《平面四杆机构的工作特性总结.pptx》由会员分享，可在线阅读，更多相关《平面四杆机构的工作特性总结.pptx（52页珍藏版）》请在taowenge.com淘文阁网|工程机械CAD图纸|机械工程制图|CAD装配图下载|SolidWorks_CaTia_CAD_UG_PROE_设计图分享下载上搜索。
1、平面四杆机构的工作特性总结平面四杆机构的工作特性总结 Suppose we wish to design a crank-rocker Suppose we wish to design a crank-rocker mechanism ABCD,in which the side link AB is an mechanism ABCD,in which the side link AB is an input crank.input crank.in in BCDBCD must must obey(obey(服从服从)the thetriangle triangle ineinequaq
2、ualitylity(不等式）If the RRR Assur group can be assembled(装配)onto the basic mechanism by the two outer revolutes B and D,the lengths of the three sides第1页/共52页 -(5-1)-(5-1)The distanceThe distance f f is is a a variablevariable(可变的)value during value during the motion of the the motion of the mechanism
3、.mechanism.第2页/共52页第3页/共52页第4页/共52页 (5-(5-2)2)(5-3)第5页/共52页 (5-(5-2)2)Again,from the inequalities(5-2),we can conclude that the sum of the shortest and the longest links must be less than the sum of the remaining(剩余)two links.This is called Grashof criterion(准则准则)or the Condition for having a crank.
4、Thus from the inequalities(5-3),we can see that the crank in a crank-rocker mechanism must be the shortest link.(5-3)第6页/共52页上述条件表明：当四杆机构各杆的长度满足杆长上述条件表明：当四杆机构各杆的长度满足杆长条件时，有最短杆参与构成的转动副都是周转副条件时，有最短杆参与构成的转动副都是周转副（A A、B B副），而其余的转动副则是摆转副（副），而其余的转动副则是摆转副（C C、D D副）。副）。于是，四杆机构有曲柄的条件是各杆的长度应满于是，四杆机构有曲柄的条件是各杆的
5、长度应满足杆长条件，且其最短杆为连架杆或机架。足杆长条件，且其最短杆为连架杆或机架。第7页/共52页第8页/共52页 The Grashof cri The Grashof criterterion can be expressed as:ion can be expressed as:L LMAXMAX+L+LMINMIN LLLb b+L+Lc c,the linkage mechanism,the linkage mechanism is an is an non-Grashof linkage mechanismnon-Grashof linkage mechanism,in whic
6、h,in which no link can rotate through 360no link can rotate through 360o o relative relative to any to any other link and all other link and all ininverversionssions(倒置倒置)are double-are double-rocker mechanisms.rocker mechanisms.第10页/共52页an an non-Grashof linkage mechanismnon-Grashof linkage mechani
7、sm第11页/共52页 In a In a non-non-Grashof linkage Grashof linkage mechanism,mechanism,no link no link can rotate through can rotate through 360360o o relative to any relative to any other link.other link.However,in a Grashof double-rocker mechanism,the coupler can rotate 360o with respect to other links
8、.第12页/共52页5.3.2 Quick Return Characteristics（急回特性）第13页/共52页曲柄摇杆机构摇杆的两极限位置是在两状态下（曲柄与曲柄摇杆机构摇杆的两极限位置是在两状态下（曲柄与连杆拉直共线和重合共线）连杆拉直共线和重合共线）用作图法画出两极限位用作图法画出两极限位置置从动件的行程角从动件的行程角运动过程分析运动过程分析工作行程工作行程和空回行程角速度不同和空回行程角速度不同（极位夹角）（极位夹角）急回特性急回特性的应用的应用如何衡量急回引入如何衡量急回引入KK0，K K11曲柄滑块机构的急回特性曲柄滑块机构的急回特性其他其他第14页/共52页 C1DC2
9、is called the angular stroke(行程)of the rocker,denoted as max.第15页/共52页 从动件位于两极限位置时，对应曲柄的一个从动件位于两极限位置时，对应曲柄的一个位置与另一位置的反向所夹的角度位置与另一位置的反向所夹的角度C C1 1ACAC2 2 ，称为，称为 极位夹角极位夹角,记为记为 .第16页/共52页 极位夹角极位夹角 可能小于可能小于9090，也可能大于，也可能大于90 90 ，一般，一般范围是（范围是（0 0 ，180 180 ）第17页/共52页 2 2、急回运动、急回运动DabdccabA主动件主动件a a时间：时间：转
10、角：转角：运动：运动：从动件从动件c c时间：时间：转角：转角：运动：运动：从动件从动件c c的平均角速度：的平均角速度：当曲柄等速回转的情况下，通常把从动件往复运动速度快慢不同的运动称为急回运动。当曲柄等速回转的情况下，通常把从动件往复运动速度快慢不同的运动称为急回运动。第18页/共52页通常把从动件往复运动平均速度的比通常把从动件往复运动平均速度的比值值(大于大于1)称为行程速比系数，用称为行程速比系数，用K表示。表示。3.行程速比系数行程速比系数K第19页/共52页 A crank-rocker mechanism with special A crank-rocker mechanis
