清华大学机械原理英文 (3).ppt

Chap 2Kinematicsfundamental 2 1BasicConcept2 2DeterminingDegreeofFreedom2 3Howdoesakinematicchainstransformintoamechanism 2 4KinematicDiagramofMechanism2 5PrincipleofMechanismConstitution 2 1BasicConceptsofMechanisms Degreeoffreedom 自由度 Link 构件 KinematicPairs Joints 运动副 MachineElement 机械零件 Fixedlink frame 机架 Crank couple rocker 曲柄 连杆 摇杆 KinematicChain 运动链 Mechanism 机构 Machine 机器 DegreeofFreedom 自由度 Thesystem sDOFisequaltothenumberofindependentparameterswhichareneededtouniquelydefineitspositioninspaceatanyinstantoftime Link 构件 isarigidbodywhichpossessesatleasetwonodeswhicharepointsforattachmenttootherlinks Joint isaconnectionbetweentwoormorelinks attheirnodes whichallowssomemotion orpotentialmotion betweentheconnectedlinks Joints alsocalledkinematicpairs canbeclassifiedinseveralways Bythetypeofcontactbetweentheelements line point orsurface Bythenumberofdegreeoffreedomallowedatthejoint Others Joint 铰链或运动副 Lowerpairs 低副 jointswithsurfacecontact Higherpairs 高副 jointswithpointorlinecontact Examplesofhigherpairs Force closedneedexternalforce DiagramofKinematicpairs 运动副简图 MachineElement 机械零件 minimumunitduringmanufacturing Binarylink Thislinkhasseveralmachineelements KinematicChain 运动链 anassemblageoflinksandjoints Iftheselinksareconnectedinsuchawaythatnomotionispossible alockedchain structure result Mechanism 机构 akinematicchaininwhichatleaseonelinkhasbeen grounded orattached totheframeofreference whichitselfmaybeinmotion thelinksinthekinematicchainmoveuponeachotherwithdefiniterelativemotion Machine 机器 acombinationofresistantbodiesarrangedtocompelthemechanicalforcesofnaturetodoworkaccompaniedbydeterminatemotion Characteristicsofamachine 1 Composedofman madeelements2 Theelementsarearrangedtotransmitmotion3 Cantransmitenergy andfulfillcertainworkforhumanbeing Constitution Partsofamachine normallytherearefourparts namely MotorsandDrivers Controller Workingpart Transmission Crank 曲柄 alinkwhichmakesacompleterevolutionandispivotedtoground Rocker 摇杆 alinkwhichhasoscillatory backandforth rotationandispivotedtoground Coupler connectingrod 连杆 alinkwhichhascomplexmotionandisnotpivotedtoground Ground 机架 anylinkorlinksthatarefixed nonmoving withrespecttothereferenceframe Notethatreferenceframemayinfactitselfbeinmotion Fullyrotatingrevolute Partiallyrotatingrevolute Planarmechanism alllinksofamechanismmoveinplanesthatremainsparalleltoeachother Otherwise itisaSpatialmechanism 2 2DeterminingDegreeofFreedom 1 DegreeofFreedomofaMechanism2 KinematicDiagramofMechanism 1 DOFofaMechanism 1 1ReviewthedefinitionofDOF1 2Degreeoffreedomofalink1 3DOFformulaofaplanarmechanism1 4PointsofAttentiontoCalculatingDOF 1 DOFofaMechanism 1 1DefinitionDOF Thenumberofindependentcoordinatesrequiredtodefineitsposition 1 2Degreeoffreedomofalink DiagramofLinksandJointsDiagramofMechanisms 1 DOFofaMechanism 1 3DOFformulaofaplanarmechanismM degreeoffreedomormobilityL numberoflinksJ1 numberof1DOFjointsJ2 numberof2DOFjoints 1 DOFofaMechanism Example 1 DOFofaMechanism 1 4PointsofAttentiontoCalculatingDOFA Multiplejoints compoundhinges B PassiveDOFC RedundantConstraints A Multiplejoints compoundhinges B PassiveDOF Howmanymovablelinks fulljointsandhalfjointsdoesithave DOFofcammechanism C RedundantConstraints 2 3Howdoesakinematicchainstransformintoamechanism Conclusion Atleastonelinkhasbeengrounded2 DOFequalstothenumberofthedrivers 2 4KinematicDiagramofMechanism 1 DefinitionandPurposeThekinematicdiagramservesapurposesimilartothatoftheelectricalschematicorcircuitdiagraminthatitdisplaysonlytheessentialskeletonofthemechanism Itembodiesthekeydimensionsthataffectitsmotionandexpressestheirconnectionbetweeneachothers Therearetwoforms asketch proportionalbutnotexactlytoscale andthescaledkinematicdiagram usuallyusedforfurtheranalysis position displacement velocity acceleration forceandtorquetransmission etc 2 Howtodraw 2Procedurefordrawingthekinematicdiagramofamechanism1 Analysetheconstruction motionofthemechanism Selectavisualangletoobservethetransmissionroutefromthedrivinglinkstooutputlinks 2 Determinethetypeandnumberofkinematicpairs 3 Chooseadrawingplanetoexpressthemechanism 4 Chooseaproperscale actuallength m lengthindiagram mm todraw Becarefulabouttherelativepositionsbetweenthepairs Forconvenientreference thelinksarenumbered whilethejointsarelettered Theinputandoutputarealsolabeled 5 Calculateandcheckthedegreeoffreedomofthemechanism DOF 3 7 2 9 2 1 2 5CompositionPrincipleofMechanism Weknow 1 Amechanismcanbedividedintotwoparts thebasicmechanism oneistheframe drivers kinematicpairs whichconnecttheframeanddrivers theothersaretheleft drivensystem 2 DOFofamechanism numberofthedriversThen DOFoftheleft 0 Assurgroups Outerpairs Innerpairs Grade Assurgroups Assurgroups Grade Assurgroups TheDOFoftheleftgroupiszero andnogroupcanbedividedfurtherintotwoormorezero DOFgroups thens