Method and device for controlling a coupling

A motor vehicle having a permanently driven axle and an axle driven from case to case, comprising a differential-speed-dependent coupling in its driveline leading to the axle driven from case to case wherein, said coupling is designed as a coupling for transmitting torque between two parts which are rotatable relative to one another, said coupling including, a friction coupling having friction elements, whose friction elements are each non-rotatably connected to the one and the other of the parts which are rotatable relative to one another, with the friction coupling being loadable by at least one movable piston which, at one end, delimits a pressure chamber which is filled with a viscous fluid, which is connected to a reservoir, a rotational housing is formed by the one of the rotatable parts and by the piston sealed relative thereto and in which there rotates a rotational member connected to the other one of the rotatable parts;

• wherein rotational faces of the rotational member, together with counter faces of a pumping and control member positioned in the pressure chamber, constitute at least one sealed shear channel which is formed by a groove laterally delimited by walls and extending circumferentially between two ends and by a surface which covers the groove and is rotatable relative thereto;
• the pumping and control member is rotatable to a limited extent between two end positions relative to the rotational housing, that the groove, by means of control apertures, in the pumping and control member which are arranged at the groove ends, is connected to the reservoir in the rotational housing and to the pressure chamber between the piston and the rotational member in such a way that in the two end positions of the pumping and control member the control aperture arranged at the respective front end of the groove in a direction of relative rotation communicates with the reservoir and that the control aperture arranged at a respective rear end of the groove in the direction of relative rotation communicates with the pressure chamber;
• and
• a pipeline for connecting the pressure chamber to the reservoir, in which pipeline there is arranged a valve which opens from the pressure chamber to the reservoir and which is provided to limit the pressure in the pressure chamber.