Hydraulic Cylinder Troubleshooting

Avoiding Rod Seal Leakage andPremature Wear

Bulletin 1242/1-GB

1Damaged piston rods orrod bearings are the mostcommon cause of rodseal failure. The usualcauses of such damageare poor alignmentbetween the cylinder andits load, resulting in sideloading; or a bent pistonrod, resulting from the useof an undersized rod in athrust application.

Cylinders are designed toprovide linear force andmotion to a guided load.The path which theguided load travels mustnot impose a significanteccentric load on the rodor the piston rod bearing,or the bearing surfaceswill be damaged and fluidleakage and reducedbearing life will result.

The maximum thrust forcewhich a cylinder shouldprovide is limited by itsrod diameter and overalllength. In long-strokecylinders, care should betaken to ensure that thepiston rod is capable ofhandling the thrust forcesgenerated in theapplication, and that thecylinder is adequatelysupported. Note, also,that a head end mountingstyle will provide greatercolumn strength than acap end mounting, due tothe smaller distancebetween mounting points.Full guidance for pistonrod size selection iscontained in Parkershydraulic cylinder productcatalogues.

SYMPTOMS: Rod sealsand bearings are worn onone side only (Fig. 1).Piston rods and cylindertubes are worn or galledon opposite sides,particularly at or near full extension (Fig. 2). Pistonrods should be straightwithin 0.15mm per metre;T.I.R. and surface finishesshould be below 0.4microns R.M.A.

SOLUTION: There is nosubstitute for precision inmachine alignment.Alignment couplers,spherical bearings andfloating flanged rod ends may help in someapplications, but they

The industrialworld has alteredits perception ofhydrauliccylinders. Pistonrod seal leaks -even small ones -are no longeracceptable.Todaysapplicationsdemand that,after anynecessaryrunning in, thepiston rod shouldshow no ring ofoil. That demandcan be met byunderstandingand applying thefollowingprinciples.

Most rod sealleaks are causedby:

l a damaged rodor rod bearing

l contaminatedfluid

l temperatureextremes

l chemical attack

l high pressure

l low pressure

Damaged Piston Rods or Rod Bearings

often require largerdiameter piston rods or longer stop tube lengthsto make up for the higherbearing stresses causedby the loss of rigiditybetween the piston rodand its moving load.Parker cylinders areavailable with substantialload bearing capabilities,and severe failures of components indicate theneed for a thorough review of the application.

Fig.1 Gland and seal showing wear resulting from excessive side loading.Brass paste on seal is amixture of gland material and hydraulic fluid.

Fig.2 Galling on this piston rod is evidence of excessive sideloading.

The load bearingcapabilities of rod bearingsare also influenced by theirmaterial. Extremely hardrod bearings are morelikely than soft bearingmaterials to cause damageto the piston rod itself.Some manufacturersemploy a hard bearingmaterial because therelative positions of theseal and the rod bearingcause the bearing to runfairly dry. The trade-off is between risk of damageto the rod vs. damage to the rod bearing. A new piston rod is amade-to-order part andreplacement requires a

complete stripdown and rebuild of the cylinder. Anew rod gland andbearing is a stock itemand is easily replaced.Parkers rod bearing andseal cartridge design has a flooded bearingsurface that does not rundry; is easily replaceable, and is made of a materialthat provides long lifewhen correctly mountedand aligned.

To improve the supportavailable to long strokecylinders, the use of additional mountings andtie rod supports should beconsidered.

Contaminated Fluid

Fig.3 Contamination damage is apparent in this rod seal, where the serrations are worn completelyaway. The seal on the right is a new one, shown for comparison.

Contaminated fluid can also cause premature rodseal failure. Abrasiveparticles suspended in thefluid can damage the sealand the piston rodsurface, while airbornecontamination can be drawn into a cylinder via afaulty wiper seal.

Water is a commoncontaminant in mineral oilsystems, affecting thelubricity of the fluid andcausing some of the mostwidely used seal materialsto age harden attemperatures above65C. Polyurethane sealsare subject to hydrolysiseffects in high water-based fluids at temperatures above50C, leading to a loss of

hardness and tensilestrength which allows fluidleakage from the rod seal.

