<P> Hartenberg & Denavit presents the definition of a kinematic pair: </P> <P> In the matter of connections between rigid bodies, Reuleaux recognized two kinds; he called them higher and lower pairs (of elements). With higher pairs, the two elements are in contact at a point or along a line, as in a ball bearing or disk cam and follower; the relative motions of coincident points are dissimilar . Lower pairs are those for which area contact may be visualized, as in pin connections, crossheads, ball - and socket joints and some others; the relative motion of coincident points of the elements, and hence of their links, are similar, and an exchange of elements from one link to the other does not alter the relative motion of the parts as it would with higher pairs . </P> <P> A lower pair is an ideal joint that constrains contact between a surface in the moving body to a corresponding surface in the fixed body . A lower pair is one in which there occurs a surface or area contact between two members, e.g. nut and screw, universal joint used to connect two propeller shafts . Cases of lower joints: </P> <Ul> <Li> A revolute pair, or hinged joint, requires a line in the moving body to remain co-linear with a line in the fixed body, and a plane perpendicular to this line in the moving body maintain contact with a similar perpendicular plane in the fixed body . This imposes five constraints on the relative movement of the links, which therefore has one degree of freedom . </Li> <Li> A prismatic joint, or slider, requires that a line in the moving body remain co-linear with a line in the fixed body, and a plane parallel to this line in the moving body maintain contact with a similar parallel plane in the fixed body . This imposes five constraints on the relative movement of the links, which therefore has one degree of freedom . </Li> <Li> A screw pair requires cut threads in two links, so that there is a turning as well as sliding motion between them . This joint has one degree of freedom . </Li> <Li> A cylindrical joint requires that a line in the moving body remain co-linear with a line in the fixed body . It is a combination of a revolute joint and a sliding joint . This joint has two degrees of freedom . </Li> <Li> </Li> <Li> A planar joint requires that a plane in the moving body maintain contact with a plane in fixed body . This joint has three degrees of freedom . The moving plane can slide in two dimensions along the fixed plane, and it can rotate on an axis normal to the fixed plane . </Li> </Ul>

Universal joint is an example of which pair