Equivalent Mechanisms:

A mechanism with higher pairs may be replaced by an equivalent mechanism along lower pairs. This equivalence is only applicable for studying the instantaneous characteristics. A mechanism along higher pair is illustrated in Figure (a) in which links 2 and 3 contain general shape and contain higher pair at P. The relative motion among links 2 and 3 consists of rolling, coupled along an uncertain amount of sliding. A & B are centres of curvature of surfaces 2 & 3 respectively, at point P. The instantaneous equivalent lower pair mechanism is illustrated in Fig (b) that has an additional link AB having two revolute pairs at A and B. Therefore a higher pair is replaced by two revolute pairs at A & B.

(a)                                        (b)

Equivalent Mechanism

 Cam and Follower Mechanism

Likewise to this mechanism, a cam and follower mechanism also contain higher pair. It is illustrated in Figure (a) and equivalent lower pair mechanism is illustrated in Figure (b). The slider link 4 is an added link and higher pair is replaced through two lower pairs that are one revolute & the other prismatic. Point C is centre of curvature of the cam surface at pt of contact. The centre of curvature of follower is at infinity & that is why one of the added lower pairs becomes prismatic. The similar procedure may be adopted to set up equivalence for other mechanisms utilizing higher pairs.