Relations between DOF of Mechanism and DOF of Links and Their Essential Differences

doi:10.3901/JME.2018.05.074

Abstract

Abstract: The essential difference of the basic conceptions between the DOF of mechanism and the DOF of links are discussed in the paper to avoid the confusion about the DOF of mechanism and the DOF of links. On the basis of the concepts of general link groups and virtual loops, the method is given to calculate the DOF of any selected link in single-loop mechanism or multi-loop mechanisms. The examples are the planar single loop mechanism with 7R, roller-follower cam mechanism, Sarrus mechanisms with different DOF and 2-3RC mechanism etc. According to the relationship between the DOF of mechanism and the DOF of all links, the mechanisms are divided into three types. The first-class is those mechanisms whose DOFs equal 1. The DOF of mechanisms equal DOF of each link. The second-class is those mechanisms whose DOFs equal the maximum DOF of links. The third-class is those mechanisms whose DOFs are greater than the maximum DOF of links. After analyzing some examples, it comes to an important conclusion that the DOF of links (including output link) is the number of independent displacement parameters with motion properties. The DOF of mechanism is only the number of all independent parameters which keep all links have definite position and orientation. It is only a natural number without the property of motion. The DOF of mechanism doesn't rely on the choice of output link. It reflects property of whole mechanism. The DOF of links reflects the property of one link in mechanism. In application, the DOF of mechanism and the DOF of links have district differences, which should not be confused.

Key words: DOF of link-group, DOF of links, DOF of mechanism, DOF of output link

CLC Number:

TH112

MU Dejun, ZHANG Yitong, ZHANG Xing. Relations between DOF of Mechanism and DOF of Links and Their Essential Differences[J]. Journal of Mechanical Engineering, 2018, 54(5): 74-83.

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