GB716022A - Improvements in or relating to pit props - Google Patents

Abstract

716,022. Pit props. GREBE, K. Sept. 25, 1950 [Oct. 1, 1949; April 25, 1950], No. 23440/50. Class 20 (2) A fluid pressure pit prop comprises at least two relatively movable sections, one of which forms a piston in a second forming a pressure chamber and elastic buffer means associated with the pressure chamber and contained in a third section (which may also be relatively movable) and adapted to take up excessive pressure on overloading of the prop, an overload valve being opened automatically during compression of the buffer and returning to its closed position when the fluid pressure has dropped sufficiently to permit the buffer to re-expand. As shown in Fig. 1 the prop comprises three relatively movable sections 3, 1, 5, section 1 forming the pressure chamber 2 and section 5 containing an elastic buffer 6 comprising. an inflatable container abutting against the closed bottom of section 1. A duct 13 with a manually operated valve 15 and a repulsion type valve 14 supp:ies high pressure oil to the chamber 2, the discharge duct 20 being provided with a valve 21 having an extension rod 23 so arranged that on compression of the buffer 6 owing to overload, the rod 23 abuts against the upper rim of the section 5 and the valve 21 opens accordingly to release the oil and reduce pressure in the chamber 2 whereupon the buffer 6 expands and the valve 21 closes. As shown in Figs. 4, 5, the section 1 is screw-threaded in the section 5 and the buffer 6 located between bottom plate and an annular collar 9a is provided with a piston head 35. Alternatively the head 35 may be independent of the buffer 6. The section 1 is bridged at its lower end by a fixed plate 40 provided with holes 39. A supply and discharge duct 41 in the head 35 is controlled bv a spring-loaded valve 21 suspended from the plate 40 so that upon compression of the buffer 6, the valve 21 is separated from the head 35. The valve 21 may be manually controlled by an eccentric 43 operated from outside and which contacts an extension rod 44 of the valve. The duct 41 is connected to supply and discharge lines 32, 33 by a spring-loaded two-way valve 47 which is normally open to the discharge pipe 33. Depression of hand lever 49 closes the valve to the discharge line 33 and opens it to the supp'y line 32 whereupon the valve 21 is opened and the pressure medium enters the chamber 2 through the openings 39 and presses the piston 3 against the roof. On overloading of the prop, the medium is pressed from the chamber 2 against the head 35 which is pressed downwardly whereupon the valve 21 which was previously closed is separated from the head 35. The medium is therefore able to flow from the chamber 2 through the duct 41 and valve 47 which is now open to discharge line 33. On release of pressure the buffer 6 expands and the valve 21 is closed. The buffer 6 may alternatively consist of elastic resin which will expand to its normal volume upon pressure release.