BASIC LINKAGE CONCEPTS

Kinematic Elements A linkage is composed of rigid-body members, or links, connected to one another by rigid kinematic elements, or pairs. The nature of those connections as well as the shape of the links determines the kinematic properties of the linkage. Although many kinematic pairs are conceivable and most do physically exist, only four have general practical use for linkages. In Fig. 3.1, the four cases are seen to include two with one degree of freedom (f = 1), one with f = 2, and one with f = 3. Single-degree-of-freedom pairs constitute joints in planar linkages or spatial linkages. The cylindrical and spherical joints are useful only in spatial linkages. The links which connect these kinematic pairs are usually binary (two connections) but may be tertiary (three connections) or even more.A commonly used tertiary link is the bell crank familiar to most machine designers. Since our primary interest in most linkages is to provide a particular output for a prescribed input, we deal with closed kinematic chains, examples of which are depicted in Fig. 3.2. Considerable work is now under way on robotics, which are basically open chains. Here we restrict ourselves to the closed-loop type. Note that many complex linkages can be created by compounding the simple four-bar linkage.This may not always be necessary once the design concepts of this chapter are applied. 

Richard E. Gustavson

Technical Staff Member
The Charles Stark Draper Laboratory, Inc.
Cambridge,Massachusetts
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