南京理工大学机械原理chapter4Planarlinkagemechanism.pptVIP

defined as the pressure angle(压力角) ? of the mechanism at that position. 4.4 Dimensional Synthesis of Four-bar Linkages 连杆机构的设计有二个任务：机构选型（称为型综合）和确定机构的尺寸（称为尺度综合）。 Usually, different problems will use different methods. Graphical method图解法 Analytical method解析法 Experimental method实验法 Dimensional synthesis(综合) of a mechanism is the determination of the kinematic dimensions necessary to achieve(获得) the required motion. 固定铰链 A、D ： 活动铰链 B、C ： 圆心 圆或圆弧 图解设计问题——作图求解各铰链中心的位置问题。 各铰链间的运动关系： 图解设计的基本原理 A B C D Bi Ci i =1、2、···、N Ei Fi (3)Enlarging a revolute pair Note:Enlarging a revolute pair in no way changes the motion relationship between any links. Crank1---eccentric偏心轮 偏心轮机构多用于曲柄销承受较大冲击，曲柄较短，需要装在直轴中部的机器 (4)Interchanging guide-bar and sliding block 变换导杆和滑块 Any link in a sliding pair can be drawn as a guide-bar, and the other link as a sliding block. The centre line of any sliding pair can be translated(平移) without changing any relative motion. 4.3 Characteristics Analysis of Four-bar Linkages 4.3.1 Conditions for having a crank a+b≤c+d (1) dc+b-a ? a+d≤c+b (2) cd+b-a ?a+c≤b+d (3) a+d ≤ b+c (1) a+b ≤ c+d (2) a+c ≤ b+d (3) (1)+(2) 得 a ≤ c (1)+(3) 得 a ≤ b (2)+(3) 得 a ≤ d 即： Grashof Criterion: 曲柄摇杆机构存在曲柄的条件 1. The crank must be shortest link. 2. The sum of the shortest and the longest links must be less than the sum of the remaining two links 最短构件与最长构件的长度之和小于等于其它两构件长度之和。 （杆长之和的条件） 即a最小时为曲柄 铰链四杆机构类型判定： （根据是否符合杆长之和条件） 如不满足杆长之和条件 如满足杆长之和条件 The shortest link is the frame. ——Double crank mechanism If the link opposite to the shortest Link is the frame. ——Double Rocker mechanism If the link adjacent to the shortest Link is the frame. ——Crank-rocker mechanism Double rocker mechanism 铰链四杆机构类型判定： 双曲柄机构 曲柄摇杆机构 双摇杆机构 杆长之和条件 N 双摇杆机构 Y 最短杆为连架杆 最短构件为机架 最短杆对边的为机架 Y Y if AB2C2D Parallel-crank mechanism AB2C’2D antiParallel-crank mechanism The centre lines of the four links