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    Guidelines for selecting pneumatic cylinders

    Machine Design

    Kenneth Korane

    Kenneth J. KoraneThu, 2011-09-29 02:08

    Pneumatic cylinders come in thousands of variations. Heres a look at different types, how tocalculate force, speed, and air consumptions, available options, and when to consider special designs.

    uthored by!Sheila CampbellProduct "anager # ctuators$orgren %nc.&ittleton, 'olo.

    (dited by Kenneth J. Koraneken.korane)penton.comKey points:* Pneumatic cylinders come in many basic versions.* ll cylinders can be tweaked to better fit an application.* 'ustom designs can perform better and save money when standard cylinders dont fit the +ob.Resources:$orgren %nc.

    Pneumatic cylinders are widely used to generate force and motionon a vast range of (" e-uipment. hey can move productsdirectly or indirectly by pushing, pulling, lifting, lowering, orrotating, and can keep them from moving by clamping them in

    place.

    /ide acceptance comes in large part because cylinders are simple,economical, durable, and easy to install. hey can producethousands of pounds of force over a broad range of velocities0cycle at high speeds without overheating0 and stall withoutinternal damage. nd they readily tolerate tough conditions suchas high humidity, dusty environments, and repetitive high#pressure wash downs.

    Pneumatic actuators come in literally thousands of styles, si1es,and configurations. his variety makes more innovative#e-uipment possible, but sorting out the best cylinder for anapplication can be a bit overwhelming. Here are some key considerations.

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    Cylinder designhe basic, rod#style industrial cylinder consists of a tube sealed by end caps. rod attached to an

    internal piston e2tends through a sealed opening in one of the ends. he cylinder mounts to amachine and the piston rod acts upon the load.

    port at one end of the cylinder supplies compressed air to one side of the piston, causing it 3and thepiston rod4 to move. he port at the other end lets air on the opposite side of the piston escape 5usually to atmosphere. 6eversing the roles of the two ports makes the piston and rod stroke in theopposite direction. 6od#style cylinders function in two ways!

    Double-acting cylinders use compressed air to power both the e2tend and retract strokes, movingthe rod back and forth. his arrangement makes them ideal for pushing and pulling loads.'ontrolling the rate at which air e2hausts determines rod speed.

    Single-acting cylinders have compressed air supplied to only one side of the piston0 the other side vents to atmosphere. 7epending on whether air is routed to the cap or rod end determines whether

    the rod e2tends or retracts. he most common type is pressure#e2tended, with an internal springreturning the piston to its original position when air e2hausts. %n other designs, gravity or an e2ternalspring powers the return stroke.

    6od#style cylinders come in various designs!

    6epairable cylinders can be disassembled to replace seals andother internal components. his e2tends a cylinders life. hesedurable cylinders are generally used in rugged, heavy#dutyapplications.

    8ealed#for#life or 9disposable: cylinders have end capsmechanically crimped to the tube. %nternal components areprelubed prior to assembly. lthough they are less e2pensive tomanufacture than comparable repairable cylinders, they cannot be taken apart to repair without destroying the housing. hesecylinders are usually used in lighter#duty applications and must be replaced when they fail.

    Compact cylinders fit into smaller spaces where only a shortstroke is re-uired. hey are used in lighter#duty applications dueto the small bearing surface on which the rod slides. hey mainlycome in single#acting versions, but double#acting styles also are available.

    Guided cylinders have guide rods and guide blocks mounted parallel to the piston rod, or dualpiston rods. hey prevent the piston from rotating and provide precise, controlled linear motion 5especially when the unit is sub+ect to high side loads. %n such cases, the guides reduce rod and piston bending and uneven seal wear. hey are recommended in applications with si1eable offset loads orre-uire that the load be guided, for e2ample, down a conveyor.

    Rack-and-pinion units convert a cylinders linear motion to angular rotation that can e2ceed ;.he rotary actuators 5 with the rack mounted on the rod 5 are often used in process industries to

    operate -uarter#turn valves.

    %n addition to rod#type cylinders, other designs include!

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    s A CG.GqD A

    where s A speed, ips0 - A airflow in standard cubic feetDminute0 and A piston area, in.C

    ther factors that might affect speed include port si1es, inlet and e2haust flow through control valves, and hose or tubing si1es 5 if they create bottlenecks that restrict air flow to or from thecylinder. &ikewise, air pressure that is barely capable of moving the load will hamper speed.

    /ith any fi2ed combination of valve, cylinder, pressure, and load, it is usually necessary to havead+ustable control over cylinder speed. @low controls at the cylinder ports let users tune speed totheir application.

    @or most applications, unidirectional flow regulators installed to restrict flow out of the cylinder andpermit free flow in give the best results. regulator in the rod#end port controls e2tension speed, andone on the cap#end port controls retraction.

    Air consumption

    'alculating a cylinders air consumption is often necessary on fast#cycling e-uipment to ensure enough supply air is available. hereare two parts to cylinder air consumption. ne is the volume thepiston displaces. he other is the unswept volume from end#covercavities, cylinder ports, connecting tubing, and valves. heunswept portion is likely to be a small percentage of the total and will vary with the installation.

    %ts best to ensure the compressor has sufficient capacity to supply pneumatic e-uipment under9worst#case: conditions. therwise, air starvation at critical times will cause performance to suffer.

    Additional considerations fter si1ing a cylinder for force and stroke, engineers have a lot of leeway in tweaking a cylinder so it best fits an application. Here are a few considerations.

    Port sizes and locations are usually dictated by bore si1e, but can be ad+usted in custom designs.

