then gears are ground and fine finished. Proper tooth profile design and geometric proportions can reduce pulsation and noise levels during pump operation. Piston pumps supply high flows at high speed. Two types of piston pumps— axial and radial—are manufactured in both fixed- and variable-displacement versions. Axial-piston pumps contain one or more pistons that convert rotary shaft motion into axial reciprocating motion. An angled cam (or wobble plate) rotates, causing pistons to reciprocate and take fluid in as they move toward the thin part of the plate. Fluid is expelled as pistons approach the thick end. In the bent-axis design, both pistons and shaft rotate, making a wobble plate unnecessary. Bent-axis pumps use the drive shaft to rotate pistons. With the longer sealing paths along the piston walls, piston pump efficiencies tend to be higher than other types of pumps. In addition, variabledisplacement pumps can provide savings by only providing the pumping necessary for the function, saving additional energy and costs. Radial-piston pumps (fixed-displacement) are used especially for high pressure and relatively small flows. Pressures of up to 10,000 psi are common. 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 ballshaped pistons. Another classification refers to porting: Check-valve radial-piston
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C OM M ON P U M P S E L E C T I O N C O N S I D E R ATIO N S HYDRAULIC PUMPS have just one role; transform incoming mechanical energy into hydraulic energy. All pumps provide flow and pressure (pressure starts at the pump, not the resistance as is commonly mistaken). With one simple task, there is surprising variability in the pump market. This FAQ should elucidate the intricacies of pump selection for you.
you with nothing more than extra speed so long as your prime mover (motor) can handle the extra horsepower. If you are looking for extra force from the same prime mover, downsizing to a smaller pump while increasing your pressure will help you achieve more force, so long as all the components used can withstand the increase in pressure. Q: Should I install a gear, vane or piston pump?
Q: Do pumps only provide flow? A: No! This is a common myth in the fluid power world. Energy can only move from an area of higher potential to lower potential. If pressure started at the restriction, as many people believe, fluid would travel backwards to the pump. Cosford’s Law states that “pressure makes it go, and flow is the rate in which you can create force.” The pump is your hydraulic system’s source for both flow AND pressure. Q: Will installing a larger pump provide my hydraulic system with more force? A: Your pump’s size represents its displacement. A higher pump displacement will provide you with more flow, all things being equal. Force is dictated by pressure, not flow. A larger pump will provide
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A: Your choice of gear, vane or piston pump depends on your ideal trade-off between budget and performance. Pumps are increasingly more expensive gear to vane, and from vane to piston. Each pump has its strengths and weaknesses, as spelled out below. Gear pumps are inexpensive but are often noisy and inefficient. Gears pumps are highly resistant to contamination and readily available from nearly every manufacturer. Vane pumps are modestly priced but not inexpensive. Vane pumps are more efficient than gear pumps but much quieter. Vane pumps are also reliable and easily repaired. Piston pumps are the most efficient, most expensive and with the highest pressure capacity. They are often loud, sometimes bulky, but if your budget can afford them, they should be at the top of your shopping list.
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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
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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).
HYDRAULIC TEST POINT ADAPTERS • Safe and simple connection of pressure gauges • Bleed trapped air from the system • Draw fluid samples for analysis • Reduce system contamination
Contact us for more details: Toll Free 866 FLANGES | anchorfluidpower.com | sales@anchorfluidpower.com 46
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REPAIR, REBUILD & MANUFACTURING
REPAIR, REBUILD &
MANUFACTURING WHEN
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fluid-power components like pumps, motors, valves and cylinders fail, the overriding goal is to get a system up and running again as soon as possible. One option is simply to purchase and install a new replacement part, but a repaired or remanufactured component is often the more economical option. Depending on the unit, almost every constituent part can be replaced or repaired, provided a suitable replacement is available. As a general rule, a repair makes sense if the cost doesn’t exceed 60 to 70% of the cost of a new component. Beyond that, the user is typically better off with a new unit. However, if a new product is not readily available and a critical or expensive machine or production line is down, repair is still practical at a higher cost. Technicians first make a full inspection and diagnose the problem. Sometimes components wear out. But premature failures typically result from contamination, cavitation, overpressurization, and excessive heat. Thus, installing a robust filtration system, keeping components cool, and following a disciplined maintenance program are critical to extending component and machine life. Finally, some components break due to incorrect application, installation or commissioning — failures that are often preventable. Technically savvy repair shops tear a unit down and try to bring it back to “as new” condition. All critical dimensions and surfaces are inspected and measured. Seals and low-cost consumable parts like springs, washers and shims tend to be replaced. Likewise, bearings will be inspected and possibly replaced. In more-serious cases, say internal wear due to contamination damage in a piston pump, lapping the surfaces might be suitable if still within acceptable tolerances. Otherwise, the technician may need to remanufacture or replace rotary barrels, pistons, and other internal parts. That can ultimately extend to replacing other major components like housings, covers, relief valves,
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FOU R T I P S F OR S E L E C T I N G T H E RIGH T M OB I L E H Y D R AU L I C R E PA I R controllers and charge pumps. In the worst case, a completely new unit can be built from parts, although that is not A REPAIR OR REBUILD performed by an genuine manufacturer’s service parts the norm. while following factory approved rework authorized service center ensures equipment Another issue is whether to specifications. will have longer life and better performance repair/remanufacture a unit with because they are performed by factory trained parts sourced from the original Factory warranty — Most reputable repair technicians with genuine service parts and hydraulic OEM or with aftermarket and rebuild companies offer a warranty in completed wtih a renewed factory warranty. replacement parts. The hydraulic keeping with the manufacturers they carry. Consider these four criteria when seeking a repair industry broadly falls into three Don’t settle for less. repair or rebuild of hydraulic components. business sectors. Hydraulic component manufacturers often tend to serve Timing — Finally, repair and rebuild should Factory trained technicians — Service large machine builders directly, with compete with new unit lead-time. The repair technicians should be factory trained and their own parts. But they often leave shop should stock the most commonly certified to perform quality repairs and user service and support to distributors used service parts, allowing factory-quality rebuilds just like the manufacturer. and large, sophisticated repair houses. component repair and rebuild to be Some use OEM parts exclusively, some performed in as little as two weeks. Genuine service parts — Select a repair do not. Further removed are smaller shop that only sells, services, and inventories 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 FLUID CONDUCTING QUICK controls and sophisticated software. Unfortunately, the complexity DISCONNECT COUPLINGS of today’s hydraulics means pure mechanical aptitude is no longer sufficient to fix many components. Working Pressures to 6,000 p.s.i. | 3/4” thru 3” Size Components with higher operating pressures and electronics Hydraulics, Inc. thread to connect couplings are designed to provide high flow and controls also tax the capabilities of test equipment at many repair low energy loss in fluid power systems. All products are built for rugged use and are facilities. Small shops will make repairs and perhaps run basic tests, but designed with a minimum 4:1 Safety Factor. Notable features include superior flow they are not capable of performing full-function tests. Larger repair characteristics and resistance to extreme pressures and systems induced shock loads. shops have made significant investments in state-of-the-art test stands The carbon steel couplings are offered with both poppet style (5TV and 6TV Series) or for qualifying dynamic open and closed-loop systems. flat face valves (TVF Series). A variety of port options and pressure ratings up to 6,000 Not surprisingly, the price of a repaired or rebuilt component psi, and proven performance in the field make these products popular in the mining, can vary widely depending on the expertise behind it. Experience oil and gas, construction, and other natural resource sectors. 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. P.O. Box 6479 · Fort Worth, TX 76115 · V. 817-923-1965 · hydraulicsinc.com
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FLUID POWER TRAINING
GOES LIVE
YOUTUBE LIVE SESSIONS featuring Carl Dyke from LunchBoxSessions.com April 28, 2020 — Pilot Pressure Control: Manual Joysticks to Electronics Carl Dyke has a working, manual pilot control stick and pedal, and also a fully operational, electro-hydraulic pilot pressure system from a mining shovel. Some stationary plant/mill applications too. You’ll experience some intricate 3D simulations and learn some troubleshooting tips! May 12, 2020 — Pilot Operated, Pressure Relief Valves: X-Port, Y-Port, External Piloting This session demonstrates working relief valves and matching simulations. Together we’ll explore external, remote piloting schemes in order to make sense of the X-port and the Y-port. We’ll measure pressure and flows as we go. May 26, 2020 — Piston Pumps: Displacement & Pressure Cutoff Adjustments Adjustments to pump controllers will be made with flow and pressures measured on live pumps and in simulation. June 09, 2020 — Particle Contamination: Seeing and Measuring is Believing Carl has digital microscopes for particle identification and measuring, sampling equipment and a laser particle counter for this lively interactive event on hydraulic fluid cleanliness.
sponsored by:
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June 23, 2020 — Servo Valves: Running Live and in 3D This session will look at the intricate and robust aspects of servo valves. You’ll experience some intricate 3D simulations and learn some troubleshooting tips! July 14, 2020 — Slip-in Cartridge Valves: Simply Logical Flow Carl will demonstrate several slip-in cartridges running live and discuss the valve surface area ratios and the pilot options on the cover plate. Live Schematic reading and 3D fly throughs included! August 25, 2020 — Load Sensing Systems: Pump Control and Valve Bank Mysteries This live training session will demonstrate several pumps running in load-sense (flow-compensating) mode. sponsored by: Together we will explore the adjustment of margin pressure. Typical proportional valve banks with load-sense shuttle valves and pressure compensators will be covered in Live Schematic form and in 3D.
