Theory of machines may be define as the branch of Engineering Science  which deals with the study of relative motion between the various parts of a machine and forces which act on them. 
A machine is an apparatus for applying mechanical power ,consisting of a number of inter-related parts ,each having a definite function , kinematics of machine deals with the relative motion between the parts , neglecting the consideration of the force .kinematic link is a resistant body or an assembly of resistance bodies which go to make a part of a machine connecting other parts which have motion relative to it. Kinematic link can be an assembly of parts forming one unit with no relative motion of the parts with respect to one another .

Sub-divisions of Theory of machines 

The Theory of machines  may be sub-divided into the following four branch 

1.Kinematics - It is that branch of Theory of machines which deals with the relative motion between the various parts of the machines.

2. Dynamics - It is that branch of Theory of machines which deals with the forces and their effects while acting upon the machine parts in motion .

3. Kinetics - It is that branch of Theory of machines  which deals with the inertia forces which arise from the combined effect of the mass & motion of the machine parts .

4. Statics - It is that branch of Theory of machines which deals with the forces and their effects while the machine parts are at rest .The mass of the parts is assumed to be negligible .


A pair is a joint of two element that permits relative motion . The relative motion between the element or links that from a pair is required to be completely constrained or successfully constrained , pairs may be classified as follow:

1. Depending upon the nature of relative motion between elements :
(i) Sliding pair                 (ii) Turning pair
(iii) Rolling pair              (iv) Spherical pair
(v) Helical pair 

2. Depending on the nature of contact between elements :
(i) Lower pairs                 (ii) Higher pairs 

3. Depending on the nature of the mechanical constraints :
(i)  Closed pairs               (ii) Open pairs 

Classification of kinematic pair

1. According to nature of contact 
Lower pair - A pair of links having surface or area contact between the member . 

Higher pair - When a pair has point or line contact between the links ,  it is known as    higher pair .


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