Fluid-Power Actuators Basics

7/30/2019 Fluid-Power Actuators Basics

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When the going gets tough,

look to fluid power.

Basics of

Fluid-PowerActuators

Hydraulic and pneumaticcontrol systems both have aplace in industrial automationequipment. Their capabilitiesoverlap to a certain extent, buteach offers specific benefits.

Hydraulics are the choicewhen an application requires ex-treme forces or torques, beyond

the capabilities of pneumatics. Pneumatics can pro-vide the needed perfor-mance at the best price.

In the end, the choice usually comes down toperformance factors such as force,precision, speed, durability and, of course, cost.

Corrosion-resistant, stainless-steel cylinders are ideal forwashdown applications. They aredesigned specifically to reducesharp edges and corners andprovide a smooth transition fromend cap to cylinder body. Rodwipers keep out contaminants andpressure-activated seals are wearcompensating for long life.

Repairablecylinders can bedisassembledto replaceseals andother internalcomponentsand extend life.

These durablecylinders aregenerally usedin rugged,heavy-dutyapplications.

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ally thousands of styles, sizes, andconfigurations. This variety makesmore-innovative equipment possible

The basic, rod-style industrialcylinders function in two ways.Double-acting cylinders usepressure to power both the extendand retract strokes, making them ide

for pushing and pulling loads. Singleacting versions supply pressure toonly one side of the piston; a springor gravity powers the return stroke.

They come in various designs.Repairable cylinders can bedisassembled to replace seals and othinternal components and extend life.These durable cylinders are generallyused in rugged, heavy-dutyhydraulic and pneumatic application

Sealed-for-life or disposablepneumatic cylinders have end caps

mechanically crimped to the tube.Internal components are prelubedprior to assembly. Although theyare less expensive to manufacturethan comparable repairable cylin-ders, they cannot be taken apart torepair without destroying the hous-ing. These cylinders are usuallyused in lighter-duty applications.

Compact cylinders fit into smallerspaces where only a short stroke is required. They are used in lighter-duty

applications due to the small bearingsurface on which the rod slides. Theymainly come in single-acting version

Sizing considerations

A key selection criterion is howmuch force a cylinder generates. Ageneral rule-of-thumb is that forvertical and high-friction applica-tions, the required force shouldbe twice the load to be moved. Insome cases additional force is nec-essary to compensate for friction.

Designers can calculate cylinderforce by multiplying the effective piston area by the working pressure. Theffective area for push force is the cylinder bore. For pull, its the bore arealess the cross-sectional area of the piton rod. Thus, theoretical push force

F = (D2/4)P

where F is force, lb; D is cylinder

Fluid-power advantages

Fluid-power devices also hold somedistinct advantages. Fluid-power cylin-ders and motors are compact, powerfuldevices that have a higher force-to-sizeratio than electric drives. So they cantransmit more power from the sameenvelope or the same power from a

smaller envelope. This is especiallysignificant where weight is a concern,say on aircraft or mobile equipment, orwhen space on a machine is limited.

Fluid-power systems also offerengineers more design flexibility,because the actuators can belocated away from the pump orcompressor. One pump or compressorcan power multiple cylinders andactuators. Thus, most designsusing fluid power take less space andare not restricted by orientation.

Another big advantage is safety, hy-draulic and pneumatic actuators canbe used in volatile atmospheres andother hazardous areas without theneed for an explosion-proof housing.

By their nature, fluid-powerdevices handle adverse environmentswell. Dust and dirt have little effecton fluid-power equipment,provided the air or hydraulic fluidsees proper filtration. Likewise, theyreadily withstand wet, humid, and

aggressive washdown conditionsand, with certain precautions, caneven operate under water. They alsoroutinely tolerate high temperaturesand frequent starts and stops.

Selecting cylinders

By far, the most widely used typeof fluid-power actuator isthe cylinder. They generateforce and motion on a vastrange of OEM equipment,for instance to push, pull, lift,

rotate, or clamp products.Wide acceptance comesin large part because cylin-ders are simple, economical,durable, and easy-to-install.They can produce thousandsof pounds of force over a broadrange of velocities; cycle athigh speeds without overheat-ing; and stall without internaldamage. They come in liter-

Compact cylinders fit into smallerspaces where only a short strokeis required.Flat-1 roundbody

cylinders (right)have a stainless-steel bodyand anodizedaluminum head.EF-1 cylindershave extruded,anodized-aluminumbodies with boresas small as 12mm.



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to determine rod column strengthand suggest design modifications.

