Abstract: It has been proposed to employ a constant mesh gearbox of the kind having pairs of gears on parallel shafts in constant mesh with each other, together with a clutch, in an automatic vehicle transmission. A microprocessor controls actuators for the clutch and throttle and hydraulic actuators for moving the selector rod of the gearbox, in order to give automatic operation of the gearbox.In accordance with the invention, a constant mesh gearbox of the kind referred is provided with actuating means for moving the selector shaft and, in at least one direction of travel of the selector rod, the actuating means acts via resilient means which is arranged to store the energy of the actuating means when the actuating means is operated but the selector rod is not free to move, the rod being moved by the energized resilient means when the selector rod becomes free.

Abstract: A valve which is arranged to output from a passage 12 a substantially constant-rate flow of fluid, notwithstanding changes in the flow rate entering the valve by passage 11, is arranged to provide an output from passage 12 which is variable with the static pressure in that passage. Thus when the valve is used to supply a controlled flow to a hydraulic control system an increased flow rate to the system is provided to compensate for increased leakage in the system resulting from a higher static pressure in the system.

Abstract: A continuously variable ratio transmission of the rolling friction type has a hydraulic control circuit including an end loading device for the transmission located upstream of a double sided piston used to balance roller reaction torque, and a valve for controlling the force exerted by the double sided piston. The valve has two outlets each feeding one side of the piston and communicating with the transmission's sump by way of a device for equalizing the flow rates from the two outlets to the sump. A feedback arrangement provides that the force exerted on the spool of the control valve is always proportional to the force exerted by the piston. The piston travel is limited by hydraulic end stops, upstream of which is provided a pressure relief valve to eliminate damaging transient overloads being exerted by the end loading device. The circuit also includes a facility whereby normal electrical/electronic control of the control valve can be overridden in an emergency.