清华大学机械原理英文 (4).ppt

Chapter3planarlinkage 3 1Typesofthefour barlinkages3 2Variationofrevolutefour barmechanism3 3Applicationofthelinkages3 4CharacteristicsAnalysisoffour barlinkages3 5VelocityanalysisbytheMethodofinstantcenters3 6Kinematicsynthesisoffour barlinkages Chapter3planarlinkage Mechanismsareusedinagreatvarietyofmachinesanddevices Thesimplestclosed looplinkageisthefour bar whichhasthreemovinglinksandfourpinjoints 4 3 1Typesofthefour barlinkages Types 1 Crank rockermechanism2 Double crankmechanism3 Double rockermechanism Crank rocker Double crank Double rocker Crane Blender 冲床 3 2Variationofrevolutefour barmechanism Transformationtechniquesorrulesthatcanbeapplied1 Replacingarevolutepairwithaslidingpair2 Takingdifferentlinksastheframe3 Enlargearevolutepair4 Interchangingguide barandslidingblock 1 Replacingarevolutepairwithaslidingpair e representseccentricoroffset Forthesamekinematicchain differentkindsofmechanismswillbegeneratedbyholdingdifferentlinksfixesastheframe Suchkindsofvariationsarecalledinversions 2 Takingdifferentlinksastheframe 3 Enlargearevolutepair 4 Interchangingguide barandslidingblock 3 3Applicationofthelinkages Canyougivesomeexamplesusedinmachinery Crank rocker Blender Crank sliderblock pistonengine punchOthers busdoordrivingmechanism crank shaperhand operatedwellpump 3 3Applicationofthelinkages ct Pleasetrytotypethequitedifferentapplicationsabove Theyrepresentthreedifferenttasks 1 Pathgeneration concernwiththepathofatracerpoint Forexample 2 functiongenerator therelativemotionbetweenlinksconnectedtogroundisofinterest Forexample 3 motiongeneration theentiremotionofthecouplerlinkisofconcern Forexample 3 4CharacteristicsAnalysisoffour barlinkages 1 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 Twokeypositions 1 LetS lengthofshortestlinkL lengthoflongestlinkP lengthofoneremaininglinkQ lengthofotherremaininglinkThenifS L P Q thelinkageisGrashofandatleastonelinkwillcapableofmakingafullrevolutionwithrespecttothegroundplane Otherwise thelinkageisnon Grashofandnolinkwillbecapableofacompleterevolutionrelativetoanyotherlink 2 TheGrashofconditionareindependentfromtheorderoftheS L P Qwhenthemechanismisassembled Discussion 1 Case1S LP Q Allinversionswillbetriple rocker nolinkcanfullyrotate Case3S L P Q Atthechangepoints theoutputbehaviorwillbecomeindeterminate Thecommonusedmechanismsareparallelogramandantiparallelogram Discussion 2 Changepoints andindeterminatebehavior Howtosolvethisproblem Table2 3Typecriteriafortherevolutefour barmechanisms 2 FourbarQuickreturn 急回特性 Thetogglepositionsaredeterminedbythecolinearityofthetwoofthemovinglinks Theyalsodefinethelimitsofmotionofthedrivenrocker 2 FourbarQuickreturn 急回特性 DefinitionoftheQuick Return 急回 Itisusedtodescribethefactthattheforwardstrokeisfasterthanthatofthereturnstroke Thetimeratio TR definesthedegreeofquick returnofthelinkage TR 2 1 3 Transmissionangle Pressureangle 传动角与压力角 WhatistherelationbetweentheFandFnor Ft WhatisthefunctionoftheFn Ft follower Transmissionangle DefinedastheanglebetweentheoutputlinkandcouplerTakenastheabsolutevalueoftheacuteangleofthepairofanglesattheintersectionofthetwolinksVariescontinuouslyfrommin tomax valueAmeasureofthequalityofforcetransmissionatthejointPressureangle Theacuteanglebetweenthedirectionsoftheforceandthevelocityofthepointreceivingtheforceonthefollower 3 Transmissionangle Pressureangle 传动角与压力角 Canyoutellthepositionwherethefour barlinkagehastheextremetransmissionangle 3 Transmissionangle Pressureangle 传动角与压力角 OneextremevalueoccurswhereLinks1and4arecolinearandnonoverlapping TheotherextremevaluewhereoccursLinks1and4arecolinearandoverlapping Pleasecalculatethemafterclass Forsmoothrunningandgoodforcetransmission keeptheminimumtransmissionangleaboveabout40o follower 3 Transmissionangle Pressureangle 传动角与压力角 4 Togglepositions Dead points 4 Togglepositions Dead points Dead points Whentherockeristhedriver thefollower crank iscolinearwiththecoupler thetransmissionangleequalsto0oor180o Theforceappliedtothefollowerpassesthroughthefixedpivotofthefollower Inthissituation themechanismcannotmovebyatorqueonthedrivingrocker Inthissense thesepositionsarecalleddead points Discussion Pros consofthedead points Pros insomecircumstances thedead pointsareusedtolocktheposition