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.docx

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.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

Hydrostaticdriveforcrawlers:

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