FLUID POWER WORLD HANDBOOK 2022

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JULY 2022

2022 fluid power handbook

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Air Preparation

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Size (HxWxD)

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5.7” x 6.5” x 4.2”

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$769.00

TAP-3000

Multiple part numbers

* Panel Mount Kit sold separately † Flow (SCFM) measured at 87psi with ∆P of 14.5psi All prices are U.S. published prices. Many other part numbers are available from all vendors. AutomationDirect prices as of 04/06/2022.

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PNEUMATI CT I PS. CO M • M O BI L E H Y D RAU L ICT IPS .C O M • H OS E AS S E M B LY T IPS .C O M • S E A L INGA ND CONTA MINAT IONT IPS . COM

contents 64

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05

fluid power overview

06 07 11 14 16 19 22 25 28 30 34 36 38 41 45

hydraulics overview cylinders filters filtration systems fittings & flanges fluids hose hose couplings hydraulic power units motors pumps repair, rebuild & remanufacturing seals sensing technologies hydraulic valves

48 49 54 57 58 60 63 64

pneumatics overview pneumatic actuators air compressors air springs frls pneumatic hose & tubing vacuum components pneumatic valves

66 68 72 74

gauges miniature fluid power controls safety shock absorbers

76

ad index

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JULY 2022 • vol 9 no 4 • www.fluidpowerworld.com

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FLUID POWER OVERVIEW

always growing, always educating welcome to the 11th edition of the fluid power handbook. This year, we’ve incorporated new content on pneumatic actuators, hydraulic valves, hose and more. We've also added new technical sidebars with frequently asked questions, sizing and specification tips and more.

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 widely used components, explaining their operation, their place in the system, and how they should be specified.

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

Over the past two years, we focused our efforts on online webinars and events, such as our Virtual Fluid Power Technology Conference, and Engineering Week. But this year, we are excited to resume in-person events, as we bring our Fluid Power Technology Conference to Detroit's Macomb Community College October 11-13. There, we will bring together industry experts, technology providers, users and integrators together for three days of fluid power programming. Visit fluidpowertechconference.com for an agenda and information on the speakers.

Although the world is still reeling from pandemic-related obstacles, our industry is charging ahead.

Manufacturers have worked to prevent some of the issues caused by disruptions in the supply chain and this continues to be a year where we're seeing strong growth (NFPA members reported all-time high orders this spring). We at Fluid Power World accept the challenge to keep growing and to bring the best knowledge in the industry to our audience in as many formats as possible. Enjoy this year's read!

Mary C. Gannon, Editor-in-Chief mgannon@wtwhmedia.com @FPW_marygannon

www.fluidpowerworld.com

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hydraulic technology overview

hydraulic technology has been helping people work for thousands of years in its various forms. Early use of water to power our world saw the discovery of the watermill, using the hydrodynamic properties of water to turn a water wheel. Water-powered technology offers methods to increase our capacity to process anything from flour or paper by grinding raw materials such as wheat or wood respectively. This early hydraulic technology paved the way for our development and use of fluid power technology as we know it today.

Today’s hydraulic systems continue to permeate our daily lives, and even despite widespread electrification, fluid power technology has only increased. However, hydraulics offers an advantage not replicated with any other technology, and that advantage is power density. No other source of motivation creates as much force in such a small space as hydraulics does.

Modern hydraulics run 3,000 psi all day, with more powerful systems approaching 6,000 psi or more. With such energy compressed into relatively small spaces, hydraulics offers a level of force not possible with any other technology. Hydraulic machinery powers the construction equipment used to build roads and cities while providing the motivation to plant and cultivate most of

the farmland in the world. If you can think of a machine that moves while achieving useful work, it’s likely powered by hydraulics.

To exploit hydraulics does not require a machine to operate on tracks or use closed-loop drive systems like much of the mobile machine industry. In fact, simple machines and objects use hydraulics to perform myriad tasks for various industries. For example, the machine tool industry uses small hydraulic power units to operate the clamp or chuck on CNC mills and lathes, while any large aircraft counts on hydraulics to operate its primary flight functions.

Even the simple bottle jack used to lift your car to install new wheels takes advantage of the high power of compressed liquid.

Hydraulics and pneumatics are both subsets of fluid power, but hydraulics differs from pneumatics in that the former uses liquid to transmit force. At the same time, the latter conveys force using compressed air. Suppliers may manufacture hydraulic fluid from base stocks of mineral oil, synthetic oil or even water-glycol solutions. The most common fluid in hydraulics is anti-wear oil derived from crude, while premium blends come from completely synthetic base stocks such as polyol esters. For applications requiring fire resistance, fluids must not be capable of supporting flame; otherwise, a hose may mimic a flame flower should it spring a leak in the presence of extreme heat. Therefore, such fluids usually

have a water base, like high-water-based fluids or the aforementioned water glycol.

Naysayers may argue that hydraulics is dirty, loud, or environmentally unfriendly.

However, that lazy argument doesn’t ring true for fluid power engineers. Those claims merely indicate that the described systems are improperly designed, installed or maintained. Coming electric and electronic technologies with hydraulics compliments a machine in many ways. Efficiency increases while maintaining power density, and the sophisticated electronic controls systems pair well in the control of electric-proportional valves, especially when paired with sophisticated transducers.

Understanding the operation of and parameters for the application is critical, as is good working knowledge of sealing and contamination control technologies. Even the practice of adding a new component to your system may introduce contamination.

Contamination destroys hydraulic components more frequently than any other cause, so you must put fluid conditioning technology at the front end of your machine design. Failure and downtime will plague your machine without the appropriate filters and coolers to keep hydraulic fluid within its ideal operating condition. The bottom line is that intelligent engineering and mindful maintenance will avoid problems in the future. fpw

POSITION SENSORS FOR HYDRAULIC CYLINDERS  Absolute detection of the cylinder stroke  No piston drilling necessary  Can also be used in telescopic cylinders  High shock and vibration resistance, IP69K

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HYDRAULIC CYLINDERS

hyd ra cou ulic c rte ylin sy o der f r a wit m in h sp dus heri trie cal s bea

ring

.

hydraulic cylinders

few things represent a fluid power system the way a hydraulic cylinder does. These true workhorses operate in industrial and mobile applications. When compared with pneumatic, mechanical or electric systems, hydraulics can be simpler, more durable and also offer greater power density. For example, a hydraulic cylinder has about ten times the power density of an electric linear actuator of similar size.

Selecting the right cylinder for an application is critical to attaining maximum performance and reliability, which means taking into consideration several design and performance parameters. Fortunately, an assortment of cylinder types, mounting methods and “rules of thumb” are available to help select the appropriate cylinder.

cylinder types

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

Tie-rod cylinders can be single acting, although they are most often powered in both directions. They have machined, square caps and heads being forced together against the barrel by high-tensile steel tie rods fastened by nuts, making them easy to disassemble and repair in the field. Welded cylinders employ a steel barrel with a cap welded to the bottom and the end treatment subsequently welded to the cap. The rod and piston assembly then

has to be assembled around the head — which uses a buttress thread for strength — and is tightened into the barrel. Finally, the 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 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. www.fluidpowerworld.com

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

Side mounts, such as side lugs, need to be well aligned and the load supported and guided.

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

Pivot mounts — Absorb force on the cylinder centerline and let the cylinder change alignment in one plane. Common types include clevises, trunnion mounts and spherical bearings. Because these mounts allow a cylinder to pivot, they should be used with rod-end attachments that also pivot. Pivoting mounts are required for many applications, such as booms and buckets, but are also most prone to rod buckling, especially as the rod reaches end of stroke.

What is the maximum pressure for the application?

The hydraulic cylinder must be rated to work within the pressure limit of the hydraulic system it is installed on. An excavator, for example, can operate at 4,000 psi or more, so light-duty snap-ring cylinders rated for 2,000 psi should be avoided. Cylinders are designed with 7 • 2022

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W W W. M O B I L E H Y D RAU L I CT I P S . C O M

bearing design for cylinders

safety factors of 2:1 to 4:1, so sometimes running slightly over-limit might be acceptable, but not double.

What stroke length will be required?

bearings are mechanical components designed to bear the load from a complimentary part, such as a shaft. Any shaft in fluid power subjected to loads perpendicular to the axis of movement requires a bearing. In most cylinder examples, one-piece bushings are best suited to support loads, offering exceptional surface area for superior alignment.

In reality, a cylinder has two bearings — one on the piston rod and the other at the piston. The piston rod bearing most commonly adopts oil-impregnated bronze to support the rod for side load and misalignment. Oil-impregnated bronze is a powered metal that releases tiny oil packets as it wears, ensuring the rod never galls against the bushing.

Although a long-wearing component, bushings require eventual replacement, so cylinder construction should offer a straightforward method to extract and replace the bearing. Some cylinder manufacturers use a thread-in gland, which allows disassembly while the cylinder remains mounted, so long as the rod end is free from attachment. Other manufacturers employ a slide-in gland held fast by a retainer, which may require tie-rod removal in some cases. A rod gland is just a bearing assembly that includes one or more seal grooves or drains.

For any cylinder bearing, its surface area offers the key to its effectiveness at supporting loads, which may come as either a longer bushing or one of larger diameter. Often a longer bushing cannot fit inside the head of standard cylinders, so that a custom option may be necessary. For example, an oversized rod provides superior column strength and better support from the larger diameter bearing.

Some gland assemblies may use replaceable wear strips on the ID, making replacing the “bearing” quick and inexpensive. However, pound for pound, bronze costs more than any other raw material in a cylinder, so the cost savings for such a solution may be extensive. Inexpensive press-in bushings reduce the investment cost of a gland assembly, but assembly requires both a small press and precise reaming tool, making this option less suitable for quick repairs.

Although pistons are rarely referred to as bearings, they aid in a cylinder’s alignment and load support nearly as much as the bushing. In addition, the distance between the rod bearing and the piston adds to the overall strength of the cylinder assembly, which is why stop tubes are used in long stroke cylinders to improve column strength.

Like with bearings, both the diameter and length of the piston help improve strength, although often the diameter is fixed for any given application. The piston wear strip is a sacrificial component that supports the piston against the tube ID. Although some cylinders have used straight cast iron against the cylinder barrel, a wear strip is a superior option that improves reliability and repairability.

Wear rings, as they’re sometimes called, come manufactured from various materials to support any application. Nylon, glass-filled PTFE, or even bronze- or lead-impregnated material makes an easily replaceable and reliable option for any fluid, temperature or load requirement. If your application may require special protection from misalignment or sideload, be sure to approach your solution holistically by selecting both bearing and piston assemblies that combine for the best approach.

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

What mounting method is being used?

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

Push or pull or both?

Any cylinder can be used as single acting, which is powered in one direction only, but it can only push or pull. When a cylinder pushes, protection against rod buckling and bending must be ensured, which can be achieved through oversized rod material or with a stop tube to prevent full extension, taking advantage of the piston’s load-bearing effect. When a cylinder pulls, there is little concern for buckling, but you should ensure your force calculations factored 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.

new cylinders are used for testing threaded connections for oil pipe lines. they are rated for loads to 5.5 million lb and pressure to 5,000 psi. courtesy of hunger hydraulics

www.fluidpowerworld.com

FLUID POWER HANDBOOK Key specifications:

Operating conditions — Cylinders must meet the requirements of the design specification, such as force, maximum pressure and mounting configuration, but consideration for operating conditions must also be heeded. Cylinders must also withstand extreme temperatures, humidity and even salt water for marine hydraulic systems. Also, when ambient temperatures rise to more than 300° F, standard Buna-N nitrile rubber seals may fail and will instead require synthetic rubber seals, such as stabilizer cylinder. courtesy of ram industries

W W W. M O B I L E H Y D RAU L I CT I P S . C O M

Viton. When in doubt, err on the safe side and choose a cylinder design capable of more of than you will ask of it.

manufacturer likes to use a different style, so it is important to replace them with a similar type and material when rebuilding.

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.

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.

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

fpw

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RAM Aluminum cylinders are designed to industry-standard dimensions, making them easy to exchange with existing cylinders on your equipment. Contact our team today!

