Crank-lever Mechanism or Crank-rocker Mechanism:

This mechanism is illustrated in given figure. In this case for each complete rotation of link 2 (called a crank), the link 4 (known as lever or rocker), makes oscillation among extreme positions O₄B₁ and O₄B₂.

The position of O₄B₁ is achieved when point A is A₁ whereas position O₄B₂ is achieved while A is at A₂. It can be observed that crank angles for the two strokes (forward and backward) of oscillating link O₄B are not similar. It can also be noted down that the length of the crank is very short. If l₁, l₂, l₃ and l₄ are lengths of links 1, 2, 3 and 4 respectively, the proportions of the link can be as follows :

(l₁  + l₂ ) < (l₃  + l₄ )

(l₂  + l₃ ) < (l₁  + l₄ )