Double Crank Mechanism is a four bar linkage. In this linkage, both the links pivoted to the fixed link rotates fully.
It is a Grashof linkage for which the sum of the lengths of the shortest link and the largest link is less than the sum of the lengths of the other two links.
s + l < p + q
s = length of the shortest link
l = length of the largest link
p and q = lengths of the other two links
In double crank mechanism, the shortest link is fixed. The position of the other three links does not affect the nature of the mechanism. It is also known as Drag Link Mechanism and Crank Crank Mechanism.