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1、 Suppose we wish to design a crank-rocker mechanism ABCD,in which the side link AB is an input crank.in BCD must obey(服从)thetriangle inequality(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)The
2、distance f is a variable(可变的)value during the motion of the mechanism.第2页/共52页第3页/共52页第4页/共52页(5-2)(5-3)第5页/共52页(5-2)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 crit
3、erion(准则)or the Condition for having a crank.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条件时，有最短杆参与构成的转动副都是周转副（A、B副），而其余的转动副则是摆转副（C、D副）。于是，四杆机构有曲柄的条件是各杆的长度应满足杆长条件，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 G
4、rashof criterion can be expressed as:LMAX+LMIN Lb+Lc,the linkage mechanism is an non-Grashof linkage mechanism,in which no link can rotate through 360o relative to any other link and all inversions(倒置)are double-rocker mechanisms.第10页/共52页an non-Grashof linkage mechanism第11页/共52页 In a non-Grashof li
5、nkage mechanism,no link can rotate through 360o relative to any other link.However,in a Grashof double-rocker mechanism,the coupler can rotate 360o with respect to other links.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（曲柄与连杆拉直共线和重合共线）用作图法画出两极限位置从动件的行程角运动过程分析工作行程和空
6、回行程角速度不同（极位夹角）急回特性的应用如何衡量急回引入K0，K1曲柄滑块机构的急回特性其他第14页/共52页 C1DC2 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，对应曲柄的一个位置与另一位置的反向所夹的角度 C1AC2，称为 极位夹角,记为 .第16页/共52页 极位夹角 可能小于90，也可能大于90 ，一般范围是（0 ，180 ）第17页/共52页 2 2、急回运动DabdccabA主动件a a时间：转角：运动：从动件c c时间：转角：运动：从动件c c的平均角速
7、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。第18页/共52页通常把从动件往复运动平均速度的比值(大于1)称为行程速比系数，用K表示。3.行程速比系数K第19页/共52页 A crank-rocker mechanism with special dimensions may not have quick return characteristics.AB=C1C2/2.=0 and k=1.This crank-rocker mechanism has no quick-return characteristics.第20页/共52页 Since =0o,
8、an in-line(对心)slider-crank mechanism has no quick-return characteristics.第21页/共52页 In the offset slider-crank mechanism,the distance C1C2 is the stroke H of the slider.C2AC1 is the angle .If the driving crank AB rotates counter-clockwise with constant angular velocity,the slider will take a longer t
9、ime 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 the follower rocker should be the working stroke(工作行程),and the clockwise stroke should be the return stroke(回程).workingstroke,then the rotation direction of the crank should be reverse
10、d(倒转).If the clockwise stroke is needed to be a第23页/共52页 In an oscillating guide-bar mechanism,two limiting positions CD1 and CD2 of the followerguide-bar CD occur when the driving crank AB is perpendicular(垂直于)to the oscillating guide-bar CD.第24页/共52页 Note:The limiting positions of the follower gui
11、de-bar CD do not occur when the driving crank AB is horizontal(水平的).第25页/共52页 D1CD2 is the angular stroke max of the follower.For this linkage mechanism,happens(碰巧)to be equal to max.The coefficient of travel speed variation K,or the time ratio,is K=(180o+)/(180o-).第26页/共52页5.3.2 Pressure Angle(压力角)
12、and Transmission Angle(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 Pressure Angle(压力角)and Transmission Angle(传动角)第28页/共52页5.3.2 Pressure Angle(压力角)and Transmission Angle(传动角)第29页/共52页5.3.2 Pressure Angle(压力角)and Transmission Angle(传动角)1、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。第30页/共52页2、传动角，F与Fr夹角，（经常用衡量机构的传动质量）第31
13、页/共52页 当 BCD90o,时 =180o-BCD.第32页/共52页 Only the tangential(切向的)component(分量)F t can create the output torque(转矩)on the driven link DC.The radial(径向的)component F r only increases pivot(枢轴)friction(摩擦)and does not contribute to the output torque.For this reason,it is desirable that is not too great or
14、is not too small.第33页/共52页 和 在运动过程中是变化的.许用压力角（最大压力角）=40o,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 and the values of and will change,if a different link is chosen as the driver.第36页/共52页 For the same kinematic chain,the positions and the values o
15、f and will change,if a 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 is an output,then the acute angle(锐角)between the coupler BC and the slider path is at that position.第38页/共52页 If the crank is an input link and th
16、e slider is an output,then the acute angle(锐角)between the coupler BC and the slider path is at that position.=90o-.第39页/共52页 The extreme(极端的)values of and ,max and min,occur when the crank AB is perpendicular(垂直于)to the slider path,i.e.,max=90o-min=sin-1(a+e)/b第40页/共52页 Obviously,the limiting positi
17、ons of the slider are different from that where min occurs.第41页/共52页 Note:the limiting position of the rocker DC is different from that where min 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：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第44页/共52页死点第45页/共52页 If a rocker or a slid
18、er is the driver,a flywheel(飞轮)on the driven crank will be required to carry the mechanism through the dead point.第46页/共52页 A flywheel on the driven crank is used to carry the mechanism through the dead point.第47页/共52页 Shown is a landing(着陆)mechanism in airplane.The mechanism is at a dead point.A small torque(转矩)on the link CD is enough to prevent(防止)the link DC from rotating.第48页/共52页l a n d i n g(着 陆)m e c h a n i s m i n a i r p l a n e第49页/共52页加紧装置 第50页/共52页Homework:Ex.5-1,5-2,5-3,5-4,5-5以上题目应严格按比例准确作图！第51页/共52页感谢您的观看。第52页/共52页