GB763603A - Improvements relating to telescopic struts such as pit props - Google Patents

Abstract

763,603. Pit props. ONIONS, J. H. Oct. 25, 1954 [Sept. 1, 1953], No. 24113/53. Class 20(2) In a telescopic strut such as a pit prop comprising a closed hollow plunger 17, Fig. 1, slidable in a cylinder 12, air pumped through an inlet 40 into the upper portion 45 of the plunger 17 forces hydraulic liquid from the lower portion 46 of the plunger 17 past a one-way spring pressed ball valve 29 into a chamber 47 in the cylinder 12, thereby extending the prop, and a manually operable external lever 38 is provided by which an axial rod 33 can be moved against spring pressure to move the valve 29 from its seat permitting release of liquid from the chamber 47 to contract the prop. The upper part 45 of the plunger 17 into which the compressed air is introduced and the chamber 46 which contains the liquid are separated by a floating piston 41. A spring loaded safety valve 30 is provided to allow liquid to return from the chamber 47 into the chamber 46 when the load on the prop reaches a predetermined safe limit. In a modification, Fig. 2, a plate 52 secured across the cylinder 12 betwen the ram and the base plate 11a, contains a one-way valve 55 which allows liquid from chamber 47 to flow into a chamber 59 when sinking of the roof causes increasing endwise pressure on the prop. The liquid entering the chamber 59 displaces a floating piston 57 which defines on its other side a chamber 60 containing a charge of compressed air, which is initially admitted through an air valve 51 at a pressure of 200 to 350 pounds per square inch, and which is further compressed as the piston 57 is moved by liquid entering the chamber .59. The high pressure thus accumulated in the chamber 60 can be subsequently used to pre-load the prop by opening the valve 55 by a rotatable cam 56 after the prop has been initially extended by pumping air in through the inlet 40. In a further modification, Fig. 3 (not shown) an additional valve is provided in the plunger 17 which is adapted to be closed, as by contact with , a resilient abutment when the plunger 17 approaches within a predetermined distance from the base of the strut, to prevent further passage of liquid from the chamber 47 into the chamber 46, thus limiting the inward stroke of the plunger. The additional valve is arranged to be manually released at the same time as the valve 29 in order to completely retract the prop for removal. The base 11 and head 21 are secured to the prop by bolts the removal of which permits disassembly of the whole prop for maintenance work.