January 25, 2019
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.
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:
- Double-crank mechanism
- Double-rocker mechanism and
- Crank and rocker mechanism
In double crank mechanism, the shortest link ‘S’ is a ground link. Both input crank and output crank rotate at 360°.
Let: ‘s’ = length of shortest link,
‘l’ = length of longest link,
‘p’ = length of one remaining link and
‘q’ = length of other remaining link.
In double-rocker mechanism, the shortest link ‘S’ is coupler link. The coupler link can rotate 360°.
In crank and rocker mechanism, the shortest link “S’ is input crank or output crank. Input crank or output crank rotates 360°.
the parallel linkage mechanism is a special case of Grashof’s criteria, where the sum of the shortest link ‘S’ and longest link ‘L’ of a planar quadrilateral linkage is less than or equal to the sum of the remaining two links ‘P+Q’.