Grashof’s Law The Grashof’s law states that for a four-bar linkage system, the sum of the shortest and longest link of a planar quadrilateral linkage is less than or equal to the sum of the remaining two links, then the shortest link can rotate fully with respect to a neighboring link.
Consider a four-bar-linkage. Denote the smallest link by S, the longest link by L and the & other two links by P and Q.
If the Grashof’s Law condition is satisfied i.e S+L ≤ P+Q,
then depending on whether shortest link ‘S’ is connected to the ground by one end, two ends, or no end there are 3 possible mechanisms. They are:
1. Double-crank mechanism
2. Double-rocker mechanism and
3. Crank and rocker mechanism
1. Double crank mechanism
In double crank mechanism, the shortest link ‘S’ is a ground link. Both input crank and output crank rotate at 360°.
Grashof’s condition for double crank mechanism: s+l > p+ q Let: ‘s’ = length of shortest link,
‘l’ = length of longest link,
‘p’ = length of one remaining link and
‘q’ = length of other remaining link.
2. Double-rocker mechanism 