GB744899A - Pit props and like struts - Google Patents

Abstract

744,899. Pit props. WATERHOUSE, W. Oct. 19, 1953 [July 18, 1952], No. 18271/52. Class 20(2). A telescopic pit prop comprises freely extendible members and a clamp carried by one of the members for frictional engagement with the other member to hold the prop against collapse, fluid pressure means being provided which applies increased pressure to the clamp in response to relative displacement between the prop members due to excess load. In the telescopic pit prop shown in Fig. 1 a moving support C of H-sectioned steel bar is locked to a fixed support A by pressure plates 10, 10<SP>1</SP> of a fluid pressure operated clamp contained in a box member 5 which bears upon fluid pressure bags 15 located upon the top of the fixed support A. The pressure plates 10, 10<SP>1</SP>, which engage opposite faces of the web 2 of the moving support C, are actuated by fluid pressure supplied by a pump P, Fig. 3, to extendible bags 11. The bags 11 are connected to the bags 15 by a oneway valve V<SP>1</SP> and the bags 15 are connected to the intake side of the pump P by a relief valve V<SP>2</SP>. The prop is extended by hand and then the pump P is operated to deliver fluid to the bags 11 and thereby impart clamping pressure to fluid to the pressure plates 10, 10<SP>1</SP>. When a predetermined pressure is reached in the extendible bags 11, valve V<SP>1</SP> opens to allow fluid to enter the bags 15. After the prop is set, an increase in roof load acts upon the bags 15 and forces fluid from these bags into the bags 11, thereby increasing the clamping action of the pressure plates 10, 10<SP>1</SP>. When a maximum clamping action is reached, upon the bags 15 becoming empty or upon the relief valve V<SP>2</SP> opening, the prop continues to yield under a constant load. The prop is released by operating by-pass valves X<SP>1</SP>, X<SP>2</SP>. The bags 11, 15 are of rubber or neoprene. In alternative constructions the prop members are of tubular form as in Fig. 8 and the extendible bags 33, 38 are annular. In Fig. 9 an annular extendible clamping bag 38 and an annular extendible bag 33 sensitive to the roof pressure, are connected by one way valves V<SP>4</SP>, V<SP>5</SP>. Fluid is supplied by the pump P<SP>1</SP> to the bag 38 and the valve V<SP>4</SP>, at a predetermined pressure, permits fluid to enter the bag 33 during operation of the pump Pl. Valve V<SP>5</SP> permits fluid to pass from the bag 33 to the clamping bag 38 when roof pressure increases. In the tubular prop of Fig. 8 the pressure plate of the clamping means consists of a helical band 43 enclosed by a band of spring metal. In other tubular props, Figs. 4 and 7 (not shown), the pressure plates are respectively annular, or in the form of arcuate segments enclosed by a band of spring metal and the gripping action may be increased by means of serrations in or protrusions on the tube. Conveniently hydraulic pistons may be used instead of extendible bags.