The lengths of the links comply with the conditions: A͞B=D͞C and B͞C=A͞D. The angles of rotation of cranks 1 and 3 are equal. All points of connecting rod 2 describe circles of a radius equal to the length of link 1. At the extreme positions (dead points) the motion of the mechanism becomes uncertain and it may be transformed into a crossed-crank mechanism.
$623$LW,PC$