When the kinematic pairs are coupled in such a way that the last link is joined to the first link to transmit definite motion or successfully constrained motion, it is called a kinematic chain.
In other words, a kinematic may be defined as a combination of kinematic pairs, joined in such a way that each link forms a part of two pairs and the relative motion between the links or elements is completely or successfully constrained.
Types of Kinematic Chain:
There are mainly two types of kinematic chain:
1. Constrained Kinematic Chain: A chain in which a single link such as AD in below figure. It is sufficient to define the position of all other links, it is then called a kinematic chain of one degree of freedom. If a definite displacement (θ) is given to the link AD, keeping the link AB fixed, then the resulting displacements of the remaining two links BC and CD are also perfectly definite. Thus, we see that in a four bar chain, the relative motion is completely constrained. Hence it may be called as a constrained kinematic chain.
2. Unconstrained Kinematic Chain: Consider an arrangement of five links, as shown in fig:
l = 5, p = 5 and j = 5
Since the arrangement of five links as shown in fig. It doesn’t satisfy the equations and left side is less than right hand side, therefore it is not a kinematic chain. Such a type of chain is called Unconstrained Kinematic Chain. The relative motion isn’t completely constrained.