Improvements in hydraulic props or struts

907,304. Hydraulic pit props. DOWTY MINING EQUIPMENT Ltd. Aug. 10, 1960 [May 22, 1959; July 23, 1959], Nos. 17549/59 and 25341/59. Class 20 (2). An hydraulic pit prop (see Fig. 1 of Provisional Specification 17549/59) comprises telescopic members 1, 2, a variable volume pressurechamber 3, 4, and a pump 7, 8 which is actuated by a rod 15 controlled by an eccentric 16, 17 and has an inlet port 31 and an outlet port controlled by a valve 29. A combined relief and release valve 13 is urged by a spring-loaded seating member 22 against a ported spigot 9 which is adapted to permit communication between the pressure chamber and the prop reservoir, the valve 13 being lifted by excess pressure in the pressure chamber or under control of the eccentric 16, 17. In a modification, (see the drawings accompanying Provisional Specification 25341/59) the outer prop unit 1, 7 slides on an intermediate member 3 and incorporates a pump plunger 8 actuated by an eccentric 16, 17, inlet and outlet valves 31, 29 in the pump chamber and a combined relief and release valve 13. The valve 13 bears on a spring-loaded seating 22 and a ported spigot 9 which is carried by the piston 8 and provided with a stop-nut 33 to limit its travel, whereupon a further lift applied to the piston-rod 15 or excess pressure on the valve 13 will serve to lift the valve seating 22 and establish communication between the pressure chamber and the reservoir. Specification 904,866 is referred to.