Red bar and yellow shaft create a joint of class II (allowing four degrees of freedom).

Red bar and green shaft create a joint of class II. Red bar and blue retainer create a spherical joint.

With this arrangement, the plane containing ball centers almost always remains in a plane that bisects the angle α between the

two shafts when α changes. See: Slider crank and coulisse mechanism 1

The video shows the transmission when α is 0 deg. and then 30 deg.

The output shaft rotates nearly regularly with max error of 1.5% at α = 30 deg.