For past and future fluid power training sessions, go to: www.lunchboxsessions.com/live
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HYDRAULIC ONE
SEALS
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.
Polyurethane
Polyurethane is an organic material whose chemical composition is characterized by a large number of urethane groups. Urethanes belong to the thermoplastic elastomers (TPE) family and close the gap between thermoplastic and elastomeric materials regarding hardness, deforming behavior and consistency. Within certain temperature limits, polyurethane possesses the elastic characteristics of rubber combined with the advantages of a rigid plastic. The composition of the material is determined by three components: polyol, diisocyanate and a chain extender. The type and amount of these materials used, and the reaction conditions, are decisive in determining the properties of the resulting polyurethane material. In general, polyurethanes possess the following properties: … … … … … …
high mechanical, tensile strength good abrasion resistance modulus of elasticity is variable wide range of hardness values, while retaining good elasticity good resistance to ozone and oxygen outstanding resistance to abrasion and tear
Acrylonitrile-Butadiene-Rubber (NBR)
NBR is a polymer of butadiene and acrylonitrile. The acrylonitrile (ACN) component affects the following properties of the NBR: … elasticity … cold flexibility … gas permeability … compression set … swelling resistance in mineral oils, greases and fuels An NBR material with low ACN content has very good cold flexibility (down to approximately –45° C) and moderate resistance to oil and fuel. In contrast, a material with very high ACN content with optimum resistance to oil and fuels, may have a cold temperature flexibility only down to –3° C. With rising ACN content, the elasticity and the gas permeability decrease and the compression set becomes worse. NBR provides: … good resistance to swelling in aliphatic hydrocarbons; greases; fire retardant hydraulic fluids of Groups HFA, HFB and HFC … good resistance to hot water at temperatures up to 100° C (sanitary fittings), inorganic acids and bases at concentrations, and temperatures which are not too high … high swell in aromatic hydrocarbons, chlorinated hydrocarbons, flame retardant hydraulic fluids of the Group HFD, esters 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.
Temperature range for use: –30 to 80° C; high performance types (compounds) up to 110° C in mineral oils (long-term exposure temperature). T E S Y O F P R E C I S I O N A S S O C I AT E S I M AG E C O U R
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HYDRAULIC SEALS
Newly developed materials (cross-lined by peroxides) have good resistance to media, which can only be tolerated to a small extent, if at all, by conventional FKM. Temperature range for use: about –20 to 200° C (for short periods to 230° C). Special grades: –50° to 200° C.
Polytetrafluoroethylene (PTFE)
PTFE is a polymer of tetrafluoroethylene. This non-elastic material is characterized by: … slippery surface that repels most media … non-toxic at working temperatures up to 200° C … low coefficient of friction against most opposing surfaces made of other materials; stiction and friction are almost the same … excellent electrical insulating properties (almost independent of frequency, temperature and weathering effects) … chemical resistance that exceeds that of all other thermoplastics and elastomers … liquid alkali metals and a few fluorine compounds attack PTFE at higher temperatures The temperature tolerance is between –200° and 260° C; PTFE has some elasticity even at extremely low temperatures; therefore it is used in many extreme cold temperature applications. Most hydraulic applications require the use of a spring or elastomeric component to energize a lip seal configuration because of the low elasticity and tendency to cold flow over time.
SIX COMMON SEAL DESIGNS Following is a list of some of the most common seal designs used in fluid power applications.
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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IMAGE COURTESY OF MFP SEALS
FKM provides: … tough resistance to high heat … excellent resistance to oil, hydraulic fluid and hydrocarbon solvents … good flame retardance … low permeability to gases … high swell in polar solvents, ketones and fire-retardant hydraulic fluids (i.e. Skydrol type)
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IMAGE COURTESY OF HALLITE
W H E N A R E STATIC S EAL S U S E D I N H Y DR AUL IC S ?
Wipers
y provide aggressive wiping force y prevent mud, water, dirt and other contamination rom entering the system y allow lubricating oil film to return to system on inward stroke y protect main sealing elements, thus increasing life of seals y often made from polyurethane, which offers high abrasion resistance y often used as a linkage pin grease seal
Rod Seals
y prevent system fluid from escaping to atmosphere y must provide sealing function at low and high pressure y require excellent extrusion and wear resistance y should provide good pump-back capability for lubricating oil film y often must withstand up to 6,000 psi
Buffer Seals
y must withstand high pressure exposure y protect the rod seal against pressure spikes y feature a pressure-relieving capability that prevents pressure build-up between seals y increase rod seal life y allow for wider extrusion gaps y require high wear resistance
Wear Bands
y prevent contact between metal parts in the cylinder y center rod and piston from housing elements y increase seal life
O-rings
y most commonly used in static applications and radial or axial deformation to maintain sealing contact force y double-acting, so seal on both sides of a component y can be used as energizing elements or as primary seals y self-acting, so do not require additional system pressure or speed to create the seal
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STATIC SEALS are used in hydraulic applications where two mating surfaces or edges require positive sealing. A static seal, by definition, is one that remains stationary and subjected to no movement and its related friction. A static seal may be exposed to hydraulic pressure on both sides or be exposed IMAGE COURTESY OF PARKER HANNIFIN to hydraulic pressure on one end and air on the other. Most often in hydraulics, static seals are used to seal a body, flange or head to another stationary tube, cap or other components. One example is the rear cover of a piston pump that must seal against the pump housing and does so with a gasket or O-Ring. The seal must only contain low-pressure case oil and prevent it from leaking unintentionally from the pump. A similar example exposed to high pressure is the flange cover of a gear pump. The case of a gear pump which contains the spur gears creating pressure must be able to withstand full pump pressure, and as such, the static seal must be rugged enough to avoid extrusion and leakage. In a hydraulic cylinder, static seals are used to seal the barrel to the head and cap in tie-rod cylinders, or perhaps the head or gland of a welded cylinder. Once again, these static seals must prevent internal pressure from escaping. Static seals used to seal a tube in tie-rod cylinders are especially prone to failure if extrusion gaps are too large, allowing tie-rod stretch to loosen gaps and provide a path for extrusion. In cartridge valve applications, the “ways” describe the ports of a valve. A 5-way valve has five ports, and in a cartridge valve cavity, each port must be positively sealed from each other, and in most cased bi-directionally. The spool itself inside the cartridge valve seals through tight clearances and full film lubrication, but without the static seals between each way, internal leakage would prevent the valve from functioning properly or even at all. As well, even the top of the cartridge valve must be sealed inside the cavity to prevent external leakage. Static seals are often just an O-Ring, but when extreme pressure is possible, harder rubber or plastic backup rings will sit behind the O-Ring to support it as it’s exposed to pressure. In places where extrusion gaps are minimal, such as a pump housing, backup rings are rare. However, most cartridge valves will use backup rings to support O-Rings in either direction, using essentially three components to achieve a static seal.
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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
IMAGE COURTESY OF SIGMA-NETICS
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
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resistance on the electrical circuit and would result in a change to the output voltage. With the aid of an analog to digital converter (ADC), the transducer’s output signal can be used in systems that require digital signals. For example, a programmable logic controller (PLC) or a programmable automation controller (PAC) can use the digital signal to monitor the pressure and take action if needed. Some pressure transducers output current rather than voltage, and are then often referred to as transmitters. These values typically fall within tens of thousandths of amps. When choosing the output of a pressure transducer, it is important to keep in mind the input requirements of the device that will be accepting the signal, the distance the signal must travel and possible interference that can be found in the environment around the system. Important performance criteria to consider are the pressure transducer’s operating pressure range, maximum rated pressure, accuracy and operating temperature range. The operating pressure range demarcates the intended pressure bounds at which the transducer has been designed to perform optimally. The maximum rated pressure is the highest allowable pressure that the pressure transducer is rated to withstand. The accuracy of the transducer is usually represented by suppliers in terms of ASME B40.1 grades: 4A (0.1%), 3A (0.25%), 2A (0.5%), A (1%), B (2%), C (3%) and D (4%) deviance from the true pressure value. A good pressure transducer is designed to operate independently of temperature; however, the operating temperature specifies a “safe” range; 7 • 2020
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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
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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.