11、m with special dimensions may not have quick return dimensions may not have quick return charactecharacterisristics.tics.AB=C1C2/2.=0 and k=1.This crank-rocker mechanism has no quick-return characteristics.第20页/共52页 Since Since =0=0o o,an,an in-linein-line(对心对心)slider-crank slider-crank mechanism ha
12、s no quick-return characteristics.mechanism has no quick-return characteristics.第21页/共52页 In the offset slider-crank mechanism,the In the offset slider-crank mechanism,the distance Cdistance C1 1C C2 2 is the stroke is the stroke H H of the slider.of the slider.C C2 2ACAC1 1 is the angle is the angl
13、e .If the driving crank AB rotates counter-clockwise with constant angular velocity,the slider will take a longer time in its rightward(向右)stroke than in its leftward(向左)stroke.第22页/共52页 In Fig.4-27,the counter-clockwise stroke of In Fig.4-27,the counter-clockwise stroke of the follower rocker shoul
14、d be the working the follower rocker should be the working strokestroke(工作行程工作行程),and the clockwise stroke should be,and the clockwise stroke should be the the return strokereturn stroke(回程回程).workingworkingstroke,then the stroke,then the rotation rotation direction of the direction of the crank sho
15、uld crank should be be reversedreversed(倒转).If the clockwise stroke is needed to be a第23页/共52页 In an oscillating guide-bar mechanism,two In an oscillating guide-bar mechanism,two limiting positions limiting positions CDCD1 1 and and CDCD2 2 of the follower of the followerguide-bar CD occur when guid
16、e-bar CD occur when the driving crank ABthe driving crank AB is is perpenperpendidicularcular(垂直于)to to the the o oscillating guide-bar scillating guide-bar CD.CD.第24页/共52页 Note:Note:The limiting The limiting positions of the positions of the follower guide-bar follower guide-bar CD do not occur CD
17、do not occur when the driving when the driving crank AB is crank AB is horihorizonzontaltal(水平的水平的).第25页/共52页 D D1 1C CD D2 2 is the angular is the angular stroke stroke maxmax of the follower.of the follower.For this linkage mechanism,happens(碰巧)to be equal to max.The coefficient of travel speed va
18、riation K,or the time ratio,is K=(180o+)/(180o-).第26页/共52页5.3.2 5.3.2 Pressure AnglePressure Angle(压力角压力角)and Transmission and Transmission AngleAngle(传动角传动角)不考虑运动副的摩擦及构件重力的影响第27页/共52页5.3.2 5.3.2 Pressure AnglePressure Angle(压力角压力角)and Transmission and Transmission AngleAngle(传动角传动角)第28页/共52页5.3.2 5
19、.3.2 Pressure AnglePressure Angle(压力角压力角)and Transmission and Transmission AngleAngle(传动角传动角)第29页/共52页5.3.2 5.3.2 Pressure AnglePressure Angle(压力角压力角压力角压力角)and and Transmission AngleTransmission Angle(传动角传动角传动角传动角)1、压力角压力角从动件上某点的受力方向与从动件上该点速度方向的所夹的锐角。从动件上某点的受力方向与从动件上该点速度方向的所夹的锐角。第30页/共52页2、传动角传动角，F与
20、与Fr夹角，夹角，（经常用（经常用衡量机构的传动质量）衡量机构的传动质量）第31页/共52页 当当当当 BCD90BCD90BCD 90o o,时时时时 =180=180o o-BCD.BCD.第32页/共52页 Only the Only the tantangengentialtial(切向的切向的)comcompoponentnent(分量分量)F F t t can create the output can create the output torquetorque(转矩转矩)on the on the driven link DC.The radialdriven link DC
21、.The radial(径向的径向的)com compoponent nent F F r r only increases only increases pivotpivot(枢轴枢轴)frictionfriction(摩擦摩擦)and does and does not connot contritribute to the bute to the output torque.output torque.For this For this reason,it is reason,it is desirable that desirable that is not too great is