High-quality aluminum body High-quality urethane seals High-strength nylon wear bands Optional pilot-operated check valve In-line or 90-degree mounting available Base fed ports with integrated oil lines Lightweight and rust resistant internal parts Light Thread-locker used for all critical connections Hollow high-strength chrome plated steel rods Alternative finishes available

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HYDRAULIC FILTERS

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 contaminationrelated issues. Thus you, as an engineer, technician or end-user, who ignores filtration does so at the peril of your hydraulic system.

cutaway of hydraulic filter element. courtesy of ohio fabricators co.

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 that can be removed www.fluidpowerworld.com

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.

Construction design of a filter assembly varies 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 pressure, and then some for safe measure. Return line filters are generally only required to handle backpressure related to flow, which increases due to flow intensification and also to pressure differential created from the clogged element itself.

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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 hydraulic filters. tests and then publishes its filtration ratings, expressing the courtesy of stauff 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 SAE 4-bolt, JIS, DIN, ISO ratio. For example, (200-1)/200 = 0.995, or standard & special adapters 99.5% efficiency. The previous example tells Socket and Butt weld, NPTF us our hydraulic filter removes particles of Grand Blanc, MI USA BSPT, ORB, BSPP, 6149, etc.

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W W W. S E A L I N GA N D C O N TA M I N AT I O N T I P S . C O M

hydraulic filtration systems when ultimate filtration is required,

an offline system is most efficient. Offline filtration requires a dedicated pump and motor to circulate tank flow through an often very fine filter with high dirt holding capacity. Often called a “kidney loop” filter, these systems run even with the machine does not, and are not exposed to unstable operating conditions related to the primary circuit. The downside is the added expense of an additional pump, motor and filter assembly.

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You can increase filtration effectiveness with an offline filter system. 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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A filter cart with very fine micron rating will clean your hydraulic system and help remove fine particles not trapped by the machine’s permanently installed filters.

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, 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. fpw

FLUID POWER HANDBOOK

W W W. H O S E A S S E M B LY T I P S . C O M

metric fittings. courtesy of stauff

hydraulic fittings & flanges

hydraulic fittings connect conductors such as hoses, pipes and tubes to the components in a hydraulic system. They allow the pressurized fluid to move through the system without leakage. Available in a variety of styles to change and direct, change or split flow, most fittings have a male and female component that join to form a connection.

They can be manufactured as unions, plugs, crosses and elbows.

It is critical to carefully identify threads on the fitting connections, as these threads can look almost identical from one standard to another.

However, because they are not mates, they will not properly engage. In addition to reviewing documents from SAE, NFPA and ISO to help identify each standard to confirm diameter and thread type, keeping thread identification kits on hand can help with this concern.

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-toconnect; 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

a variety of hose fittings and flanges. courtesy of adaptall

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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.

HYDRAULIC FITTINGS & FLANGES

when should you use banjo hose ends?

there isn’t much in the fluid power world taking a unique spin on a familiar concept. Pumps, valves and actuators operate and look much the same as decades ago, especially if you ignore electronic advancements. Hydraulic components like the internal gear pump and electrohydraulic actuator are certainly worth your time to learn, but sometimes it’s the everyday applications that surprise you the most.

Banjo fittings bring the “why didn’t I think of that?” mind frame to hydraulic hose technology. The bolt, fitting and two washers combine to offer a hydraulic hose end with a 360° range of motion and high-pressure operation. In addition, the cross-drill in the bolt combined with the internal groove in the fittings provides a two-way flow path sealed by the washers regardless of installation direction.

Other hose connections and adaptors may offer permanent or live connections that allow full rotation, but the low-profile advantage of the banjo fitting is hard to beat. In addition, a 90° JIC female hose end allows the technician to connect the hose at any angle or even loosen the fitting to pivot the hose at any time afterward.

One example of minimum installed height compares ½ in. ORB banjo hose end to ½ in. JIC hose end. The banjo assembly comes in at 37.5 mm (about 1.5 in.), while the JIC low profile comes in at over 50 mm (about 2 in.) when you include the hose height.

It should be said that banjo fittings and hose ends find themselves primarily on European and Asian applications, so BSPP and metric are more common thread types. If you’ve done the brakes on your own car, and especially if you’ve had to remove the calipers, you may have found banjo bolts as the fitting of choice for your brake lines.

Like ORB, JIC or NPT, hose end sizes may differ from the fitting size. For example, you may find ¼ in. NPT male on the end of a ½ in. hydraulic hose.

However, that combination makes for a less common hose end. Fluid power distributors and hose shops often stock separate ferrules and stems to combine as needed to match the hose size with the fitting size. Stocking one ferrule SKU for each hose ID allows the technician to simply choose the stem with the fitting for their application. Those stems are less expensive than their one-piece counterparts.

Banjo bolts and hose ends seal at the base and the top using crush washers, usually made from copper or bonded seals as the bolt is torqued into the port, which seals each end while locking the fitting in place. However, copper crush washers flatten over time, so some technicians prefer bonded seals using a metal backing with synthetic rubber soft seal in the ID. The banjo fitting is a rare animal, especially here in North America.

However, banjo fittings are functional and versatile, so they deserve more attention from technicians and designers. I see them especially useful as workports in cylinders and motors, where their slim profile offers up a clean installation.

FLUID POWER HANDBOOK

W W W. H O S E A S S E M B LY T I P S . C O M

cad assembly of a swivel fitting. courtesy of super swivels

O-ring face seal, SAE straight thread and ISO 6149 fittings have a seal, normally Buna N, contained within a groove to seal the fluid. It is important for the seal to be compatible with the fluid and the operating temperature range. An elastomeric seal greatly reduces the possibility of leakage caused by vibration, thermal cycling and pressure cycling.

SAE J518 split flange fittings are used on larger line sizes, starting at 1⁄2 in. (-8) but coming into predominance at 2 in. (-32) and above. A flange head with an O-ring groove on its face is attached to a conductor (hose, tube or pipe) and is secured to the port, which could be a flat-face fitting or a pad on a pump, valve or cylinder, by a clamp with four bolt holes. The clamp can be whole, but is often split so that a quarter of the diameter of the flange head is on either side of the centerline of the bolt holes to help minimize torque on the clamp. The screws used are tightened to a high torque value to avoid problems with fatigue. In many cases, using pipe or tubing, the flange connections have operated within their specified working pressure for decades.

Flare fittings, such as the JIC 37˚, are fittings with a conical end face and the seal is formed when this seat is forced against a mating seat, generally by torquing a swivel nut on one fitting, engaging with a threaded portion of the mating fitting. The angle of the seat and face for most JIC

fittings in the North American market is 37°, and it is popular enough that the 24° and 45° versions are rarely used. The fittings can be designed to clamp onto a tube by means of a sleeve or ferrule, and care needs to be taken so that the correct size is used because inch and metric tubing sometimes have sizes that are close to overlapping.

The quick disconnect allows multiple reconnections of the assembly without causing excess wear or concern for thread damage. Some fittings allow disconnection and reconnection under pressure; others do not. Disconnects hold fluid pressure by way of a ball or poppet, which is spring offset to remain closed when the lines are unattached. Upon reattachment, the balls or poppets push against each other, lifting themselves from their seats and allowing fluid flow. Standard plug and socket configurations, such as the Pioneer coupling, are prone to trapping contamination, which was addressed with the advent of flat-face couplers, which have no recess to collect contamination.

Staple and band fittings are low-pressure fittings. Band fittings are attached to the hose by a barbed or beaded end being inserted into a hose and a band clamp securing the connection. This method is only for extremely low pressures. Staple fittings have a cylinder with an O-ring and a bead further up on it that slides into a socket. The connection is secured by a staple that goes through both sides of the connection behind the bead, although it is still typically used for low pressure or suction lines. fpw

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HYDRAULIC FLUIDS

common fluid types and their benefits all hydraulic fluid is designed to transmit

fluid power energy through a system, and for the most part, they do the job the same way.

But that doesn’t mean all oil is the same. In fact, there are five common types of hydraulic fluid, each with its benefits and downsides whose use depends on the application best suited for them.

Anti-wear hydraulic fluid enjoys the comfortable throne atop the hydraulic fluid kingdom. Anti-wear oil derives from petroleum oil, offering excellent lubricity, superior corrosion protection and great oxidation resistance. AW oil generally offers a good viscosity index and heat transfer as well but fairs poorly in flame-resistant applications, being quite flammable.

AW oil is the jack of all trades product that suits the majority of hydraulic systems and machinery in a wide range of temperature applications. Additives easily improve the base oil to increase performance, and AW oil is offered in a wide range of viscosity offerings.

Biodegradable hydraulic fluid gets its base stock from renewable and natural oil

sources, such as seed sources like soy or canola. These fluids benefit not only from their renewable source but their safer impact on the local environment should they spill or leak. Bio oils also offer excellent lubricity and corrosion protection, but their viscosity is more likely to change with temperature.

Granted, the overall effective temperature range of most bio-oil is very good.

Bio-oil makes its home in any hydraulic application where the possibility of contamination exists in a sensitive environment. Forestry machinery does well to be offered with bio-oil, which does not contaminate soil after a leak. Food-safe applications do well to take advantage of the fully ingestible nature of bio-oil since cross-contamination results in a less daunting cleanup effort after a spill.

Synthetic fluid offers good performance across the board in most categories; viscosity index, lubricity, oxidative stability, corrosion protection and thermal transfer capacity.

Depending on the base oil, it may also offer fair fire resistance.

With a quality additive package, a synthetic blend offers the best choice for most hydraulic fluid requirements. Expect to have excellent all-weather performance, superior lubrication and corrosion protection.

Phosphate ester was created as a synthetic form of fire-resistant fluid, used primarily in steel mills or aircraft hydraulics.

If your hose springs a leak, the last thing you need is a flame thrower should a spark intercept the jet of oil. Phosphate ester also enjoys good overall oxidative stability, corrosion protection and thermal transfer properties.

However, its viscosity tends to be more affected by temperature than other fluids.

Water glycol offers some excellent properties as a hydraulic fluid; mainly being water, it has an outstanding viscosity index, fire resistance and thermal transfer capacity.

However, as you would imagine, it’s merely okay regarding lubricity and oxidation while being the poorest of the top five with corrosion resistance.

Water glycol has replaced phosphate ester in many steel mill applications, being a much less aggressive fluid on seals, paint and personnel. However, it tends to be a high maintenance fluid, requiring frequent testing to confirm its specific gravity suits specification. Typically, the ratio is 60:40 or 65:35 glycol to water, and as you imagine, the water evaporates over time and requires replenishing. fpw

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Hydraulic Live Swivels Inline & 90°

FLUID POWER HANDBOOK

what are environmentally acceptable lubricants (eals) in hydraulics?

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environmentally acceptable lubricants, as far as the hydraulics industry is concerned, are fluids offering a reduced

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impact on the environment. EAL fluids are especially a concern for machinery operating on or around waterways, where the disastrous impact of potentially

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hazardous and toxic oil is most severe. Many environmental regulations require fluids to be EAL compliant, such as the 2013 VGP, a US regulation to prevent the discharge of toxic lubricants into US waterways.

An EAL hydraulic fluid must meet three primary criteria — they must be biodegradable, not bioaccumulative and low toxicity. In addition, each additive to the fluid must also meet those three criteria, such as viscosity improvers, antiwear chemicals, or detergents.

Biodegradability defines the capacity of a material to eventually break down into elemental components in the presence of microorganisms. If the base material is something bacteria like to eat, the material will likely break down into water, C02 and some minerals over time. Ideally, those remaining minerals would also be non-toxic.

Unfortunately, in some cases, the base material may be manufactured from chemicals not suited for environmental protection, even if bacteria still enjoy it as a meal.

Furthermore, EAL fluids are likely constituted from more pure base oils. The most popular bio-oils are made from vegetable seed oil base-stock, which is naturally biodegradable and safe. Soy or canola stock is popular and inexpensive, relatively speaking, making them the non-toxic choice for EAL Fluids. That doesn’t mean synthetic fluids aren’t an option because paraffin-based oils also offer low toxicity.

