Device for controlling a coupling

A device for transmitting torque between two parts rotatably mounted relative to one another comprising:

• a friction coupling having friction elements rotationally fixed to each of two rotatable parts, said friction coupling being loadable by at least one movable piston which, on one side, delimits a pressure chamber filled with a viscous fluid, said pressure chamber being connected to a reservoir, wherein said pressure chamber being formed by a rotational housing and said piston, and the rotational housing is connected to one of said rotational parts, a rotational member rotates in said pressure chamber and is connected to said other rotatable part, with rotational faces of said rotational member, and counter faces of a control member which are positioned in said pressure chamber, constituting at least one shear channel formed by a shear groove laterally delimited by walls and extending between two ends in the circumferential direction and by a surface covering said shear groove and being rotatable relative thereto, said control member being rotatable to a limited extent between two end positions relative to said rotational housing and said shear groove being connected to said reservoir and to said pressure chamber by control apertures in said control member at said end of said shear groove, said connection being effected, in said two end positions of the control member, by a control aperture being arranged at a front end of said shear groove 20 seen in the direction of relative rotation--communicating with said reservoir, and a control aperture arranged at a rear end of said shear groove--seen in the direction of relative rotation--communicating with said pressure chamber,
• characterized in
• that at least two separate, axially movable annular pistons delimit said pressure chamber, and on the basis of different characteristic functions, said pistons convert pressure in said pressure chamber to an axial force acting on said friction coupling.