# Kinematic Pair – Its types and examples

Kinematic Pair is the combination of two kinematic links ( a link is a resistant body that constitutes part of the machine ) that have relative motion with respect to each other and relative motion between them is completely or successfully constrained.

Examples of Kinematic Pairs are :-
i) Rectangular bar in a rectangular hole.
ii) Wheel rolling on a flat surface.
iii) Ball and Socket Joint

Kinematic Pair can be classified into different types based on the following criteria:-

1.Type of relative motion between the different elements of the pair.
2. Type of contact
3. Type of Mechanical Constraint

### 1. Type of relative motion between the elements of pair.

Based on the relative motion between the elements of kinematic pair are classified into following types :-
i) Sliding Pair
ii) Rolling Pair
iii) Turning Pair
iv) Screw pair
v) Cylindrical Pair
vi) Spherical Pair

i) Sliding Pair:-
In sliding pair, the two elements of the kinematic pair in which each elements has sliding contact with respect to the other element.

Eg – i) Piston inside a cylinder
ii) Rectangular bar in rectangular hole.

ii) Rolling Pair:-
The kinematic pair in which elements of the pair have rolling contact with each other is known as Rolling Pair.

The main examples of rolling pair are:-
i) Ball bearings
ii) Belt and pulley
iii) Wheel rolling on a flat surface.

iii) Turning Pair:-
In a kinematic pair if one element of pair can only turn or revolve about a fixed axis of another element, then this type of kinematic pair is known as turning pair.

Example of turning pair are:-
i) Shaft with a collar at both ends in a circular hole.
ii) Cycle wheel revolving about their axles.

iv) Screw Pair:-
When two elements of a kinematic pair have both turning as well as sliding motion between them by means of screw threads, then this type of kinematic pair is called screw pair.

Examples of screw pair are:-
i) Nut and Bolt
ii) Lead Screw of Lathe and Nut

v) Cylindrical Pair:-
Cylindrical Pair is a kinematic pair in which the elements of pair undergoes rotational as well as translational motion with respect to each other. These pairs have 2 degrees of freedom.

An example of a cylindrical pair is a Cylindrical bar inside a hollow shaft.

vi) Spherical Pair:-
The kinematic pair in which one of element of the pair is spherical and turns inside the other element which is stationary element is called spherical pair. A spherical pair has 3 degrees of freedom.

An example of a spherical pair is a ball and socket joint.

### 2. Type of contact-

Based on the type of contact the kinematic pair is classified into two types:-
i) Lower Pair
ii) Higher Pair

i) Lower Pair:-
When two links of a kinematic pair have surface contact between them and two links which have surface contact slides over each other is known as Lower Pair.
Eg – i) Rectangular bar in a rectangular hole.
ii) Nut and bolt.
iii) Bolt and Socket Joint.
iv) Piston inside a reciprocating cylinder.

ii) Higher Pair:-
When only one point or line contact is responsible to form a kinematic pair, then this type of pair is called higher pair.
The elements of a higher pair is always curve in its shape. Higher pairs are generally found in cylinders or spheres of the same or different radius and their axes are parallel to each other.
Eg -i) Contact between cam and its follower is a higher pair with line contact.
ii) Pivot on a plate is a higher pair with point contact.

### 3. Type of Mechanical Constraint:-

Based on Mechanical Constraint, kinematic pairs are classified into following types:-
i) Self Closed Pair
ii) Force Closed Pair

i) Self Closed Pair:-
When two elements of a kinematic pair have direct mechanical contact even without the application of external force in such a way that only required type of relative motion occurs then it is called Self Closed Pair.

Example – All lower pairs are self closed pairs.

ii) Force Closed Pair:-
When two elements of a kinematic pair are kept in contact by the application of external forces then this type of kinematic pair is called force closed pair.

Example:-
i) Cam and spring-loaded follower pair.
ii) Ball and roller bearings.