A deltoid linkage is a four bar mechanism which consists of two pairs of equal and adjacent links.
It is a grashof linkage for which the sum of the lengths of the shortest link and the longest link is equal to 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
The behavior of this linkage depends on whether the shortest link is fixed or the longest link is fixed.
Case 1: The longest link is fixed.
The mechanisms obtained in this case are two crank rocker mechanisms.
Both the mechanisms differ in the sense of the rotation of their cranks. In one, the crank rotates clockwise, and in the other, it rotates anticlockwise.
Case 2: The shortest link is fixed.
The mechanism obtained in this case is double crank mechanism.
In this mechanism, the shorter link revolves twice per revolution of the longest link. That is, in the time the longer link complete one revolution, the shorter links completes two revolutions.