As one can see, for a seeminglysimple device there are a number ofconsiderations when sizing cylinders.For that reason, cylinders may be themost overspecified component inthe average fluid-power system. An

oversized cylinder has a healthy extramargin of force that can override a bitof misalignment, binding, or overload.But a larger-than-necessary cylinderalso increases cost, air and energyconsumption, along with size andweight, and can slow actuation time.

Fortunately, leading manufacturersoffer online configuration tools andenergy-saving calculators that helpdesigners size components to pre-cisely meet application re-quirements. This

online sizingsoftware, suchas Bimbas ver-sion available atwww.bimba.com

simplifies the selec-tion process, ensures accuracy,and lets engineers find the rightcylinder for a job and test it withdifferent application variables.

Extending cylinder life

While fluid-power cylinders arerugged and built for rapid cyclingand long life even under less-than-ideal conditions theyre not in-destructible. Here are some tips tohelp ensure trouble-free operation.

Mounting. Its said that more cyl-inders are ruined by improper mount-ing than by any other type of abuse.Poor mounting produces excessiveside loads on the rods that can pre-maturely wear the seals, bend therods, or bind the load. An important

design consideration is to keep cylin-der thrust as close as possible to thecenterline of the piston rod and freefrom misalignment or off-center loads.

Cylinders with fixed or pivotmounts that absorb force on thecenterline are considered best forstraight-line force transfer. Thrustor tension forces are distributeduniformly about the cylinder andbearing side loading is minimized.

bore, in.; and P is pressure, psi.Theoretical pull force is:

F = (D2/4 - d2/4)P

where d is piston-rod diameter, in.When estimating the relative

force of cylinders with different

bore sizes, remember that thrustincreases with the square of the di-ameter. In other words, doublingthe bore will quadruple the thrust.

Speed is another consideration,affecting productivity, longev-ity, and controllability. For ex-ample, calculate the stroking speedof a pneumatic cylinder from:

s = 28.8 q/A

where s is speed, in./sec; q is airflow

in scfm; and A is piston area, in.2

Other factors that might affectspeed include port sizes, inlet andexhaust flow through control valves,and hose or tubing sizes if they cre-ate bottlenecks that restrict flow toor from the cylinder. Likewise, pres-sure that is barely capable of movingthe load will hamper speed. A goodrule of thumb is that for high-speedapplications (>20 in./sec), the cylin-der should be sized to supply twice

the calculated force requirements.Calculating a cylinders air con-sumption is often necessary on fast-cycling equipment to ensure enoughsupply air is available. This shouldinclude not only the volume the pis-ton displaces, but also the unsweptvolume from end-cover cavities, cyl-inder ports, connecting tubing, andvalves. The unswept portion is likelyto be a small percentage of the totaland will vary with the installation.

Most cylinders are designed to

have ample rod strength. But forlong cylinders (above approximately20-in. strokes) used in compression(thrust) applications, the rod acts likea slender column and must have ad-equate strength to prevent buckling.Standard rods may be too small, andsome mounting styles aggravate theproblem. When ordering cylinderswith long strokes, cylinder suppliersusually offer engineering assistance

Non-repairablepneumatic cylindersare an economicalchoice for manyapplications, End capsare mechanically crimped

to the tube, with internalcomponents prelubedprior to assembly.



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applications to low-friction U-cupsand other pressure-energized seals.

Common materials include BunaN rubber for pneumatics and manyhydraulic cylinders. Its good for tem-peratures up to 200F with air, mineraoil, and water glycol. When syntheticfluid or high heat is encountered,

Viton seals are often a better bet. Vitoworks at temperatures of 400 to 450with air and most any hydraulic fluid.Many other seal materials such asNeoprene and Teflon are also availabl

Lubrication. Hydraulic cylinders aby their nature, self-lubricating. Mostpneumatic cylinders are assembledwith a coating of grease on the boreof the barrel and seals for service withnonlubricated air. If the compressedair supply is clean and dry, the greasewill give the seals a long life without

adding oil through an airline lubrica-tor. However, contaminated air willgradually compromise the originalgrease lubricant and shorten seal life.Lubricated air will extend the life ofthe cylinder, but it will also wash outthe original lubrication. So once lubricated air is introduced, it must alwaysbe used, and the lubricator should beregularly checked and maintained.

Cushions. Cylinders that move athigh speed need some sort of decelera

tion method to keep them from slam-ming when they reach end of stroke.Otherwise, the impact loads can quickdestroy the cylinder or machine. Inmost cases, standard cushions builtinto cylinders work well by takingadvantage of airs compressibility orcontrolling airflow exiting the cylin-der. If the cylinder sees high forces, orchanging loads and speeds, externalshock absorbers are another option.

