In order to protect the spindle (2) displaced in the valve by a spindle nut (6) rotating in a housing (4), two friction surfaces (11, 13) facing each other are pressed against each other when a given threshold force on the spindle (2) is exceeded. One friction surface (13) is attached to rotate with the spindle nut (6) and the other friction surface (11) with the housing (4). The invention calls for at least one (13) of the two friction surfaces to have a free angle ( PHI ) for the rotary attachment. This measure makes it possible to loosen the spindle nut (6) again using a smaller torque than the braking moment which acts in the event of a control failure.
