This law is applicable to planar four bar linkage, is to ensure that the input crank can make a complete revolution.
Grashof’s law states that to be continuous relative rotation between two members, the sum of the lengths of shortest and longest link cannot be greater than the sum of lengths of remaining two links. The shortest link will rotate relative to other three links only if
s + l ≤ p + q
s = length of shortest link
l = length of longest link
p, q = lengths of remaining two links
If the condition is not satisfied, no link will make complete revolution.
If smallest link (s) is grounded (fixed) at one end we obtain crank-rocker mechanism.
Crank-crank mechanism is obtained, when smallest link is fixed.
The smallest link is kept free to obtain rocker-rocker mechanism.
If s + l ˃ p + q, then the mechanism is of rocker-rocker type.