Topic: Theory of Machines
A planar four bar mechanism with all revolute joints has link lengths AB=30 mm, BC=120 mm, CD=100 mm and AD=80 mm. The link to be fixed to obtain a double crank mechanism is
- A. AB
- B. BC
- C. CD
- D. AD
Answer: A. AB
Explanation: By Grashof's law, Shortest + Longest (S+L) = 30 + 120 = 150 mm. The sum of the other two links (P+Q) = 100 + 80 = 180 mm. Since S+L < P+Q, it is a Grashof linkage. To obtain a double crank mechanism, the shortest link (AB) must be fixed.