Because of its lower follower speed in the working stroke and the higher follower speed in the return stroke

quick-return planar linkage mechanisms can raise produceing efficiency. The quick-return characteristics of planar linkage mechanisms have closely relationship with its crank angle θ between the two limiting positions. The definition of the angle θ in the present text books restricts the widely use of the formula about the coefficient k of travel speed variation and the θ existed in these books. By using the definition of the θ presented in this paper

this problem is soluted

and the results of two examples show that the definition of θ is right and practicable.