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BendingfatiguetestsonametallicwireropeforaircraftrescuehoistsMarcoGiglio,AndreaManesDepartmentofMechanics,PolitecnicodiMilano,ViaLaMasa34,20158Milan,ItalyReceived29July2002accepted24August2002AbstractDuringnormaloperation,theendsofhelicopterrescuehoistropes,towhichahookisattached,canbesubjecttobendingstresscausedbyvibrations.Thishappensintheeventofpartialortotalrecoveryofthehookintoitslodgingwithoutaspringloadedblockingsystem.Theswingingoftheropeendconsequentlycausesbendingoftheropeneartheterminal,withresultantfatiguestressthatcanleadquicklytobreakageordamageoftherope.Aseriesofalternatingbendingfatiguetestsusingaconstantload,similartothoseknownasBoSbendingoversheavetests,werecarriedout.Thesesimulatetheeffectiveworkingconditionsofarescuehoist.Thispaperdescribesmodificationstotestmachinery,thetestmethodandthedataobtainedstatisticalanalysisofthisdataenabledustomakearopelifepredictionwhichwasthenexperimentallyconfirmedbyrunoutsontestropes.2002ElsevierScienceLtd.Allrightsreserved.KeywordsWireropesSheaveFatigueHoistsAircraft1.IntroductionWireropes,likeropesingeneralandalsochains,arecommonlyusedfortractionloadtransmission.Despiteitswidespreaduse,wireroperemainsanextremelycomplexandlittleknownpieceofequipment.Itsconstruction,theconsiderableanddiverseinternalcontactforces,thewearduetotheseforcesandtocontactwithdrums,pulleysorsheaves,allmakeacompletelyanalyticalapproachtoreliabilityextremelydifficult.Eventoday,experimentaltestingplaysanimportantroleintheanalysisofropesubjectedtoaparticularload,especiallyasregardsfatigueanalysiswherethebreakingofstrandsresultsfromvaryingtypesoftestandload.Thepurposeofthispaperistoinvestigatethebehaviourofropeusedonahelicopterrescuehoist.Weanalysethecircumstanceinwhichthehookattachedtotheendoftheropehasbeentotallyorpartiallyrecoveredintoitslodgingandisleftfreetoswing.Incorrecthooklodgingorinsufficientpreloadingofthespringblockingsystemresultinbendingoftheendsectionoftherope,whichmaythenhit13506307/03/seefrontmatter2002ElsevierScienceLtd.Allrightsreserved.PIIS1350630702000432EngineeringFailureAnalysis102003223–235www.elsevier.com/locate/engfailanalCorrespondingauthor.Tel.390239947354fax39070638377.Emailaddressmarco.gigliopolimi.itM.Giglio.theguidewheelsofthewinchseeFig.1.Breakageanddamagetoropewithouttheapplicationofanytractionload,andthereforenototherwiseexplainable,havebeenattributedtothisparticularoperatingcondition.Thiswirerope,likeallaeronauticalcomponents,mustbeoptimisedforweighttheropehasasafetyfactorof5onthestaticfailureloadlessthanropesforcivilandindustrialusebutmustbereplacedafter1500completelowerraisecyclesunderoperatingloads.Itis,therefore,reasonabletoassumethatwearanddamageduetovibrationalswinging,evenwhenthehoistisnotinuse,canleadtoprogressiveandmarkedweakeningoftheresistantsection.Assumingaconstantpreloadequaltotheweightofthehookmultipliedbyanappropriatefactor,whichtakesintoaccountinertiaforces,westudiedthebendingfatiguebehaviourofwireropeduringcyclicallyrepeatedswingingontheguidewheels,atafixedalternatingangle,inordertoestablishwhetherthereexistsanangularvariationbelowwhichnodamagetotheropeoccurs.Theinstrumentationusedwasdesignedandconstructedspecificallyforthesetestsitreproducestheswingingoftheropeendandtheconsequentalternatingbendingfatiguestressoftheropeatthesheave.2.BackgroundWefoundnopreviouspublishedstudiesconcerningbendingtestscarriedoutwiththemethodsusedinthisexperiment.Inpublishedresearchonbending,ropehasbeenwoundoffandontoapulleyofagivendiameterandthusassumesthebendingradiusforadefinedlength.Intheexperimentaltestsperformed,thebendingradiusoftheropeonthesheavediffersfrompointtopoint.Moreover,thealternatingbendingoftheropeendinducesavariablesignbendingloadingineverysectionoftheropeineverycycle,thusdeterminingalternatingfatiguestressofthewiresincontactwiththesheaves.FatiguetestsonpulleysareforeseenundertheMilW84140standard1forhelicopterrescuehoistropes.Theropeinourcase,aropewithanominaldiameterof3/16inches,suitablyconditioned,mustFig.1.Terminalbending.224M.Giglio,A.Manes/EngineeringFailureAnalysis102003223–235perform150,000cyclesonapulleywithathroatdiameterof1.781inches45.23mm,withapreloadof22.5pounds100.08N,andmustsubsequentlyhaveresidualmechanicalcharacteristicstoallowatensilestrengthofover2000pounds8896.44N.Asimilartestintermsoftestsystem,aimedatobtainingmoredetailedinformationondeformationsinthewiresmakinguptherope,isthatcarriedoutbyRidgeetal.2,3.Inthisarticle,measurementsaretakenusingstraingaugesplacedonthewiresthickerthan1mmofacomplexsectionropeduringwindingoveratestpulley.Thediameterofthepulleyis18timesthatoftheropeandthepreloadis20ofthetensilestrengthoftherope.Itcanbeseenhow,initially,whentheropeisonlyaxiallyloaded,wiresthatshouldgivesimilarmeasurementsactuallydisplaydifferentstrainsprobablyduetoimperfectionsintheropeconstruction,andhowthesedifferencestendtodisappearquicklyduringthebendingfatiguetests.Theamplitudeandtheshapeofthestraincyclesrecordeddonotchangesignificantlyduringthefatiguetest.Theauthorsshowexperimentallyhowdegradationandimpairedqualityduetowronguse,wearandthemanufacturingprocess,consideringtheconfigurationandloadusedbyRidgeetal.,haveonlyalimitedinfluenceonbendingfatiguelife,whichdependsmainlyontheropesnominalconstructionandontherope/sheavediameterratio.NabijouandHobbs4analyse,inawidevarietyofcases,thefatiguebehaviourofaropewoundonsmalldrum,subjectedtoapreloadproportionaltoitstensilestrength40–60.Theirstudylooksintotheinfluenceofparametersincludingthegeometryofthegrooveinwhichtheroperests,thetypeofworkingofthepulleythroatandthetreatmentcarriedoutonthewiresoftherope,aswellastheeffectofthediameteroftheropeandofanylubricatingorcontaminatingagentssuchaswaterorsand.Itisinterestingtonotehowtwoidenticalsheavesmadebytwodifferentmanufacturersproducedifferentropefatigueendurance.Thisconfirmsthatfatiguelifecanalsothereforedependonthemicroscopiccharacteristicsofthesurfaceswithwhichtheropecomesintocontact.FromNabijouandHobbswork,itcanbeseenthatforlowloadandhighsheavecurvaturecomparedtotheropediameter,thepredominantformofbreakageisthatduetowearcausedbycontactwiththepulley.Inthecaseofhighloadandsmalldiametersheave,ontheotherhand,thefirsttypeofweartoappearisthatduetoslidingbetweenwires.Theinternalslidingbetweenwires,insuchaconfiguration,hasalsobeenobservedanalyticallybyNabijouandHobbs5.ThetwogroupsofarticlesoutlinedabovearenotconflictingbuthelpprovideamorethoroughunderstandingofthecomplexphenomenonofBoS.ThefatiguelifeandtheconsequentfailureofaBoStestedropedepend,unequivocally,onfrettingwire–wire,wire–sheave.Fatiguelifeseems,however,tobeinfluencedmainlybythenominalloadconditionsanddiameterratios,andonlyslightlybyparticularunfavourableinitialconditionsorprogressiveinitialyielding.TheseconditionsunderlineasubstantialdifferenceoftheBoSfromtheaxialfatigueloadwherenotchesandwearresultinaconsiderablereductioninthefatiguelife.TheimportanceofwearduetofrettingwasfirsthighlightedbyStarkeyandCress6.Theirstudywas,infact,thefirsttoidentifytheexistenceoftwodifferentcriticalregionsinaropethefirstonerelativetothelinearcontactbetweenadjacentstrandsinthesamelayer,thesecondrelativetolocaland,therefore,moredangerouscontactbetweentwowiresbelongingtotwoadjacentlayers.Instatictests,theeffectofwearhasaconsiderableinfluenceontensilestrength.DeSilvaandFong7analysethesignificantresidualloadreductioninropessubjecttowearbothinthelaboratoryandduringactualoperation,andcomparetheirresultswithcurrentstandardsonthereplacementofropesubjecttowear,inparticularISO4309.Inasimilarway,Kuruppuetal.8analysethesamereductioninmechanicalcharacteristicsandreachsimilarbutlessdrasticconclusionstothoseofthestudybyDeSilvaandFong.Inbothstudies,however,itcanbeseenhowametallicarealossofafewpercentagepointsresultsintensilestrengthreductionsofabout10.Aspreviouslystated,publishedresearchcontainsnostudiesonfatiguebehaviourwhenthewindingangleoftheropeonthesheaveislimitedandwhenthereisaverylowconstantpreload,inotherwords,undertheconditionsappliedinthistestproject.M.Giglio,A.Manes/EngineeringFailureAnalysis102003223–2352253.TheropetestedTheropetestedcorrespondstoMilW83140specificationsasTypeI,19C27stainlesssteel,preformed,nonrotatingforhelicopterrescuehoists.Cablesofthisconstructionconsistofonecoreofsevenstrandsofsevenwireseachforatotalof49wiresandanouterlayerof12strandsofsevenwireseachforatotalof84wires.Theinnercoreislanglay,leftlaytheouterlayerisregularlay,rightlay.Thetotalnumberofwiresinthecablesis133.Theropediameteris3/16inch4.76mm.4.Thetestmachine4.1.DescriptionofthemachinebeforemodificationThemachineusedforthetests,theMETROCOMT6ALtestfrequency0.5Hz,maximumangularvariation180C14,wasmadetoperformalternatingtorsionfatiguetests.Thetestropesareplacedbetweentwoclamps,oneofwhichturnsarounditsaxistheotherisfixedseeFig.2.Themachinewasmodifiedtobeabletocarryoutthealternatingbendingtests.Themachinesmobileclampingheadwasreplacedbyaropeterminalsupportspeciallydesignedforthetests,andaservicestructure,necessarytocreatethedesiredtestconditions,wasadded.4.2.RopeterminalsupportThesupportallowsbothanoptimallockingoftheropeterminalandthecreationofactualloadconditions.ThesupportFig.3wasdesignedwiththeaimofmaintainingthemachinesabilitytoperformrotationswithamaximumangularvariationofC6180C14.Twosheavesaremountedonthesupport,symmetricallyplacednearthemachinedriveshaftaxis.Theyareanexactreproductionofthesheavesofahelicopterrescuehoistand,thankstoanassemblywithtwodegreesoffreedom,theirpositionisthesameasthatwhichtheyhavewheninoperationduringbending,theropeliesoverthesesheavesseeFig.4.Thesupportisfixedtothetestmachinebymeansofahubshaftcouplingwithtangentialkeyandblockingscrew.ThetwosheavesbetweenwhichtheroperunsaremountedonshaftswithTeflonbearingsinordertolimittheeffectsofwearthesheaveshavebeenhardenedandpolishedtoavoidthepossibilityofsurfacedamageseeFig.5.AsurfacehardnesstestgaveanHRCof63.ThesupportwasdesignedandbuiltwithtolerancestoguaranteeC15parallelismbetweendriveshaftandsheaveaxisC15perpendicularitybetweentheaxisofthecableandthatofthesheavesC15flatnessofthesupportfixingholeaxis.Fig.2.METROCOMT6ALbeforemodification.226M.Giglio,A.Manes/EngineeringFailureAnalysis102003223–235
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