A four-bar mechanism is shown below.  For the mechanism to be a crank-rocker mechanism, the length of the link PQ can be 

A four-bar mechanism is shown below.  For the mechanism to be a crank-rocker mechanism, the length of the link PQ can be  Correct Answer 80 mm

Concept:

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 links 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.

Calculation:

Given:

For getting a crank rocker mechanism, any of the adjacent links of the shortest link should be fixed and S + L ≤ P + Q.

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Here fixed link is PS = 400 mm which is not the shortest link.

Let's start with incorrect options first.

1. Now consider the length of PQ = 350 mm
   • S + L = 300 + 600 = 900
   • P + Q = 400 + 350 = 750
   • S + L > P + Q.
   • Grashof’s Law not satisfied
2. Now consider the length of PQ = 300 mm
   • S + L = 300 + 600 = 900
   • P + Q = 400 + 300 = 700
   • S + L > P + Q
   • Grashof’s Law not satisfied
3. Now consider the length of PQ = 200 mm
   • S + L = 200 + 600 = 800
   • P + Q = 400 + 300 = 700
   • S + L > P + Q
   • Grashof’s Law not satisfied
4. Now consider the length of PQ = 80 mm

• S + L = 80 + 600 = 680
• P + Q = 400 + 300 = 700
• S + L < P + Q
• Grashof’s Law is satisfied
• Shortest link is PQ (80 mm). Link adjacent to shortest link is fixed i.e. PS. So it will give crank rocker mechanism.