The patent application discloses a mechanical speed variator configured to transmit rotational motion from a prime mover (1) to a driven member (50). The mechanical speed variator includes a variable speed device, a plurality of actuated two way grippers (100) and a driven member (50). The actuated two way grippers are operable to sequentially grip and release the driven member over a predetermined range of rotation of each actuated gripper to rotate the driven member around a driven member axis of rotation (38). Also disclosed is the functioning of the variable speed device.