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PISTON PUMPFAILURE ANALYSISA Guide to Analyzing AxialPiston Pump Failures1TYPICAL FAILURE MODES2 PowerInnovative FluidCONTAMINATIONl PISTON SEIZED IN BORE, PULLS SHOE OFF. l PISTONS SHOW FINE SCRATCHES,DULL FINISH.ContaminationFluid IssueOver PressurizationImproper Inlet ConditionCase To Inlet DifferentialMiscellaneousl EXCESSIVE WEAR ON SWASHBLOCK FACE,SHOE FACE AND VALVE PLATE FACE.l EXCESSIVE WEAR ON SADDLE BEARINGS.l HYDRO-DYNAMIC BEARING WORN.l CONTROL UNSTABLE: PISTON STICKING,COMPENSATOR SPOOL STUCK OR WORN,ORIFICE IN CONTROL PISTON PLUGGED.3 PowerInnovative Fluid4 PowerInnovative Fluid1
CONTAMINATION EXAMPLESA Clear Indication of ContaminationVertical scratches on thepiston barrel.Note Metal Transfer5 PowerInnovative Fluid6 PowerInnovative FluidSHOE WEAR PLATE SHOWINGSIGNS OF WEAR DUE TO DIRTDull and scratchy finishNormal Appearance:A Mirror FinishScratchy UnevenWearAbnormal Appearance:A Steel Wool Look7 PowerInnovative Fluid8 PowerInnovative Fluid2
FLUID ISSUEViscosity too Low, Operating Temperature too High,Not a Hydraulic Fluid, Fluid Breaking DownllllllVALVE PLATE RUN ON A BADFLUIDPISTON SEIZED IN BORE, PULLS PISTON SHOESHOE FACES AND OR VALVE PLATE FACESMEAREDBALL WORN THROUGH SHOE RETAINERSHAFT SEAL LEAKSCAVITATION, AIR ENTRAINMENTEXCESSIVE SADDLE BEARING WEARNote theSmearedRunningSurface9 PowerInnovative FluidWORN FULCRUM BALLTypical Failure onLow LubricityFluidThe fulcrum ballis between therotating group andthe shoe retainerplate.11 PowerInnovative Fluid10 PowerInnovative FluidWORN SADDLE BEARINGNote the material has delaminated12 PowerInnovative Fluid3
OVER PRESSURIZATION/SPIKESSpike Relief Always RecommendedlExcessive swashblock face, cylinder to valve plate wearlPistons broken at neckslBroken shaft where cylinder rideslBroken tail shaft (dual)lControl pin brokenlCylinder cracked between kidneyslControl O-ring, gasket failure (PVW)EXCESSIVEWEAR ON CYLINDERFACECracksWouldAppearBetweenKidneys13 PowerInnovative Fluid14 PowerInnovative FluidTORSIONAL FATIGUE:BENDING FATIGUE FAILURENote Cone Shaped FailureNote Straight Break15 PowerInnovative Fluid16 PowerInnovative Fluid4
IMPROPER INLET CONDITIONSCAVITATION ON COMPRESSIONBRIDGEWhat are the improper inlet conditions for piston pumps?lCAVITATION ON VALVE PLATE FACElNOISE (MARBLE SOUND)17 PowerInnovative Fluid18 PowerInnovative FluidDIFFERENTIAL CANNOTEXCEED 10 PSICASE TO INLET DIFFERENTIALCase to inlet differential refers to the pressure differencebetween the inlet pressure (vacuum) and the case drainpressure.The pressure in the case has to be vented back to the reservoirbelow fluid level. As the pump creates flow the inefficienciesare drained into the case. If the case drain line or fittings areundersized this will cause the case pressure to increase.Examples of high case pressure differential:lSHOE EDGES ROUNDEDlSHOES LOOSE ON BALLSlSWASHBLOCK WEAR, HALF MOON SHAPElEXCESSIVE WEAR ON BACK OF SHOEFLANGESl19 PowerInnovative FluidSEAL RETAINER BENT20 PowerInnovative Fluid5
SHOE LOOSE ON BALLThis is an example of ashoe that became looseon the ball and withrepeated high casepressure fatigued theslipper and finallycracked . This is calledShoe Lift, the soundassociated with it wouldbe constant machine gunnoise typically worse atfull stroke.21 PowerInnovative FluidTHE FINAL STAGE OF SHOE LIFTSTAGES OF DAMAGE DUE TOSHOE LIFT3rd Stage2nd Stage1st Stage22 PowerInnovative FluidHALF MOON SHAPED MARKSA sure sign of shoelift23 PowerInnovative Fluid24 PowerInnovative Fluid6
ROLLED SHOES: THE TELLTALE OF SHOE LIFTWEAR ON SHOE RETAINER FROMSHOE FLANGES25 PowerInnovative FluidBENT SEAL RETAINERAs can be seen in the picture the retainer is clearly bent, thatis from the case pressure pushing the seal against retainer.The surface on the shaft where the seal rides will have agroove cut in it making unusable again.27Innovative Fluid Power26 PowerInnovative FluidSEAL RETAINERAfter removal of the shaft assembly, the seal retainer isaccessible. The seal retainer is placed between the outboardside of the pump and behind the bearing.28 PowerInnovative Fluid7
SEAL RETAINERSEAL RETAINER REMOVEDSeal retainer in positionon the pump shaft.29 PowerInnovative FluidOnce the seal retainer has been removed the shaft seal isexposed for removal. Investigate the seal closely and youprobably notice that it has been pushed out of the pumphousing a little distance.30Innovative Fluid PowerMISCELLANEOUSlFRETTED DRIVESHAFTINPUT SHAFT BROKEN MISALIGNMENTlCONTROL INSTABILITY AIR62l 1998 Commercial Intertech Corp.PUMP VIBRATES MISALIGNMENT62. This splined drive shaft has been worn by frettingcorrosion. To prevent this damage the drive coupling should becleaned regularly followed with the application of a light coat ofsynthetic grease.31 PowerInnovative FluidInnovative Fluid Power8
HELPFUL HINTS FORANALYZING BASKET CASESKEYS TO FAILURE ANALYSISlThe last piece to fail will have the least amount of damage.lTry to piece together the broken parts. This may seem like alwaste of time but many times you will observe things whichcan help you determine the original cause of failure.Don’t go into a problem with a preconceivedidea about the cause.lDon’t assume anything, verify everything.lDon’t overlook the obvious.lGet the history of events: What recently changed ? What was going on just prior to failure ? How long was pump running for? Talk to the operators.33 PowerInnovative Fluid34 PowerInnovative Fluid359
PISTON PUMP FAILURE ANALYSIS A Guide to Analyzing Axial Piston Pump Failures 2 Innovative Fluid Power3 TYPICAL FAILURE MODES Contamination Fluid Issue Over Pressurization Improper Inlet Condition Case To Inlet Differential Miscellaneous Innovative Fluid Power4 CONTAMINATION l PISTON SEIZED IN BORE, PULLS SHOE OFF.
