Pink, yellow and green links create a parallelogram mechanism.
Length of the pink and green cranks are equal to center distance of the internal gear drive. Line connecting centers of the gears O1O2 (playing role of a crank of the parallelogram mechanism) must be parallel to pink and green cranks.
Z1: tooth number of blue gear (= 30).
Z2: tooth number of internal gear (= 60).
Pink, green and line connecting gear centers are always parallel.
Pink crank and blue gear rotate in opposite directions with the same velocity.
Yellow link and blue gear rotate around the same axis in opposite directions with the same velocity.