GB767012A - Improvements in or relating to pit or like props - Google Patents

Abstract

767,012. Fluid pressure pit prop. COAL INDUSTRY (PATENTS), Ltd. Nov. 12,1954 [Nov. 12, 1953], No: 31376/53. Class 20(2) [Also in Group XXIX] A pit prop comprises two telescopic members 1, 2 between which is formed a variable-volume pressure chamber 10, the telescopic. members 2 constituting a liquid pressure accumulator associated with an externally controllable valve 15 for admitting liquid under pressure from the accumulator to the chamber 10 to extend the prop, a bleed-relief valve being provided for permitting escape of liquid from the chamber 10 to release the prop. In Fig. 1, the outer telescopic prop member 2 is divided by a wall 8 into a lower chamber 9 which constitutes a liquid pressure . accumulator wherein liquid is maintained under pressure by compressed air within a flexible bellows 12, and an upper chamber 10 of variable volume one wall of which is constituted by the inner end 3 of the inner telescopic prop member 1 which forms a liquid reservoir 11. A valve 15, carried by the wall 8, includes a fixed valve seat 21, Fig. 2. against which a ball valve 18 is held by pressure within the accumulator when the prop is out of use. To set the prop against the roof, the ball valve 18 is lifted from the seat 21 by a needle 30 which is in screw-threaded engagement with the wall of the prop for external actuation by rotation of a knob 33, thereby permitting liquid to flow from the chamber 9 into the chamber 10. When a roof load acts upon the props, pressure built up within the chamber 10 forces the ball valve 18 against a seat 17 in a slidable valve block 16 which prevents return of liquid to the chamber 9, and also displaces the block 16 against a spring 22 so that the position of the block 16 is a function of the pressure in the chamber 10. At a predetermined position of the block 16 the ball valve 18 is pushed from the seating 17 by a needle 23 to permit bleedrelief of liquid from the chamber 10 to the chamber 9. The pressure at which bleedrelief occurs is. predetermined by axial adjustment of the needle 23 by means of a screw thread 24. To release the prop a valve 4 between the chamber 10 and reservoir 11, is opened by actuating an external operating knob 7. When setting the prop, the valve 4 may be opened to permit the prop to be first extended by hand from roof to floor, after which the valve 4 is closed and the valve 15 opened. In a modification (Fig. 3, not shown) the bellows 12 is replaced by a floating piston, and an additional pressure relief valve is provided between the chamber 10 and reservoir 11, which in the event of failure of the valve 15 to open sufficiently permits liquid to escape directly to the chamber 11 from the chamber 10 in the event of pressure in the latter rising beyond a predetermined limit. The air pressure point for the accumulator may incorporate a blow-off valve to operate at a predetermined pressure in order to ensure that the pressure in the air chamber 12 does not rise beyond a desired limit