Correct Answer - Option 4 : S + L > P + Q
According to Grashof's criterion of movability, a four-bar mechanism can be classified into the following three types:
Double crank mechanism: A mechanism in which both the driving and driven links make a complete rotation is called a double crank mechanism.
Double rocker mechanism: If both crank and follower links make only oscillation and none of them makes a complete rotation, such a mechanism is termed as a double rocker mechanism.
Crank rocker mechanism: A mechanism in which the driving link makes complete rotation and the driven link makes the only oscillation, such a mechanism is commonly referred to as crank rocker mechanism
In a four-bar linkage ‘S’ is the length of the shortest link, ‘L’ is the length of the longest link and ‘P’ & ‘Q’ are the length of other links
According to Grashof’s Law:
(1) S + L ≤ P + Q (Class I mechanism)
There will be at least one complete revolution between the two links.
- If the shortest link is fixed: Double Crank Mechanism
- If any of the adjacent link of the shortest link if fixed: Crank rocker mechanism
- If the link opposite to shortest link is fixed: Double Rocker Mechanism
(2) S + L > P + Q (Class II mechanism)
- Only a double rocker mechanism is possible, i.e. no links will fully rotate