IMAGE COURTESY OF IMI NORGREN
POSITIONING SENSING position feedback or data of a cylinder’s position. The sensor converts the position into a proportional analog or digital signal. Hall-Effect Transducers use a magnet that communicates with the Hall chips, which then give an output to the internally built microprocessor. The output from the microprocessor is converted to a signal required by the user interface such as voltage, current, PWM or digital output. Using Hall-effect technology on linear position sensors allows manufacturers to make small, compact designs that can be mounted internally or externally to a cylinder. Hall-effect technology is well suited to mobile applications as it is highly resistive to shock and vibration. It is used commonly on steering and depth applications. LVDTs or Linear Variable-Distance Transducers are durable and resist shock and vibration while offering high repeatability. These absolute linear position/displacement transducers convert a linear displacement into an analog electrical signal. Their design includes transformer coils wound around non-magnetic coils.
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LVITs — Linear Variable Inductive Transducers — are contactless position sensing devices, with sensing ranges up to 30 in. or more. Most www.fluidpowerworld.com
IMAGE COURTESY OF ROTA ENGINEERING
Several technologies exist to provide
designs feature an inductive probe surrounded by a conductive tube. This is attached to the moving object to make the reading. These contactless position sensing devices use eddy currents developed by an inductor in the surface of a conductive movable element to vary the resonant frequency of an L-C tank circuit. Modern electronics using
SENSING TECHNOLOGIES
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.
U N D E R STA N D I N G S E N S O R O UTP UT SENSORS most often output a variable
voltage or variable amperage signal. The most common outputs are 0-5 VDC, 0-10VDC and 4-20mA. Sensors taking advantage of Industry 4.0 operate using CAN networks, itself a variable voltage output. Going a step further, some sensors now employ wireless technology such as Bluetooth.
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 The output signal from a sensor connects attached to the axis to be measured. The to your PLC, if you have one, to help the magnet does not touch the sensing rod, machine recognize the state of the parameter so no parts can wear out. The sensing its measuring. It may use a pressure signal to rod mounts along the motion axis to trigger the next sequence of operation or turn be measured and the position magnet on a cooling fan when it measures excessive attaches to the member that moves. An fluid temperature. electronics module sends an analog or digital position reading to a controller The sensor outputs may tell a smart relay to or other receiving device. Also within activate an auxiliary function. A differential the electronics housing is the electrical connection interface, either an integral connector or cable and visual diagnostic LEDs to ensure proper wiring, power, and magnet positioning. Encoders are used heavily on mobile machinery. Available with capacitive, optical or magnetic sensing and with incremental or absolute position sensing, these devices accurately determine the stroke of the cylinder. Rotary and linear devices are available, but rotary styles are most common. These devices measure position and resolution in pulses per revolution. Some rotary encoder designs use a wire-actuated sensor mounted directly inside the cylinder. Others are mounted externally. Others are based on Hall-effect, magnetostrictive, or inductive technologies. 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.
pressure sensor relays to a warning lamp telling the operator that the hydraulic filtering is nearing its bypass pressure and provides maintenance with the opportunity to change the element. Sensors may offer a visual display, either with integrated LCD/LED screens, or by sending its output to a secondary display screen. Maintenance parameters, such as contamination level, temperature, fluid level or backpressure, offer data on machine health critical to reliability.
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PRO GRADE REELS SOLUTIONS FOR:
AIR / WATER | HYDRAULIC | PNEUMATIC | VACUUM | WELDING | POWER AND MORE
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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. 56
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By using a small pilot valve to control the movement of the larger, mainstage spool, the size (and flow) of the valve is nearly limitless. Directional valves are often described by the number of “ways” fluid can travel through them, and also by the positions available to be shifted into. The ways are equal to the number of work ports, so a 4-way valve will have Pressure, Tank and A and B work ports. Positions are equal to the number of positional envelopes. For example, one would describe a double-acting single monoblock valve as “4-way, 3-position,” or simply a “4/3 valve.” Directional valves are available in monoblock or sectional valves, common to the mobile-hydraulic industry, as well as subplate mounted industrial type valves such as ISO style D03, D05 and so on. Also common to both mobile and industrial markets are cartridge valves installed into manifold blocks. Cartridge valve manufacturers offer many unique products and allow high levels of creativity with limitless available valve combinations.
PRESSURE CONTROLS A pressure valve is any component designed to limit pressure. Most pressure valves are based on a poppet being pushed against a seat with an adjustable spring, although pressure valves can be a simple ball and spring configuration or use spools for high flow circuits. Their operation is simple: a spring pushes the poppet against a seat, and when pressure from the system is strong enough to counteract the force of the spring, the valve will open, bleeding off fluid to limit pressure. A relief valve controls maximum pressure for either the entire system or a sub-circuit of it, the lowest spring pressure of a system being the one to open up first. Most other pressure valves are based
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HYDRAULIC VALVES
on the relief valve’s simple spring-loaded ball or poppet. Sequence, counterbalance and brake valves are all forms of relief valves with added utility or functionality, such as reverse flow checks or pilot operation built in. The pressure-reducing valve differs from the other pressure valves because it limits pressure downstream of itself rather than upstream. It is used in applications where sub-circuit pressures need to be lower, without sacrificing any performance in the rest of the system.
FLOW CONTROL VALVES Flow control valves control or limit flow in one way or another. They are often just a needle valve, which is just a variable restriction, adjusted by a screw or knob much like pressure valves, to limit the energy potential to create flow. When installed with reverse flow check valves, we change the name to flow control. Flow control valves can sometimes have multiple ports, such as with a priority flow control. They provide controlled, fixed flow to one part of the circuit (sometimes at the sacrifice of another part) and only if input flow is high enough for its priority demand. Flow controls are (ideally) pressure compensated, which allows the valve to maintain its set flow regardless of 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 NG the valve and coil when ERI DO F O only a couple amps are SY RTE U required to achieve full CO GE flow anyway. Additionally, IMA 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.
W H AT I S A R E L I E F VA LV E ? A RELIEF VALVE is a device used to
limit pressure in one or more locations in a hydraulic circuit. A relief valve uses a spool or poppet engaged to the closed position by a spring. A spool is a cylindrical piece of machined steel that slides within a machined body. A poppet is a flat piece of machined steel attached to a stem, and the face of the poppet rests against a seat to provide superior sealing. A spool often provides better metering characteristics, but has significantly more leakage than a poppet design. When pressure rises in the portion of the circuit where the relief valve is installed, force acts upon the spool end or poppet face; the force applied by the spring opposes the force on the spool end or poppet face to keep the valve closed. Valve spring compression force is often variable — compression height can be reduced by
an adjustable screw — although effective range is limited (for example, a spring might be effective between 1,000 and 5,000 psi or 100 and 1,200 psi, but rarely between 100 and 5,000 psi). As hydraulic pressure continues to rise in the circuit where the valve is installed, force against the spool or poppet starts to overcome the opposing force of the spring, opening a flow path to the tank. As pressurized fluid exits the relief valve, energy is diverted (in the form of heat) until downstream pressure equals the spring value force, which could be drops of flow or all of pump flow, depending on the application and state of the circuit. In short, a relief valve is a hydraulic component designed to limit pressure in an entire system or subcircuit by diverting pressurized
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flow to the reservoir. They are most often installed directly downstream of the pump to control system pressure, but can be used in other parts of the circuit to protect isolated components.
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Smart IoT Compressed Air Device Delivers Advanced System Diagnostic and Energy Efficiency Saving energy is easier than ever before thanks to the MSE6-E2M. Achieve your energy efficiency and sustainability targets while optimizing process equipment performance. Intelligent assembly features include: • • • •
Zero compressed air consumption in standby mode Monitors the system for leaks Ensures maintenance in the event of leaks Enables effective real-time monitoring of relevant process data www.festo.us
PNEUMATICS OVERVIEW
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 applicationspecific devices such as grippers or air springs. Vacuum systems, also a part of the pneumatic realm, use vacuum generators and cups to handle delicate operations, such as lifting and moving large sheets of glass or delicate objects such as eggs. Engineers commonly use pneumatics in industries such as medical, packaging, material handling, entertainment and even robotics. What’s more, pneumatics can be useful in very specific applications where hazards are critical — for example, in a mine or on an offshore oil platform — where a single stray spark could mean total disaster and lost lives. By its nature, air is easily compressible, and so pneumatic systems tend to absorb excessive shock, a feature that can be useful in some applications. Most pneumatic systems operate at a pressure of about 100 psi, a small fraction of the 3,000 psi that many hydraulic systems experience. As such, pneumatics is generally used when much smaller loads are involved. A pneumatic system generally uses an air compressor to reduce the volume of the air, thereby increasing the pressure of the gas. The pressurized gas travels through pneumatic hoses and is controlled by valves on the way to the actuator. The air supply itself must be filtered and monitored constantly to keep the system operating efficiently and the various components working properly. This also helps to ensure long system life. In recent years, the control available within pneumatic systems (thanks to advanced electronics and componentry) has increased greatly. Where once pneumatic systems could not compete with many comparable electronic automation systems, the technology today is seeing a renaissance of sorts. More and more, pneumatics is being used in interesting ways that would have been unthinkable a decade or two ago. Creative applications from soft robotics to pneumatic muscles are consistently making the news, showing not only the creativity of the engineering community, but also the inherent flexibility and adaptability of this important technology.
