A rotary drive arrangement for rotating a camshaft so that the speed of rotation oscillates during a revolution of the camshaft comprises a driving sprocket wheel driving a follower mounted on a square-section shaft driven by the sprocket wheel. The follower is slidable on the shaft transversely and is held in contact with a cam surface of a stationary hollow cam member. A pin offset gyro axis fixed to the follower and, as a driving member, is drivingly engaged in an aperture in a drive arm fixed to the camshaft, so that there is imparted to the camshaft a rotational motion which oscillates during a revolution of the driven member in accordance with the sliding motion of the follower resulting from its contact with the cam surface. |