Abstract: The invention relates to improved mechanism systems for controlling and assisting the operating effort of a mechanical member and is especially applicable to automatic assisted steering systems for automobile vehicles. The said servo-mechanism comprises essentially a steering shaft including two coaxial sections, the first section being coupled to a steering wheel for its rotation about the common axis of the two sections, while the second section is coupled to the mechanical member to be servo-controlled, this second section being fixed on an end sleeve which is coaxial and surrounds one part of the first, which is provided with a pin mounted in one of its diametral planes, the wings of said pin being identical and projecting over the same length from each side of the common axis, the second section co-operating with the first by means of the wings of said pin and an elastic member interposed between the pin and the second section.