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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. There are many styles of pneumatic actuators including diaphragm cylinders, rodless cylinders, telescoping cylinders and through-rod cylinders.
CYLINDERS The most popular style of
IMAGE COURTESY OF PENINSULAR CYLINDER CO.
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.
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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
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less than the resultant force, the piston and rod will extend and move the machine element. Reversing the directional control valve will provide compressed air to the retract port, allowing exhaust to escape the extend port, and the cylinder will return back to its home position. Pneumatic actuators are at the working end of a fluid power system. Upstream of these units, which produce the visible work of moving a load, are compressors, filters, pressure regulators, lubricators, on-off control valves and flow controls. Connecting all of these components together is a network of piping or tubing (either rigid or flexible) and fittings. Pressure and flow requirements of the actuators in a system must be taken into account when selecting these upstream system components to ensure desired performance. Undersized upstream components can cause a pneumatic actuator to perform poorly, or even make it unable to move its load at all.
WHY DO AIR CYLINDERS LEAK? While a leaking pneumatic cylinder does not represent the environmental and safety catastrophe that a leaking hydraulic cylinder does, it’s still a serious situation that you should pay attention to. In
PNEUMATIC ACTUATORS
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 nonrepairable type, and if this is what you are dealing with, the entire cylinder will have to be replaced.
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COMPACT CYLINDERS Engineers are sometimes confronted with situations where space and weight are limited, but high force is still a requirement. In these types of circumstances, compact cylinders may be best suited in for the job. Common applications include medical devices, robotics, packaging, and semiconductor, among others. These lowprofile components have been shortened relative to standard pneumatic cylinders. They may be up to 50-60% shorter than the normal cylinder, but still maintain the capacity to exert the same force as their larger counterparts. Important parameters for the proper selection of a compact cylinder can be broken up into general, dimensional, performance, material and features.
Originally called the “Pancake cylinder,” these miniature cylinders were first invented in 1958 by Al Schmidt, to fill a need for force in a tight, enclosed space. The basic intent was to get the most stroke in a short overall length using common machined parts and seals. Over the years, this design has been further developed, with many additional features and options to satisfy a variety of customer applications. This round body cylinder has a smooth, clean outside diameter for ease of machinery cleaning. Even though initially used for strokes less than 1 in., manufacturing methods have allowed increased strokes to as much as 4 in. Non-metallic rod bushings and piston bearings can accommodate extreme or unforeseen loads for long-term durability. Other compact or mini cylinders vary quite a bit. They can be rectangular or square shaped, offer numerous mounting features
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and can be placed with adjacent cylinders at a close center-to-center dimension. Piston bearings, materials, hard anodized bore and chrome plated rods can enhance cylinder capability for unexpected side loads and long-term durability. Up to 6-in. strokes can be accomplished with extruded body material. Other features may include metric dimensions, extruded sensor mounting and non-rotating styles. They are available in single-acting and double-acting versions.
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COMPACT CYLINDER REDUCES DOWNTIME, MAINTENANCE FOR BUILDING MATERIALS MANUFACTURER A NATIONAL BUILDING materials manufacturing company was needing to replace cylinders on a shingle palletizing machine three times per year. To make sure the shingles are straight and lined up, the side-pusher cylinders are activated as a set of shingles is added to a pallet. These cylinders had a male thread rod end that would corrode the bolt and break, making the cylinder economically unrepairable. Also, while the load on the cylinders was within specifications, high
temperatures and contaminants attributed to only a 4-month life span. The customer turned to Motion Industries, Tampa, Fla., for a solution to help prevent unplanned downtime and expensive maintenance costs. Motion Industries and SMC reviewed the application and recommended switching to the MGPM50TN-50AZ compact guide cylinder, which has a shorter rod and thinner plate, resulting in a weight reduction. Designed for high side-load applications found in material handling, lifting and stopping, the cylinder utilizes an ultra-compact design by incorporating the IMAGE COURTESY OF SMC cylinder body as part of the guide body. As the stroke length increases, so does the bearing length, thus enhancing the cylinders’ load capacity. With a female threaded rod end, there is no breakage. The customer has three cylinders which have been in service for over a year without any issues, saving the customer expensive downtime and maintenance. Not only has the MGPM50TN-50AZ outperformed the original cylinder by three times, it also is a more economical product replacement part, saving over $475 per cylinder for a total of $4,288.50 per year. Motion Industries | motionindustries.com SMC Corp. | smcusa.com
PNEUMATIC RODLESS & CYLINDER SLIDES When an application calls for power and linear motion while also supporting side loads, pneumatic rodless and cylinder slides (also known as guided cylinders) are up to the task. Unlike standard pneumatic cylinders, which are unable to hold the position of the piston rod, pneumatic slides can stabilize and hold a load because there is no rotating rod to cause side loads. Because they feature a non-rotating platform to mount other actuators and tooling, they are ideal for automation applications where there is repeated pick-and-place of parts.
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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. Be aware of several considerations when selecting the best type of pneumatic cylinder slides. These include: • •
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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 rodstyle pneumatic cylinder, which is attached to the body of the slide, or may be integral to the slide. In either case, the cylinder piston
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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. 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
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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.
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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 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 pressure. They can further be 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 rotors remesh on the opposite side of the air end. Thus, the air is compressed and moved to the discharge port. For lubricated compressors, cooling fluid is injected into the housing, which mixes with the air to seal, lubricate and remove the heat generated by compression. This fluid forms a thin film between the rotors that virtually eliminates metal-tometal contact and wear. The fluid is separated from the compressed air, cooled, filtered and returned to the injection point. The compressed air passes through an after-cooler and water separator to reduce its temperature and water content so it is ready for the air treatment equipment. R TE SY IM AG E C O U
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AIR COMPRESSORS 6 COM P R E S S E D A I R PR O BL E M S I G NS IS YOUR COMPRESSED air system in trouble — and if it was, how would you know? Fortunately, there are some common signs that will give you some hints as to the health of your system. Here are six things to watch for. 1. Low Pressure. Having adequate pressure is most important in an air system. If there is a constant low pressure, or periods of time where the pressure falls — only to recover again a short time later, these are bad signs. Have your service professional check on the compressor settings to ensure they are set and coordinated properly. If this does not fix the issue, then long-term monitoring with data loggers may tell you the cause. Often times, pressure differentials in filters or piping can cause pressure issues, or the lack of compressor capacity. 2. Hot temperatures. Is the ambient temperature in your compressor room noticeably hot? Is the discharge air pipe of the compressor too hot to touch? These are signs that there is a cooling issue that needs attention. Blocked filters, clogged coolers or even improper ventilation design can all be causes. High temperatures prematurely age compressor components and lubricants — and they overwhelm air drying systems. If you have temperature problems, you should get them fixed right away.
3.High or low compressor amps. Have a qualified person check the amps of your compressors. If the input amps when fully loaded is significantly higher or lower than the nameplate rating of the main compressor motor, this is a sign of a problem. Too high will burn out the motor and may be a sign of high internal pressures. Too low shows the compressor may not be putting out full capacity, a sign of inefficiency or internal problems. 4. Long unloaded run times. Does one (or several) of your compressors spend many hours running unloaded? Check the compressor hour meters to find out. Long unloaded run times are a sign of poor compressor sizing (too big) or improper coordination. Sometimes this problem can be solved by simply placing the compressor in “auto mode” (a push of a button) which may save thousands of dollars per year. 5. Water or oil in your pipes. Do you have free water or oil in your compressed air pipes? This contamination will work its way into your expensive compressed air equipment and tools — or worse, it will ruin your finished products. Failed filters or drains, temperature problems, overloaded dryers or internal compressor problems can all cause these problems. Have a qualified service person look at the issue and repair it. 6. Many audible leaks. Take a walk through your plant during a period of low production. Can you hear audible leakage? Typically, between 15 to 30% of the compressed air made by your compressors will be wasted due to leakage. If you have no leak management program, this number could be above 50%. And in worst case scenarios, more than 80% of the compressed air will be wasted. Save yourself some energy costs by declaring a ban on air leaks — and taking action to fix all the ones you can hear! FIGURE 1. THE DIFFERENTIAL GAUGE ON THIS FILTER INDICATES A PRESSURE LOSS PROBLEM. PHOTO BY RON MARSHALL.
