Chapter3planarlinkage3.1Typesofthefour-barlinkages3.2Variationofrevolutefour-barmechanism3.3Applicationofthelinkages3.4CharacteristicsAnalysisoffour-barlinkages3.5VelocityanalysisbytheMethodofinstantcenters3.6Kinematicsynthesisoffour-barlinkagesChapter3planarlinkageMechanismsareusedinagreatvarietyofmachinesanddevices.Thesimplestclosed-looplinkageisthefour-bar,whichhasthreemovinglinksandfourpinjoints.43.1Typesofthefour-barlinkagesTypes:1)Crank-rockermechanism2)Double-crankmechanism3)Double-rockermechanismCrank-rockerDouble-crankDouble-rockerCraneBlender冲床3.2Variationofrevolutefour-barmechanismTransformationtechniquesorrulesthatcanbeapplied1.Replacingarevolutepairwithaslidingpair2.Takingdifferentlinksastheframe3.Enlargearevolutepair4.Interchangingguide-barandslidingblock1.Replacingarevolutepairwithaslidingpaire:representseccentricoroffsetForthesamekinematicchain,differentkindsofmechanismswillbegeneratedbyholdingdifferentlinksfixesastheframe.Suchkindsofvariationsarecalledinversions.2.Takingdifferentlinksastheframe3.Enlargearevolutepair4.Interchangingguide-barandslidingblock3.3ApplicationofthelinkagesCanyougivesomeexamplesusedinmachinery?Crank-rocker:Blender,Crank-sliderblock:pistonengine,punchOthers：busdoordrivingmechanism，crank-shaperhand-operatedwellpump3.3Applicationofthelinkages(ct)Pleasetrytotypethequitedifferentapplicationsabove.Theyrepresentthreedifferenttasks:1)Pathgeneration:concernwiththepathofatracerpoint.Forexample?2)functiongenerator:therelativemotionbetweenlinksconnectedtogroundisofinterest.Forexample?3)motiongeneration:theentiremotionofthecouplerlinkisofconcern.Forexample?3.4CharacteristicsAnalysisoffour-barlinkages1.TheGrashofcondition(曲柄存在条件)2.FourbarQuick-Return（急回特性）3.Transmissionangle&Pressureangle（传动角与压力角）4.Togglepositions&Dead-points（极限位置与死点）5.Mechanicaladvantage（机械增益）1.TheGrashofcondition(曲柄存在条件)Itisusedtopredicttherotationbehaviororrotatabilityofafour-barlinkage’sinversionsbasedonlyonthelinklengths.Twokeypositions1)LetS=lengthofshortestlinkL=lengthoflongestlinkP=lengthofoneremaininglinkQ=lengthofotherremaininglinkThenifS+L<=P+Q,thelinkageisGrashofandatleastonelinkwillcapableofmakingafullrevolutionwithrespecttothegroundplane.Otherwise,thelinkageisnon-Grashofandnolinkwillbecapableofacompleterevolutionrelativetoanyotherlink.2)TheGrashofconditionareindependentfromtheorderoftheS,L,P,Qwhenthemechanismisassembled.Discussion(1)Case1S+L
P+Q:Allinversionswillbetriple-rocker,nolinkcanfullyrotate.Case3S+L=P+Q:Atthechangepoints,theoutputbehaviorwillbecomeindeterminate.Thecommonusedmechanismsareparallelogramandantiparallelogram.Groundthelinkoppositetheshortest*Double-rockerGroundtheshortestlinkDouble-crankGroundeitherlinkadjacenttotheshortestCrank-rockerFrameMechanismDiscussion(2)Changepoints,andindeterminatebehavior.Howtosolvethisproblem?Table2-3Typecriteriafortherevolutefour-barmechanismsCrank-rockerAdjacenttotheshortestlinkDouble-rocker(withfullyrotate)OppositetotheshortestlinkDouble-rocker(withoutfullyrotate)orTriple-rockerDouble-crankTheshortestlinkChange-pointNon-GrashofGrashofS+L=P+QS+L>P+QS+L
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