Transmission device comprising a gearbox having multiple input axles and method using such device

A transmission device for application to a vehicle is disclosed. The transmission device includes a planetary gear (10) device including means (20) for restricting degrees of freedom and a gearbox (100) having at least two independent input axles. The root axle of the gearbox is functionally connected either to the planetary shaft of carrier or to the planetary shaft of the ring gear. The first axle of the gearbox has at least one first gear. An end portion of the first axle is functionally connected to the root axle via a synchronizing device and the other end portion either to the driving axle of the vehicle or to the internal combustion engine. The second axle of the gearbox has at least one second gear which maintains as engaged with said at least one first gear. An end portion of the second axle is a free end and the other end portion is connected to the root axle in such a way that the second axle has a direction of rotation opposite to that of the root axle. The third axle also has an end portion functionally connected to a planetary shaft in such a way that the third axle has a direction of rotation opposite to that of the connected planetary shaft.