984,033. Variable-ratio gears. AUTO TRANSMISSIONS Ltd. Feb. 5, 1964 [Feb. 5, 1963], No. 4593/63. Heading F2D. An over- or under-drive gear following a main vehicle change-speed gear comprises a three element epicyclic train having a reaction element selectively clutchable to one of the other elements for direct drive and stationable by a brake for drive at a planetary ratio; the lower ratio drive is automatically engaged when reverse is operative in the main changespeed gearing, and torque responsive means are provided to augment clutch or brake engagement under these conditions, biasing means being provided opposing the force generated by the torque responsive means so that the force of clutch or brake engagement will not be augmented until the bias has been overcome. The bias provided is equivalent to the maximum force that could be generated by the torqueresponsive means during over-run conditions so that if the planetary gearing is changed during over-run conditions from the higher ratio to the lower ratio the clutch or brake engagement is not augmented. As shown in Fig. 1, the planetary gearing comprises an input planet carrier 16, an output annulus gear 19 and a reaction sun 20 which is splined to a clutch-brake member 26 which is urged by springs 33 into frictional engagement with the annulus 19 and is moved by an annular fluidoperated piston 36 in opposition to the springs 33 to disengage the clutch and engage the member 26 with a brake surface 30 to station the sun 20. The sun, planets and annulus have helical teeth such that when the driving shaft 10 is driven in reverse the sun 20 is moved to the right against the bias of a spring 23 to increase the clutch engaging force provided by the springs 33. The sun 20 acts upon the member 26 by way of a circlip 47, axial clearance being provided between the circlip and the member 26, the rating of the spring 23 and the axial clearance being such that the circlip cannot abut the member 26, when that member is clutched to the annulus, until the spring 23 has absorbed an end thrust substantially equal to the maximum end thrust that the sun gear 20 can generate during over-run. Alternatively the axial clearance may be reduced and the spring 23 be pre-loaded to give the same result. As a modification two planet pinions may be mounted together on the same spindle, one meshing the sun gear and the other meshing the annulus so that only the sun and its associated planet need have helical teeth. In another alternative the gears have straight teeth and the sun gear has a helical splined connection with the member 26. In the modification shown in Fig. 4 the torqueresponsive sun 20 acts upon the clutch-brake member 26 in either direction irrespective of whether unit or planetary ratio is engaged. The sun is allowed to move in either axial direction and springs 23, 50 and circlips 47, 52 are provided. The springs 23, 50 can be preloaded, the axial clearances between the circlips 47, 52 and the member 26 in this case being just sufficient to allow the member 26 to change its engagement without also moving the sun 20. Alternatively the sun may be axially fast with the member 26 and be allowed sufficient free axial movement along the input shaft to prevent it from interfering with ratio changes, the pre-loaded springs 23, 50 being retained by, for instance, washers and circlips axially fast with the input shaft. The modification shown in Fig. 5 differs from that shown in Fig. 1 principally in respect of the brake which comprises an axially movable annular piston 55 movable by fluid pressure into engagement with a friction surface 28 of the clutch-brake member 26 to station it and move it axially against the force of springs 33 to disengage it from frictional engagement with the annulus 19. When the brake cylinder is exhausted the piston 55 is retracted by springs 58. Clutch engagement is assisted by rightward movement of the sun 20 when the bias of a spring 23 is overcome as in the embodiment of Fig. 1. Reference has been directed by the Comptroller to Specifications 739,126 and 952,371.