Bioaccumulative is likely a new word to fluid power designers. It refers to how some

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chemicals may accumulate in the cells of organisms, which may pass those chemicals up the food chain, where they eventually become toxic. Even if the chemical is not initially toxic, such as cadmium or lead, years of accumulation result in harmful or fatal effects on the animals at the top of food chains. Imagine plankton that absorbs lead, sardines that eat plankton, herring that eat plankton, and then tuna eat the herring while accumulating the lead initially absorbed in the plankton.

Lubrication manufacturers commonly know bioaccumulative chemicals, so those additives are avoided for EAL fluids. However, to be an EAL certified fluid doesn’t mean your hydraulic oil comes shipped in a 200-gallon drum of rainbows. EAL fluid must contain at least 90% readily biodegradable base oil, up to 5% non-biodegradable compounds that are not bioaccumulative, and the remainder inherently biodegradable. Readily biodegradable compounds must return to within 60-100% of their natural state within 28 days, while inherently biodegradable compounds must do so within 20-60% of their natural state in the same period.

Although currently a requirement to waterways, I would expect that the future of all hydraulic fluids will reside within the EAL camp. Like all bio-oils, EAL lubricants are expensive compared to conventional mineral-based oils. When crude oil is no longer extracted from the Earth one day, we will rely entirely on renewable sources for all lubricants, making EAL fluids less expensive and more readily available.

FLUID POWER HANDBOOK

W W W. H O S E A S S E M B LY T I P S . C O M

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 fluidpower 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

tensioning hose assembly, with 26,000 psi working pressure. designed for flexibility and courtesy of spir star 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 offer good flex life, superior chemical and concentration, as well as substances that may corrosion resistance and can handle high attack the hose cover. Hose selection must temperatures. Thermoplastic hydraulic hose ensure compatibility if it is to convey special offers tight minimum bend radii and excellent oils or chemicals. The same holds for hose kink resistance. Metal hoses handle high exposed to harsh environments. Substances temperature flow and often higher pressures. such as UV light, ozone, saltwater, They can be either stiff or flexible. chemicals and pollutants can Flexible hoses are easier to route and cause degradation and premature install, compared with rigid tubing and pipe. failure. For in-depth fluid They lessen vibration and noise, dampen compatibility data, consult pressure surges and permit movement the manufacturer. between parts. In addition, increasing While hydraulic hose demands for higher productivity, efficiency is usually constructed and environmental compatibility are forcing of multiple materials, hose manufacturers to improve product the most commonly integrity — hoses now withstand higher used primary materials pressures, extreme heat and cold and include elastomers, accommodate a range of fluids including fluoropolymers and silicone, today’s “green” variants. thermoplastics, metal, Most hoses are manufactured to SAE and composite or J517, European Norm (EN) or ISO Standards. laminated structures.

These standards predominate in the Americas, Elastomeric or rubber Europe and Australia, and are also used hydraulic hose are often throughout Asia. selected for their flexibility.

Fluoropolymer hose fpw

braided hose and couplings. courtesy of kurt hydraulics 22

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HYDRAULIC HOSE

electronic control for hose crimping as the rest of the industry moved towards electronic control, so too did the hose crimping machine.

Traditional dial-micrometer was hard to read and adjust, and often found itself out of calibration.

Linear position sensors replaced the limit switches, and then the adjustment option went digital. A small LCD screen shows the crimp setting, which increased accuracy and reduced the chance for error. The precision of the linear transducer all but guarantees perfect, repeatable crimps.

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Some manufacturers have produced semi-automated hose assembly stations. One such machine requires only that the operator load the parts into the machine. Operators load the stems and ferrules separately, two at a time, and then inserts the hose ends into the machine.

They start the sequence that inserts the stems into the pre-cut hose ends along with the ferrules. They unclamp the hose assembly, and if it’s long enough, simultaneously insert each

end into the automatic crimper. If the hose length isn’t long enough to span the gap and into the two openings, the ends are done individually but in parallel. The insertion and crimping functions are completed while the technician works on the opposing operation.

crimpers get smart

The industrial world continues to find new and creative ways to utilize Industry 4.0 concepts. Busy hose shops require speed and versatility, leaving little time for thumbing through catalogs looking for crimp specs.

Many top crimp manufacturers offer high-end machines with touch screen HMIs employing wireless links to the manufacturer database. A quick selection of hose, dash size and stem results in readily available crimp specs populated right into the crimper settings.

Crimping technology will continue to advance.

Augmented reality will identify hose and fittings visually, such as with QR codes, then automatically populate crimp specifications.

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HYDRAULIC HOSE COUPLINGS

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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 onehanded operation required?

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

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

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

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

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

There are three types of O-ring seal designs: O-ring boss, flat-face O-ring seal and O-ring flange. In the boss design, straight threads make the connection www.fluidpowerworld.com

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 highpressure 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, largediameter connections. A port is bored with a center outlet, surrounded by a smooth flat face, which has four tapped holes and four mounting bolts that tighten down onto flange clamps. There are no threads on this coupling.

The flange itself has the groove for the O-ring.

There are several SAE and ISO standards that cover the performance requirements of hydraulic hose assemblies.

Included are the J517, J516 and J343 standards.

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hq series flat face coupling conforms to iso 16028. courtesy of holmbury inc.

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

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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

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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. fpw

H90/HQ90 Series

HDB Series

EZFFC Series

HDSL Series

FFC Series

FLUID POWER HANDBOOK

W W W. M O B I L E H Y D RAU L I CT I P S . C O M

hydraulic power units

nearly every single hydraulic system shares one thing in common — they’re run by a hydraulic power unit. Although some units are multitaskers, like the engine on a tractor, most often they’re purpose built for the single task of converting mechanical energy into hydraulic energy. The scope of a power unit ranges from fractional horsepower electric units to monstrous constructions in the hundreds of horsepower.

The power unit exists to supply the machine with hydraulic energy in the form of pressure and flow, without which you have idle components. You must first calculate the pressure and flow required by the actuators in the systems. You may calculate this step more than once as you balance performance with economy, as very few machinery OEMs have no limits, financial or otherwise.

After you arrive at your pressure and flow requirements, you specify the pump type and size. Pump cost and complexity are vast but this dictates the level of performance you can expect to achieve with your actuators. The type of pump used correlates with the direction you must take with reservoir design, filtration and complexity of pumping. A gear pump, for example,

p ort ab c o u r l e h y d r au tesy of dalic p ower kota fluid unit. p owe r

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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 rest of the power unit can be built around the pump. You must choose the size of your reservoir. Although opinions vary, you can’t go wrong with sizing it as large as possible. Limitations will exist for cost and footprint, but on average, expect to need at least three times pump flow at minimum, to ideally five times if it can be achieved. Every multiple of pump flow provides a precious extra minute of fluid dwell time. Reservoir size is critical for many reasons.

A large volume of hydraulic fluid relative to pump size gives time for fluid to cool before being drawn back into the circuit where heat soaks in once again. Large tank volume means large tank surface area, and in addition to the first point, this large surface provides a radiation layer to improve cooling. Additionally, with more fluid, particles settle more effectively than if they immediately re-enter the circuit, as with smaller tanks. Opposite to the settling of particles, air bubbles are given more time to rise, reducing the potential for cavitation-related damage from aeration.

After you calculate tank volume, you must now consider the reservoir construction type. Reservoir style plays an important role in ensuring the pump inlet conditions are ideal, preventing conditions favorable to cavitation. Economics are also primary here, ranging from the vertical type at the low end, to the L-shaped at the upper end.

The former is compact but difficult to service, while the latter is highly serviceable but large and expensive.

Highly complex hydraulic systems consist of many components — some related to the function of the circuits, like manifolds, directional valves and pressure valves — and other components required for fluid conditioning and monitoring. Filters, heat exchangers and pressure gauges are components added to ensure safe and reliable power unit operation. Because of ease and convenience, as many components as possible should mount to the reservoir. As such, these components command much real estate, and reservoirs are oversized to accommodate.

Use your hydraulic schematic as the first step in the power unit creation process. A hand-drawn schematic helps you choose actuators and major components, but a detailed drawing helps with the visualization of component layout. A circuit drawing should be modified to include every component that will exist on the power unit, not only for the assembly technician to understand how to install and plumb all the components, but for future troubleshooting and repair.

Experienced hydraulic designers know what a power unit needs, but seeing the circuit helps spot gaps where less obvious components should be drawn, and then subsequently added to the bill of materials.

Test points, ball valves, bellhousings, drive couplers etc., are all

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HPUS

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important and should be included. Once a schematic is complete, a bill of materials (BOM) can be created from it.

The most important set of components is the pump/motor assembly. It includes the chosen pump, a motor of adequate power capacity, a pump-motor mount and drive coupler set. The pump/motor mount — often called the bellhousing — rigidly fixes the C-Face electric motor to the pump and provides a gap to install the couplers. A coupler slides on the motor shaft, its mate slides on the pump shaft and then a synthetic rubber insert is placed between them before the couplers are pushed together and fixed in place with set screws. Be sure to select a coupler set rated for the required horsepower and pressure spike potential.

Once together, the pump/motor assembly is mounted to the reservoir, preferably with isolation mounts. These mounts are either welded or bolted to the reservoir and consist of two metal plates galvanized onto either side of a chunk of rubber. This isolator prevents excessive pump/motor vibration from resonating through the steel plates of the reservoir; an important requirement because hydraulic power units are already prone to nasty harmonic noise pollution.

In succession, the remaining “fixed” components are mounted to the reservoir.

Accumulators mount off the side, valve banks to the top, filtration inside or on top, level/ temperature indicator located to the side, ball valves (very important!) hard plumbed to the suction port, and any other component either welded or bolted to the tank are now installed.

With all fixed components now complete, you can fabricate the power unit’s plumbing.

Most designers prefer tube but it is timelier to fabricate. It is semi-permanent and is more reliable in the long run. Hose can be used for plumbing as well, although hose invariably fails.

Hose has its place, however, especially if noise and vibration is a concern. Tube transmits vibrations more readily, but hose can often dampen it. Most frequently, you plumb a power unit using a combination of hose and tube.

At the end of fabrication, the unpainted components are removed, and the power unit is cleaned and prepped for paint. Epoxy paint is best because it resists oil and provides a durable finish. When the paint dries, the technician re-assembles the unit, and the final stages of power unit fabrication begin.

The electrician now wires the electrical components, and this includes adding any electrical enclosures, transducers, switches, etc. Wiring of the motor itself will often occur on sight at commissioning, but the electrician wires the valves and control boxes when the power unit is manufactured, to allow for testing. Once electrical is complete, the technician fills the tank with oil and jogs the motor from the electrical panel to ensure the motor is turning the correct direction. The technician turns on the pump, checks for leaks and tests the HPU at pressure. The final step, after successful testing, includes draining the tank, installing on a skid and wrapping up for shipping. fpw

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FLUID POWER HANDBOOK

hydraulic motors

W W W. M O B I L E H Y D RAU L I CT I P S . C O M

motor designs

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, they require no electricity and can withstand dusty and dirty environments, extreme heat, and even submersion. Hydraulic motors have exceptional power-to-weight ratios. In terms of power capacity, an electric motor can weigh 20 times more than an equivalentrated hydraulic motor.

Some hydraulic motors offer high speed capabilities, such as in fan drives. Others, for instance winches, move heavy loads at low speeds. They are used in industrial applications such as augers, conveyors, mixers, and rolling mills, thanks to their robust nature and resistance to heat.

They are also often the primary drive in off-highway equipment. Hydrostatic drive systems transmit engine power to the drive wheels with exceptional versatility and reliability. Hydraulic wheel motors’ speed control and 30

FLUID POWER WORLD

500-series motor adapts the geroler gear set design with high speed distribution of flow and high pressure. courtesy of fluidyne

smooth reversibility are perfect for backhoes, skid-steers and wheeled loaders. The high power density of hydraulic motors let them achieve substantial torque in a relatively small package for use in tracked vehicles such as excavators and bulldozers.

Hydraulic motors are rated according to parameters such as torque capacity, speed range, pressure limitations, efficiency, and displacement. Displacement is the amount of fluid needed to turn the output shaft one revolution; it is rated in terms of cc/rev or cu.in./rev.

The units can be fixed- or variable-displacement and operate bidirectionally or unidirectionally.

With input flow and operating pressure constant, fixeddisplacement 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.