22、not too great or or is not too is not too small.small.第33页/共52页 和和 在运动过程中是变化的在运动过程中是变化的.许用压力角（最大压力角）许用压力角（最大压力角）=40o,许用传动角（最小传动角）许用传动角（最小传动角）=50o.第34页/共52页 min=min min,min.第35页/共52页 For the same kinematic chain,the positions For the same kinematic chain,the positions and the values of and the values
23、 of and and will change,if a will change,if a different link is chosen as the driver.different link is chosen as the driver.第36页/共52页 For the same kinematic chain,the positions For the same kinematic chain,the positions and the values of and the values of and and will change,if a will change,if a di
24、fferent link is chosen as the driver.different link is chosen as the driver.and must be drawn on the driven link!第37页/共52页 If the crank is an input link and the slider If the crank is an input link and the slider is an output,then the is an output,then the a acucute anglete angle(锐角锐角)between the co
25、upler BC and the slider path between the coupler BC and the slider path is is at that position.at that position.第38页/共52页 If the crank is an input link and the slider If the crank is an input link and the slider is an output,then the is an output,then the a acucute anglete angle(锐角锐角)between the cou
26、pler BC and the slider path between the coupler BC and the slider path is is at that position.at that position.=90o-.第39页/共52页 The The extremeextreme(极端的极端的)values of values of and and ,maxmax and and minmin ,occur when the crank AB is,occur when the crank AB is perpenperpendidicularcular(垂直于垂直于)to
27、the slider path,to the slider path,i.e.,max=90o-min=sin-1(a+e)/b第40页/共52页 Obviously,the limiting positions of the slider Obviously,the limiting positions of the slider are different from are different from that where that where minmin occurs.occurs.第41页/共52页 Note:Note:the limiting position of the ro
28、cker DCthe limiting position of the rocker DC is is different from different from that where that where minmin may occurs may occurs.第42页/共52页4.3.3 死点位置第43页/共52页死点位置：死点位置：机构在运动过程中，当从动件的传动角机构在运动过程中，当从动件的传动角=0=0时，驱动力与从动件受力点的运动方向垂直，其有效分力等于零，这时机构不能运动，称此位置为死点位置时，驱动力与从动件受力点的运动方向垂直，其有效分力等于零，这时机构不能运动，称此位置为死
29、点位置第44页/共52页死点死点死点死点第45页/共52页 If a rocker or a slider is the driver,a If a rocker or a slider is the driver,a flywheelflywheel(飞飞轮轮)on the driven crank will be required to carry on the driven crank will be required to carry the mechanism through the dead point.the mechanism through the dead point.第4
30、6页/共52页 A A flywheelflywheel on the driven crank is used to carry on the driven crank is used to carry the mechanism through the dead point.the mechanism through the dead point.第47页/共52页 Shown is a Shown is a landinglanding(着陆着陆)mechanism in airplane.mechanism in airplane.The mechanism is at a dead
31、point.A small The mechanism is at a dead point.A small torquetorque(转矩转矩)on on the link CDthe link CD is enough to is enough to preventprevent(防止防止)the link DCthe link DC from rotating.from rotating.第48页/共52页landing(landing(着陆着陆)mechanism in airplane mechanism in airplane第49页/共52页加加加加紧紧紧紧装装装装置置置置 第50页/共52页Homework:Homework:Ex.5-1,5-2,5-3,5-4,5-5Ex.5-1,5-2,5-3,5-4,5-5以上题目应严格按比例准确作图！以上题目应严格按比例准确作图！第51页/共52页