平面四杆机构的运动特性planarlinkagemecha

平面四杆机构的运动特性planarlinkagemechanisms主讲人：周会娜目录CONTENTS1234曲柄存在的条件Conditionsfortheexistenceofacrank急回运动和行程速比系数Quickreturnmovementandtravelspeedratiocoefficient压力角和传动角Pressureangleandtransmissionangle死点位置Dead-centerposition1曲柄存在的条件ConditionsfortheexistenceofacrankONE铰链四杆机构的三种基本形式的区别在于连架杆是否为曲柄。Thedifferencebetweenthethreebasicformsofhingefourbarmechanismsliesinwhethertheconnectingrodisacrank.曲柄摇杆机构Crank-rockermechanism空间连杆机构Spatiallinkage一、曲柄存在的条件1.Conditionsfortheexistenceofacrank双曲柄机构Doublecrankmechanism曲柄摇杆机构:1根连架杆能作整周360°回转。Crankrockermechanism:Oneconnectingrodcanrotate360°throughouttheentirecycle.曲柄摇杆机构Crankrockermechanism一、曲柄存在的条件1.Conditionsfortheexistenceofacrank双曲柄机构:2根连架杆能作整周360°回转。Doublecrankmechanism:Twoconnectingrodscanrotate360°throughouttheentirecycle.双曲柄机构Doublecrankmechanism一、曲柄存在的条件1.Conditionsfortheexistenceofacrank双摇杆机构:连架杆均无法作整周360°回转。Doublerockermechanism:Eventheframerodcannotrotate360°throughouttheentirecycle.双摇杆机构Doublerockermechanism一、曲柄存在的条件1.Conditionsfortheexistenceofacrank一、曲柄存在的条件1.Conditionsfortheexistenceofacrankcrank-rockermechanism曲柄摇杆机构Doublecrankmechanism双曲柄机构double-rockermechanism双摇杆机构曲柄存在的条件：Conditionsfortheexistenceofacrank:▶杆长之和条件：最短杆与最长杆长度之和小于或等于其它两杆长度之和。Theconditionforthesumofpolelengthsisthatthesumofthelengthsoftheshortestandlongestpolesislessthanorequaltothesumofthelengthsoftheothertwopoles.▶最短杆条件：连架和机架中必有一杆是最短杆。Shortestpoleconditionisthattheremustbeonepoleintherackandframethatistheshortestpole.一、曲柄存在的条件1.Conditionsfortheexistenceofacrank判断由不同杆作机架时四杆机构类型的方法：Themethodtodeterminethetypeoffourbarmechanismwhenusingdifferentrodsastheframe:一、曲柄存在的条件1.ConditionsfortheexistenceofacrankYNa、b、c、d(杆长a≤b≤c≤d)双摇杆机构Double-rockermechanisma+d≤b+c以最短杆相邻杆为机架Usingtheshortestadjacentpoleastheframe曲柄摇杆机构Crank-rockermechanism以与最短杆相对的杆为机架Usingthepoleoppositetheshortestpoleastheframe以最短杆为机架Usingtheshortestpoleastheframe双曲柄机构Doublecrankmechanism双摇杆机构Double-rockermechanisma、b、c、d（Rodlengtha≤b≤c≤d）例:平面四杆机构中，若各杆长度分别为a=40、b=60、c=60、d=70。Example:Inaplanarfourbarmechanism,ifthelengthsofeachrodarea=40、b=60、c=60、d=70.(1)当以a为机架，则此四杆机构为_____.

(1)Whenaisusedastheframe,thenthisfourbarmechanismis(2)当以a对面的杆为机架，则此四杆机构为_____

(2)Whenthepoleoppositeaisusedastheframe,thenthisfourbarmechanismis(3)当以a的相邻杆为机架，则此四杆机构为_____.(3)Whentheadjacentbarsofaareusedasframes,thenthisfourbarmechanismis一、曲柄存在的条件1.ConditionsfortheexistenceofacrankA.双曲柄机构DoublecrankmechanismB.曲柄摇杆机构Crank-rockermechanismC.双摇杆机构Double-rockermechanismD.曲柄滑块机构CrankslidermechanismAACBCB以最短杆相邻杆为机架Usingtheshortestadjacentpoleastheframe以与最短杆相对的杆为机架Usingthepoleoppositetheshortestpoleastheframe以最短杆为机架Usingtheshortestpoleastheframe曲柄摇杆机构Crank-rockermechanism双曲柄机构Doublecrankmechanism双摇杆机构Double-rockermechanism40+70≤60+60一、曲柄存在的条件1.Conditionsfortheexistenceofacrank2急回特性Quick-returncharacteristicsTWO二、急回特性2.Quick-returncharacteristics曲柄

crank摇杆rocker曲柄摇杆机构Crank-rockermechanism二、急回特性2.Quick-returncharacteristics摇杆逆时针运动过程中，运行速度慢;摇杆顺时针运动过程中，运行速度比较快。这就是急回特性。Duringthecounterclockwisemovementofthejoystick,theoperatingspeedisslow;Duringtheclockwisemovementofthejoystick,therunningspeedisrelativelyfast.Thisisthequickreturnfeature.曲柄摇杆机构Crank-rockermechanism二、急回特性2.Quick-returncharacteristics牛头刨床机构运动简图Kinematicdiagramofthecowheadplanermechanism二、急回特性2.Quick-returncharacteristics曲柄摇杆机构Crank-rockermechanism二、急回特性2.Quick-returncharacteristics工作行程Stroke空回行程