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Motor types include gear, vane and piston units. They are usually similar in construction to the analogous hydraulic pumps.

Gear motors are probably the most popular designs, and they come in several versions.

External gear motors feature a matched pair of spur or helical gears enclosed in a housing. One is the driven gear — which is attached to the output shaft — and the other an idler gear. Their function is simple: high-pressure oil is ported into one side of the meshing gears and forces them to rotate. Oil flows around the gears and housing to the outlet port. It is a constant-displacement motor.

A second type of gear motor is the gerotor, or internal-gear, motor. The internal gear has one less tooth than the outer gear, and it rotates and seals against the outer component to minimize bypass leakage. The inner gear connects to the output shaft, and speeds and power density of the unit can be quite high.

Another variation is the roller-gerotor motor, where rollers replace the lobes of the outer gear to minimize friction. They tend to provide smooth, lowspeed 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.

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Vane motors operate in the medium-pressure and cost range.

Torque develops by pressure acting on exposed surfaces of vanes that slide in and out of slots in the rotor, which connects to the output shaft.

As the rotor turns, vanes follow the surface of a cam ring and carry fluid from inlet to outlet. Vane motors are fixed-displacement types.

Piston motors are also available in a variety of styles, including radial, axial and other less common designs. Radialpiston motors feature pistons arranged perpendicularly to the crankshaft ’s axis in barrels that radiate out from the drive shaft.

Fluid pressure moves the pistons linearly and causes the crankshaft to rotate. This reciprocating action against a lobed cam ring can produce extremely high torques with very low to moderate speeds.

Axial-piston designs feature a number of pistons arranged in a circular pattern inside a housing (cylinder block, rotor or barrel).

This housing rotates about its axis by a shaft that is aligned with the pumping pistons.

There are two designs of axial-piston motors. The first is the swashplate design where the pistons and drive shaft are parallel. The second is the bentaxis 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

FLUID POWER HANDBOOK

W W W. M O B I L E H Y D RAU L I CT I P S . C O M

what type of flow divider is best for my application? flow dividers provide a unique solution to a specific problem in hydraulic applications — splitting

flow while limiting its tendency to take the path of least resistance. Without a flow divider, when you wish to split flow to two or more downstream subcircuits, the path with the lowest pressure will steal most (if not all) the flow. Common designs include spool or gear type that input a single flow path and output two or more at a given ratio. Others include radial piston and volumetric types.

Spool-type flow dividers use a Y-shaped ported body and a clever method of pushing the spool toward the outlet port with the lowest pressure. This form of pressure compensation will choke the flow on the side, attempting to take the path of least resistance, thereby equalizing the pressure drop through each of the outlet ports. However, the method is not entirely accurate, allowing up to 15% or more of the flow to take the low-pressure path.

Spool flow dividers are particular in their operation, requiring they are specified for their ideal pressure and flow range. Should you operate with flow less than their specified minimum, they become unstable and imprecise. Conversely, with too much flow or too much differential pressure, the flow divider may lock up one flow path only.

Because flow dividers do not offer absolute accuracy, you can expect hydraulic cylinders to drift out of synchronization, primarily when they’re not provided with the opportunity to reach the end of stroke. Spool flow dividers may use a pressure compensated orifice to divert flow from a stalled cylinder to the active one, allowing for synchronization. Should your flow divider come with no inherent synchronization capability, cross port relief valves installed on the two work ports offers the same effect.

Flow dividers aren’t limited to 50:50 flow division, and with spool-type, you may find ratios such as 60:40 or 33:66. Custom ratios would be available, but the most common is still 50:50.

Motors benefit from unequal flow split, allowing two motors of the same displacement to rotate at the different rpm, or perhaps two motors of different displacement to rotate at the same speed.

Gear type flow dividers are essentially hydraulic pump/motors connected by a common shaft. That single shaft ensures each gear section rotates at the same speed, thereby each flowing equally. Gear-type flow dividers tend to be more efficient than spool-type flow dividers. Less fluid is lost from one outlet port to the other, whereas with spool-type flow dividers, the clearances are larger to allow for lubrication of the spool. Gear type come with up to eight sections or sometimes more, an advantage over spool-type, which primarily offer just two outlet ports. However, for geartype flow dividers to synchronize, you must install relief valves at each outlet port to send the fluid back to the flow divider inlet from a stalled actuator.

Most flow dividers are also combiners, meaning they will accept flow in retraction and extension, for example. In combining mode, synchronization still occurs and helps to keep multiple actuators in synch for both directions at the end of stroke. Gear flow dividers are inherently combiners, but spool-type dividers must be specified as such.

Two additional styles, radial piston type and volumetric dividers, offer high synchronization.

Radial piston types divide a single flow into two output flows offer high volumetric efficiency and low speed capability. Volumetric dividers work only by volume dosage and the exchange between volume synchronizer and working cylinders. These are best suited for high-precision applications that require low pressure drop.

jahns mto small gear flow divider with integrated relief valves. courtesy of ic fluid power

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high-speed hydraulic motor. courtesy of hof hydraulic solutions

can be fixed-displacement or variabledisplacement, 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

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 important. Operating fluid and tolerance for contamination are other considerations.

In broad terms, gear motors tend to be suited for medium flows and pressures, and are the most economical. Vane motors offer medium pressure ratings and high flows, with a mid-range cost. At the most expensive, piston motors offer the highest flow, pressure and efficiency ratings.

Cost is clearly a factor, but initial cost and expected life are just one part of the equation. Users must know the motor’s efficiency rating. Also, 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 size and weight, as this will impact the size and weight of the system or machine with which it is being used. fpw

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FLUID POWER HANDBOOK

piston 2 axial5 s ie r e a10v s e pump. ne variabl tesy of fluidy cour

hydraulic pumps are used in literally every single hydraulic power transmission system. A hydraulic pump converts mechanical energy into hydraulic energy, which is a combination of pressure and flow. A hydraulic pump can be any device that you can input force into to create pressure, which in turn creates flow.

Most hydraulic pumps have a mechanical input from an internal combustion engine or electric motor. These prime movers input their mechanical power to the hydraulic pump in a rotational fashion. The input shaft of the pump will be connected to gears, vanes or pistons of pump, where they will rotate or reciprocate to transfer pressure (force) to the hydraulic fluid. As long as the force (pressure) created 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. Positivedisplacement pumps can be further divided into two categories: fixed- and variabledisplacement. Fixed-displacement pumps provide a single, specific volume displacement per revolution. In variable-displacement pumps, displacement per cycle can vary from zero to maximum volumetric capacity. Some 34

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hydraulic pumps

of the more widely used types of positivedisplacement 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, 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 (area over the crescent) is relatively long, internal gear pumps offer lower noise levels than some other types of pumps.

Gears are made of special steel and are often case- and quenchhardened. They are ground and fine finished. Proper tooth profile design and geometric proportions can reduce pulsation and noise during operation. www.fluidpowerworld.com

Piston pumps supply high flows at high speed. Axial and radial types are manufactured in fixed- and variable-displacement versions.

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

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

Radial-piston pumps (fixed-displacement) are used 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 truck vane pump. courtesy of hof hydraulic solutions

HYDRAULIC PUMPS

c40v axial-piston pump. courtesy of hawe hydraulik

in such a way that the plungers can be switched off one by one, so that a sort of variable-displacement pump is obtained.

Radial-piston pumps are characterized by a radial piston arrangement within a cylinder block.

As pistons reciprocate, they convert rotary shaft motion into radial motion.

One version has cylindrical pistons, while another uses ball-shaped pistons.

Another classification refers to porting: Check-valve radial-piston pumps use a rotating cam to reciprocate pistons; pintle-valve pumps have a rotating cylinder block, and piston heads contact an eccentric stationary reaction ring.

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

Some types of vane pumps can change the center of the vane body, so that a simple adjustable pump is obtained. These adjustable vane

pumps are constant pressure or constant power pumps. Displacement is increased until the required pressure or power is reached and subsequently the displacement or swept volume is decreased until equilibrium is reached.

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

FLUID POWER HANDBOOK

W W W. M O B I L E H Y D RAU L I CT I P S . C O M

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

Depending on the unit, almost every constituent part can be replaced or repaired, provided a suitable replacement is available. As a general rule, a repair makes sense if the cost doesn’t exceed 60 to 70% of the cost of a new component. Beyond that, the user is typically better off with a new unit. However, if a new product is not readily available and a critical or expensive machine or production line is down, repair is still practical at a higher cost.

Technicians first make a full inspection and diagnose the problem. Sometimes components wear out. But premature failures typically result from contamination, cavitation, over-pressurization, and excessive heat.

Thus, installing a robust filtration system, keeping components cool, and following a disciplined maintenance program are critical to extending component and machine life. Finally, some components break due to incorrect application, installation or commissioning — failures that are often preventable.

Technically savvy repair shops tear a unit down and try to bring it back to “as new” condition. All critical dimensions and surfaces are inspected and measured. Seals and lowcost consumable parts like springs, washers and shims tend to be replaced. Likewise, bearings will be inspected and possibly replaced. In more-serious cases, say internal wear due to contamination damage in a piston pump, lapping the surfaces might be suitable if still within acceptable tolerances. Otherwise, the technician may need to remanufacture or replace rotary barrels, pistons, and other internal parts. That can ultimately extend to replacing other major components like housings, covers, relief valves, controllers and

spill gate cylinders before, top, and after, below, being overhauled. courtesy of hunger hydraulics

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OFCO-F

REPAIR, REBUILD & MANUFACTURING

charge pumps. In the worst case, a completely new unit can be built from parts, although that is not the norm.

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

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

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

Components with higher operating pressures and electronics controls also tax the capabilities of test equipment at many repair facilities. Small shops will make repairs and perhaps run basic tests, but they are not capable of performing full-function tests. Larger repair shops

have made significant investments in state-of-the-art test stands for qualifying dynamic open and closed-loop systems.

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

The quality of the repair shouldn’t be driven solely by price, either.

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

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hydraulic seals FLUID POWER HANDBOOK

one of the arguments often used against hydraulic systems is that dreaded word, leakage. But with proper sealing, leakage won’t be a problem with your system.

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. They possess the following properties: y y y y

high mechanical, tensile strength good abrasion resistance modulus of elasticity is variable wide range of hardness values, while retaining good elasticity y good resistance to ozone and oxygen y outstanding resistance to abrasion and tear Temperature range for use: –30 to 80° C; high performance types (compounds) up 38

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to 110° C in mineral oils (long-term exposure temperature).

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

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

FKM provides: y tough resistance to high heat y excellent resistance to oil, hydraulic fluid and hydrocarbon solvents y good flame retardance y low permeability to gases y high swell in polar solvents, ketones and fire-retardant hydraulic fluids 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: y slippery surface that repels most media y non-toxic at working temperatures up to 200° C y low coefficient of friction against most opposing surfaces made of other materials; stiction and friction are almost the same y excellent electrical insulating properties (almost independent of frequency, temperature and weathering effects)

Temperature range for use (depending on the blend composition): –40 to 100° C and for short periods up to 130° C (material hardens at higher temperatures). For special blends, 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. www.fluidpowerworld.com

780 double-acting piston seal in a 5-part assembly is designed for one piece pistons in a wide range of medium duty applications. courtesy of hallite

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FLUID POWER HANDBOOK

epdm seals epdm rubber is a synthetic polymer used in fluid power applications to seal static applications, such as pump housing seals or cylinder barrel end seals. Although rarely used as a dynamic seal for hydraulic applications, EPDM has benefits making it perfect for various applications. EPDM stands for ethylene propylene diene monomer and offers a resilience not

It may be easier to describe the applications in which EPDM does not work well, and unfortunately, for most hydraulic applications, the offender is mineral oil. So EPDM is not appropriate for most hydraulic machinery, but there are plenty of applications where it’s effective. Water-based hydraulic fluids are in the wheelhouse of EPDM, so expect water-glycol fluid to primarily power these applications. Although not relevant to hydraulic applications, EPDM has excellent chemical resistance, should you need it for a process application.

