Four bar chain mechanism is the most common type of mechanism. To Make a kinematics chain, we need at least four links or four kinematics pairs to establish a completely constrained motion for a mechanism in which links moves relative to each other.

The simplest Mechanism built by using four elements is called four bar chain or four bar mechanism. Total four kinematics pair would be involved in this mechanism which makes the moment of elements in the mechanism completely constrained.

The links in the mechanism can have any length but they have to follow a certain law called Grashof's law.

Grashof's law says that the sum of shortest and the longest four bar mechanism should be less than the sum of length of other two links.

         s+l < p+q

In the mechanism, One link should be considered to have complete revolution so that it can be used to provide input to the mechanism. The shortest link should have the complete revolution.

Also Read:

Crank Slider Mechanism and its inversion

Double Slider mechanism and its inversion

Inversions of four bar chain mechanism

First we should understand what is inversion. In a mechanism which has several links, when the different links are fixed one by one then we will get different types of mechanisms from that particular mechanism. this different types of mechanisms are called inversions of the mechanism.

In the figure of four bar mechanism the shortest link AB which has the complete Revolution is called Crank. other link connected with crank BC is called connecting rod. The link CD will have partial revolution and it is called Lever.

The different types of inversions are given below

Crank and Lever mechanism

When of link 1 and 3 is fixed. The link 2 will have complete rotation. This complete rotation will cause link 4 to oscillate. This mechanism is called crank Rocker mechanism.
Crank and lever Mechanism

Double crank mechanism

In this the shortest link is to be fixed. The two links joined at the ends of the fixed link will have the freedom to rotate completely. So there seems to be two cranks in this mechanism. Thus this mechanism is called double crank mechanism.
Double Crank Mechanism

Double Rocker mechanism

In this mechanism link 4 is fixed. The two links attached with the fixed link are free to move but none of them will have the complete rotation. Both the links will oscillate. That is why it is called double rocker or double Lever mechanism.
Double Rocker Mechanism