机械设计专业毕业设计文献翻译履带牵引装置的液压传动Word格式.docx
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机械设计专业毕业设计文献翻译履带牵引装置的液压传动Word格式.docx
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Somedesignadvantages
Hydrostaticoperationofvehiclessuchasdozersandloadersimpliesuseofinenginetooperateahydraulicpumporpumps.Highpressurehosesareusedtotransmitthefluidpressureandflowtoseparatedrivemotorswhichcanbelocatedsomedistancefromengineandpump.Theenginecanberunatitsmostefficientconditionswithareversible,variableswashplatepump.Thehydraulicmotorscanalsomakeuseofvariableswashplates,simplifyingbothhydraulicdesigns.Onlythedesignadvantagesofsuchasystemarecoveredhere,thoughserviceprocedures,lifeexpectancy,andoperatorsallcanbenefitfromotheraspectsofsuchdrives.
Amajordesignobjectiveintheconstructionequipmentmanufacturingbusinesssincethe1960shasbeenmoreefficientuseofinstalledhorsepower.Lowefficiencymeansenergywastedasheatwhich,initsturn,usuallyrequiresextrapowertodrivefansandmaintaintemperature.Themoreinefficientthemachine,thehotteritgets,soheatservesasameasuringscaleforoverallefficiency.
Withinamachine,enginehorsepoweristransformedintoforce(psi)andspeed(gal/mm).Forpurposesofcomparingdifferentsystemsonecanusetheexpression:
Enginehp∝ForcexSpeedwhereforcemeansdrawbarpull(lbs)andspeedmeanstravelspeed(mph).
Sinceallmachineshaveinternallosses,theidealforce/speedhyperbolacannotbeachieved.However,themoreefficient,thecloseramachinesystemapproachestheidealcurve,andthemoreworkitcanproducefromagivenamountofinstalledenginehorsepower(Fig.1).
Toappreciatetheefficiencyofahydrostaticdrive,itisnecessarytolookatthepowerutilizationofconventionaldrives
.
Mechanicaldrive-Inamechanically-drivencrawlerdozer,gearsmustbeswitchedfordifferentspeedsatvaryingresistances(drawbarpull).Onceingear,thereisapositive,non-variabledriveconnectionbetweentrackandengine.Infact,speedinacertaingearcanonlybevariedbychangingtheenginespeed.Therefore,thedrawbarpullandefficiencyineverygearspeedparallelthatoftheengine.
Theadvantageofamechanicaldriveisthat,atacertaintravelspeedineachgear,itsefficiencywillbehigherthanthatofahydrostaticdrive.Thusifacrawlertractorcouldworkatthosespeedsonly,therewouldbenoneedforotherdrivesystems.Infact,agriculturalandforestusessometimesofferapossibilityforadjustingtheresistancetofitthemechanicaldrive,i.e.,depthofplowingornumberofblades.Toimprovetheiradaptabilityfurther,thesespecializedmachinesareusuallyequippedwithmanygearspeedswhichareclosetogether.
Disadvantagesofthesedrivesare:
Themachinesarenotverywellsuitedfornormaltractorapplicationswithconstantlyvaryingresistances.
Theirefficiencydropsconsiderablyassoonasthetravelspeedchanges.
Speedselection,andwithitenginehorsepoweruse,isleftcompletelytotheoperator,s
judgement.AsshowninFig.2,atacertainresistance,manyofseveralspeedsmaybechosen
-influencingefficiencyandhorsepowerusage.
Torqueconverter
Torqueconverters(liquidclutches)arenon-positivepowertraincomponents,andrunattheirmaximumefficiencyatonlyoneratioofoutputrpm(n2)toinputrpm(n1).Wheneverthisratiochangesduetovaryingdrawbarpullrequirements,efficiencydropsrapidly(Fig.3).
Thisfigurealsoshowstheefficiencycurveofahydrostaticdriveforcomparison.Atthepeakofthetorqueconvertercurveitisalsopossiblethatitsefficiencyisactuallyhigherthanthatofthehydrostaticdrive.Tousethis,torqueconvertersworkincombinationwithapower-shifttransmissionandtheindicatedefficiencycurveofthetorqueconverterrepeatsitselfwitheverygear-speedstep.Theindicatedhydrostaticdrivecurvecoversthemachinescompletetravelspeedrange.
Itshouldalsobenotedthatduringmachineaccelerationfromastandstillordirectionalchange,thehydrostaticdrivebecomesmuchmoreefficientmorerapidly.
Power-shifttransmission-Combinationofatorqueconverterwithapower-shifttransmissionimprovesenginehorsepoweruseconsiderablycomparedtoamechanicaltransmission.Butduetotheefficiencybehaviorofthetorqueconverter,power/speedcurveswillstillnotparalleltheidealcurve.Again,useoftheinstalledenginehorsepower(efficiency)dependsagreatdealontheoperatorwho,atagivenresistance(drawbarpull),mayselectanyofseveralpossiblespeeds(Fig.4).
Torquedividertransmission(lock.upconverter)--Toimproveitsenginehorsepoweruse,thetorquedividertransmissionusestheadvantagesofboththepowershiftandmechanicaltransmission.Since,incertainspeedbrackets,themechanical(direct)driveoffersalittlehigherefficiency,thetorqueconverterwillloc
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