EPDM has a wide usable temperature range and works well anywhere from –65° to 300°F –54° to 150°C). This extensive operating range allows machinery to function in the coldest or hottest environments typical of machinery — from forestry to steel mills. Still, mills offer a considerable challenge to traditional seal polymers, so EPDM excels in using waterbased fluids in these high heat conditions. In addition, steel mills like to avoid flame-supporting hydraulic fluid — imagine a pinhole leak in a hydraulic hose near a white-hot steel plate, and you’ll imagine a flame thrower if that fluid is oil.

There are other seal materials capable of withstanding either end of the temperature extreme, but rarely both at once. Viton, for example, is happy to seal away at 400°F (200°C) but won’t like being less than –20°F (–29°C).

Conversely, some low-temperature urethane polymers work well down to –65°F (–54°C) but might fail prematurely above 230°F (110°C). Forest firefighting trucks face the most extremes of temperature, where they may fight residential fire in the winter or find themselves deep inside a burning forest six months later. The EPDM’s natural resistance to water-based fluids and temperature extremes will benefit their hydrostatic drive or hydraulic booms.

Aerospace applications take advantage of the versatile nature of EPDM, controlling various functions on most jet-powered aircraft. The temperature resistance has been established, above, but it’s important to note how easily EPDM handles more exotic hydraulic fluids, such as phosphate esters.

Used primarily as a fire-resistant fluid, this option has been known to break down most traditional seal compounds but has no adverse effect on ethylene propylene diene monomer.

Whether environmental extremes or possible extreme fluids, EPDM offers fluid power designers a versatile option for static seals in demanding fluid power applications.

FLUID POWER WORLD

y chemical resistance that exceeds that of all other thermoplastics and elastomers y liquid alkali metals, some fluorine compounds attack at higher temperatures 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

found in other seal materials.

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

Piston Seals y y y y y

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

Wipers y y y y y y

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

Rod Seals y y y y y

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

Buffer Seals y y y y y y

must withstand high pressure exposure protect the rod seal against pressure spikes pressure-relieving capability prevents pressure build-up between seals increase rod seal life allow for wider extrusion gaps 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 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; do not require system pressure or speed to create the seal

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SENSING TECHNOLOGIES

sensing technology 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.

positioning sensing Several technologies exist to provide position feedback or data of a cylinder’s position. The sensor converts the position into a proportional analog or digital signal.

Hall-Effect Transducers use a magnet that communicates with the Hall chips, which then give an output to the internally built microprocessor. The output from the microprocessor is converted to a signal required by the user interface such as voltage, current, PWM or digital output. Using 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-Displacement Transducers are durable and resist shock and vibration while offering high repeatability. These absolute linear position/displacement transducers convert a linear displacement into an analog

draw-wire encoder in hydraulic cylinder. courtesy of siko products inc.

electrical signal. Their design includes transformer coils wound around nonmagnetic coils.

LVITs — Linear Variable Inductive Transducers — are contactless position sensing devices, with sensing ranges up to 30 in. or more. Most designs feature an inductive probe surrounded by a conductive tube. This is attached to the moving object to make the reading. These contactless position sensing devices use eddy currents developed by an inductor in the surface of a conductive movable element to vary the resonant frequency of an L-C tank circuit. Modern electronics using microprocessors and small component size makes high performance possible, achieving linearity errors of less than ±0.1% and temperature coefficients of 50 ppm/°F, along with either analog or digital outputs.

attached to the component in motion and the head-end of a sensing rod that is attached to the axis to be measured. The magnet does not touch the sensing rod, so no parts can wear out. The sensing rod mounts along the motion axis and the position magnet attaches to the moving member.

An electronics module sends an analog or digital position reading to a controller or receiving device. The electrical connection interface can be either an integral connector or cable and visual diagnostic LEDs to ensure proper wiring, power, and magnet positioning.

Encoders are used heavily on mobile machinery.

Available with capacitive, optical or magnetic sensing and with incremental or absolute position sensing, these devices accurately determine the stroke of the cylinder. Rotary and linear devices are available, but rotary styles are most common. These devices measure position and resolution in pulses per revolution.

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

Linear encoders are available with resistive, capacitive, optical or magnetic sensing with incremental or absolute position sensing to accurately determine the stroke of the cylinder. fpw

Magnetostrictive Transducers measure the distance between a position magnet

lightweight pin-mounted transducer. courtesy of rota engineering

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FLUID POWER HANDBOOK

W W W. M O B I L E H Y D RAU L I CT I P S . C O M

pressure transducers

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pressure transducers,a subset of pressure sensors, can be any number of devices that sample and record the pressure in a system. They convert a pressure measurement into an analog electrical output signal, which can be used by sensing instrumentation such as microprocessors and computers. This is accomplished through physical deformation or mechanical deflection. Criteria to consider when selecting a pressure transducer are mechanism type, input and output, and performance specifications.

Common types are strain gauge, and thick/thin film.

Strain gauge 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.Input voltage can vary but typically falls under 10 V, while output is typically in the hundreds of thousandths of volts. A change in system pressure would cause a change in the transducer’s resistance on the electrical circuit and would result in a change to the output voltage. With the aid of an analog to digital converter (ADC), the transducer’s output signal can be used in systems that require digital signals, such as a programmable logic controller (PLC) or a programmable automation controller (PAC).

Some pressure transducers output current rather than voltage, and are referred to as transmitters.

These values typically fall within tens of thousandths of amps. When choosing the output of a pressure transducer, 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. fpw

pte7100 pressure sensor. courtesy of sensata 42

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HYDRAULIC VALVES

hydraulic valves

hydraulic valves, used in conjunction with actuators, help make hydraulics unique in its control of force, torque and motion. Valves

govern the direction, pressure and flow of hydraulic fluid, enabling smooth, safe control of actuators.

A valve’s purpose may be as simple as relieving pressure to protect your pump and actuator or as complex as electronically controlling both pressure and flow with a proportional valve. A valve circuit may contain a single lever valve or an extensive complexity using a dozen valves per function, as seen in custom manifolds.

directional control valves

The directional control valve comes available in myriad configurations and is named appropriately to its primary function, which is to control the path of fluid flow in some way. Directional control valves manage fluid by blocking, diverting, directing or dumping. Their complexity varies immensely (just like their cost), as does the integration method.

Valve construction runs the gamut from cartridge valves to monoblock valves or subplate mounted valves to inline valves.

The operation of a directional valve depends on the mcd25 manual sectional directional control valve. courtesy of hof hydraulic solutions

industry application in which they are typically applied. For example, directional control valves for log splitters represent the economical and straightforward end.

In contrast, servovalves controlling flight simulators perform well at the precise yet expensive niche.

You’ll find valves to operate every possible combination of pressure and flow, although extreme combinations of simultaneous pressure and flow are rare.

The most basic directional valve is the check valve; it allows flow into one work port and blocks flow coming back through the opposite port. Alternatively, directional valves with complex construction are also common, such as with the pilot-operated valve, which uses a small valve to control a larger one.

A standard solenoid spool valve has one directly operated component (the spool) that controls fluid direction when it shifts.

As flow increases, the force upon the spool also increases, and these flow forces can prevent a spool from actuating, can be the case with direct-acting electric coils. By using a small pilot valve to control the mainstage spool's movement, 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 into which the valve may shift. The ways are equal to the number of work ports, so a 4-way directional valve contains 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. Very few valves offer more than three positions, although the snowplow float valve is one such animal.

Directional valves are available in monoblock or sectional valves, common to the mobilehydraulic industry, as well as subplate mounted industrial type valves such as ISO style D03, www.fluidpowerworld.com

oid so l e n ition o3 valves. s o p d 3 ional d , 2 an t 4 wayated direc fluidyne oper urtesy of co

D05 and so on. Also common to 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. Inline valves offer a standalone combination of valve with a ported body, which must be plumbed as a separate component not directly interfaced with any other manifold or body.

pressure controls

In its most concise description, a pressure valve offers designers an option to limit pressure.

Most pressure valves use spring-energized poppets pushed against a seat with some form of adjustment screw to modify the spring’s pretension. Pressure valves often use a simple ball and spring configuration or spools for high flow circuits. Relief valve 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 slowly open, proportionally bleeding off fluid to limit pressure.

A relief valve limits maximum pressure for either the entire system or a sub-circuit, with the lowest pressure parallel valve opening first. It is critical to understand that pressure takes the path of least resistance. Therefore, selecting a pressure valve downstream with a lower setting than the main system relief valve will see all pump flow dump through the valve with the lower setting, thereby leaving no hydraulic energy source for other actuators.

Most other pressure valves are based 7 • 2022

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FLUID POWER HANDBOOK on the relief valve’s simple spring-loaded ball or poppet construction. Sequence, counterbalance, and brake valves are variations of the relief valve but with added utility or functionality, such as reverse flow check valves or integrated pilot operation. The pressure-reducing valve differs from the other pressure valves because it limits pressure downstream rather than upstream. Reducing valves are used when sub-circuit pressures need to be lower without sacrificing pressure performance in the rest of the system.

Sequence valves operate much as their name suggests — they are pressure valves that remain closed until upstream pressure overcomes the valve’s spring setting. At this point, it simply opens to pass flow to a downstream subcircuit. Sequence valves may be specified with a reverse flow check valve that allows the flow to bypass in the reverse direction. Sequence valves, or any valve with pressure at all work ports simultaneously, should contain a method to drain the spring chamber of trapped pressure. Without a drain, pressure becomes trapped in the spring chamber, which is additive to the spring valves.

The result is

W W W. M O B I L E H Y D RAU L I CT I P S . C O M

a valve that opens later than intended or not at all.

Counterbalance and brake valves are motion control valves designed to safely limit and control loads with overrun potential.

Essentially any cylinder that may pull or drop under load, or any motor with constant tension, should use a motion control valve. Installed on the actuator port where loadinduced pressure occurs, these are essentially relief valves that require extensively more than load pressure to open directly. More commonly, they take a pilot signal from the opposing work port to open the counterbalance or brake valve using a fraction of the load-induced pressure. This method is safer and more efficient and has the effect of limiting the actuator to a velocity dictated by pump flow rather than load-induced acceleration.

Pressure reducing valves remain open until downstream pressure rises above the valve’s setting, an effect much different from other pressure valves, which remain closed until cracked open by pressure. Pressure reducing valves may reduce pressure on either work port or the primary pressure port and also perform best when their spring chambers can drain to tank. They operate by modulating incoming flow rate, which in turn reduces downstream pressure. lx-6 load-sensing directional valve. courtesy of hydac

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directional valve bank combines different valves for operating independent consumers. courtesy of hawe hydraulik

flow control valves

Flow control valves control or limit flow by one of various methods. They are often just a needle valve, which is just a variable restriction, adjusted by a screw or knob much like pressure valves, to restrict the cross-sectional area to reduce flow.

When installed with reverse flow check valves, we change the name from needle valve to flow control.

Flow control valves can sometimes have multiple ports, such as a priority flow control. They provide controlled, fixed flow to one part of the circuit (sometimes at the sacrifice of another part), but only if input flow is high enough for its priority demand.

Flow controls are (ideally) pressure compensated, allowing 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 to meter flow and control the direction in which flow is metered.

Proportional valves use pulsewidth modulation to vary current while they maintain voltage.

Varying the current modifies the

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force of the magnetic field and subsequently how far the spool or poppet moves within its body, changing the size of the opening for fluid to take, which of course, limits flow. A simple variable resistor can limit current, but it is inefficient and cannot provide a PWM controller’s benefits.

An electronic valve controller can provide adjustable minimum and maximum settings.

A minimum current value is needed to move the spool past its “dead zone” overlap where it “starts” to flow. Also, a maximum current value prevents too much electric juice from fatiguing the valve and coil when only a couple of amps are required to achieve full flow anyway. A proper controller and driver provide a dither signal to the valve, which vibrates the spool so that static friction doesn’t allow the spool to stick inside 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. fpw

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FLUID POWER HANDBOOK

W W W. P N E U M AT I CT I P S . C O M

pneumatics overview pneumatics is a fluid power technology similar to hydraulics in that it transmits force through a pressurized medium to create useful work.

However, pneumatic technology differs from hydraulics in every critical fashion — compressibility. To be fair, hydraulic oil compresses as well, but only 0.4 percent every thousand psi or so. However, in the simplest explanation, air will compress to half its volume when subjected to double the pressure.