Air is often overlooked asa fluid contaminant, butaerated oil can causephysical damage to pistonrod seals. Pressureshocks in systems with

high cycling speeds cancause air bubbles tobecome highly chargedwith heat energy, acondition often referred toas dieselling. Thiscondition is particularlypronounced in vertical, rod-up applications where arapid increase in hydraulic

pressure can cause intense,localised heating of thebubbles at the lip of theprimary seal. The presenceof air in the fluid can alsointensify the transmission ofvibration which, in turn, canlead to other forms ofsystem failure.

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(continued)

Contaminated Fluid (continued)

Fig.5 Piston seals damagedby air in oil.

Fig.4 Particle contaminationhas scored the surface of thiscushion bush, resulting inloss of cushioning effect.

SYMPTOMS: Moderateto extreme leakage, withhydrostatic leakage.Particle contaminationappears as score markson the seal (Fig. 3) androd bearing, and on thecushion sleeve (Fig. 4).Water in the oil may causesome seal materials tocrumble or disintegrate athigh temperatures (Fig. 7).Air bubble damageappears as small scoremarks on the innersurface of the highpressure side of the seal,and the heel of the sealmay appear to benibbled (Fig. 5). Thetrumpet-shaped score iscaused by expansion ofthe bubble as it works itsway through the seal. Airbubbles may also attackthe outer diameter lip of arod seal, or may burn ahole through the heel ofthe seal.

SOLUTION: Particlecontamination may bereduced throughappropriate filtrationmethods, provided thefluid circulates from thecylinder to the reservoir inan efficient manner. Todetermine if a cylinder willefficiently flush oil andcontaminants to thereservoir, compare thevolume of oil contained inthe cylinder to the volumeof oil contained in thehose between the cylinderport and the directionalcontrol valve. If the hoseholds more oil than thecylinder, particlecontamination probablywill not flow to thereservoir to be filtered out.Instead, it will remain inthe cylinder and scour theseals. One solution is torelocate the directionalvalve closer to the cylinderto reduce the line volumeto less than that of thecylinder. This allows oil tobe flushed from thecylinder to the reservoir, tobe filtered and cooled.Other solutions to particleand water contaminationinclude the installation ofmetal rod scrapers or rodbellows to slow theingress of contaminants.Paying close attention tothe alignment of thecylinder on installation willalso reduce the rate atwhich the cylinderproduces its own fluidcontaminants (Fig. 1).

All hydraulic oil containsdissolved air. Dissolved aircannot be bled out, and,in fact, does not cause aproblem. The symptomsdescribed above occurwhen air comes out ofsolution in the form ofbubbles. Conditions thatcause air to come out ofsolution are: rapiddecompression, high flowthrough an undersizedport (turbulent flow) orover-running loads. If acylinder that was properlyinstalled and bledcontinues to haveproblems with air, ie: if itseems to generate air, itis likely that one of thesethree conditions isoccurring. This isparticularly true ofapplications involvingwater glycol fluids, sincethese are capable ofdissolving more air thanmineral oil fluids.

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Extreme temperatureapplications pose twochallenges. First, thetemperature itself may limitthe choice of sealmaterials and geometries,eg: polyurethane sealsshould be restricted to amaximum of 50C, inorder to avoid hydrolysisof the seal compound.Second, the fluids used insuch applications oftenhave less lubricity thanmineral oil-based fluids.

Extreme Temperatures

SYMPTOMS: Seals thathave been overheated orexposed to too low atemperature have thesame appearance: hard,cracked and brittle (Fig. 6).

SOLUTION: Parkercylinders are available withhigh and low temperatureseal options. In someapplications, temperaturecontrol of the cylinders oroil must be provided toensure effective operation.

Fig.6 The cracked outer sealing lip of this wiper seal showsdamage caused by heat.

