Patent GB-580858-A (GB)
Title: Improvements in hydraulic servo motors
IPC: F15B11/12
Filed: 1944-03-02
Abstract: 580,858. Fluid-pressure servomotorcontrol systems. HOBSON (AIRCRAFT & MOTOR) COMPONENTS, Ltd., H. M., and SIMPSON, T. March 2, 1944, No. 3941. [Class 135] A hydraulic servomotor especially suitable for changing the gear ratio of the supercharger of an aircraft engine comprises a control valve 10 actuated by a pressure-sensitive device 11 and having at least three control positions, a servo-piston 34 or like power device having corresponding operative positions, at least two shuttle valves 26, 31 movable between alternative end positions under the control of the valve 10 in such manner that the shuttle valves are reversed one at a time and in succession as the control valve moves to successive control positions and restriction-free conduits such as A, B, C between the shuttle valves and the servo-cylinder or the like 32 by which pressure liquid is applied to the servo-piston 34 and to a stroke limiting means 36 in the same cylinder. A three-position arrangement with two shuttle valves is shown in Fig. 1. The first shuttle valve 26 is reversed on changing from the first to the second control position and the second shuttle valve 31 is reversed on changing from the second to the third control position. In the first control position shown in Fig. 1, both shuttle valves 26, 31 are raised and the servopiston 34 is at the extreme right of its cylinder. In the intermediate position, the servo-piston 34 occupies a central position abutting the stroke limiting piston 36, but maintained in spaced relation thereto so as not to obstruct the conduit C by means of stops 40. In the third and final position, both pistons 34, 36 occupy the left-hand end of the cylinder 32. The pressure-sensitive device 11 comprises a stack of capsules exposed to atmospheric pressure and acted on at one end by a datum varying cam 13 adjusted by a lever 15 connected to the pilot's lever so as to be varied by the latter in accordance with the selected boost pressure and engine speed as described in Specification 577,910. The control valve 10 is of the two-land type and operates in a sleeve 12 having a pressure oil inlet 17 and two exhaust outlets 20, 21. The sleeve 12 also has two pairs of ports 22, 23 and 29, 30 respectively in staggered axial relationship so that as the valve 10 descends due to expansion of the capsule 11 at higher altitudes, the pressure and exhaust connections through conduits 24, 25 to the opposite ends of the first shuttle valve 26 are first reversed and subsequently corresponding the pressure and exhaust connections through conduits 27, 28 to the opposite ends of the second shuttle valve 31 are reversed. The result of the first shuttle valve 26 moving to its upper position is that the conduit A is connected to pressure and conduit C is connected to exhaust. The pistons 34, 36 then move to the central position. The movement of the piston 36 to the right is limited by a flange 38 on its piston-rod 37 co-operating with a stop 39. Due to the piston-rod 35 of the main piston 34 being of greater diameter than the piston-rod 37, the piston 36 has a greater area of its outer face exposed to pressure. When the second shuttle valve 31 moves to its upper end position, the conduit B is connected to exhaust and the conduit C is cut off by the land 49 of the valve 31. Both pistons 34, 36 then move to the left of the cylinder 32 and any leakage past the piston 34 is exhausted by passages in the piston 36 which are then opened by a relief valve 51. The invention may be applied to a .rotary vane type of servomotor. Specification 577,912 also is referred to.