Compressibility is an asset for pneumatic systems, allowing for extremely high acceleration and inherent shock absorption. As compressed air moves from high pressure to one of lower pressure, the expansion accelerates the air. The acceleration occurs at the loads of the actuators to provide the snappy movement pneumatic systems are known for. But, of course, the compressibility of air prevents it from being the first choice for load-holding applications. Still, air-over-oil systems provide a versatile solution when hydraulic power units are unavailable.

Pneumatic systems get their power from a compressor that takes air from atmospheric pressure and reduces it to an eighth or less of its original volume. A compressor’s role is singular, and unless your machine uses every cubic inch of compressor air as exists, the compressed air must be stored. Specialized air tanks called receivers act as a buffer between the compressor and the downstream sub-systems. This buffer stores the compressed air to reduce the compressor’s load and provides extra capacity when overall demand exceeds compressor capacity.

Pneumatic systems operate between 80-120 psi, which depends on the requirements of the loads and actuators in the system. Pneumatic systems compromise the force and speed advantages of higher pressure with the efficiency challenge of high compression. In other words, above 120 psi, the energy required to further pack air into a smaller space is somewhat of a challenge. Higher compression results in more heat, which adds to the system’s complexity as multiple stages and coolers may be required. High-pressure pneumatic systems also experience a higher risk of leaks and failures, sometimes catastrophic.

Actuators cannot use air directly from a compressor or receiver.

Air must first be conditioned to suit each subcircuit or function. Most pneumatic tools, motors, cylinders, effectors and valves prefer clean, dry air with a stable pressure. Therefore, the primary compressor set comes with (or should be optioned with) a pressure regulator set higher than all others in the plant, a dryer large enough to peak demand and a filter to prevent any compressor-generated particles from travelling downstream.

Because each subcircuit may have a separate conditioning requirement, individual branches come fixed with their own conditioning components. Filters, regulators and sometimes lubricators 48

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offer tailored control over the air quality for each function. For example, a work cell using pneumatic drills and impact guns require reduced-pressure clean air and air tool oil, which combine to improve performance and reliability. On the other hand, an automated paint booth requires a very high volume of clean, dry air with no lubrication, so understanding the requirements of each air system branch prevents costly design mistakes.

Pneumatic applications are diverse, to say the least. Air processes systems, such as sandblasting, are not traditionally a fluid power function since air does not achieve work. Pneumatic actuators are incredibly varied — cylinders, motors, clamps, grippers and rotary actuators offer unique solutions for any designer.

Air cylinders offer quick acceleration for light loads or automated processes, such as packaging equipment, pick and place robots, ejectors or clamps. Air motors offer a high-speed, clean choice to replace light duty electric or hydraulic motors, such as those requiring an inexpensive explosion-proof option.

Grippers used as robot end effectors provide an inexpensive, rapid method to clamp material for manipulation. Manufacturers offer myriad pneumatic clamping solutions, such as two-jaw parallel grippers or a single pneumatic cylinder against a toggle for high clamping force.

Pneumatics offer manufacturers an unmatched combination of performance and economics. Pneumatic actuators and the various pressure, flow and control valves used to control them may be creatively integrated to perform in various industries. For example, food & beverage is served by stainless and aluminum components resistant to washdown while offering finishes resistant to bacteria or fungal growth. Likewise, the medical and pharmaceutical industries benefit from the rapid cycle times provided to robots pressing, punching and ejecting millions of pills or parts.

No matter the industry, pneumatic systems find ways to help.

Even using a vacuum to move delicate objects without harm offers another trick in the pneumatic hat. But, as creative engineers and designers know, the possibilities for air-powered machines are limited only to your imagination.

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PNEUMATIC ACTUATORS

end lock pneumatic cylinders from camozzi automation are fitted with automatic mechanical end stroke locks which guarantee the safe and secure holding of the cylinder rod in both the fully retracted and fully extended positions.

pneumatic actuators pneumatic actuators take advantage of compressed air to provide your machine with quick and powerful actuation of nearly infinite possible functions. Pneumatic actuators may take the form of cylinders, motors, slides, grippers and rotary actuators, each of which receives pressurized and compressed air to create linear or rotational force.

The pistons or vanes of pneumatic actuators produce force when delivered with pressurized air. Because of the compressibility of air, pneumatic actuators are sensitive to both back pressure and flow restrictions. To get around the complexities of variability, we express the forces applied via pneumatic actuators in terms of differential pressure rather than the inlet pressure. In other words, the force generated by a pneumatic actuator is defined by the difference between the pressure at the actuator’s inlet port compared to its outlet port.

Because of the relatively clean air operating pneumatic actuators, any leaks result in a loss of performance and reduction in efficiency rather than a possible hydraulic oil leak that makes a mess at best and contaminates the environment at worst. However, many pneumatic actuators require lubrication for performance and longevity, so be sure to study your product documentation to check the requirements of your actuator.

pneumatic cylinders

The cylinder is the most popular pneumatic actuator — consisting of a piston and rod assembly installed into a barrel and capped on either side to contain both working and load pressure. When one of two ports has directed air from an upstream valve, the cylinder will extend or retract

with force equal to the previously mentioned pressure differential factored with the piston’s surface area. A cylinder with a larger bore will have more surface area and, subsequently, more force when applied with a given pressure differential. Cylinders may also be single-acting, pressuring in one direction only while employing gravity or springs to retract.

Cylinder bodies must include the mounting method, which is fixed to the machine’s frame or support. Conversely, the cylinder’s rod end attaches to the machine component which moves under load. Should that loaded component move through a radius or arc, the cylinder’s pivot method must be included, such as with a trunnion, clevis or eye.

Pneumatic cylinders come in various construction styles; NFPA tierod, extruded body ISO, pancake style and compact round line, to name a few. A technology unique to pneumatic actuators is the rodless cylinder, which uses a magnet or mechanical coupling that attaches the rodless piston to the mounting platform. Rodless cylinders typically operate as non-rotating slides for positioning applications in automation.

air motors

Some pneumatic actuators operate with rotational torque rather than linear force. Air motors are efficient, high-speed actuators used for drilling, milling, conveying and tool operation. Air motors have low inertia, which allows them to accelerate and decelerate rapidly while spinning quickly to high speed. Air motors may stall with no trouble, but a rotary actuator makes a great choice if you require partial rotation for your application. Vane-type rotary actuators offer snappy action between two discrete machine positions. Still, rack and pinion style actuators use dual air cylinders to offer up to a thousand degrees of rotation or more.

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PNEUMATIC ACTUATORS

rotary actuators

rotary actuators are devices used to transmit torque through

a limited rotational arc. Rather than pneumatic motors, which rotate continuously, pneumatic rotary actuators move a load through a limited range of motion. Some actuators rotate through two discrete positions, while others are capable of positioning at any angle around the arc.

The two most common rotary actuator construction types in pneumatic fluid power systems are the vane and rack and pinion. In addition, a diaphragm-style rotary actuator is popular in the process equipment industry, and the Scotch Yoke style of actuator provides high torque for valve actuators in oil & gas. Still, both applications are less common in fluid power.

The vane rotary actuator looks much like an air motor, with a shaft protruding from its mounting face. Two air ports connect internally to the common chamber separated by the vane. Pressure at one port forces the vane towards the opposing port with force equal to the vane’s surface area multiplied by the pressure differential at that chamber. The vanetype rotary actuator makes an excellent two-point operation for industrial automation requiring a load to move quickly between two positions.

Although most models offer stroke adjustment, the function pertains

norgren offers rotary actuators in rack and pinion (left) and rotary vane variants (right).

mainly to fine-tuning the end-of-stroke position.

Rack and pinion rotary actuators are like two air cylinders working in opposite directions and joined at the center by the rack and pinion. As each cylinder strokes, the horizontal motion of their racks forces the center pinion to rotate. The torque created is a combination of the cylinders’ forces and the diameter of the pinion gear. Rack and pinion style actuators offer superior side load resistance. The reinforcement from its bearing and abutment of the racks help ensure the load is well-supported.

The rack and pinion rotary actuator works well cycling to the end of the stroke and at any point in between. It is not capable of supporting a load mid-stroke in the same fashion a hydraulic application could.

However, with position feedback (like an encoder) and sophisticated control valves, rapid and accurate positioning is possible.

Vane-type rotary actuators rarely achieve more than 270 degrees of rotation because their action is like a swinging door. However, rack and pinion actuators are limited only by the stroke of the air cylinders powering them. So it’s not unreasonable to expect a thousand degrees or more of rotation.

Accessories improve the function of any rotary actuator. Position switches located on the body of an actuator may provide a signal for endof-stroke confirmation for either vane or rack and pinion styles. Many position sensors, such as magnet-sensing reed switches along the length of the rack-and-pinion cylinders, may allow discrete positioning anywhere along stroke length. Furthermore, if infinite positioning suits the process, LDTs may be installed to provide an accurate position signal to be relayed to the PLC. Popular applications for the rotary actuator are rotary tables, which are used in automation to position workpieces at various work stages as part of a process. In addition, pick and place applications with partial rotation are suitable for vane-type actuators. Other typical applications are tilting, braking, tensioning and lifting.

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

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

when the air pressure is removed.

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

In a typical application, the actuator body is connected to a support frame, and the end of the rod is connected to a machine element that is to be moved.

A directional control valve is used to provide a path of compressed air to the extend port while allowing the exhaust air to escape through the valve to the www.fluidpowerworld.com

d2-series double-acting, nfpa heavy-duty tie rod air cylinders are for use where abusive conditions exist. courtesy of automationdirect

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FLUID POWER HANDBOOK atmosphere. The difference in pressure on the two sides of the piston results in a force equal to the pressure differential multiplied by the surface area of the piston.

If the load connected to the rod is less than the resultant force, the piston and rod will extend and move the machine element. Reversing the directional control valve will provide compressed air to the retract port, allowing exhaust to escape the extend port, and the cylinder will return back to its home position.

W W W. P N E U M AT I CT I P S . C O M

Pneumatic actuators are at the working end of a fluid power system. Upstream of these units, which produce the visible work of moving a load, are compressors, filters, pressure regulators, lubricators, on-off control valves and flow controls. Connecting all of these components together is a network of piping or tubing (either rigid or flexible) and fittings.

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

why do air cylinders leak?

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

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

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

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pneumatic rodless & cylinder slides

when an application calls for power

•

and linear motion while also supporting side loads, pneumatic rodless and cylinder slides (also known as guided cylinders) are up to the task. Unlike standard pneumatic cylinders, which are unable to hold the position of the piston rod, pneumatic slides can stabilize and hold a load because there is no rotating rod to cause side loads. Because they feature a non-rotating platform to mount other actuators and tooling, they are ideal for automation applications where there is repeated pickand-place of parts.

Rodless cylinder slides are different from standard pneumatic cylinders because they have no piston rod that extends outside of the cylinder body. This makes them ideal for long-stroke applications or where space is limited. The rodless design eliminates common problems from side loads, such as rod bending and overhang, among others. Instead of the rod, a magnetic or mechanical coupling system connects an internal piston to an external carriage.

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

Be aware of several considerations when selecting the best type of pneumatic cylinder slides. These include: •

•

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

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

•

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

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

Specification and sizing software allows users to select the proper slide required for various applications.

The idea of applying a load to a linear actuator is common, and there are a number of types of cylinder slides that can be used for these applications. The first basic style of powered slide is commonly known as a “thruster” or cantilever type unit. This type of guided slide is typically powered by a rod-style pneumatic cylinder, which is attached to the body of the slide, or may be integral to the slide. In either case, the cylinder piston rod is attached to a tool plate providing power and motion.

The tool plate is supported by a bearing mechanism, and together they are able to carry any loads that are attached, rather than transferring the load to the cylinder rod. This type of slide is designed to carry an overhung load known as a cantilevered load.

Gantry slides can handle heavier loads while

traveling longer distances with a higher thrust than most other designs. Similar to thruster slides, they use a moving or reciprocating carriage between two fixed bars for their motion.

The second basic type of cylinder slide is called a saddle slide or base slide.

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

Another type of slide is a rodless slide.

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

Other pneumatic cylinder slides use profile rails with reciprocating ball carriage bearings. The profile rail bearing systems provide long life with minimum deflection.

