Patent ID: 8220598

Claim:
An apparatus comprising: at least a pair of members of which one of the members is relatively movable with respect to another of the members under a directional force applied by an external stimulus, the relative movement of the members causes a relative displacement of a pair of friction surfaces so as to convert the external stimulus into a friction force so as to control the movement of said one of the members; a prestressing means for pressing on said pair of friction surfaces, the members being cooperative with said prestressing means, the directional force controlling an amount of the relative displacement, a force in a direction opposite said directional force causing a blocking of the relative displacement, the members being rotationally movable with respect to each other; a means for transforming the rotational movement into a rectilinear movement; an annular element intercalated between the members, said annular element being blocked from rotation on one of the members so as to create a frictional torque between said annular element and the other member, said annular element being a coaxial assembly of two rings, each of said two rings having a friction surface; and a ring element blocked in rotation in at least one direction of rotation, said ring element positioned on the immobilized member, said annular element positioned on balls received respectively in cavities in said ring element and coincident cavities in said annular element, said cavities distributed uniformly around a periphery of said annular element and uniformly distributed around a periphery of said ring element, said cavities in said annular element having a ramp shape such that during a relative rotation of said annular element with respect to said ring element the balls rest on the ramp shape of the cavities so as to cause a separation of the rings.