Kinematic Pair
When two kinematic links are connected in such a way that their motion is either completely or successfully constrained, these two links are said to form a kinematic pair.
Kinematic pairs can be classified according to:
Kinematic pairs according to nature of contact:
a. Lower Pair:
A pair of links having surfaced or area contact between the members is known as a lower pair. The contact surfaces of two links are similar.
Examples: Nut turning on a screw, shaft rotating in a bearing.
b. Higher Pair:
When a pair has a point or line contact between the links, it is known as a higher pair. The contact surfaces of two links are similar.
Example: Wheel rolling on a surface, Cam and Follower pair etc.
Kinematic pairs according to nature of contact:
a. Closed Pair:
When the elements of a pair are held together mechanically, it is known as a closed pair. The two elements are geometrically identical; one is solid and full and the other is hollow or open. The latter not only envelops the former but also encloses it. The contact between the two can be broken only by destruction of at least one of the members.
b. Unclosed Pair:
When two links of a pair are in contact either due to force of gravity or some spring action, they constitute an unclosed pair. In this, the links are not held together mechanically, e.g. cam and follower pair.
Kinematic pairs according to Nature of Relative Motion:
a. Sliding pair:
When two links have a sliding motion relative to another; the kinematic pair is known as sliding pair.
b. Turning pair:
When one link is revolve or turn with respect to the axis of first link, the kinematic pair formed by two links is known as turning pair.
c. Rolling pair:
When the links of a pair have a rolling motion relative to each other, they form a rolling pair.
d. Screw pair:
If two mating links have a turning as well as sliding motion between them, they form a screw pair.
e. Spherical pair:
When one link in the form of sphere turns inside a fixed link, it is a spherical pair.