These can be incorporated in both thruster and saddle type slides. fpw

dgc rodless linear actuator courtesy of festo

www.fluidpowerworld.com

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FLUID POWER HANDBOOK

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.

Various definitions that relate to compressor capacity should be understood: 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.

W W W. P N E U M AT I CT I P S . C O M

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.

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-to-metal 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.

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 air ends to compress air without oil in the compression chamber, yielding true oilfree air. These typically two-stage units are more costly than lubricated types. Oil-free rotary screw air compressors are available as air-cooled and water-cooled, with both load/ unload and variable speed control options.

They also offer the same flexibility as oilflooded rotaries when oil-free air is required.

Reciprocating air compressors use a piston within a cylinder as the compressing and displacing element. Single-stage compressors are generally used for pressures in the range of 70 to 100 psig and two-stage compressors are generally used for higher 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.

They are available as air-cooled or water-cooled in lubricated and non-lubricated configurations.

dynamic air compressors courtesy of adobe stock

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www.fluidpowerworld.com

Dynamic compressors use the mechanical action of rotating impellers to transfer pressure to the air. The centrifugal air

AIR COMPRESSORS

how do you measure your compressed air’s air quality? many industrial sites measure their compressed air quality, or think they do. When asked to show the dew point of a system with refrigerated air dryers, the compressor operators will typically point to the air dryer display, not knowing that the reading is not really dew point, but the temperature within the air dryer itself. Quite a few things can affect the level of dryness besides the temperature of the air — for example, a condensate drain failure (Fig. 1).

The most challenging sites in which to monitor and detect air dryer failure are ones with multiple parallel air dryers. Installation of dryers in parallel has its benefits, the resulting pressure loss is usually lower, saving power, and the system more reliable, but when one unit of a multiple set of dryers fails it is often difficult to identify a problem and narrow down the exact one causing the issues.

A particular issue is the fact that the dew point reading from a transducer placed at the outlet of a compressor room will measure the average dew point. For example, on a system of five parallel dryers, with one failed unit in the mix, the reading on a main dew point meter will often only read slightly higher under normal conditions — even though one of the dryers has completely stopped functioning. Therefore, it pays to closely

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 oilfree compressor by design. The oil-lubricated running gear is separated from the air by shaft seals and atmospheric vents.

monitor and trend the output of the compressor room, and even place dew point monitors on each individual dryer, if air quality is critical.

This was the recent experience at a large industrial plant producing abrasives. The site had all refrigerated dryers in parallel, and each one showed normal dew point on the local control monitor. However, frequent complaints from production areas about water in the compressed air challenged the compressor room operators. Many checks of the dryers and associated condensate drains showed no problems. The problem wasn’t solved until the Facilities Superintendent arranged to purchase and use a dew point monitor.

The output of the main compressor room showed only slightly elevated levels; however, when the dew point readings of the individual dryer outputs were monitored, one-by-one, it became obvious that one dryer was the culprit. Further testing found an unexpected internal leak in the heat exchanger.

Many new, more accurate, and affordable air quality measurement instruments have been developed that can help monitor not only dew point, but lubricant content, hazardous gas contamination, air temperature, pressure, and even particulate size and volume.

Add these types of instruments to your monitoring system to know for sure our compressed air quality is to your specification.

CUSTOM TESTING AT YOUR DISPOSAL.

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A L WAY S A DA P T I N G .

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tompkinsind.com 800-255-1008

ALA Industries Limited is a leader in engineering. Leadership in engineering can be seen through teamwork not only within ALA Industries but with our manufacturing companies that we represent.

We aim to create a seamless flow from our manufacturers to our distributors. The combined effort of all affiliated companies help ALA to better ourselves and provide excellent service and products. We focus on delivering outstanding communication, accountability, and transparency with all companies that we work along. ALA Industries is a leader in engineering due to the goal and ability to create and maintain long-term relationships with our distributors and the manufacturers that we represent.

ALA provides a prestigious product line throughout the Americas that aids the maximization of the end users hydraulic systems. The overall goal of ALA Industries Limited is to focus our time and energy not only

ALA INDUSTRIES LIMITED 3410 Delta Dr, Portage, IN 46368 (219) 762-2059 • alaindustrieslimited.com

on the success of our products, but also to be able to acknowledge areas of improvement and fix them within a timely manner. ALA Industries also seeks to expand their product market to help sustainability efforts, such as pitch control valves for wind turbines.

Within ALA Industries, our employees are dedicated, dependable, and respectful to their fellow employees and clients. They have a focus on servant leadership. Overall, ALA Industries Limited has positioned themself as a leader in engineering due to their ability to have open communication with clients alongside providing premier products.

AIR SPRINGS

smart sensor air spring integrated solution combines a height and pressure sensor with the air spring. courtesy of continential

air springs

air springs have been used in heavy-duty vehicle suspension systems for nearly a century, where they have been able to

provide usefulness by taking advantage of the compressed air required for vehicle braking systems. Air springs have provided a two-fold advantage over mechanical leaf or coil springs.

One advantage with air suspension is the extra comfort provided by being able to vary the air pressure inside the spring, which changes the spring rate, and therefore, ride quality.

Additionally, because variable control over air pressure adjusts the deck or trailer height, aligning loading docks to the level of the deck is possible when dock plates are unavailable.

The usefulness of air springs or actuators didn’t go unnoticed in the industrial machine industry, and it was clear they could offer unique solutions for various applications. Air actuators have seen duty as shock absorbers,

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

As air is directed into air springs, the bladders allow them to expand in a linear fashion. This permits them to be used as force developing actuators — like pneumatic cylinders — and as such, rod attachments are available to mimic the function of them. Most often, however, an air actuator is simply two end plates connected by a bladder, and as they are pressurized, force pushes the plates away from each other. As linear actuators, they can provide up to 35 tons of force, making them www.fluidpowerworld.com

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

The two major types of air springs are the rolling lobe (sometimes called reversible sleeve) and the convoluted bellow. The rolling lobe air spring uses a single rubber bladder, which folds inward and rolls outward, depending on how far and in which direction it is moved. The rolling lobe air spring is available with high usable stroke length — but it is limited in strength because of its tendency to bulge, and therefore has limited force capacity.

The convoluted bellow type air spring uses one to three shorter bellows, with the multiple units being reinforced by a girdle hoop.

Convoluted air springs are capable of ten times the force of a rolling lobe version and twice the life cycle rating, but have less usable stroke to work with. fpw

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W W W. P N E U M AT I CT I P S . C O M

frls

compressors generate pressurized air, but that exiting air typically contains dirt

and water. Before it can travel downstream to valves and actuators, it must be filtered, regulated and sometimes lubricated.

Otherwise, left untreated, it can damage products, cause premature component wear, attack seals and cause them to leak, and permit rust and corrosion in tools and piping — all leading to faster breakdowns and higher maintenance and operating costs.

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

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

Pressure regulators reduce and control fluid pressure in compressed air systems.

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

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

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nitra a series of frls for air preparation. courtesy of automationdirect

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 www.fluidpowerworld.com

quantity of atomized lubricant is supplied to the air operated equipment, and the only maintenance required is a periodic refill of the lubricator reservoir. Adding lubrication to a system also “washes away” compressor oils that travel through the system in vapor form.

Mineral oils added to the system prevent synthetic compressor oil build-up on system components. When lubricators are not used in a system, a coalescing filter should be installed to remove compressor oil aerosols.

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FLUID POWER HANDBOOK

W W W. P N E U M AT I CT I P S . C O M

pneumatic hose & tubing ado esy of court

be sto

ck

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.

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Manufacturers generally rate tubing by measuring the burst pressure at 75° F, and then divide it by an appropriate safety factor (typically 3:1 or 4:1) to determine the maximum working pressure. Keep in mind that published burst-pressure ratings are only for manufacturing test purposes, and in no way indicate that a product can safely handle pressure spikes or otherwise operate above maximum working pressure. Also note that some products handle vacuum to approximately 28 in.-Hg without collapse.

Thermoplastic tubing is made from several common materials. 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 www.fluidpowerworld.com

•

•

•

formulated for higher-pressure pneumatics, and offers 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.

PNEUMATIC HOSE & TUBING

why can compressed air hose reels be a leakage source? it is very common to find retracting style hose

reels installed in locations that use hand tools. These items are installed for convenience and make it easy to put the air hoses away, reducing trip hazards — but they have some commonly found problems that can increase compressed air costs and lead to performance problems: •

•

The compressed air is transferred to the hose through a special rotating seal. This seal will wear out and start leaking after a few years of use. If the reel is ceiling-mounted, the leak may not be easy to detect.

The reels often hold very long lengths of hose, which can cause considerable pressure loss to the tool depending on the required air flow.

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. fpw

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•

• •

For a fairly large air tool a 50 foot, ¼-in. hose will have a 50 psi pressure loss at 15 cfm flow, causing very poor tool performance. Always size for the largest tool.

Hoses are often put away nice and neat, but pinholes will develop over time due to rough handling. These are hard to detect on a rolledup hose.

Often times, shut off valves are missing, making repairs difficult.

It is common to install quick connect couplers on the inlet of the hose reel, this can cause extra pressure loss. Always try to minimize the number of connectors.

quality matters. every time.

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ALA Industries Limited

3410 Delta Dr • Portage, IN 46368 Tel: 877-419-8536 Fax: 219-762-2066 Web: www.alaindustrieslimited.com

VACUUM COMPONENTS

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 intake below atmospheric pressure and the output at atmospheric pressure. They draw in a fairly constant volume of air, which is mechanically shut off, expanded, and then ejected. The main feature of vacuum pumps of this type is that they can achieve a high vacuum with low flow rates. Types include reciprocating piston, rotary vane, diaphragm and rotary screw. They are often suited for precision industrial applications.

Kinetic vacuum pumps cause gas particles to flow in the delivery direction by applying additional force during evacuation.

Rotary blowers, for example, operate according to the impulse principle: a rotating impeller transfers kinetic energy by impacting air molecules. In operation, air is drawn in and compressed on the suction side by the impeller blades.

These vacuum pumps generate a relatively low vacuum, but at high flow rates (high suction capacity).

They are usually suited for handling extremely porous materials, such as clamping cardboard boxes.

Among the advantages, typical positive-displacement ejgov vacuum pump controller with cloud connectivity and control for industrial vacuum pumps and systems. courtesy of edwards www.fluidpowerworld.com

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 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. fpw

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pneumatic valves W W W. P N E U M AT I CT I P S . C O M

Controlling pneumatic actuators in requires safe and precise functionality. The execution of control is different in many ways than with a liquid but there is still 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, to prevent excessive pressurization. Relief valves are often called safety valves and are not typically appropriate for use anywhere but 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. Regulators are 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. Most regulators are capable of relieving downstream pressure, which prevents it 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. 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. Differential pressure regulators can maintain a set pressure differential between the two ports, rather than just maintaining downstream pressure. Note that all pressure regulators are adjustable, most often with screws or knobs.

flow controls

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

Flow control valves are quite simple, usually available in two configurations used in two different ways. One configuration is as 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 aladco pilot-operated pneumatic check valves 64

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aventics series advanced valve systems with opc ua. courtesy of emerson

valve, which allows free flow in one direction and restriction in the opposing direction.

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

A method of increasing cylinder velocity, typically for double-acting or spring-return cylinder retraction functions, is to add a quick exhaust valve to the cap side work port.

Because cylinders retract faster than they extend as a result of differential air volumes, it is harder to evacuate the cap side air volume without oversized valves or plumbing. A quick exhaust valve vents directly to air from the cap side work port and massively reduces the backpressure created upon retraction, permitting rapid piston velocity.

directional control valves

Pneumatic directional valves are available in many sizes, styles and configurations. At the basic end of the spectrum is the check valve, which allows free flow in one direction and prevents flow in the reverse direction. They 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

PNEUMATIC VALVES envelopes and the number of work ports in the valve, and 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. 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.

Poppet valves use a spring to push a face of the poppet down on its seat. Construction can be metal-to-metal, rubber-to-metal or even with diaphragms. Poppet valves can often flow in one direction, just as a check valve, but need to be energized to flow in reverse. They are limited to two- or three-way port configurations, but can mimic four- or five-way valves when used in parallel. They offer high flow conductance for their size, and resist to contamination.

