Double Crank Mechanism:

The links 2 & 4 of the double crank mechanism make full revolutions. There are two forms of this mechanism.

Parallel Crank Mechanism:

In this, lengths of links 2 and 4 are equivalent. Lengths of links 1 & 3 are also equivalent. It is illustrated in given figure.

The familiar instance is coupling of the locomotive wheels where wheels behave as cranks of equivalent length and length of the coupling rod is equivalent to centre distance among the two coupled wheels.

Drag Link Mechanism:

In this also links 2 & 4 make full rotation. Since the link 2 & 4 rotate sometimes link 4 rotate quicker and sometimes it becomes slow in rotation.

The proportions of this mechanism are following

l₃ > l₁; l₄  > l₂

l₃ > (l₁  + l₄  - l₂ )

and      l₃  < (l₂  + l₄  - l₁ )

It can be observed from given figure that length of link 1 is smaller than other links.