机械毕业设计英文外文翻译492无级变速器.docx

机械毕业设计英文外文翻译492无级变速器.docx

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机械毕业设计英文外文翻译492无级变速器

附录A

ABSTRACT

HighclampingforcelevelsreducetheefficiencyoftheContinuouslyVariableTransmission（CVT）.However,highclampingforcelevelsarenecessarytopreventslipbetweenthebeltandthepulleys.Ifasmallamountofslipisallowed,theclampingforcelevelcanbereduced.Toachievethis,slipinaCVTisinvestigated.Frommeasurementsonanexperimentalsetup,Tractioncurvedataandefficiencymeasurementsarederived.AmodeldescribingslipinaCVTisverifiedusingmeasurementswithabeltwithincreasedplay.Itisfoundthatsmallamountsofslipcanbecontrolledinastablewayonthesetup.ThetractioncurvewasmostlydependentontheCVTratio.Efficiencyisfoundtobehighestfor1to2%slipdependingontheratio.Themodelisinreasonableagreementwiththemeasurements.

1.Introduction

ApplyingaContinuouslyVariableTransmission（CVT）inanautomotivedrivelinehasseveraladvantages.ACVTcanoperateatawiderrangeoftransmissionratios,thereforetheenginecanbeoperatedmoreefficientlythanwithasteppedtransmission.Also,aCVTdoesnotinterruptthetorquetransmissionwhenshifting.Thisgivesamoresmoothridethanasteppedtransmissiondoes.AV-beltbasedContinuouslyVariableTransmissionusesabeltorachaintotransmittorquefromadrivingsidetoadrivensidebymeansoffriction.ThelayoutoftheCVTandtheV-beltareshowninfigure1.Thevariatorconsistsoftwopulleyswhicharewedgeshaped.BychangingthepositionofthepulleysheavestheratiooftheCVTcanbeadjusted.TheV-beltconsistsofblockswhichareheldtogetherbytworingsthatinturnexistofasetofbands.Toachievetorquetransmissionsufficientlyhighclampingforcelevelsareneededtopreventslipinthevariator.Becausethetorquelevelisnotexactlyknownatalltimes,sincenotorquesensorisusedduetocostconsiderations,asafeclampingforcelevelbasedonthemaximumpossibleloadismaintainedatalltimes.Thissafetylevelisbaseduponassumedmaximumshockloadlevelsfromtheroad,likebumps,andtheenginetorque.Inordertomaintainthesesafetylevelshigherclampingforcelevelsaremaintainedthenneeded.HigherclampingforcelevelscausemorelossesintheCVT.Theselossesarecausedbyincreasesinpowerconsumedbythehydraulicpump,byincreasesinthelossesduetoslipinthebeltifapushbeltisused,andbyincreasesindeformationinthebeltandinthepulleys.Furthermorewearisincreasedandfatiguelifeisreduced.Inordertoreducetheseclampingforcelevelsamethodisneededtodetectslipinthevariatorfastenoughtopreventslipfromreachingdestructivelevels.Amethodtodetectandcontrolslipisthereforeneeded.InthispapermeasurementsarepresentedofthetractioncurveinaV-beltCVT.

Figure1.LayoutofaCVTandametalpushbelt

2.Tractioncurve

TheV-belttypeCVTutilizesfrictiontotransmitpowerfromtheprimarypulleytothesecondarypulley.Thetractioncurveisthedimensionlessrelationshipbetweentransmittedtorqueandtheslip.ThemaximuminputtorquethatcanbetransmittedbytheCVTisdependentontheappliedclampingforce.Thetractioncoefficientisthereforechosentobeadimensionlessvalue.Thetractioncoefficientμisdefinedas:

（1）

Inwhich

representstheinputtorque,

representsthesecondaryrunningradiusofthebeltonthepulley,

representsthesecondaryclampingforceand

isthepulleywedgeangle.

Figure2.CVTtorquetransmissionscheme

Thesecondvariableinthetractioncurveistheslipinthevariator.

Slipisdefinedas:

（2）

Where

istheangularspeedofthesecondaryaxle,

istheangularspeedoftheprimaryaxleand

isthegeometricalratio,whichisdefinedby:

（3）

istherunningradiusontheprimarypulley.

2.1Tangentialslip

Slipisdefinedinequation2.WhentheCVTtransmitspoweracertainamountofslipcanbemeasuredalmostlinearwiththeappliedtorque.ThisiscalledthemicroslipregimeoftheCVT,becausetractionisstillincreasinginthisregimewithincreasingslip.Themicroslipiscausedbygapsbetweentheblocksontheidlepartofthedrivingpulleyasshowninfigure3.Onthedrivingpulleyanidlearcexistswherenoslipoccurs.Alsoanactivearcexists（seefigure2）,whereslipoccursrelativetothetotalplayinthebeltandtheactivearclength.However,whenthemaximumtorquecapacityoftheCVTisreachedslipwillincreasedramatically.Thissituation,macroslip,isnotstableduringnormaloperationoftheCVT,becausethetractioncoefficientdecreaseswithincreasedslipspeed.Itisassumedthatthetotalgapdtisevenlydistributedalongtheidlearcofthedrivingpulley.Thetraction

Figure3.Gapsinthebelt

curve（figure5）showsthattorquetransmissionincreasesalmostlinearlywithanincreaseinslip,untilacertainmaximumtorqueisreached.dtcanbeestimatedbyaddinganinitialgapdototheincreaseinbeltlengthduetotheinternalstressesinthebandsandadecreaseinlengthoftheblocksduetothecompressiveforces.

