Inversions of mechanism: A mechanism is one in which one of the links of a kinematic chain is fixed. Different mechanisms can be obtained by fixing different links of the same kinematic chain. These are called as inversions of the mechanism. By changing the fixed link, the number of mechanisms which can be obtained is equal to the number of links.
Excepting the original mechanism, all other mechanisms will be known as inversions of original mechanism.
The inversion of a mechanism does not change the motion of its links relative to each other.
Four bar chain:
One of the most useful and most common mechanisms is the four-bar linkage. In this mechanism, the link which can make complete rotation is known as crank (link 2). The link which oscillates is known as rocker or lever (link 4). And the link connecting these two is known as coupler (link 3). Link 1 is the frame.
Inversions of four bar chain:
Crank-rocker mechanism: In this mechanism, either link 1 or link 3 is fixed. Link 2
(crank) rotates completely and link 4 (rocker) oscillates. Drag link mechanism. Here link 2 is fixed and both links 1 and 4 make complete rotation but with different velocities. Double crank mechanism. This is one type of drag link mechanism, where, links 1& 3 are equal and parallel and links 2 & 4 are equal and parallel. Double rocker mechanism. In this mechanism, link 4 is fixed. Link 2 makes complete rotation, whereas links 3 & 4 oscillate