Abstract: A pinion shaft has thereon a sun gear for imparting rotational motion to or to be rotated by a pair of planetary gears which rotate about space axes extending parallel to and lying in a first plane containing the axis of said shaft. Each of a pair of planocentric compound cycloidal discs has an internal set of gear teeth constantly meshing with the teeth of the planetary gears, and an external set of cycloidal teeth engaged with the cylindrical teeth of a ring gear that is rotatable coaxially of the shaft. The cycloidal discs rotate eccentrically about the axis of the shaft, and respectively, about a second pair of axes extending parallel to and equi-spaced from diametrally opposite sides of said shaft, and lying with the axis of the shaft in a second plane that extends normal to the first plane.