Cooling takes place inside the compressor package, so the rotary compressor is a continuous duty, air-cooled or water-cooled compressor package. These compact designs provide smooth, pulse-free air output and high output volume. They are also easy to maintain and operate. Oil-free rotary screw air compressors use specially designed air ends to compress air without oil in the compression chamber, yielding true oil-free air. Oil-free compressors are typically two-stage units and are more costly than lubricated types. Oil-free rotary
screw air compressors are available as aircooled and water-cooled, with both load/ unload and variable speed control options. They also offer the same flexibility as oil-flooded rotaries when oil-free air is required. Reciprocating air compressors use a piston within a cylinder as the compressing and displacing element. Single-stage and two-stage reciprocating styles are commercially available. Single-stage compressors are generally used for pressures in the range of 70 to 100 psig and twostage compressors are generally used for higher www.fluidpowerworld.com
pressures in the range of 100 to 250 psig. These types of units are most often used for smaller systems. Typically, these compressors are not rated for continuous duty due to limited cooling methods and should be operated at duty cycles of 60% of full capacity or lower, or equipment damage may result. The reciprocating air compressor is singleacting when the compressing is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double-acting. 7 • 2020
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Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations and provide a wide range of pressure and capacity selections.
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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 oil-lubricated 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.
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AIR
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,
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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 singleacting, 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
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SPRINGS
F SY O I M AG E C O U R T E
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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.
FRLS
FRLS
IMAGE COURTESY OF EMERSON
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. A lubricator adds controlled quantities of oil or other lubricant into a compressed air system to reduce the friction of moving components. Most air tools, cylinders, valves, air motors and other air-driven equipment require lubrication to extend their useful life. The use of an air line lubricator solves the problems of too much or too little lubrication that arise with conventional lubrication methods, such as either grease gun or direct oil application. Once the lubricator is adjusted, an accurately metered quantity of atomized lubricant is supplied to the air operated equipment, and the only maintenance required is a periodic refill of the lubricator reservoir. Adding lubrication to a system also “washes away” compressor oils that travel through the system in vapor form. Mineral oils added to the system prevent synthetic compressor oil build-up on system components. When lubricators are not used in a system, a coalescing filter should be installed to remove compressor oil aerosols.
COMPRESSORS
generate pressurized air,
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.
IMAGE COURTESY OF CLIPPARD
but that exiting air typically contains dirt and water. Before it can travel downstream to valves and actuators, it must be filtered, regulated and
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WH AT ARE SI ZI N G C O N S I D E R AT I O N S FO R AN FR L? CAREFUL CONSIDERATION to the sizing of the components feeding an end use is important. If not done correctly, the performance of the device can be negatively affected. Too often, low-pressure problems will occur because a plant has selected a filter, regulator, lubricator (FRL) combination that is a standard size for all devices in the facility. This saves inventory costs because all the spare parts are the same. However, depending on the characteristics of the demands, there could be performance issues caused by excessive pressure drop across undersized components. If the compressed air device has a “flow static” characteristic, then the pressure only needs to be held above the minimum required level at the end of an operation. For example, a cylinder is moved from one position to another to hold a piece in a clamp. In this case the pressure needs to be adequate only when the clamping action takes place, with minimal flow, not while the cylinder is moving positions. A “flow static” application is something that requires the minimum pressure to be maintained at the point of use at the same time as peak flow. An example of this might be a cylinder with a constant load that might need to stroke from one position to another in one half a second. For the flow static application, smaller components can be
Standard Parts. Winco.
Pressure Equalization Plugs
used — it doesn’t matter much what pressure drop occurs in the filter regulator, as long as the proper pressure occurs at the end of the stroke to apply the required force. For the flow dynamic application, careful design must take place to ensure the combined total of all the pressure drops across the filters, regulator, lubricator, supply hoses and connectors does not allow the pressure to fall below the minimum required pressure at the point of use. One common mistake is to assume an incorrect average flow per minute and wrongly size the components too small. Consider a flow dynamic load that strokes a cylinder with a volume of 0.1 ft3 at a pressure of 60 psi, 4x per minute at 0.5 sec per stroke. This load would consume 0.5 ft3 of free air per stroke. Operating the cylinder one way, 4x per min would consume about 2 ft3 of air in one minute. If an FRL were sized for this flow, an excessive pressure drop would occur. It must be sized for the dynamic flow. The dynamic flow of the cylinder would be the flow rate in the 0.5 sec operating time. Since the 0.5 ft3 of free air flows in one half a second the dynamic flow during the stroke would be 1 ft3 per second (0.5/0.5 sec) or 60 cfm. Thus, the flow requirement would be much higher, requiring larger components. Like all compressed air components, each element of the FRL will have a pressure loss characteristic. You first need to know the characteristic of the end used (flow static or flow dynamic), the flow, and the minimum required pressure. Then you must know the minimum input pressure from your compressed air system. Then it is an exercise in mathematics in selecting the components that will result in proper end use pressure. Each component will have a pressure loss curve you can consult to find the pressure loss at your stated flow. For example, at an inlet pressure of 100 psi on an end use we find the following results from our research:
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www.jwwinco.com Inch and metric sizes available — Explore our full product range online
Item
1/4-in.
3/8-in.
1/2-in.
Supply pipe - 10 ft
6
2
0.05
Filter
6
2
0.05
Regulator droop
3
2
2
Lubricator
2
1
0.02
Connectors
6
2
0.05
Total Dp
23
9
2.17
Remaining pressure
77
91
97.83
Then an adequate component size would be the 3⁄8in. size.
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SHUTTLE VALVES
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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 spiral-wound 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
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3:1 or 4:1) to determine the maximum working pressure. Keep in mind that published burst-pressure ratings are only for manufacturing test purposes, and in no way indicate that a product can safely handle pressure spikes or otherwise operate above maximum working pressure. Also note that some products handle vacuum to approximately 28 in.-Hg without collapse. Thermoplastic tubing is made from several common materials. Typical tubing materials used in pneumatic applications include: •
• •
•
•
Polyurethane tubing is strong, flexible, kink and abrasion resistant, and it withstands contact with fuels and oils. It’s commonly used in pneumatic actuation and logic systems, robotics and vacuum equipment, and 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 flexuralfatigue 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
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. 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.
P N E U M AT I C F ITTIN GS A N D C O U PL IN GS WHEN LOOKING FOR connectors for your pneumatic systems, it is important to understand what type of connection is being made. Fittings can be used to connect pneumatic tube, pipe or hose and each connection requires specific fittings. Pipe fittings, push-in fittings, barb adapters, compression fittings, and quick-connect fittings are some of the most common types available. They can be manufactured in plastic, brass, steel, stainless steel and other materials. It is critical the fitting and tubing or hose are compatible with each other. Other important considerations for fittings include pressureholding capacity, ease of installation, size and weight, corrosion resistance and, of course, cost. With hose barb terminations, the tubing is pushed over a barb slightly larger than the tube. Compression fittings use a ferrule. This ferrule is slid over the tube to make the connection and then is secured by compressing the assembly together. In push-to-connect fittings, the tubing is inserted into the fitting end. Push-to-connect are often quick-connect fittings, which offer easy connections and safe, dry-break release. They are available in plastic and all-metal, stainless steel designs for pneumatic applications.
