Abstract: A rotary actuator suitable for a robotic arm or servo drive, which is capable of large loads and large speed reductions is disclosed. The device includes an input shaft that drives an epicyclic transmission integral with the articulation. The transmission is contained in a fixed arm housing and an end housing. An input shaft has eccentrics that rotate two primary external spur gears which are engaged with two primary internal gears. The two primary external gears are fastened on both faces of the external secondary gear. The external primary gears and external secondary gear rotate about the axis of an eccentric, and orbit the input shaft axis. A secondary external gear transmits its rotary motion to an internal secondary gear affixed to an output arm. Large speed reductions are possible due to the differential action of the primary and secondary gears.