A kinematic pair is a combination of two kinematic links that have relative motion with respect to each other.
When two links (or elements) in a machine are in contact with each other, they form a pair. When the relative motion between these two links is completely or partially constrained, then the links are said to form a kinematic pair.
Types of Kinematic Pairs:
Kinematic Pairs can be broadly classified into many types based on three main criteria. The classification of kinematic pairs is listed and described below:
- Based on the nature of contact between the pairing elements
- Based on the type of mechanical constraint (or mechanical contact)
- Based on the type of relative motion between the elements of the pair
A kinematic pair is said to be a lower pair if the links in the pair have surface or area contact between them.
Along with a cylinder, a piston forms a lower pair
A higher pair is a kinematic pair in which the links have point or line contact. Ball bearings, cam and follower are examples of higher pair.
A pair is said to be self closed if the links in the pair have direct mechanical contact, even without the application of external force.
A kinematic pair is said to be force closed if the links in the pair are kept in contact by the application of external forces. A good example of this type of pair is ball and roller bearings.
As the name suggests, a sliding pair is a kinematic pair in which each element has sliding contact with respect to the other element. Some good examples of sliding pairs are piston inside a cylinder, spur gear drive and square bar in a square hole.
Square Bar in a Square Hole – An example of Sliding Pair
In a rolling pair, one element undergoes rolling motion with respect to the other.
Wheel rolling on a flat surface – An example of Rolling Pair
In a turning pair, one link undergoes turning motion relative to the other link. Example is a shaft with collars in a circular hole.
A screw pair consists of links that have both turning and sliding motion relative to each other.
Bolt and Nut – An Example of Screw Pair
A cylindrical pair is a kinematic pair in which the links undergo both rotational and translational motion relative to one another.
Example: A solid cylindrical bar inside a hollow shaft.
In a spherical pair, a spherical link turns inside a fixed link.
A spherical pair has three degrees of freedom.