机械毕业设计英文外文翻译无级变速器CVTWord文档格式.docx
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机械毕业设计英文外文翻译无级变速器CVTWord文档格式.docx
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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.Fromfigure10and11canbeseenthatinputspeedalsohasaninfluence
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