Forexampletheclampingdevice landingmechanisminairplane Cons thedead pointsarenotgoodtothetransmission Butwecanuseeithertheflywheelonthedrivencrank orprovidingtheduplicatelinkage900outofphasetoovercomethedead points 5 Mechanicaladvantage Mechanicaladvantage mA canbedefinedas mA Fout Fin 3 5VelocityanalysisbytheMethodofinstantcenters VelocityanalysisAnalyticalandGraphicalmethod 3 5VelocityanalysisbytheMethodofinstantcenters Definition a Apointinbothbodies b Apointatwhichthetwobodieshavenorelativevelocity c apointaboutwhichonebodymayconsideredtorotaterelativetotheotherbodyatagiveninstant Quantityofinstantcenters N n n 1 2Howtofindtheinstantcenters Graphicalmethod Positionsoftheinstantcenters Byinspection justusethedefinitionoftheinstantcenter ByKennedy srule anythreebodiesinplanemotionwillhaveexactlythreeinstantcenter andtheywilllieonthesameline Example Graphicalmethod Rjoints Pjoints Higherpairs Roll Roll slide PositionsoftheICsfortwolinksdirectlyconnectedtogether If K2equals K3 thepointKmustbeonthelineoftheP12P13oritsextension PositionsoftheICsfortwolinksnotconnectedtogether Applications TheICsofthefour barlinkage Graphicalmethod UsingAssurgrouptoanalyzethevelocityofthelinkages Analyticalmethod 3 6Kinematicsynthesisoffour barlinkages Synthesis QualitativesynthesisandQuantitativesynthesis 定性综合和定量综合 TypeSynthesisisaformofqualitative Itreferstothedefinitionofthepropertypeofmechanismbestsuitedtotheproblem 型综合 DimensionalSynthesisofalinkageisthedeterminationoftheproportions lengths ofthelinksnecessarytoaccomplishthedesiredmotions 尺寸综合 图解法 解析法 Threetypesofsynthesistasks 1 Example 汽车转弯机构 FunctionGenerationisdefinedasthecorrelationofaninputmotionwithanoutputmotioninamechanism 函数发生器 Threetypesofsynthesistasks 2 PathGenerationisdefinedasthecontrolofapointintheplanesuchthatitfollowssomeprescribedpath 轨迹生成器 Crane Blender Threetypesofsynthesistasks 3 MotionGenerationisdefinedasthecontrolofalineintheplanesuchthatitassumessomeprescribedsetofsequentialposition 运动生成器 Threetypesofsynthesistasks FunctionGenerationisdefinedasthecorrelationofaninputmotionwithanoutputmotioninamechanism 函数发生器 PathGenerationisdefinedasthecontrolofapointintheplanesuchthatitfollowssomeprescribedpath 轨迹生成器 MotionGenerationisdefinedasthecontrolofalineintheplanesuchthatitassumessomeprescribedsetofsequentialposition 运动生成器 Two positionsynthesis Twoconditions RockeroutputandCoupleroutputRockerOutput Twopositionwithangulardisplacement FunctionGeneration Exp Designafour barGrashofcrank rockertogive450ofrockerrotationwithequaltimeforwardandback fromaconstantspeedmotorinput RockerOutput Twopositionwithcomplexdisplacement MotionGeneration Exp DesignafourbarlinkagetomovelinkCDfrompositionC1D1toC2D2CouplerOutput Twopositionwithcomplexdisplacement MotionGeneration Exp DesignafourbarlinkagetomovelinkCDfrompositionC1D1toC2D2 withmovingpivotsatCandD Exp Designafour barGrashofcrank rockertogive450ofrockerrotationwithequaltimeforwardandback fromaconstantspeedmotorinput Analysisthecharacteristicsofthefour barlinkagewithequaltimeforwardandback fromaconstantspeedmotorinput Designthefour barlinkage pp84figure3 4 RockerOutput Twopositionwithangulardisplacement FunctionGeneration 2 RockerOutput Twopositionwithcomplexdisplacement MotionGeneration Exp DesignafourbarlinkagetomovelinkCDfrompositionC1D1toC2D2 UserockertomovethelinkCD UsecouplertomovethelinkCD 3 Output Twopositionwithcomplexdisplacement MotionGeneration Exp DesignafourbarlinkagetomovelinkCDfrompositionC1D1toC2D2 withmovingpivotsatCandD UsecouplertomovethelinkCD Three positionsynthesiswithspecifiedmovingpivots pp89Exp 3 5 Three positionsynthesiswithalternatemovingpivots pp91Exp 3 6 Three positionsynthesisknowncranklengthwithspecifiedfixedpivotsThree positionsynthesiswithspecifiedfixedpivots pp93Exp 3 7 Three positionsynthesis Three positionsynthesisknowncranklengthwithspecifiedfixedpivots FourbarQuick Return FourbarCrank ReturnLinkageforSpecifiedTimeRatio Exp Designafour barGrashofcrank rockertogive450ofrockerrotationwithtimeratioof1 1 25 fromaconstantspeedmotorinput Drawacrank rockermechanismandfinditstogglepositionwhenthecrankisthedriver thengivethedefinitionofthetimeratio Quiz Homework 3 18 Readbook pp40 49 pp55 60 pp76 100 pp169 172 pp188 196 pp241 pp249 261 2 32 2 35 Sketchits