Patent GB-673378-A (GB)
Title: Improvements in hydraulic servo motors
IPC: B64C13/24
Filed: 1949-06-10
Abstract: 673,378. Fluid-pressure servomotor-control systems. HOBSON, Ltd., H. M., and MOTT, L. F. June 9, 1950 [June 10, 1949], No. 15502/49. Class 135. A hydraulic servomotor has at least three hydraulic jacks arranged to act in parallel on an output member 16, the control devices of the jacks being linked to a manually operated input member and the movable members of the jacks being directly coupled to the output member through yielding connections each of which can yield on seizure of the corresponding jack piston and under the force exerted on the output member by the still effective jacks to free the output member from the piston of the seized jack and simultaneously lock the pistons of the effective jacks positively to the output member. The control devices are normally valves but in some cases variable stroke hydraulic pumps may be used. As applied to the control of an aircraft control surface, four jacks are arranged in pairs in a common housing 10 attached to brackets 12 forming part of the aircraft structure. The output member which displaces the control surface (not shown) through an arm 17 and a shaft 18 is formed as a block 16 with parallel bores to receive the piston rods 15, 115, 215, 315 of the four jacks. One pair of jack cylinders 13, 113 are supplied with liquid under pressure through a pipe 19 and the other pair from a separate source through a pipe 119. The jacks are controlled by valves such as 20, 120 connected to a common yoke member 39. The input member (not shown) is connected at 35 to a floating lever 34 which is pivoted at 38 to the yoke 39 and at 36 to a link 37 connected to the arm 17. Each piston rod has four spaced grooves 21, 22, 22, 22, the groove 21 being of round section and the grooves 22 of square section. The grooves 21 are staggered in the four planes A, B, C, D and corresponding to each is a plunger 23 arranged in the block 16. Each plunger 23 has a rounded end 24 normally engaged in the appropriate groove 21 and a square end 25 normally opposite but clear of a groove 22. Each plunger 23 also has a conical collar 26 engaged by a pair of spring pressed bolts 28 having square ends 30 normally opposite but clear of grooves 22 in the two remaining piston rods. When the jack piston corresponding say to the piston rod 215 seizes, the other three jacks move the rounded end 24 out of the groove 21. The plunger 23 therefore moves to place its square end 25 in the groove 22 of the piston rod 115. The plunger 23 is automatically locked in this position by a spring- loaded plunger 32 engaging a groove 31. At the same time, the conical collar 26 spreads apart the bolts 28 which engage the grooves 22 in the piston rods 15, 315 with their square ends 30. The three piston rods 15, 115, 315 are thus positively locked to the block 16. Spring boxes 40 between the control valves and the yoke 39 permit the yoke 39 and the remaining control valves to move in the event of any control valve seizing. In this case the pressure and exhaust connections are reversed by the operation of the input member and the piston corresponding to the seized valve is driven in reverse by the other three jacks, the displaced fluid circulating through the other cylinder say 113 connected to the common supply line 19. Spring-loaded valves 46 set to open at a pressure greater than the normal supply pressure but less than that necessary to cause the yielding connections to break connects the cylinder passages 44, 45 with the exhaust outlet 43. These allow a jack piston to be displaced when its control valve seizes in the mid-position. In a modification each pair of jacks has a single control valve connected to the input member in parallel with a collapsable member as described in Specification 673,369 which yields when the control valve sticks. A spring catch is thereby released and this permits a valve mechanism as described in Specification 673,370 to cut off the pressure supply and connect the two ends of the jack cylinder together. The two collapsable members may be interlocked by a device as described in Specification 673,372. In this form the movement of the input linkage is limited by stops. Reference has been directed by the Comptroller to Specification 617, 375.