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根据机构组成原理,平面连杆机构可认为由主动件、基本杆组及机架组成。基本杆组由不能分离的、自由度为零的构件组台。按此条件,基本杆组中构件数 n 和低副数 p_L 应满足:3n-2p_L=0或 n=2pL/3(1)由于 n 和 p_L 总是整数,则从上式可知,n 和 p_L应分别为2和3的倍数,即:=2n,4,6,……,p_L=3,6,9,……由2个构件和3个低副构成的基本杆组称为Ⅱ级杆组,它共有五种类型,如图1所示。由四个构件和六个低副组成的基本杆组可为Ⅲ级杆组(图2a)或Ⅳ级杆组(图2b)。在Ⅲ级杆组中,BCE为一个闭合形——三角形。而在Ⅳ级杆组中有两个三角形 ABC、DEF 和一个四边形 BCDE,其闭合形的最多边数为4。基本杆组的级别就是由其中最多边闭合
According to the composition of the principle, the plane linkage can be considered by the active parts, basic pole group and rack. The basic pole group consists of components that can not be separated and have zero degrees of freedom. According to this condition, the number of members n and p L in the basic rod group should satisfy: 3n-2p_L = 0 or n = 2pL / 3 (1) Since n and p_L are always integers, Shall be multiples of 2 and 3, respectively: = 2n, 4,6, ..., p_L = 3,6,9, ... ... A basic rod set consisting of two members and three low pairs is called a grade II rod There are five types of groups, as shown in Figure 1. The basic rod group consisting of four members and six lower subgroups can be either the rod group III (Figure 2a) or the rod group IV (Figure 2b). In the III pole group, BCE is a closed-triangle. In the class Ⅳ rod group, there are two triangles ABC, DEF and a quadrilateral BCDE, the maximum number of sides closed is 4. The basic pole group level is closed by the most of them