    Envelope dimensions . he ational Fluid !ower Assn . and "nternational Standards#rgani$ation have established standards for many cylinder dimensions, letting engineersinterchange cylinders from different manufacturers. "any models also have uni-ue dimensions.

    Mounting configuration refers to how a cylinder attaches to the ad+acent e-uipment. he largenumber of standard mountings 5 both rigid and articulated 5 usually ensures a cylinder can e2ecutethe specific movements an application re-uires.

    Cylinder materials . he operating environment is the ma+or factor that governs material choice.Pneumatic cylinders are typically made of steel, aluminum, stainless steel, brass, or engineeredplastics. 8ome models combine several materials.

    Seal materials . 'ylinder manufacturers use a variety of methods to seal the end caps and rod.7esigners can specify alternative seal materials for applications that operate in e2treme high or lowambient temperatures or are e2posed to caustic chemicals.

    Position feedbac . "agnetic cylinders have a band of magnetic material around the circumference ofthe piston and a nonmagnetic cylinder barrel. he magnetic field travels with the piston as the rod

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    strokes in and out. "ounting reed switches on the outside of the barrel 5 one at each end, fore2ample 5 generates a signal each time the piston completes a stroke. "ore#advanced versions use& 7 transducers or &6 linear potentiometers to provide continuous indication of piston position.

    Cus!ions" %f the piston makes metal#to#metal contact with the end covers, the result is noise andpotential mechanical damage. 'ushions in cylinders prevent such contact. d+ustable cushions let

    operators control the rate at which cylinders decelerate at the end of stroke.

    8ome cylinders have integral fi2ed cushions. hey have a preengineered fi2ed cushion orifice thatrestricts e2haust airflow to slow the piston at the end of stroke. he amount of cushioning isrepeatable but cannot be altered in the field.

    nly use noncushioned cylinders at slow speeds. o operate noncushioned cylinders faster, installe2ternal stops with shock absorbers. Position these to prevent contact between the piston and endcovers.

    Cautionary tips

    n important design consideration is to keep cylinder thrust as close as possible to the centerline ofthe piston rod and free from misalignment or side thrust. 'ylinders are normally intended to pushand pull without e2cessive side loads. ff#center loads can substantially reduce the service life of rod bearings and seals.

    ff#center and side loading are caused by improper mounting, cylinder deflection under load,machine frame deflection, and rod bending or sagging 5 as well as by poor machine design.8ometimes adding an optional internal stop tube can reduce a cylinders bearing load. stop tube isa spacer between the piston and rod#end head. %t increases the distance between the piston bearingand rod bearing when the rod fully e2tends. his configuration also helps in long#stroke applications.

    %f the piston rod sees compressive a2ial loads, care must be taken to ensure its length, diameter, andload are within safe limits to prevent the e2posed rod from buckling.

    "ost pneumatic cylinders are assembled with a coating of grease on the bore of the barrel and sealsfor service with nonlubricated air. %f the compressed air supply is clean and dry, the grease will givethe seals a long life without adding oil through an airline lubricator. However, contaminated air willgradually compromise the original grease lubricant and shorten seal life.

    &ubricated air will e2tend the life of the cylinder, but it will also wash out the original lubrication. 8oonce lubricated air is introduced, it must always be used, and the lubricator should be regularlychecked and maintained.

    %ngineering custom actuators

    8tandard catalog components often cannot give (" designersthe level of performance they re-uire. %n these cases, customdesigns may be warranted. 8ometimes it involves combiningstandard or modified components in a novel configuration,other times it means inventing something completely new.'onsider this approach when!* (2act off#the#shelf hardware is not available.* he re-uired combination of components doesnt e2ist.* n application re-uires particularly high efficiency.

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    Complex motion control is needed. Equipment design constraints dictate special configurations.

    For instance, Norgren s Engineering Advantage team uses a full range of motion and fluid-controltechnologies to solve problems and create products that offer a competitive advantage. he resultsare often better performance, reliabilit!, and uptime, as "ell as lo"er overall equipment and

    operating costs. #ere are a fe" examples.

    Baghouse cylinder. Air-pollution-control equipment that uses fabric tubes, envelopes, orcartridges to capture, separate, or filter dust is $no"n as a %baghouse.& A po"er-generation plantneeded to routinel! close off chambers in its baghouse "ith a pneumatic-c!linder that providedquic$, repeatable response, could adapt to various applications, and include all the requiredpneumatic and electronic components.

    Engineers combined a standard 'F(A c!linder "ith a directional valve, air filter, pressureregulator, and electrical "iring "ith )unction box to build the baghouse-c!linder assembl!. he unit "as factor! assembled and tested. he customer onl! needed to run control "ires directl! to the

    terminal bloc$ * saving time b! eliminating the need to "ire all individual components.

    A mechanical loc$out lets the c!linder isolate individual chambers "ithin the baghouse formaintenance. +ounting the valve and electronics directl! to the c!linder provides quic$ responseand reduces air consumption. n addition, the space-saving combination leaves room for othercomponents in the air-pollution-control s!stem.

    Unit-air assembly. Combining a c!linder, valve, and base in a single, compact unit produces fastresponse, saves space, reduces air consumption, and simplifies installation and maintenance. hepac$age can be used in industrial applications such as pac$aging, material handling, printing, andpaper converting.

    Reciprocating-air motor . his c!linder valve combination has stro$e-signal valves that serve asexternal pilots. t is used to pump media such as lubricants, fiberglass, foams, resins, hot-melt glue,liquid pol!mers, "aste gases, and other liquids. he c!linders quic$ response time assistsdispensing. he self-contained pac$age simplifies installation.

    /011 (enton +edia, nc.

    Source URL: http2 machinedesign.com ne"s guidelines-selecting-pneumatic-c!linders

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