railwayconsiststheeldtoinherentlyandotherscomputervisionguidance5,6.ThesetruckshavebettervisibilityfordriversbuttheyarenotfundamentalsolutionstoovercomeMechanismandMachineTheory45(2010)18921896ContentslistsavailableatScienceDirectMechanismandMachineTheorytheshortagesofthegeneralmaststructures,andtheproblemofhighselfweighthasnotbeencrackedcompletelyeither.Inordertoimprovethedrivingcomfortandsafetyforthedriversandtoreduceenergyconsumption,thispaperfocusesontheinnovativedesignoftheliftingmechanismforforklifttrucks.Aspatialmulti-linklift-guidancemechanismisproposed,andthenanewkindofliftingmechanismbasedonitispresented.Thismechanismnotonlyexpandsthevisioneldofthedriverbutalsoreducesthewholeweightofthevehicle.2.Propositionofalift-guidancemechanismExpandingthedriversvisionhasalwayschangingtheformsofthemast2,3,whileAsiswellknown,aplanarRRR-open-chaSupposethattwosuchplanarRRR-linkagesRRR-open-chainlinkagesareconnectedwithCorrespondingauthor.Fax:+861062788308.E-mailaddress:0094-114X/$seefrontmatter2010ElsevierLtd.doi:10.1016/j.mechmachtheory.2010.08.002beenanimportantresearchproject.Somekindsoftruckshaveimprovedvisibilityviautilizethevisualguidancemethodsofmobilecamera-spacemanipulation4orcomponentswillbadlyaffectthedriversbetweenpedestriansandtrucksaredueanimportantroleinsupportingtheloadsenoughanditsself-weighthastobeincreasedtheunnecessarywasteofenergy.stations,warehouses,portsandfactoriesforloading,unloadingandconveying.Aofachassisandaworkdevicewhichcanbetiltedandliftedvertically.followingmajordisadvantages.First,themastsystemcomposedofseverallargeofvisionbecauseitlocatesinfrontofthedriver.Manyaccidentsinvolvingcollisionsbadvisibilitiesoftheforklifttrucks1,2.Inaddition,themastsystemplaysguidingtheforkframetoliftvertically.Soitsstrengthandstiffnessmustbehigh.Theweightoftherearequilibratorisincreasedasaresult,whichsurelyimproves1.IntroductionForklifttrucksareusuallyusedatgeneralweight-balancedforklifttruckHowever,thegeneralforkliftshaveShortcommunicationInnovativedesignoftheliftingmechanismsforforklifttrucksJian-YiWang,Jing-ShanZhao,Fu-LeiChu,Zhi-JingFengDepartmentofPrecisionInstrumentsandMechanology,TsinghuaUniversity,Beijing100084,PRChinaarticleinfoabstractArticlehistory:Received17June2010Receivedinrevisedform3August2010Accepted4August2010Availableonline1September2010Forklifttruckisoneofthemostimportanttoolsinlogistics.However,thegeneralmastsystemofaforklifttrucknotonlyrestrainsthedriversvision,butalsoincreasesthewholeweightofatruckanddecreasesthefueleconomy.Therefore,thispaperfocusesontheinnovativedesignofanewliftingmechanismforforklifttruck.Firstly,aspatialmulti-linklift-guidancemechanismisproposed.Andthen,undertheconstraintsofthismechanism,themobilityoftheforkandforkframeisinvestigatedintheory.Lastly,anewliftingmechanismbasedonitispresentedandcomputersimulationisusedtodemonstratethefeasibilityofmotion.Thismulti-linkliftingmechanismtakesadvantageofexiblecabledriveandrigidbodyguidance,whichnotonlyprovidestheoperatorwithawidereldofvisionbutalsoreducestheequilibrateweightofavehicleandthereforeimprovesthefueleconomy.2010ElsevierLtd.Allrightsreserved.Keywords:ForklifttruckSpatialmulti-linkmechanismRigidbodyguidanceFlexibledrivingmethodjournalhomepage:/locate/mechmtinlinkagethatconsistsofthreerevolutepairsandtwolinksgeneratesplanarmotions.areplacedintwoplaneswithacertainnonzerosubtendedangle.TheendsofthetwoasamerigidbodythroughrevolutepairsshowninFig.1.Link1isthebaseandrigid(J.-S.Zhao).Allrightsreserved.1893J.-Y.Wangetal./MechanismandMachineTheory45(2010)18921896body4isconnectedwiththebasethroughtwoparallelRRR-kinematicchains,ABCandDEF.Itiseasytondthat,thesetwokinematicchainsarebothplanarRRR-kinematiclinkages.Thereforethetrackofrigidbody4mustbeparalleltobothofplaneandplane.Thatis,rigidbody4canonlymakeafreetranslationparalleltotheintersectionline,mn,ofthetwoplanes.Asaresult,thetrackofrigidbody4isastraightline.Slider-crankmechanismisacommonmechanismthatappliesRRR-open-chainstructure.ItiswidelyusedandexamplesofitsFig.1.Spatialmulti-linkstructure.applicationsareeasilyfoundingasolineanddieselengines7.However,intheapplicationsofslider-crankmechanism,deadpointspossiblyleadtoactuatingfailure.Inordertoimprovethestabilityandavoidthedeadpoints,thispaperreplacestheRRR-kinematicchainswithRPR-kinematicchains,whicharecomposedoftworevolutepairsandoneprismaticpairandcanalsoonlygenerateplanarmotionswithoutdeadpoints.SupposethetwoverticalplanesthatthesupportblocksandtheirRPR-kinematicchainslocatedaresymmetricalwithrespecttothelongitudinalsymmetricalplaneofthevehicle.AsisshowninFig.2,thesubtendedanglebetweenthetwoplanesisdenotedby(0bb180).TwoRPR-kinematicchainsareplacedinthetwoplanes,respectively.EveryRPR-kinematicchainincludestworevolutepairsandoneprismaticpair,aconnectingrodandaslideway.Theconnectingrodandtheslidewayareconnectedthroughaprismaticpair.TheforkframehastwoextrudedrevolutepairsconnectedwiththeendsofthetwoRPR-kinematicchains.Asisdiscussedabove,insuchaspatialmulti-linkstructure,thetrackoftheforkframeisinastraightline.Inaddition,onecanimprovethestrengthandstiffnessbyaddinganothersimilarkinematicchainineachverticalplane,inwhichthesupportblockslocated,toconnecttheforkframeandthetruckchassis.Meanwhile,theupperandlowerkinematicchainsareconnectedbyconstraintrods.Asthetrackoftheupperandlowerkinematicchainsareparallel,theconstraintrodsprovideredundantrestraintsandimprovethestiffnessandstabilityofthelift-guidancemechanismandtheload-carryingcapacity8.Nowthekinematicprinciplesofthespatialmulti-linklift-guidancemechanismcanbeanalyzedintheory.Inordertoinvestigatethedegreeoffreedom(DoF)oftheforkframe,onecanestablishaCartesiancoordinatesystem,wherexoy-planeistheplanedeterminedbytheaxesofrevolutepairsAandD,theoriginofcoordinatesystemistheintersectionpointofaxesofAandD,x-axisistheaxisofrevolutepairA.ThecoordinatesystemisshowninFig.3.AssumethatthesubtendedangleoftheaxesofrevolutepairDandrevolutepairAisdenotedby(0obb180o),thedistancefromrevolutepairsAandDtotheoriginofcoordinatesystemareequalanddenotedbya.ThecoordinatesofAandDarea00andacosasin0,respectively.Inaddition,thecoordinatesofCandFcanbeexpressedasayCzCandxFyFzF,individually.Itisnotdifculttondthat,theaxesofrevolutepairsAandCareparallel,andsoaretheaxesofrevolutepairsDandF.s1=100TdenotesthedirectionvectoroftheaxesofrevolutepairsAandC,s2=cossin0TdenotestheunitdirectionvectoroftheaxesofrevolutepairsDandF,s3=0yCzCTdenotesthedirectionvectorofprismaticpairBands4=xFbcosyFbsinzFTdenotesthedirectionvectorofprismaticpairE.Accordingtothescrewtheory9,theterminalconstraintscrewmatrixofakinematicchaincanbeobtainedbysolvingthereciprocalscrewequation$TE$=01whereThetheterminalFig.2.Anewlift-guidancemechanism.1fork；2forkframe；3extrudedrevolutepair；4connectingrod；5constraintrod；6slideway；7supportblock.1894J.-Y.Wangetal./MechanismandMachineTheory45(2010)18921896000001inaccordancewithEq.(1),onecanobtaintheterminalconstraintscrewmatrixcomposedofasetofbasescrewsofconstraintscrewsystem.$ABC=1000000000102435T31000zCyCTherefore,$isakinematicscrewmatrixand$istheterminalconstraintscrew.kinematicscrewmatrixofakinematicchain,ABC,canbeexpressedas$ABC=$A$B$CC138=1000000000yCzC2435T2Fig.3.Cartesiancoordinatesystem.Itproves1895J.-Y.Wangetal./MechanismandMachineTheory45(2010)18921896Undertheconstraintsofthelift-guidancemechanism,theforkframecanbeliftedupanddownperpendiculartotheground.3.ImplementThissimulateAswhileisshownoftheworks,InraisedmechanisperpendicuThisCable-driveweightrequiremMeanwhilthiskindthembecomes$CF=000001T6thattheforkframehasonlyoneDoFalongz-direction,i.e.,thetrackoftheforkframeisrestrictedtoastraightline.AccordingtoEq.(1),solongas0bb180,thekinematicscrewmatrixoftheframeCFcanbesolelyobtained$CF=$ABC$DEFC2C35Consequently,theterminalconstraintscrewmatrixonCFcanbeexpressedas$DEF=cossin0000000sincos0000001454Similarly,theterminalconstraintscrewmatrixofkinematicchainDEFisobtained:23TFig.4.Implementationofthenewliftingmechanismandcomputersimulations.1forkframe；2frontpulley；3frontsupportingrods；4backsupportingrods；5cables；6backpulley.ationofanewliftingmechanismsectionwillpresentanewliftingmechanismforforklifttruckbasedonthelift-guidancemechanismproposedaboveanditsmotioninacomputer.isdiscussedabove,thelift-guidancemechanismrestrainsthetrackoftheforkframeinastraightline.Inordertoliftgoodsnotaffectingthedriverseldofvision,onecanusewindlassandsoftsteelcablestolifttheforkandforkframe.ThestructureinFig.4(a).Onecansetawindlassattherearoftheforklifttruckandplacessomepulleysandsupportingrodsonthetopcab.Oneendofeachcableisconnectedwiththeforkframeandtheotherendisxedtothewindlass.Whenthewindlassthecableswillliftordroptheforkframe.ordertoverifywhetherthiskindofforklifttruckcanachievethedesiredmovements,especiallyinsuretheforkframetobevertically,Pro/engineersoftwareisutilizedtosimulateitsmotion.Fig.4(a),(b)and(c)showdifferentpositionswhenthemliftsth