Rod covers. Consider protectingexposed rods from abrasion and corro

sion that could damage the rod surfacand, in turn, the rod seal. In especiallydirty environments, protect the rod wa cover such as a rod boot or bellows.

Custom designs

Its no secret that some of the biggefluid-power manufacturers offer literthousands of different kinds of cylin-ders. But that can carry a take-it-or-leave-it attitude: If one doesnt fit you

Side-mounted cylinders do notabsorb forces along their centerlines.They produce a turning moment underload that tends to rotate the cylinderabout its mounting bolts. If the cylinderis not well secured to the machine andthe load is not well guided, side loadwill be applied to the rod gland and

piston bearings. To avoid this problem,ensure side-mounted cylinders havea stroke length at least as long as thebore size and machine frame membersare strong enough to resist bending.

Sometimes, even properly mountedcylinders face side loading of the rods. Ifoffset loads cannot be avoided, guidedcylinders have guide rods and guideblocks mounted parallel to the pistonrod, or dual piston rods. They pro-vide precise, controlled linear motion,

especially when the unit is

subject to high side loads, andreduce rod and piston bend-ing and uneven seal wear.

Filtration. Reliability is oneof the strongest reasons to usefluid power, and proper filtra-tion is the key to maximizingreliability and longevity. Airexiting a compressor can carrycondensed water, oil carryoverfrom the compressor, and solidimpurities from internal wear

and scale and rust within pipelines. The contaminants cancause problems at every pointof use, and should be removedby installing suitable filters.

Likewise, hydraulic fluidwill become contaminated over timewith water, dust and dirt ingression,and wear particles from moving partswithin the system. Thus, installing andmaintaining the proper hydraulic fil-ters for a given system is a must. Hightemperatures and extreme pressures or

operating conditions can also degradethe fluid itself, so testing the fluid ona regular basis is also recommended.

Seals. Cylinders are exposed to avast range of temperatures, pressures,fluids, and environments that all affectseal performance and life. Fortunately,many seal designs and materials areavailable to meet most any applicationrequirement. Typical seals range frominexpensive O-rings for low-pressure

Guided cylinders such as this TESeries linear thruster have guiderods and guide blocks mountedparallel to the piston rod. Theyprovide precise, controlledlinear motion despite side loads,minimizing rod bending and unevenseal wear.



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movements an application requires.Materials. The operating environ-

ment is the major factor that governsmaterial choice. Cylinders are typi-cally made of steel, aluminum, stain-less steel, brass, or engineered plas-tics, along with protective coatings,plating, paint, or anodized finishes.

Seal materials. Cylinder manu-facturers use a variety of methods toseal the piston and rod. Designers canspecify alternative seal materials for ap-plications that operate in extreme highor low ambient temperatures or are ex-posed to caustic chemicals or radiation.

Combo units. Sometimes its simplerfor OEMs to specify cylinders, valves,filters, regulators, wiring, and othercomponents as a single, factory built andtested package. It ensures the compo-nents are properly sized and assembled,

and simplifies and speeds installation.When considering a custom design,

first find a manufacturer that will workclosely with you throughout the process;present fresh ideas and suggest work-able alternatives when necessary; have adedication to quality solutions executedwith quick delivery; and take a strong in-terest in the outcome of the final project.

A suitable manufacturer also ad-heres to established industry-widequality control standards when ap-

plicable including ISO 9001 cer-tification, which provides a uniformframework for quality assurancethat is recognized worldwide.

application, you have to redesign yourmachine or downgrade performance.

Other manufacturers have a dif-ferent mindset, more willing to tailorbasic lines of products to precisely fit acustomers needs. Theyve set up theirengineering and manufacturing op-erations to offer application assistance

and streamline production prototypesfor quick delivery at little or no ad-ditional cost over off-the-shelf parts.

Engineers may equate custom withexpensive. But the results are often bet-ter performance, reliability, and uptime,as well as lower overall equipmentand operating costs. Consider customunits when standard catalog compo-nents cannot give the requisite level ofperformance, complex motion controlis needed, or equipment design con-straints dictate special configurations.

Sometimes it involves combining stan-dard or modified components in a novelconfiguration, other times it meansinventing something completely new.

Engineers have a lot of leeway intweaking a cylinder to best fit an ap-plication. Considerations include:

Port sizes and locations canbe adjusted in custom designs.

Dimensions. NFPA and ISO haveestablished standards for many cyl-inder dimensions, but many mod-

els also have unique dimensions.Mounting configuration. A spe-cial rigid or articulated mounting en-sures a cylinder securely attaches to amachine and can execute the specific


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Fluid-Power Actuators Basics