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VACUUM
COMPONENTS VACUUM
is pressure that is lower than atmospheric — 14.7 psia at sea level. In a vacuum system, the difference between atmospheric and vacuum pressure creates the ability to lift, hold, move and generally perform work. There are two types of vacuum applications: closed, or non-porous; and open, or porous. In a closed system, removing air progressively decreases the air density within the sealed, confined space and creates a vacuum. In an open system, a vacuum unit must remove more gas molecules than are able to leak back into the system. Vacuum is typically divided into three areas of application, depending on the level of vacuum required. Low-level vacuum applications are typically those requiring high flows and low force. These systems are primarily serviced by blowers. Screen printing on cloth is one application that falls into this range. The majority of industrial vacuum falls within the range of 6 to 29.5 in.-Hg. Application examples include pick-and-place and thermoforming. Scientific or process applications encompass the deepest levels — approaching a near-perfect 29.92 in.-Hg. Flow in this range is minimal. Examples of applications are ion implantation and space simulation. The vacuum generators that evacuate air and create the required low pressure come in an extensive array of types, sizes, designs and efficiencies to suit widely ranging applications. Two basic types are electric-motor-driven vacuum pumps and vacuum ejectors. Vacuum pumps. Mechanical vacuum pumps generally fall into one of two different types: positive-displacement and dynamic/ kinetic. Displacement vacuum pumps essentially operate as compressors with the 76
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G IMA
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 www.fluidpowerworld.com
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materials, such as clamping cardboard boxes. Among the advantages, typical positivedisplacement 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. 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
VACUUM COMPONENTS
gap between the jet-nozzle exit and the entry to the receiver nozzle. Here, expanded compressed air from the jet nozzle creates a suction effect at the gap which, in turn, creates a vacuum at the output vacuum port. Vacuum ejectors come in two basic versions, single and multi-stage. A single-stage ejector includes a jet nozzle and one receiver nozzle; a multi-stage ejector has a jet nozzle and several nozzle stages, each of which has a larger diameter in proportion to the falling air pressure. Air 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. IMAGE COURTESY OF PIAB
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KE Y U S E S O F VAC U U M C U P S VACUUM CUPS, or suction cups, are often used as grippers in manual or automated handling applications. They can secure and help move a wide range of products — everything from bottles and bags to bricks and wooden boards, and sheet metal, pipes and glass windows. In essence, they’re the interface between a vacuum system and the workpiece. Typical vacuum handling systems are a mainstay in many industries, including packaging, food, beverage, woodworking, metalworking, automotive, semiconductor and electronics. Vacuum cups hold several advantages in such applications, including the fact that they are relatively simple, compact, light, inexpensive and require little maintenance. They are capable of firmly gripping parts in high-speed motion applications, as well as providing gentle handling of fragile parts. Here are some basics on how they work. Technically, a suction cup does not attach itself and grip the surface of a product. Instead, when a suction cup contacts the workpiece surface, it activates a vacuum generator (such as a vacuum ejector, blower or pump) and draws out air from the cup interior and creates a vacuum. Given that air pressure inside is then lower than that outside of the cup, atmospheric pressure holds the workpiece against the cup. The greater the difference between ambient pressure and vacuum pressure
inside the cup, or the larger the effective area of the cup acting on the workpiece, the greater the holding force pressing the cup onto the workpiece. Ideally, a suction cup should mate against a smooth, nonporous surface. Then, when generating vacuum, the cup rim completely seals against atmospheric air and the interior air is quickly evacuated, resulting in a firm grip on the workpiece. However, non-ideal conditions are many times the norm because materials are often permeable, rough or uneven. In these cases, the cups cannot completely seal and outside air constantly enters the system. That’s termed a leaking system. Designers must compensate for leaking systems by using high-flow vacuum generators or using smaller cups to reduce the potential for leaks. Types of suction cups range from simple, circular types to those designed for special applications like handling candy, greasy sheet-metal panels, or porous wood and cardboard. They come in two general shapes, flat and bellows. Flat suction cups are suited for handling workpieces with flat or slightly curved surfaces, such as metal and glass plates, plastic sheets and wooden boards. Flat cups have a small inner volume and, thus, evacuate quickly and can grip in a very short time. Properly designed, they have good stability to handle high shear forces and can withstand forces and accelerations from fast automated-handling movements.
IMAGE COURTESY OF SMC
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IMAGE COURTESY OF SMC
Bellows suction cups, on the other hand, have one or more accordion-like convolutions. This lets them compensate for varying workpiece heights and handle parts with uneven surfaces. Evacuating the bellows also creates a lifting action which can be useful to lightly grip fragile parts, like electronic parts or even chocolate candy. Bellows versions are typically used for handling curved parts like car body panels, pipes and tubes, injected molded plastic parts, and nonrigid packaged goods or shrink-wrapped products. Both types come in a number of shapes, including round and oval. Various sizes make them suited for handling products weighing from a fraction of an ounce to several pounds. And they come in many different rubber and elastomer materials to suit specific application requirements, from FDA compliant cups for handling food, abrasiveresistant materials for moving bricks to oilresistant types in metalworking operations.
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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 Pressure relief valves will control pressure at their inlet port by exhausting pressure to atmosphere. Relief valves are typically used only in receivers or air storage devices, such as accumulators, as a means to prevent excessive pressurization. As such, relief valves are often called safety valves and are not typically appropriate for use anywhere but the air preparation stage. Pressure regulators in pneumatic systems limit pressure downstream of the unit by blocking pressure upstream at the inlet. Regulators are used in the air preparation stage, as well as in control of cylinders and motors. The letter R in the acronym FRL stands for regulator, which is installed downstream of the receiver tank, but before the circuit they are regulating pressure for. Sometimes multiple stages of pressure reduction are required, especially with a large centralized compressor and receiver feeding various workstations. A regulator can control pressure within the main grid of distribution plumbing, but sometimes air is piped directly to an FRL at each workstation or machine. Pressure at this main header could be 120 psi or more, but a branch circuit could be regulated at 90 psi, for example. Most regulators
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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 adjustment to
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PNEUMATIC VALVES
IMAGE COURTESY OF FESTO
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.
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PNEUMATIC VALVES FRLS
DIRECTIONAL CONTROL VALVES Pneumatic directional valves are available in many sizes, styles and configurations. At the basic end of the spectrum is the simple check valve, which allows free flow in one direction and prevents flow in the reverse direction. These can be installed anywhere from right after the receiver to within a flow control valve itself. As directional valves grow in complexity, they are specified under a general naming practice related to the number of positional envelopes of the valve and the number of work ports in the valve, and specifically in the order described. For example, if it has five ports, port 1 will be for pressure inlet, ports 2 and 4 for work ports, and 3 and 5 for the exhaust ports. A valve with three positions will have a neutral condition, extend condition and retract condition. Putting it all together, this describes a five-way, three-position valve, also referred to as a 5/3 valve. The common configurations seen in pneumatics are 5/3, 5/2, 4/2, 3/2 and sometimes 2/2 valves. Also part of the description of a directional valve is its method of both operation and positioning. The valve operator is the mechanism providing the force to shift the valve between its positions. The operator can be a manual lever, electric solenoid, air pilot or cam mechanism, to name a few. Some valves are a combination of these, such as a solenoid pilot valve, which is a tiny valve providing pilot energy to move the main-stage valve. Positioning of any valve is achieved by either a spring, such as with a 5/2 spring-offset valve, or with detents, as in 5/2 detented valves. A 5/2 spring-offset valve will return to its starting position when energy is removed from its operator, like de-energizing the coil, or removing pilot pressure. A 5/2 detented valve will stay in the position it was last activated to until the operator switches it again. Pneumatic valves are manufactured in various incarnations. Poppet valves are simple, using a spring to push a face of the poppet down on its seat. Construction can be metal-to-metal, 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 twoor three-way port configurations, although they can mimic four- or five-way valves when used in parallel. They offer typically high flow conductance for their size, and are generally resistant to contamination. Spool valves use a notched metal cylinder that slides within a precisely machined body,
IMAGE COURTESY OF ALADCO
drilled with three to five ports, or seven ports if the valve is pilot operated. Low-end valves consist of only a spool and body, and are prone to internal leakage. Better valves use seals in the body or spool to prevent leakage between ports. High-end spool valves are constructed with precision, often requiring fine lapping procedures during manufacturing, and with their tight tolerances, often require few seals, improving reliability and longevity. Other forms of high-end valves use a sliding block of metal or ceramic, which is efficient and extremely resistant to contamination.
MOUNTING CONSIDERATIONS Pneumatic directional valves come in both standard and 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
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gauges measure a fluid’s intensity. They ensure reliable operation and reduce the risks of pressure spikes or changes that could cause damage to the system. In addition, they prevent leaks by alerting personnel of unusual changes in system pressure.
