CONTINUOUSLY VARIABLE TRANSMISSION DEVICE AND METHOD

A variable transmission device and method are disclosed. The variable transmission device comprises: radially inner and outer races, each comprising two parts spaced along an axis; planetary members arranged in rolling contact with said inner and outer races; a transmission ratio controller operable to vary a separation along said axis of said two parts of one of said inner and outer races resulting in a radial displacement of said planetary members to vary a transmission ratio; and a clamping force controller coupled to at least one of said two parts of another of said inner and outer races and to an input shaft, said clamping force controller comprising opposing surfaces operable rotate relative to each other in response to a torque applied to said input shaft, at least one of said opposing surfaces being defined by a ramp having a non-linear lead profile, said relative rotation of said opposing surfaces causing a non-linear change in separation along said axis of said two parts of said another of said inner and outer races to accommodate said radial displacement of said planetary members and to control clamping forces applied between said planetary members and said inner and outer races proportionately to said torque applied to said input shaft and said different transmission ratios. Providing a non-linear ramp enables the clamping force generated between the races and the planets to be varied dependent on the transmission ratio of the device. Accordingly, rather than simply generating a clamping force which is proportionate to the torque applied to the input shaft, the clamping force generated is proportionate to both the input torque and the transmission ratio of the device. In this way, only the required amount of clamping force is generated, which is suited to the particular transmission ratio of the device. This helps to ensure that the loading on the components is reduced, the amount of friction and heat generated within the device is minimized which reduces wear and the efficiency of the device is increased.