Abstract: A differential drive mechanism comprises a cage, which is rotatable about a first axis and represents the input, two coaxial output shafts which are rotatable with respect to the cage about the first axis, a coupling which is connected eccentrically to the two output shafts to transmit relative contra-rotational movement between them by connections which permit relative rotation of the coupling and the output shafts about a second axis parallel to the first axis and a restraint member which is coupled to the cage and to the coupling such that the coupling is rotatable with respect to the cage about a third axis perpendicular to the axis and capable of reciprocation in a direction perpendicular to the first axis but prevented from movement in a direction parallel to the first axis. The eccentric connections are constituted by a respective eccentric hole in the inner end of each output shaft in which the associated end of the coupling is received.