# In a four-bar linkage if ‘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, th

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In a four-bar linkage if ‘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, then the criteria for getting a triple rocker mechanism in which no links will fully rotate is.
1. S + Q > P + L
2. S + L < P + Q
3. S + L = P + Q
4. S + L > P + Q

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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 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