Transmission device for self-propelled equipment, includes a housing having a driving input shaft transmitting movement to an output shaft, the input shaft bearing a receiving pulley around which pulley a belt transmission is wrapped. The input shaft also bears a component that rotates as one with the shaft and a member for controlling the movement of the pulley along the input shaft in terms of the pulley movement towards or away from the component so that the pulley, which is mounted with freedom to rotate on the input shaft, can be moved from a disengaged position when it is away from the component, into an engaged position when it is close to the component, the pulley having a groove of constant predetermined width for accepting the belt so that when the belt is taut inside the groove, the belt tension is maintained as the pulley moves along the input shaft.