Abstract: A rotary mechanism having two members, the relative rotary positions of which are to be controlled, the mechanism including:A rotatable drive shaft on which the two members are co-axially mounted;An eccentric fixed to the drive shaft;An internally toothed ring gear on one of the two members and co-axial with the drive shaft;An externally toothed pinion on the other of the two members and co-axial with the drive shaft; andAn intermediate gear rotatably mounted on the eccentric and having external teeth in mesh with the ring gear and internal teeth in mesh with the pinion, the internal and external teeth on the intermediate gear being selected to permit an eccentric movement of the intermediate gear about the pinion and within the ring gear thereby to cause a driving inter-action between the two members upon rotation of the drive shaft.