Spool valves use a notched metal cylinder that slides within a precisely machined body, drilled with three to five ports, or seven ports if the valve is pilot operated. Low-end valves consist of only a spool and body, and are prone to internal leakage. Better valves use seals in the body or spool to prevent leakage between ports. Highend 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 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.

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. fpw

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FLUID POWER HANDBOOK

pressure gauges

pressure 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. They also prevent leaks by alerting personnel of unusual changes in system pressure.

Hydraulic pressure gauges can measure up to 10,000 psi, though typical hydraulic systems operate in the 3,000 to 5,000 psi range. They 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.

They are now 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

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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 absolute, gauge and vacuum. Absolute pressure is a measure of actual pressure including ambient air, which is zero-referenced with a perfect vacuum, but can be as high as 14.7 psi at sea level. Absolute pressure readings are considered in applications interacting with ambient air, such as the compression ratio calculation for flow (cfm) requirements. Gauge pressure is zero-referenced against ambient pressure and is used in most applications operating in, but not with, ambient air, such as in fluid power systems. Disconnected from equipment, gauge pressure will read zero.

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

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.

Bourdon tubes take pressure and convert it into mechanical energy, which moves a dial in the gauge, displaying the pressure in the system. Bourdon tube gauges have different configurations such as curved, helical and spiral. The different style of tubing, the size of the tube and the material it is made out of all vary based on the pressure range.

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

Bourdon tubes consist of a semicircular and flat tube

300 series brass case liquid-filled pressure gauges are rated for vacuum and compound ranges through 0 to 15,000 psi.

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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 pulls away from the lever, which being connected to the needle, 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 test-point fitting attaches to the gauge, which can be screwed onto the test points throughout the circuit, allowing you to connect under pressure to measure throughout the system. Most gauges are 21⁄2 in. in diameter, and can be top-mount or panel-mount styles.

Common threats to reliability are vibration, pulsation and pressure spikes. Look for gauges designed for hydraulic applications to reduce costly downtime.

A forged brass case prevents resonant frequencies from destroying internal components; a liquid-filled case protects the gauge from vibration and extreme pressure cycles; and a restrictor prevents damage from pressure spikes. When choosing between dry, water- or glycerin-filled, consider temperature range, needle response time required, changes in pressure and expected vibration.

Gauge accessories, such as restrictors, piston snubbers or diaphragm seals, prevent premature gauge failure.

Finally, gauges should be calibrated at least once a year to ensure accuracy, and to prevent wear and tear, failure and unexpected maintenance. Inaccurate gauges can cause safety concerns, adding the risk that products can be manufactured with defects and damage. Some industries require specific standards and regulations be met, such as meeting ISO standards. fpw

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PRESSURE GAUGES

a flow meter is a mechanical

flow measurement

or digital device manufactured to measure and indicate flow.

The most common flow meter is the mechanical variable area construction type, which employs a spring-loaded orifice-equipped piston. As flow passes the piston, the pressure differential moves the piston assembly against the spring, displaying the flow rate reading.

A flow meter should be installed in a hydraulic circuit to provide operational information for health and performance. They may be permanently installed or used for troubleshooting during periods of machine failure. Which installation option employed depends on the flow meter for pump health monitoring. courtesy of aw-lake Safeguard Your System Fluid Power Ad - Half Pg.ai 1 6/30/2022 3:01:24 PM

initial machine build (and its budget) or machine specifications’ depth.

Flow meters are not inexpensive, so their location must be thoughtful, especially when large units are required. Their cost rises exponentially with size, so often, only the maintenance team uses them for troubleshooting. Should the budget exist, a flow meter provides valuable data.

Installing a flow meter in the primary pressure line(s) of the pump(s) is the first step to understanding your system in breakdowns. I open the conversation to multiple pumps because a grouping run in parallel provides difficulty diagnosing failure when you’re unsure which, if any, of the pumps are not functioning.

The first step to diagnose an unknown stoppage of machine function is first to confirm flow exists. A flow meter installed after a pump confirms pump flow, but also of pump health should the flow reading be reduced. Recording periodic pump flow

provides insight. If the flow reading drops over time, it may be a sign of pump wear.

Furthermore, adding a second flow meter to the pump’s case drain line provides the maintenance team with additional insight. As a pump wears, its case flow accelerates as more fluid bypasses internally. Regular checks and recordings of the case flow rate will provide clues to pump condition, enable you to change or repair the pump before it fails entirely.

Lastly, flow meters work well in subcircuits with critical functions requiring predetermined flow, especially those related to cycle time performance. High volume production machinery requires 100% throughput. The installation of flow meters before a motor or cylinder circuit allows the production team to confirm that the flow supports the cycle rate. If machine performance drops, the flow meter confirms whether flow to the circuit is the culprit.

Safeguard Your System with Flow Meters from AW-Lake

If you could avoid costly system downtime by simply installing a flow meter, wouldn’t you do it?

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Variable Area Flow Meters are a popular choice for monitoring pump performance, as they can identify required maintenance without interrupting operation.

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AW-Lake offers a robust line of VA meters in numerous materials, including aluminum, brass, and stainless steel. Whether you need a basic meter, high temperature, bi-directional, or a meter specifically designed for case drains, AW-Lake has a low-cost solution to keep your system online and your pump health in check.

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FLUID POWER HANDBOOK

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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.

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associated with pressuredriven flow dictates that high pressures are required to permit high flow rates through microsized 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 reengineered hydraulic components to work on a “miniature” scale.

As one example, Bieri Hydraulik, a unit of Hydac International, 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

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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 offers a complete range of fixed and variable displacement micropumps; 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. makes an array of miniature, highperformance fluid control components, including Lee Plug

www.fluidpowerworld.com

a selection of miniature plugs, valves, restrictors and safety screens. courtesy of the lee co.

expansion plugs, solenoid valves, flow restrictors, safety screens, relief and check valves, and shuttle valves. The company’s 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 one-way 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

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FLUID POWER HANDBOOK

W W W. F LU I D P OW E R WO R L D. C O M

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 highpressure versions won’t burst or collapse at pressures of 7,500 psid, even if fully clogged.

SFC Koenig offers a range of plugs, flow controls, check valves and related components. The Expander plugs, for instance, reportedly seal drilled holes with excellent reliability. They come in sizes from 0.093 to 0.875 in. and are rated to 6,500 psi (450 bar) for push-type units and 7,200 psi (500 bar) for pull versions.

Its stainless-steel Restrictor units provide precise flow control in fluid systems and are available in sizes as small as 0.093 in. (4 mm) in expander and threaded styles, and handle pressures to 2,900 psi (200 bar). Orifice can be specified to achieve desired flow rates. Check valves, 0.216 in. (5.5 mm) diameter, handle forward or reverse flow,

have a cracking pressure of 2 to 29 psi (0.14 to 2 bar) and maximum working pressure of 4,352 psi (280 bar).

Takako Industries, a member of the KYB Group, claims to make the world’s smallest axial-piston pump. The square TFH-040 unit measures only 1.18 in. (30 mm) wide by 3.0 in. (77 mm) long and is rated for a maximum working pressure of about 2,030 psi (140 bar). Displacement is 0.4 cc/rev, input speed is to 2,000 rpm, with a flow rate of 0.8 lpm.

The unit is part of a family of micro-pumps which feature a hybrid drive system that combines the benefits of hydraulics with the controllability of an ac-servomotor and inverter to satisfy a range of specifications with a small-volume pump. Typical applications, Takako says, include a pump for valve controls, mold switching equipment for forming machines, hydraulic clamps, and crimping presses. fpw

miniature pneumatics

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

Sometimes referred to as precision pneumatics, miniature pneumatics consists of components that have been miniaturized for use in lightand medium-duty applications with low-tomedium 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.

Miniature pneumatic products encompass a range of scaled down parts, including valves, 70

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cylinders, fittings, manifolds and tubing.

Specialized components, such as nozzles and screens, are also seen on occasion.

For example, the evolution of medical equipment has expanded beyond hospitals and toward home care and ambulatory uses. 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. www.fluidpowerworld.com

an assortment of miniature pneumatic technologies. courtesy of clippard

This type of design can be used for compact, battery-powered pneumatic instruments such as portable oxygen delivery systems, environmental gas samplers and other OEM flow switching devices.

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

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Nexen’s spring-engaged, air-released rod locks are linear holding/locking devices that supplement air cylinders and guide rods for holding in power-off/e-stop situations.

fluid power safety

understanding where stored energy exists in any fluid power system is critical to safe machines and personnel. It is important to keep components plumbed properly, but also have the correct levels of machine

safeguarding in place — from properly labeled lockout/tagout systems to safety valves to ensure redundancy and safe shutdown. It is critical to evaluate the entire system and its complete schematics, including the electrical portion, to minimize exposure to unnecessary risk. Systems are rated based on the weakest link in the control chain.

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

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is subject to the same rules for classifying safety integrity. Thus, properly specified machine safeguarding systems include provisions for pneumatic valves, including:

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

Providing control reliability with fluid power is not quite the same as with electrical www.fluidpowerworld.com

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

Two general abnormal conditions can affect valve safety. The first is similar to an electrical-control fault, such as when a relay might be stuck in the open or closed position.

The second abnormal condition is when a valve develops diminished performance, such as when it becomes sticky or sluggish. In such cases, the valve reaches the proper position, but slower shifting affects safe stopping distances or precise timing. The ANSI B11.192010 standard mandates a monitoring system

SAFETY

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 selfmonitoring, Category-3 or -4 valve, designed to detect both conditions.

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

Two standard air valves, whether in parallel or in series, cannot perform the same safeguarding function as a double valve providing this critical function. By simply incorporating two standard air valves into the circuit, no provision is made to sense the abnormal operation of one side of the valve or, even more preferable, diminished performance such as slow shifting. In addition, there is no provision for inhibiting further operation of the circuit until the valve is repaired. If one valve actuates abnormally, the second one continues to function and redundancy is lost. The circuit doesn’t recognize lost redundancy, nor would it halt operations as a warning that redundancy has been compromised. Then, if the second valve also actuates abnormally, there is no back up, and control integrity no longer exists.

Double valves are appropriate for pneumatic and hydraulic equipment anytime reliability is an issue.

Typical applications include e-stop, two-hand-control, light curtains, safety gates, pneumatic locking devices for safety gates, hydraulic brakes, air brakes, amusement rides, hoists, elevators, pinch-point applications, or any other application where control system integrity depends on valve operation.

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FLUID POWER HANDBOOK

shock absorbers

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

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

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

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

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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 conditions, including the weight and velocity of the moving mass and how frequently the

shock is loaded. For simplicity, let’s look at a linear-motion application and use Imperial units for the calculations.

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.

The next step is to calculate the total energy, Et (lb-in.) per cycle, shown as: Et = Ek + Ew Again, if this exceeds the model’s energyabsorbing capacity, select a larger unit and recalculate the work energy. Otherwise, the shock’s temperature may rise beyond rated limits and critical internal components like hydraulic seals could fail.

If the application uses more than one shock

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SHOCK ABSORBERS

ACE Motion & Vibration Control when performance matters Industrial shock absorbers

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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:

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.

Etc = Et(C)

Standard products available

g = (Fp – Fd)/W

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.

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Calculate this g-load from:

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:

Industrial gas springs

The above calculations help ensure that a given shock absorber meets all operating parameters.

Again, make certain that the selected model matches or exceeds requirements for energy absorbed per cycle and per hour, as well as the shock force. Otherwise, it will likely cause damage or fail prematurely.

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

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

There are also a number of important shock absorber features to consider.

Adjustable shock absorbers allow the stiffness of the response to be monitored and finetuned. 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.

Tools Sizing & specification ▪ Online calculations & product selection ▪ Chat function for application assistance

Resources Online & downloadable ▪ ACE CAD database ▪ ACETips video tutorials ▪ Technical blog & case studies

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HAWE Hydraulic.............................. 44

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Texcel Rubber.................................. 24

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Dakota Fluid Power......................... 29

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Ultra Clean Technologies................. 39

Delta Computer Systems................. 42

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Veljan Hydrair.................................... 9

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Panolin America............................... 21

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