Airreturnstroke摇杆摆角RockerswingangleΨΨ曲柄转速Crankspeedωω曲柄转角Crankangleφ1=180°+θφ2=180°-θ运动时间Motiontimet1t2C点的平均速度AveragespeedatpointCV1V2φ1>φ2t1>t2V2>V1曲柄摇杆机构crank-rockermechanism二、急回特性2.Quick-returncharacteristics急回运动特性可用行程速比系数K表示TherapidreturnmotioncharacteristicscanberepresentedbythetravelspeedtocoefficientK连杆机构输出件具有急回特性的条件:Theconditionsfortheoutputcomponentsofthelinkagemechanismtohavequickreturncharacteristics:▶原动件作等角速整周转动;Theoriginalcomponentrotatesatanequalangularspeedinafullcircle;▶输出件具有正、反行程的往复运动;Theoutputcomponenthasreciprocatingmotionwithforwardandreversestrokes;▶极位夹角θ>0，θ角越大，K值就越大，机构的急回特性就越显著。Polarangleθ>0,θthelargertheangle,thegreatertheKvalue,andthemoresignificantthequickreturncharacteristicofthemechanism.二、急回特性2.Quick-returncharacteristics3压力角和传动角PressureangleandtransmissionangleTHREE在不计摩擦力、惯性力和重力的条件下，机构中驱使输出件运动的力与输出件上受力点的速度方向间所夹的锐角，称为压力角。Theacuteanglebetweentheforcedrivingtheoutputcomponentinamechanismandthevelocitydirectionoftheforcepointontheoutputcomponent,regardlessoffriction,inertia,andgravity,iscalledthepressureangle.FVcα三、压力角和传动角3.Pressureangleandtransmissionangle三、压力角和传动角3.Pressureangleandtransmissionangle压力角α的余角为传动角γTheremainingangleofpressureangleαisthetransmissionangleγγγF2F1机构运转时，压力角α和传动角γ是变化的。为了保证机构传动的动力学性能良好，设计机构时应限制其最小传动角。Duringtheoperationofthemechanism,thepressureangleαandtransmissionangleγischanging.Inordertoensuregooddynamicperformanceofthemechanismtransmission,theminimumtransmissionangleshouldbelimitedwhendesigningthemechanism.设计时，一般应使:Whendesigning,itisgenerallynecessaryto:γmin≥40°对于高速、大功率机械应使:Forhigh-speedandhigh-powermachinery:γmin≥50°三、压力角和传动角3.Pressureangleandtransmissionangle三、压力角和传动角3.Pressureangleandtransmissionangle曲柄摇杆机构的最小传动角γmin出现在曲柄与机架共线(即曲柄转角φ=0°或φ=180°)的位置。MinimumtransmissionangleofcrankrockermechanismγMinappearscollinearbetweenthecrankandtheframe(i.e.crankangleφ=0°orφ=180°）position.4死点位置Dead-centerpositionFOUR当摇杆为主动件，连杆和曲柄共线时，过铰链中心A的力，对A点不产生力矩，不能使曲柄转动，机构的这种位置称为死点位置。Whentherockerisanactivecomponentandtheconnectingrodandcrankarecollinear,theforcepassingthroughthehingecenterAdoesnotgeneratetorqueonpointAandcannotcausethecranktorotate.Thispositionofthemechanismiscalledthedeadcenterposition.曲柄摇杆机构的死点位置Thedeadcenterpositionofthecrankrockermechanism四、死点位置4.Dead-centerposition在工程上，为了使机构能够顺利通过死点而正常运转，必须采用适当的措施，如发动机上安装飞轮加大惯性力，或利用机构的组合错开死点位置。Inengineering,inorderforthemechanismtosmoothlypassthroughthedeadcenterandoperatenormally,appropriatemeasuresmustbetaken,suchasinstallingaflywheelontheenginetoincreaseinertiaforce,orusingacombinationofmechanismstostaggerthedeadcenterposition.四、死点位置4.Dead-centerposition缝纫机踏板机构Sewingmachinepedalmechanism四、死点位置4.Dead-centerpositiondeadcenter飞机起落架A