Question

In a four-link mechanism, the sum of the shortest and the longest link is less than the sum of the other two links. It will act as a rocker-rocker mechanism if:
1. the shortest link is fixed
2. shortest and adjacent to the shortest both are fixed
3. the link opposite to the shortest link is fixed
4. any link adjacent to the shortest link is fixed

Answer 1

Correct Answer - Option 3 : the link opposite to the shortest link is fixed

Concept:

Grashof’s Law states that a four-bar mechanism has at least one revolving link if the sum of the lengths of the largest and shortest links is less than the sum of lengths of the other two links.

In a four-bar mechanism let us denote the smallest link by S, the longest link by L, and the other two links by P and Q.

Grashof’s Law condition is satisfied

If S + L ≤ P + Q

Following Points are important about Grashof's law

Consider a mechanism

(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 links of the shortest link if fixed: Crank rocker mechanism
• If the link opposite to the shortest link is fixed: Double Rocker Mechanism

(2) S + L > P + Q (Class II mechanism)

• Only double rocker mechanism is possible