东华大学机械原理课件 平面连杆机构及其设计第一节.ppt

Chap.3 Planar? Linkage and Its Design Sun Zhihong Zhsun@dhu.edu.cn Room307,No.3 Teaching Building Today’s topics What is planar linkage mechanisms ( 连杆机构)? Characteristics of planar linkage mechanisms (连杆机构的特点) Classification and applications of four-bar linkages（四杆机构的分类及应用）? Planar Linkage Mechanism Definition: Linkage mechanisms are lower-pair mechanisms, in which all links are connected with each other by lower kinematic pairs (revolute pairs or sliding pairs/prismatic pairs). Properties of linkage mechanisms： .连杆机构中的运动副均为低副（lower pair）。低副运动副元素为面接触，压强较小，故可承受较大的载荷；且有利于润滑，磨损较小；运动副元素几何形状简单（平面或圆柱面），便于加工制造，成本低。 Properties of linkage mechanisms 在连杆机构中，当原动件的运动规律不变，可用改变各构件的相对长度来使从动件得到不同的运动规律。 在连杆机构中，连杆上各点的轨迹是各种不同形状的曲线（称为连杆曲线），其形状还随着各构件相对长度的改变而改变，从而可以得到形式众多的连杆曲线，我们可以利用这些曲线来满足不同轨迹的设计要求。 Disadvantages of linkage mechanisms 由于连杆几的运动必须经过中间构件进行传递，因而传递路线较长，易产生较大的误差积累，同时，也使机械效率降低。 在连杆机构运动过程中，连杆及滑块的质心都在作变速运动，所以产生的惯性力难于用一般平衡方法加以消除，因而会增加机构的动载荷(dynamic load)，所以连杆机构不宜用于高速运动。 Four-bar linkages Four-bar linkage is the simplest planar linkage whose DOF is 1. There are four links and four lower pairs in a four-bar linkage mechanism. Types: Revolute four-bar mechanism(铰链四杆机构) One sliding pair four-bar mechanism(含有一个移动副的四杆机构) Two sliding pair four-bar mechanism(含有两个移动副的四杆机构) Revolute four-bar mechanisms Coupler(连杆) Classification of revolute four-bar mechanisms According to the type of the two side links, the types of four-bar mechanisms can be divided into: Crank-rocker mechanism(曲柄摇杆机构) Usually, crank is the driving link, sometimes not. Revolute four-bar mechanisms Double-crank mechanism(双曲柄机构) Both side links can completely rotate 360 relatively to the frame. If one crank rotate at a constant angular velocity, the other one will rotate at a varying speed. Double-rocker mechanism（双摇杆机构） Both the side links can only rock through a limited angle relatively to the frame. The driving rocker oscillates at a constant angular velocity, the driven rocker rocks at a va