AutomatedGearPrognosticsforHelicopterTransmissionsAvinashV.SarlashkarKennethBarlowWangmingLu,Ph.DMichaelJ.Roemer,Ph.DImpactTechnologies,LLCNavalAirSystemsCommand200CanalViewBlvd.,Suite300AIRRochester,NY14623PatuxentRiver,MDABSTRACTGearfailuresinhelicopterpowertransmissionsareverycritical,bothfromtheviewpointofthesafetyoftheoccupantsaswellastheabilityoftheassettocompletethemission.Twoprominentfailuremodesexistforspiralbevelgearscommonlyusedinhelicoptertransmissions.Thesefailuremodesarea)geartoothseparationfromahigh-cyclefatiguecrackoriginatinginthetoothfilletandb)surfacedamageresultingfromscuffingdamageongeartoothcontactingsurfaces.Anautomatedprocesstoperformgearprognosticshasbeendevelopedthatencompassesthecriticalelementsnecessaryforreliableestimationoffailureprogressionratesinspiralbevelgearsandprovideassociatedconfidencebounds.TheSpiralBevelGearPrognosticSystem(GPS)describedhereinautomatesthegenerationofgeargeometriesandassociatedfiniteelementmodels,estimationoftoothcontactloads,3-Dfracturemechaniccalculations,leadingtopredictionofremainingusefullife.ThesoftwaresystemisdesignedtorunonaPCrunningMicrosoftWindowsandseamlesslyusesthefiniteelementprogramANSYSandthe3-DfracturemechanicsprogramFranc3D.AlsopresentedaretheresultsofscuffingtestsforthegearmaterialAISI9310.Inparticular,testresultsarepresentedwithrespecttodegreeofscuffingdamageandtheassociatedfeaturessuchasoildebrisconcentrationandchangestocoefficientoftraction.Keywords:Prognostics,HCF,SpiralBevelGears,FractureMechanics,Franc3D,ANSYSINTRODUCTIONSpiralbevelgearsareusedextensivelyinhelicopterpowertransmissions.Therefore,overallreliabilityandsafetyofhelicoptersisdictatedbythereliabilityoftheseindividualspiralbevelgears.Therearetwoprimarymodesofgearfailures.FirstmodeistheHighCycleFatigue(HCF)drivencrackinitiationfollowedbycrackpropagation.Crackstypicallyinitiateathighlystressedfilletregionatthebaseofageartooth.Itisalsopossiblethatacrackcouldinitiatefromatoothsurfaceasaresultofsurfacedamagesuchaspitting,or,alternately,acrackcouldalsoinitiatefromamachiningmarkorothersurfacedamagethatcouldoccurduringassembly.Theworstoutcomeinvolvescrackpropagationthatultimatelyleadstotoothseparationandsubsequentlossoffunctionalityofthegearbox.Thesecondfailuremodeinvolvessurfacedeteriorationthatcanbetheresultofoneormoreofthefollowing:wear,pittingandscuffing.Typicalsymptomsassociatedwiththesurfacefailureareincreaseinvibrationandnoiselevel.Inthespiralbevelgearprognosticsystem(GPS),scuffingandsubsequentwearwereevaluatedsince,comparedtootherformsofsurfacedamage,itismoresevereandthereforecanrapidlyreducetheremainingusefullife(RUL).Scuffingisabnormalwearoftoothsurfacesduetocombinationofhighcontactpressuresandslidingvelocitiesattheinterface.Highcontactpressuresalongwithhighslidingvelocitiesbetweencontactingsurfacescanresultinrepeatedlocalizedweldingandfractureleadingtoseveresurfacedeterioration.Calculationofremainingusefullifeofanin-servicespiralbevelgearsystemdependsonthemodeoffailure.Forthefirstfailuremodedescribedabove,thetotalusefullifeiscomposedoftwoparts:crackinitiationlifeandcrackpropagationfatiguelife.Forthesecondfailuremode,servicelifeisdefinedbysurfacedurability,i.e.,thetimerequiredtoaccumulatecertainamountofsurfacedamagesuchas,forexample,depthofwearonthecontactsurfaces.Surfacedurabilitydependsonavarietyoffactorssuchasrollingandslidingvelocities,contactpressure,typeoflubricantandtypeoflubrication,materialandassociatedmaterialheattreatmentetc.DevelopedbyImpactTechnologies,LLCanditscontractorsunderaPhaseIISBIReffort,thisprototypeGPSisaimedtodemonstratearobustphysics-basedtoolthatmodelsphysicsofdifferentfailuremodesforspiralbevelgearsandatthesametimehastheabilitytousevibrationandothersystem-levelfeaturestoprovideenhancedprognosticsforthespiralbevelgearsystems.Thisprototypesystemincludesseveralmodulesfor:a)Automatedgenerationofspiralbevelgeargeometriesandtheassociatedfiniteelementmodels,b)Geartoothcontactanalysis,c)Crackinitiationand3-Dcrackpropagationcalculations,d)Gearsurfacescuffingmodel,ande)AnexampleofintegrationwithHealthandUsageMonitoringSystem(HUMS)data.AttheheartGPSisthegearprognostickernelthatinteractswithdifferentmodules.Theseindividualmodulesareresponsibleforspecificautomationtasks.Figure1showsoverallorganizationofGPS.Figure1Architectureofthespiralbevelgearprognosticsystem(GPS)TheprogramisdevelopedtorunonaPCrunningMicrosoftWindowsandseamlesslyinteractswiththecommercialfiniteelementanalysisprogramANSYS,3-DfracturemechanicsprogramFranc3D,andGleasonsgeartoothcontactanalysisprogramT900.ANSYSisprimarilyusedasthemainstructuralanalysisandpost-processingmodule,Franc3Disusedfor3-DcrackpropagationanalysisandtheT900programisusedforgearfiniteelementmodelgenerationandgeartoothcontactanalysis.OutputofGPSistheestimatedremainingusefullife,measuredastimeinoperatinghours.Basically,inputdataincludesthespiralbevelgeargeometry,torque,materialproperties,andothersystem-levelfeaturedatathatcanbeusedto“tune”thepredictedanalyticalresults.Mainly,therearetwotypesoftestdata:vibrationsignalbasedfeatures,andexperimentallyobtainedscuffingprogressioninformation.Fusionofnumericallypredictedresults(ANSYS,Franc3D,T900)withthosefromtestandreal-timemeasurementdata,ifprocessedproperly,cansignificantlyimprovetherobustnessandaccuracyofRULpredictions.Inthefollowingsections,basicconceptsbehindthedevelopmentofGPSarepresentedonamodule-by-modulebasis.Inthenextsection,geargeometryandfiniteelementmodelgenerationmodulesarepresented.Thisisfollowedbypresentationofthegeartoothcontactanalysismodule.BothmodulesusetheT900programdevelopedbyGleasonCorporation.Amodifiedversionofthe3-Dfracturemechanicprogram,Franc3D,developedbyFractureAnalysisConsultant,Inc(FAC)isusedtoperform3-Dcracksimulation.ThismodifiedversionofFranc3DistightlycoupledwithANSYStoimprovecomputationalefficiency.TheGPSkernelcollectsinformationfromallsourcesandthen“fuses”thisinformationtogeneratebestRULestimateforthein-serviceasset.Thebasicideainfusionistouseasmuchavailableevidence,suchasinspectiondata,toimproveonthequalityoftheresultsthatwouldbeproducedbyanalyticalmodelsalone.AUTOMATICGENERATIONOFGEARGEOMTRYANDFEMODELSGenerationofgeargeometryandassociatedfiniteelement(FE)modelsservesasthestartingpointforGPS.AcustomversionofGleasonsT900programisusedtogeneratethisinformationinanautomatedfashion.Figure2showsgear/pinionfiniteelementmodelsgeneratedusingGleasonsprogram.ThefiniteelementmodelsareimportedinANSYSforsubsequentanalysispurpose.Figure2Fin