（4）

Tocalculatetheslipcausedbythesegapswecanusethefollowingequations:

（5）

（6）

Inequation5,aistheidlearc,disthewidthofabeltelementanddtisthetotalgapbetweentheelementsinthebelt.Tocalculatetheamountofslipthetotalgapdthastobeknown.Thiseffecthasaninfluenceonthetractioncoefficientinthemacroslipregime.Whenmacroslipoccursthetractionwilldecreasewithincreasingslip.TheStribeckeffectismodelledusingequation9.

（7）

（8）

（9）

Equation7givesavalueforthefrictioncausedbyviscousfrictioncomponent.Equation8givesavalueforthecoulombfrictioncomponent.a0,1,c0andv1arecoefficientswhichcanbechosentomatchthemeasuredvalues.Withtheseequationswecanderiveslipandtractionfrommeasureddataasshowninsection4.WithAsayama[1995]wecanobtainthetensionandcompressionforcedistributionneededtocalculatethelengtheningofthebelt.Also,wecancalculatetheidlearcfromthismodel.Fromtheidlearc,thelengthofthebeltandtheinitialgapwecancalculateanestimateforslipinthebeltforagivenload.

2.2Radialslip

Notonlyslipintangentialdirectionoccurs,butalsoslipinradialdirection.Thefirstreasonforradialslipisspiralrunning.Whenthebeltrunsalongthearcofcontacttheradiusatwhichitrunsisnotconstant.Thiseffectiscausedbypulleydeformation.Onetypeofdeformationisthebendingoftheaxlebetweenbothpulleysheaves.Thebeltisnotfullywrappedaroundthepulley,thereforetheresultingnormalforceoftheblocksonthepulleyisnotaxial.Thiscausesabendingmomentintheaxle.

Asecondeffectisthebendingofthepulleyitself.Thiseffectismostlydependentonthelocalnormalforceexertedonthepulleybytheblocks.Thiseffectissmallwhenthebeltisrunningonasmallrunningradius,butonalargerunningradiusthiseffectissignificant.Thesecondreasonforslipinradialdirectionisduetoshifting.WhentheCVTisshiftedtoadifferenttransmissionratio,radialslipisforced.Thisisdonebychangingtheclampingforceratio.Theamountofradialslipthatisforceddependsontheshiftingspeedandthe（primary）angularspeed.

3.Experimentalsetup

Intheexperimentsthegeometriccvtratioisfixedandtheclampingforcesareconstant,thetractioncoefficientthendependsonlyontheslipinthesystem.Thetractioncurvecanbeconstructedfromoutputtorqueandslipmeasurements.Thetestrigmotorsdeliveramaximumtorqueof298Nmwithamaximumspeedof525rad/s.BothmotorsareequippedwithaHeidenhainERN1381incrementalrotaryencoderwith2048pulses/rev.ThetorqueatbothsidesismeasuredusingaHBMT20WNtorquesensor.Themaximumallowabletorqueis200Nmwithspeedsupto1050rad/s.Aseparatehydraulicunitisusedtoprovidetherequiredflowandpressurefortheclampingforces.Figure4givesaschematicoverviewoftheexperimentalsetup.

4.Experimentalresults

ThegeometricratiooftheCVTwasfixedduringtheexperimentsusingaso-calledratioringandthelimitsoftheprimarypulley.Thisratioringlimitthemovementofthepulley.Primaryandsecondarypressurewasheldconstant（clampingforceswereheldconstant）duringtheexperiments.

Figure4.Experimentalsetup

4.1Tractioncoefficient

Thetractioncoefficientwasmeasuredatdifferentratios,atdifferentprimaryspeedsandatdifferentpressures.Infigure6and7canbeseenthatthetractioncoefficientdependslittleonprimaryspeedorsecondaryclampingpressure,butmostlyonthetransmissionratio,ascanbeseeninfigure5.Anincreaseinclampingforcecausesmoreslip（seefigure8）.Thisiscausedbyanincreaseintensioninthebandsandthereforeinanincreaseinlengthofthebelt.Thiscausestheplaytoincrease.

Figure5.Tractioncoefficientat300rad/s,ratiolow（0.4）,Medium（1.1）andoverdrive（2.26）

4.2Efficiency

Theefficiencydependsonpressureandonratio.Fromfigure12canbeseenthatanincreaseinpressurecausesadecreaseinefficiency.Thiseffectiscausedbytheinternalfrictioninthebelt.Slipbetweentheblocksandthebandsalsocausesastrongdependencyonratio（seefigure9）.Efficiencyisclearlyhigherinmediumthaninoverdriveorlow.Inmediumnoslipoccursbetweentheblocksandthebands,butinoverdriveorlowthebandsslipovertheblocks.Athighclampinglevelsthiseffectisgreater,becausethenormalforcesactingbetweentheblocksandthebandsincreaselinearlywithanincreaseinclampinglevel.Fromfigure10and11canbeseenthatinputspeedalsohasaninfluenceon