bearing in casing 18 which controls the axial position of the shaft 14 the servo motor 48 comprises a casing or cylinder 56 which may be integral with the bearing casing 18 and a piston 58 which concentrically and loosely surrounds the shaft 14 the front end thereof is rigidly fastened to the shaft positioning control bearing in casing 18 the piston 58 has a central flange 60 which divides the cylinder 56 into two separated hydraulic pressure chambers 72 and 76 a hydraulic pressure medium is maintained within the pressure chambers by a pilot valve 66 to create sufficient axial forces to maintain the shaft 14 and the rotatable grinding member 24 in their desired positions the pilot valve 66 is described in detail for example in u s pat nos 2 971 704 and 4 073 442 it is rigidly secured to the casing 18 and is actuated by an arm 68 with a set screw 70 working against the end of the piston of the pilot valve the arm is attached to the servo motor piston 58 and moves axially therewith the pilot valve 66 is actuated by the axial movement of the rotating shaft through the servo motor piston 58 and creates simultaneous changes of the hydraulic pressures in the opposing pressure chambers 72 and 76 of the.