GB748374A - Improvements in hydraulic pit props - Google Patents

Abstract

748,374. Hydraulic pit props. DOWTY HYDRAULIC UNITS, Ltd., [formerly DOWTY AUTO UNITS, Ltd.]. Sept. 16, 1954 [Sept. 24, 1953], No. 26349/53. Class 20(2). [Also in Group XXVIII] A hydraulic pit prop has a valve conduit 13 which is disposed con. centrically of the ram tube 18 and extends between the cylinder space 17 at the inner end of the ram 11 and a release valve housing 21 at the outer end of the ram 11, and a pump for extending the prop includes a large area piston 61 for rapid initial extension of the prop and a small area piston 49 for creating the desired setting pressure of the prop, both pistons being arranged concentrically of the ram tube 18, the large area piston 61 having a relief valve 67 for rendering the large area piston ineffective upon the attainment of a predetermined pressure in the cylinder space 17. The piston 49 : is constituted by the lower end of a tube surrounding the conduit 23 which extends into a high pressure pumping chamber comprising an annular space 48 formed between the conduit 23 and an upstanding wall 47 on a closure member 15 of the ram. The closure member 15 contains a non-return valve 59 through which pressure fluid passes into the space 48 when the tube 49 moves upwardly and a non-return valve 60 through which fluid passes from-the space 48 into the cylinder space 17 when the tube 49 moves downwardly. The piston 61 .fits within the tubular part 18 of the ram and is carried with annular clearance by the tube 49 between shoulders 62, 63, spaced to permit limited relative axial movement between the piston and the shoulders so that the piston 61 functions as a clack valve which opens when the tube 49 moves upwardly and closes when the tube 49 moves downwardly whereby fluid is delivered by the piston 61 through one way valves, Fig. 3 (not shown), in the closure member 15 into the cylinder space 17. The tube 49 has an extension 51 carried by a slide block 52 which is adapted to be reciprocated by a crank mechanism operable by an external handle, in order to operate the pump. The valve housing 21 has a passage 22 opening into the conduit 23 and a passage 31 opening into the reservoir space within the ram 11, communication between the passages 22 and 31 being controlled by a manually operable pressure balanced release valve 33 in a passage 32. An automatic relief valve 36 which operates to limit the pressure which can be built up in the cylinder space 17 is disposed between passages 34 and 35 in the manual relief valve 33. Specification 636,568 is referred to.