Hydraulic pressure gauges are available to measure up to 10,000 psi, though typical hydraulic systems operate in the 3,000 to 5,000 psi range. Hydraulic gauges are often installed at or near the pump’s pressure port for indication of system pressure, but can be installed anywhere on the machine where pressure needs to be monitored — especially if sub-circuits operate at a pressure rate different from pump pressure, such as after a reducing valve. Often, pressure-reducing valves have a gauge port to tap into, allowing you to directly monitor its downstream pressure setting. Pressure gauges are now more routinely designed with hydraulic friendly pressure connections (such as SAE/Metric straight threads) to prevent system leaks. Analog gauges with custom scales are more common and digital pressure gauges with customizable firmware allow process measurement of pressure-based measurement of leaks or other parameters like torque, load, force and hardness. Pressure is measured in many locations throughout pneumatic and compressed air systems. It is measured at receiver(s), as well as every system FRL or stand-alone regulator and sometimes at pneumatic actuators. These gauges can be rated up to 300 psi. Pressure is measured in three ways—absolute, gauge and vacuum. Absolute pressure is a measure of actual pressure including ambient air, which is zero-referenced with a perfect vacuum, but can be as high as 14.7 psi at sea level. Absolute pressure readings are considered in applications interacting with ambient air, such as the compression ratio calculation for flow (cfm) requirements. Gauge pressure is zeroreferenced against ambient pressure and is used in most applications operating in, but not with, ambient air, such as in fluid power systems. Disconnected from equipment, gauge pressure will read zero. Finally vacuum “pressure” is expressed in Torr, or referenced against ambient pressure, as with “in.-Hg” (inches of mercury) units, which measures pressure below ambient. The pressure range at which a hydraulic gauge will be working is a primary selection factor for the type of material used to make the gauge. Gauges operating at higher pressures generally tend to be made of materials such as steel; when operating at lower pressures, they tend to be made of bronze. The most common gauges are Bourdon tubes and bellow gauges. Bourdon tubes take pressure and convert it into mechanical energy. This energy moves a dial in the gauge, displaying the pressure in the system. Bourdon tube gauges have different configurations such as curved, helical and spiral. The different style of tubing, the size of the tube and the material it is made out of all vary based on the pressure range. One important characteristic to note is the cross section of the tubing changes with increasing pressure. Generally, as the working pressure of
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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 gauge reliability are vibration, pulsation and pressure spikes. Therefore, it’s best to look for gauges designed specifically for hydraulic applications to reduce costly downtime. A forged brass case prevents resonant frequencies from destroying internal components; a liquid-filled case protects the gauge from vibration and extreme pressure cycles; and a restrictor prevents damage from pressure spikes. When choosing between a dry, water- or glycerinfilled gauge, it is also important to consider temperature range, needle response time required, changes in pressure and expected vibration. Gauge accessories, such as specialized restrictors, piston snubbers or diaphragm seals, may be used to help prevent premature gauge failure.
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WHEN SHOULD YOU USE LIQUID-FILLED GAUGES?
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LIQUID-FILLED GAUGES are used to damp vibrations and pulsations and minimize their effect on the gauge dial pointer. They are used primarily in dynamic and rugged applications where sudden shocks or pressure spikes might occur. They help to ensure the gauge maintains accurate readings for its rated lifecycle. This longer lifecycle means longerterm cost savings, as gauges do not fail and need to be replaced as often. Because they are already filled with fluid and sealed, liquid-filled gauges are not impacted by condensation, thus cannot be obscured by moisture and ambient air ingress, as can happen with dry gauges. Most liquid-filled gauges use glycerin for its high viscosity to damp the pulsations, though in more extreme environments, silicone or mineral oil may be used to withstand temperature extremes. This liquid also serves to protect the internal components of the gauge, preventing friction and wear by adding a layer of lubrication. This in turn reduces corrosion by serving as a barrier to other contaminants that may come in contact with the gauge.
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MINIATURE
FLUID POWER CONTROLS
MICROHYDRAULICS Fluid power systems are noted for their high power density — permitting high force and torque output from relatively small components when compared to electromechanical systems. Microhydraulics makes it feasible to obtain a significant amount of force from a minimal power source within a very restricted space envelope. Thus, it can provide a straightforward solution to problems that are often beyond the limits of traditional mechanical options. In many cases, these systems are ideally suited for wide-ranging applications like medical orthotic and prosthetic equipment, human-assist lifts, exoskeletons, hand tools, rescue robots, aircraft and missiles, race cars and oceanographic instrumentation. Engineers might be tempted to simply downsize typical commercial components when the need arises to control motion and force in very small powered systems. However, the reality is a bit more complex because scaling laws are not intuitive, according to researchers Jicheng Xia and William Durfee of the University of Minnesota. For example, they note that in a cylinder, force is proportional to area (L2) while weight is proportional to volume (L3). On the other hand, the thickness and weight of a cylinder wall required to contain a fixed pressure goes down with bore size. Thus, the final weight of a hydraulic system at small scale cannot be determined by proportionally scaling a large system. Also, the fundamentals associated with pressure-driven flow dictates that high pressures are required to permit high flow rates through micro-sized channels. In laminar-flow conditions, an order-of-magnitude decrease in the hydraulic diameter of a channel increases by two orders of magnitude the pressure difference required to maintain a constant average flow velocity. Another critical barrier for increased hydraulic power density at reasonable efficiency is the seals. Surface effects such as friction drag of seals and viscous drag of gaps become significant in small bores and that impacts overall efficiency. Too tight and friction dominates; too loose and the pressurized fluid will leak past the seal. Cost and power consumption are also important considerations. Fortunately, a number of manufacturers have designed or re-engineered hydraulic components to work on a “miniature” scale. 86
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As one example, Bieri Hydraulik, a unit of Hydac International based in Liebefeld, Switzerland, makes six standard versions of Type AKP micro-axial piston pumps designed with three or five pistons. For instance, the 5-piston Size AKP36 pump measures only 1.4 in. (36 mm) in diameter by 3.9 in. (99 mm) long. It features a displacement of 0.36 cm3/rev with 250 bar maximum pressure and 4,000 rpm max speed. The Size AKP103 measures 1.9 in. (50 mm) in diameter x 3.8 in. (98 mm) long. It has 3 pistons, displacement of 0.1 cm3/rev, 500 bar max rated pressure, and runs at speeds to 5,000 rpm. A 5-piston version offers displacement of 0.3 cm3/rev. The quiet-running units reportedly offer high volumetric efficiency even at minimum speed of 100 rpm. They are valve controlled on pressure and suction side, so are not suitable as motors. The small units are used in offshore and oil and gas applications, in metering systems, and general hydraulics systems with small displacements. Hydro Leduc, Azerailles, France offers a complete range of fixed and variable displacement micro-pumps; micro-motors (speeds from 350 to 6,500 rpm); check, pressure-relief, solenoid and pilot valves; and complete micro-power packs for operating in widely varied working environments. For example, its PB32 fixed displacement micro-pump has a body diameter of only 1.28 in. (32 mm) with displacement as small as 0.0007 in.3, maximum speed of 5,000 rpm continuous and 6,000 peak, and maximum pressure of 4,350 psi continuous and 5,075 psi peak. A slightly larger PB33 HP version has a 0.0027 in.3 displacement and a maximum continuous pressure rating of 13,050 psi and max peak of 14,500 psi (1,000 bar). The Lee Company., Westbrook, Conn., makes an array of miniature, highperformance fluid control components, including Lee Plug expansion plugs, solenoid valves, flow restrictors, safety screens, relief and check valves, and shuttle valves. The company’s
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flow controls, to cite one typical product, offer metered flow in one direction and free flow in the opposite direction. It’s also called a oneway restrictor. In the smallest size, diameter is only 0.18 in., yet nominal system pressure rating is up to 3,000 psi. Other similar-size products include poppet-style check valves that can flow one gpm at 25 psid and have a nominal system pressure rating up to 4,000 psi; and pressure relief valves that have cracking pressures from 20 to 100 psid and, in some versions handle nominal system pressures up to 5,000 psi. Some miniature check valve and restrictor models can even handle system pressures up to 8,000 psi. Likewise, safety screens as small as 0.13 in. diameter help protect orifices, relief valves, and other sensitive hydraulic components. Critical components are often relatively immune to low levels of small-size contaminants, but a single large particle can cause sudden failure — possibly with catastrophic effects. While filters maintain fluid cleanliness during operation, safety screens provide an added level of
protection. The units come with hole sizes from 0.0008 in. to 0.062 in., and high-pressure versions won’t burst or collapse at pressures of 7,500 psid, even if fully clogged. IMAGE COURTESY OF THE LEE COMPANY SFC Koenig, North Haven, Conn., Takako Industries, a member of the KYB offers a range of plugs, flow controls, Group based in Kyoto, Japan, claims to make the check valves and related components. The world’s smallest axial-piston pump. The square Expander plugs, for instance, reportedly seal shape TFH-040 unit measures only 1.18 in. (30 drilled holes with excellent reliability. They mm) wide by 3.0 in. (77 mm) long and is rated for come in sizes from 0.093 to 0.875 in. and are a maximum working pressure of about 2,030 psi rated to 6,500 psi (450 bar) for push-type units (140 bar). Displacement is 0.4 cc/rev, input speed and 7,200 psi (500 bar) for pull versions. is to 2,000 rpm, with a flow rate of 0.8 lpm. The company’s stainless-steel Restrictor The unit is part of a family of micro-pumps units provide precise flow control in fluid which feature a hybrid drive system that systems and are available in sizes as small as combines the benefits of hydraulics with the 0.093 in. (4 mm) in expander and threaded controllability of an ac-servomotor and inverter styles, and handle pressures to 2,900 psi (200 to satisfy a broad range of specifications with bar). Orifice can be specified to achieve desired a small-volume pump. Typical applications, flow rates. Check valves, 0.216 in. (5.5 mm) according to the company, include a pump diameter, handle forward or reverse flow, have for valve controls, mold switching equipment a cracking pressure of 2 to 29 psi (0.14 to 2 bar) for forming machines, hydraulic clamps, and and maximum working pressure of 4,352 psi crimping presses. (280 bar).
