GB912759A - Thrust applying mechanism - Google Patents

Abstract

912,759. Screw-and-nut gearing; step-bystep mechanisms. AUDCO Ltd. Oct. 14, 1960 [Oct. 27, 1959], No. 36345/59. Class 80 (3). [Also in Group XXIX] In a pneumatic or hydraulic system for imparting, automatically according to load, either a continuous or a step-by-step axial movement to a non-rotatable screw 12 operating a gate valve (not shown), the screw is moved either by rotation of a nut 14 coupled to gears 15, 18 driven through a fluid coupling 20, Fig. 3, or, if the load exceeds a predetermined amount, by limited and, if necessary, repeated axial movement of the nut 14 in its housing 17 by means of one or other of two annular pistons 22 engaging the ends of the nut, the axial movements of the nut being followed by rotary return movements and the pistons being actuated by the fluid pressure developed in the coupling 20 when rotation of its output gear 18 is resisted. The nut 14 is slidably keyed in a gear 15 driven by a worm 18, or alternatively by a pinion, on the output shaft of a gear pump coupling 20 having an input shaft 21, driven by hand or by electric motor, and two fluid output conduits 25. The two conduits 25 lead, respectively, to upper and lower valve pairs 24, 29 controlling the respective annular pistons 22. Normally, the valves 24 are so positioned by springs 28 that the two piston chambers 23 are connected through passages 26, 27 to a reservoir so that no force is exerted by the pistons on the nut 14. Rotation of shaft 21 in one or other direction then rotates worm 18 and nut 14 to move the screw 12, the nut reacting against the housing 17. Should rotation of worm 18 in one or other direction be resisted, the coupling 20 develops a pressure in one or other conduit 25 which shifts the upper or lower valve 24 so that the corresponding chamber 23 is connected to the pressure conduit 25 and causes joint axial movement of the nut and screw. The axial movement continues either (a) until the resistance is sufficiently diminished for the rotary drive to be effected, in which case the nut is screwed back to its normal position, the pressure in conduit 25 dropping and the valve 24 being returned by its spring 28, or (b) until the piston 22 reaches the end of its permitted stroke, whereupon the valve 29, which has been caused by its spring 30 to follow the piston, connects the pressure conduit 25 to the adjacent end of the valve 24 so that the latter is returned by its spring 28 to re-connect chamber 23 with the reservoir passage 27 and allow nut 14 to be rotated (under no load) back to its normal position. If movement of screw 12 is still resisted the sequence is repeated so that a step-by-step axial movement is imparted to the screw by alternate axial and rotary movements of the nut 14. A valve 32, Fig. 3, ensures that for either direction of rotation of shaft 21 the suction side of coupling 20 is connected to the reservoir by a passage 34. Pressure operated cut-out switches 35, Fig. 2, prevent the pump 20 developing excessive pressures.