contact and leakage characteristics in the embodiment of fig1 the springs 7 and 8 provide a fixed asymmetric closing force which is suitable for applications where the pressure is constant and of known value for applications where pressure varies or is not known it is desirable to have the total combined closing force vary with the pressure difference across the seal face with reference to fig3 the seal face 36 of stator 34 is urged toward face 35 of rotor 33 by hydraulic actuators 31 and 32 having pistons 41 and 42 respectively the actuators are connected with the interior of the housing 37 by means of conduits 38 so that the pistons 41 and 42 are subjected to system operating pressure the areas a 1 and a 2 of the pistons 41 and 42 are selected to provide the desired asymmetric closing force of the stator against the rotor with the area a 1 of one piston 41 being larger than the area a 2 of piston 42 to apply a corresponding higher closing force f h to a portion of the seal face where the relative velocity vector is directed from a region of higher to lower fluid pressure than to the opposite portion where a lower force f l is applied the total combined area a 1 a 2 of piston 41 and.