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Some fluids react withseal materials and attackthem chemically. Fluidmanufacturers often listcommon seal materials ascompatible withoutdefining the suitability ofthe material for a dynamicseal. It may be acceptablefor a static seal such as acylinder body O-ring toswell by 10% but, if adynamic rod seal swellsby 10%, the result can bea dramatic rise in frictionand accelerated wear.One common mineral oil-based fluid has anadditive that shrinks awidely-used seal materialby 10%. Again, the resultis a leak, particularly indynamic applications.

Chemical Attack

SYMPTOMS: Seals feelsticky or gummy in somecases, or may appear dryand cracked (Fig. 7).Shrunken seals may rattlein their cartridge grooves,while swollen seals mayprove difficult to removefrom the cylinder.

SOLUTION: Ask yourParker representative forhis recommendations for a particular fluid. He willconsult with the fluidmanufacturer and sealsupplier to resolve theproblem. Often, a simpletest can be performed todetermine the effects of a particular fluid/sealcombination.

This type of leakage israrely encountered with the latest generation ofpolyurethane seals. Withrod seals of other materials,high pressure leakage isusually the result ofinadvertent pressureintensification across thepiston. A severe meter-outflow restriction at the headend of a cylinder with anoversized piston rod canexpose the rod seal to aback pressure equal totwice the system pressure.Continued operation underthese conditions cancause rapid seal wear due to excess friction,extrusion of the primaryseal or, in extreme cases,

High Pressure Leaks

failure of the rod cartridgeor the retainer.

SYMPTOMS: Extremeamounts of leakage occur.Rod seals may be wornuniformly around theirentire inner sealingsurface. Extrusion damagemanifests itself as flashor feathered edges at theinner diameter of the heelof the seal (Fig. 8).Mechanical failure of sealsupport lands in rodcartridges or of cartridgeretainer bolts can alsooccur.

Fig.8 Wiper seal showing highpressure extrusion damage.

Fig.7 Before and after. Thispolypak rod seal has beendamaged by water, acommon contaminant inmineral oil systems.

SOLUTION: If excessivepressure is caused byintensification due to aflow restriction, therestriction should beremoved. If the condition persists, a special glandand seal design should be

considered to reduce theextrusion gap, permittingthe seal to withstand thehigh pressures.

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Conclusions

The key to a successful dry rod cylinder application is the ability to control or

eliminate undesirable conditions, or to select a cylinder with rod seals that can

tolerate those conditions. While there is no universal seal, Parkers engineers

will help you to select the right combination of

seals and rod bearing materials to ensure

dry rod performance and long service life in

your application.

Problems with leakage atlow operating pressuresare uncommon withpolyurethane rod seals dueto their low energisingpressure requirement.With other classes of rodseal material, low pressureleaks may occur inapplications that have nometer-out flow control atthe rod end of the cylinder.With the rod extending, thedrive pressure may be 200bar, but the back pressurein the cylinder may be lessthan 20 bar. Someelastomeric lipseals requirea minimum pressure levelto become fully energised,and only work at fullefficiency with backpressures above 30 bar.Below that level, the sealmay hydroplane and fail toseal against therod surface.

Low Pressure Leaks

Fig.9 The rodwiper mayextrude (top) butthe primary rodseal looks perfect(right), so noleakage occurswhen the cylinderis hydrostaticallytested.

SYMPTOMS: The rodruns wet, with a collar ofoil. The rod wiper mayextrude, (Fig. 9) but theprimary rod seal isundamaged (Fig. 9, inset)so that no leakage occurswhen the cylinder ishydrostatically tested.This condition should notbe confused with therunning in periodrequired by some lowfriction seals, which maynot give full dry rodperformance until around100 cycles have been completed.

SOLUTION: Raise theback pressure to above30 bar, or substitute a rodseal with better lowpressure sealingcharacteristics. Note thatsome loaded seals mayexhibit excessive frictionlevels or may otherwisedisrupt the normaloperation of a primary andsecondary sealing system.

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Hydraulics 498

Hydraulic Cylinder TroubleshootingDamaged piston rods or rod bearings Contaminated FluidExtreme TemperaturesChemical attackHigh pressure leaksLow pressure leaksCylinder Division sales offices