Patent GB-547438-A (GB)
Title: Improvements in or relating to fluid-pressure-operated relays
IPC: F16K17/00
Filed: 1941-02-26
Abstract: 547,438. Fluid-pressure servomotor-control systems; hydraulic machines. SMITH, R. J. Feb. 26, 1941, No. 2641. [Classes 69 (ii) and 135] [Also in Group XXIV] A piston control or relay valve for a fluid pressure servomotor with a double-acting power device is arranged so that when the valve has been moved to an intermediate point in its travel the pressure conditions set up cause the valve to snap-over to one of its extreme positions. Fig. 1 shows the invention incorporated in a servomotor system for operating the change-speed mechanism for two-speed supercharger used on aircraft in accordance with the varying altitude of the craft. The piston 11 coupled to the change-speed mechanism control is mounted for reciprocation in a cylinder having passages 27, 28 at each end for the supply and exhaust of pressure fluid which may be the oil circulated by the scavenge oil pump and having additional exhaust ports 25, 26 uncovered by the piston when near the end of its stroke to permit a continuous circulation of pressure fluid. The passages 27, 28 and ports 221, 231 leading to the ports 25, 26 lead to a ported liner having a further port 20 leading to the supply pipe 21, and two exhaust ports 22, 23 connected by a passage 241 leading to an exhaust outlet 24. A piston valve 13 housed in the liner comprises a central solid part 16 of less axial depth than the width of the supply port 20 and two tubular parts 17, 18 adapted to control the ports 22, 221, and 23, 231 and so arranged that the part 18 opens the ports 23, 23<SP>1</SP> before the valve 17 completely closes the the ports 22, 221 so that there is always a path connecting the supply and exhaust passages. In the position shown oil from the inlet 21 passes through port 20, holes 30 in the tubular part 18, passage 28 to the underside of the piston escaping to exhaust through ports 25, passages 221, 241 to the exhaust outlet 24. At the same time the upper end of the cylinder is connected to exhaust by way of passage 27, holes 29 in the part 17, port 22 and passage 241. Pressure acting on the lower face of the solid part 16 holds the valve in its raised position. As the valve is moved downwards and reaches the position in which its lower face is opposite the lower edge of the port 20 the upper surface becomes subject to pressure and the valve snaps over to its extreme lower position, so altering the fluid connections that the piston moves to its lower position. This valve may be actuated manually but in the arrangement shown it is actuated by a pair of plungers 31, 32 working in opposed cylinders 33, 34 connected by pipes 35, 36 with a piston control valve actuated by an aneroid capsule. The aneroid capsule comprises a base 40 screwed at 41 into the base of an air chamber 42 connected through an opening 46 to the space behind the carburetter chokes or directly to atmosphere. The base is connected by a bellows part 44 to a cup-shaped top 45 and exhausted or filled with air at atmospheric pressure in which case the chamber 42 is jacketted to maintain its temperature constant. The valve has a spindle 48 passing through a packing 49 and having its lower end held in contact with the bellows top 45 by a spring 47 arranged between a collar 50 on the spindle and the top of the chamber 42. The valve has four lands 51, 52, 53, 54, the lands 52, 53 being solid while the lands 51, 54 have axial holes 56. The valve works in a liner having a supply port 58, ports 62, 63 connected respectively to the pipes 35, 36 and exhaust ports 59, 59<SP>1</SP> connected by a passage 60 leading to an exhaust outlet pipe 61. When moving upwards the land 53 uncovers the port 62 before the land 51 uncovers the port 59 that pressure fluid exhausting through passage 63 acts on the lower surface of land 52 to give a snap-over action. A similar sequence occurs when the valve is moving in a downward direction. The lands 52, 53 are spaced further apart than the ports 62, 63 so that the land 52 crosses the port 63 at a different altitude from that at which the land 53 crosses the port 62.