Kinematic Pairs:
Kinematic pair is a connection between two physical objects that imposes constraints on their relative movement (kinematics). The kinematics pairs may be classified according to the following considerations:
1. According to the type relative motion between the elements:
i. Sliding Pair: When the two elements of a pair are connected in such a way that one can only slide relative to the other, the pair is known as a sliding pair. The piston and cylinder, cross-head and guides of a reciprocating steam engines, ran and guides in shaper, tail stock on the lathe bed etc are the examples of a sliding pair. A little consideration will show that a sliding pair has a completely constrained motion.
ii. Turning Pair: When the two elements of a pair are connected in such a way only turn or revolve about a fixed axis of another link, the pair is known as turning pair. A shaft with collars at both ends fitted into a circular hole, the crankshaft in a journal bearing in an engine, lathe spindle supported in head stock, cycle wheels turning over their axles etc are the the examples of turning pair. A turning pair also has a completely constrained motion.
iii. Rolling Pair: When the two elements of a pair are connected in such a way that one rolls over another fixed link, the pair is known as rolling pair. Ball and roller bearings are examples of rolling pair.
iv. Screw Pair: When the two elements of a pair are connected in such a way that one element can turn about the other by screw threads, the pair is known as screw pair. The lead screw of a lathe with nut and bolt with a nut are examples of a screw pair.
v. Spherical Pair: When the two elements of a pair are connected in such a way that one element turns or swivels about the other fixed element, the pair formed is called a spherical pair. The ball and socket joint, attachment of a car mirror, pen stand etc are the examples of a spherical pair.
2. According to the type of contact between the elements:
i. Lower Pair: When the two elements of a pair have a surface contact when relative motion takes place and the surface of one element slides over the surface of the other, the pair formed is known as lower pair. It will seen that sliding pairs, turning pairs and screw pairs form lower pairs.
i. Higher Pair: When the two elements of pair have a line or point contact when relative motion takes place and the motion between the two elements is partly turning and partly sliding then the pair is known as higher pair. A pair of friction discs, toothed gearing, belt and rope drives ball and roller bearing and cam and follower are the examples of higher pairs.
3. According to the type of of closure:
i. Self-closed Pair: When the two elements of pair are connected together mechanically in such a way that only required kind of relative motion occurs, it is then known as self-closed pair. The lower pairs are self-closed pair.
ii. Force-closed Pair: When the two elements of a pair are not connected mechanically but are kept in contact by the action of external forces, the pair is said to be a force-closed pair. The cam and follower is an example of force-closed pair, as it is kept in contact by the forces exerted by spring and gravity.