The link lengths of a planar four bar mechanism are AB = 100 mm, BC = 25 mm, CD = 75 mm and DA = 90 mm

Q. The link lengths of a planar four bar mechanism are AB = 100 mm, BC = 25 mm, CD = 75 mm and DA = 90 mm. For achieving the full rotation of both the input (crank) as well as the output (follower) links, the link that needs to be fixed is

Ans: BC

Sol:

S = 25 mm, L = 100 mm, P = 75 mm and Q = 90mm.

∵ L + S = 100 + 25 = 125 mm, and P + Q = 75 + 90 = 165

∴ L + S < P + Q, Grashof’s criterion satisfies..

∴ For Double-Crank Mechanism, fixed link is the shortest link i.e. link BC.