Kinematic Chains:
It was described in earlier units that in kinematic chain, four links are needed which are connected with each other through the help of lower pairs. These pairs may be revolute pairs or prismatic pairs. A prismatic pair may be thought of like the limiting case of a revolute pair.
To show this let us consider the two mechanisms illustrated in Figure. The curved slider in figure behaves similar to the revolute pair. If radius of curvature 'ρ' of the curved slider becomes infinite, the angular motion of the slider modify into linear displacement and the revolute pair R4 transforms to a prismatic pair. Based on different kind of kinematic pairs, four bar kinematic chain may be classified into three categories:
(a) 4R-kinematic chain that has all of the four kinematic pairs as revolute pairs.
(b) 3R-1P kinematic chain that has three revolute pairs & one prismatic pair.
(c) 2R-2P kinematic chain that has two prismatic pairs and two revolute pairs.
