Transmission systems for vehicles

Claim 1

A transmission coupling for a vehicle including: a substantially tubular body; a respective set of helical gear teeth provided circumferentially around the interior of the body at each end of the body; a shaft rotatably mounted in the body, coaxially with the plurality of teeth; helical spline means extending along the shaft; a sliding gear rotatably mounted on the shaft, the sliding gear having a plurality of internal helical gear teeth in engagement with the helical spline means on the shaft and a plurality of external helical gear teeth; the sliding gear being so arranged to be movable along the shaft between a first position, where the external gear teeth engage a first set of teeth in the body to provide a driving coupling between the shaft and the body in one direction, a second position where the external gear teeth are dis-engaged from both sets of teeth in the body to decouple the shaft from the body, the shaft and the body being capable of rotating at different rotational speeds, and a third position where the external gear teeth engage the other set of teeth in the body to provide a driving coupling between the shaft and the body in the opposite direction; wherein: the sliding gear is moved from the first position to the second position when the rotational speed of the body is greater than the rotational speed of the shaft, the body forming the output side of the coupling; the sliding gear is moved from the second position to the third position, engaging the other set of teeth, when the shaft is rotated in the opposite direction, and the spline means on the shaft and the internal gear teeth on the sliding gear are of one hand, and the teeth in the body and the external gear teeth on the sliding gear are of opposite hand, the sliding gear locking the shaft to the body when the external gear teeth on the sliding gear engage the sets of teeth in the body.