MINIATURE PNEUMATICS
IMAGE COURTESY OF CLIPPARD
The area of miniature pneumatics is a specialized niche that sees a lot of use in applications such as medical/dental instruments, test equipment, analytics, pharmaceuticals, entertainment/animatronics, semiconductor, HVAC systems, aerospace, down-hole oil tools, machine tools, ink-jet printing and process control systems. 88
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Sometimes referred to as precision pneumatics, miniature pneumatics consists of a range of components that have been miniaturized for use in light- and medium-duty applications with low-to-medium pressure ranges. Size and weight constraints matter in these systems and the need for precision is high. System pressures of 20 psi are not uncommon in miniature pneumatic applications. Miniature pneumatic products encompass a range of scaled down parts, including valves, cylinders, fittings, manifolds and tubing. Specialized components,
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MINIATURE FLUID POWER CONTROLS
such as nozzles and screens, are also seen on occasion. For example, the evolution of medical equipment has expanded beyond the hospital environment and toward the home care and ambulatory environments. This has made portable battery-operated variants of traditional stationary equipment more attractive. In today’s culture, the persistent demand for reliable and innovative products compels companies to integrate new and more advanced technology into smaller packages. The medical market is the major sector for growth in miniature pneumatics right now. Industry experts expect that in the near future, this will remain the case, especially for small solenoids. A magnetically latched solenoid valve is suitable for reducing power consumption in applications where conventional, higher power valves have been used in the past. This type of design can be used
for compact, battery-powered pneumatic instruments such as portable oxygen delivery systems, environmental gas samplers and other OEM flow switching devices. Similarly, properly designed miniature solenoid valves can improve patient comfort by reducing actuation noise. A typical solenoid valve has an inherent clicking sound when energized, which is caused by the metal-to-metal contact of the moving armature and stationary IMAGE COURTESY OF AUTOMATIONDIRECT core. Quieter operational design found in some miniature pneumatics uses so-called whisper technology to greatly reduce sound levels. These valves are used for medical applications flowing gas or air, such as dialysis machines, patient monitors, ventilators/respirators and other bedside medical devices.
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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.
… must be functionally redundant … must be monitored for faults (including diminished performance faults, which may create the loss of redundancy), without depending on external machine controls or safety circuitry … must return to a safe position in the event of a loss of pressure or other such event … must be able to inhibit further operation upon detection of a fault condition until such condition is corrected … should have a dedicated, specific function-reset input and should prohibit the ability to perform a reset by simply removing or re-applying pneumatic or hydraulic power, and must not automatically reset. Providing control reliability with fluid power is not quite the same as with electrical controls, however. For instance, plain redundancy in a safety circuit requires the equivalent function of four valve elements, not just two. Two of the four valve elements handle the inlet function while the 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 90
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Several standards (including ISO 13849-1:2006, ANSI/ASSE Z244.1-2003 [R2008] and ANSI/PMMI B155.1-2011) define the control system as including not only input, sensing and interlock devices, but also output devices such as pneumatic and hydraulic valves. The function of a fluid control valve mimics that of an electrical-control relay and, therefore, is subject to the same rules for classifying safety integrity. Thus, properly specified machine safeguarding systems include provisions for pneumatic valves, including:
FLUID POWER SAFETY
position. The second abnormal condition is when a valve develops diminished performance, such as when it becomes sticky or sluggish. In such cases, the valve reaches the proper position, but slower shifting affects safe stopping distances or precise timing. The ANSI B11.19-2010 standard mandates a monitoring system that detects these conditions for critical applications and the ANSI/PMMI B155.1-2011 standard requires diminished performance monitoring if stopping time can be affected. An easy solution is to use a self-monitoring, Category-3 or-4 valve, designed to detect both conditions. The use of double valves remained relatively unheard of for many years, except in a few select industries, such as stamping presses, which first initiated control reliability requirements. Double valves provide dual internal functions (redundancy) so that an abnormal function of one side of the valve does not interfere with the overall normal operation. At the same time, the double valves sense abnormal operation on either side of the valve and then inhibit further operation until
the problem has been corrected and the valve deliberately reset. This sensing and inhibiting function is commonly referred to as monitoring. Two standard air valves, whether in parallel or in series, cannot perform the same safeguarding function as a double valve providing this critical function. By simply incorporating two standard air valves into the circuit, no provision is made to sense the abnormal operation of one side of the valve or, even more preferable, diminished performance such as slow shifting. In addition, there is no provision for inhibiting further operation of the circuit until the valve is repaired. If one valve actuates abnormally, the second one continues to function and 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
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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.
Fluid Power Technology Conference
VIRTUAL SERIES PRODUCED BY
EVERY OTHER TUESDAY BEGINNING JULY 2020
Session: July 21, 2PM EST
SPONSORED BY
JOSH COSFORD speaks on FROM COMPLEX TO SIMPLE: SIZING AND SELECTION OF FLUID POWER SEALS
Future Session: August 4, 2PM EST THOMAS WANKE, CFPE speaks on INTERPRETING PUMP AND MOTOR PERFORMANCE DATA
Future Session: August 18, 2PM EST DONNA RITSON speaks on TRENDS AND ADVANCES IN FOOD PACKAGING AND PROCESSING
Visit fluidpowertechconference.com for current information.
Sponsorship opportunities are available for future Fluid Power Technology Conference programs. For more information, contact Mike Ference, 216.386.8903, mference@wtwhmedia.com
SHOCK ABSORBERS
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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, however, users first need to determine the relevant operating www.fluidpowerworld.com
conditions, including the weight and velocity of the moving mass and how frequently the shock is loaded. For simplicity, let’s look at a linearmotion application and use Imperial units for the calculations. Determine kinetic energy in the system from:
Ek = W/(722)(V2)
where Ek = kinetic energy, lb-in.; W = weight of moving mass, lb; and V = velocity of moving mass, in./sec. This equation represents the amount of kinetic energy that the shock absorber will convert to thermal energy on each impact. Next calculate the work energy in the application, defined as the amount of energy an external device generates to move the load: Ew = Fd(S) where Ew = work or drive energy, lb-in.; Fd = drive force, lb; and S = stroke of the shock absorber, in. Note that Fd should not exceed the unit’s maximum rated propelling force. If it does, select a larger size and recalculate the work energy.
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The next step is to calculate the total energy, Et (lb-in.) per cycle, shown as: Et = Ek + Ew Again, if this exceeds the model’s energy-absorbing capacity, select a larger unit and recalculate the work energy. Otherwise, the shock’s temperature may rise beyond rated limits and critical internal components like hydraulic seals could fail. If the application uses more than one shock absorber, divide the total energy Et by the number of shocks to determine the total energy per shock. Then determine the total energy a unit must convert in one hour. That’s because even though a shock might absorb an acceptable amount of energy in a single impact, it might not be able to dissipate the generated heat if the cycle rate is too fast. Here, multiply Et by C, the total number of cycles per hour: Etc = Et(C) The device’s hourly capacity must exceed this calculated amount. If not, choose a larger absorber (and recalculate Ew if the stroke changes) or, possibly, add an external oil tank or a cooling device to help dissipate the heat. Finally, consider the shock force, Fp (lb) in the application. Shock force, in essence, is the resistive force required by the shock absorber to stop the moving load: Fp = Et/(Sη) 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.
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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 ....................... 87
Flow Ezy Filters .......................... 14
Panolin America......................... 41
Adaptall ..................................... 89
FluiDyne Fluid Power ...............IBC
Peninsular Cylinder .................... 63
Aggressive Hydraulics ................. 3
HAWE Hydraulic ........................ 45
Prince Manufacturing ............... 77
Aladaco ..................................... 81
Hunger Hydraulics ..................... 13
RAM Industries Inc............... 19, 20
Anchor Fluid Power ................... 46
HYDAC International ................. 79
Rota Engineering ....................... 26
AutomationDirect ........................ 1
Hydraulics Inc ............................ 48
RYCO Hydraulics ....................... 36
Clippard .....................................BC
Hydro Leduc N.A. Inc ................ 15
SMC Corporation of America .... 65
Coxreels..................................... 55
J.W. Winco, Inc. ......................... 72
Super Swivels............................. 30
DMIC ......................................... 27
Kuriyama .................................... 37
Texcel Rubber ............................ 29
Doering Co. ............................... 23
Main Manufacturing .................. 68
The Lee Company ..................... 73
Dura-Bar .................................... 85
Motion Industries....................... 69
Tompkins Industries Inc. .....IFC, 51
Emerson ASCO ......................... 82
MP Filtri USA ............................. 31
Veljan Hydrair ............................ 95
Festo .......................................... 58
OEM Controls ............................ 39
Yates Industrial........................... 11
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