574,147. Fluid-pressure servomotorcontrol systems. DODSON, E. Jan. 6, 1944, No. 251. [Class 135] In a hydraulic servomotor control system operating under a relatively high pressure, say 1,000 lbs. per square inch or more, and having follow-up mechanism between the servopiston and the control valve for terminating the movement of the servopiston by returning the control valve to its neutral position, the control valve 55 is operated by a further relay operating at a relatively low pressure, say 300 lbs. per square inch or less, this relay having a piston 45 connected to the control valve, the piston being controlled by a pilot valve 42. In this way sticking of the control valve under high pressures is avoided. Built into the same unit is a pressure-reducing valve 48 for regulating the relatively low pressure, a byepass valve 72 which automatically places the ends of the main jack in communication when the pump pressure fails, and a further valve 91 which operates to move the jack to a predetermined position when the pressure in a remotelycontrolled signal line fails. In Fig. 1, a pump 11 feeds oil from a reservoir through a pipe 12 to a sender unit 13 as described in Specification 574,148. The latter is controlled by a,pilot's lever 15 which is operated to control the oil pressure through duplicate pipes 14 to a receiver unit 16. Both ends of the pipes 14 have cut-off valves 23 as described in Specification 482,935 to cut off the oil pressure in one of the pipes in the event of it leaking or being damaged. Oil pressure from the pipes 14 acts through a connection 27 on the underside of a piston 28, Fig. 2, loaded by adjustable springs 30 which act through a knife edge 34 on a double V block 35 mounted on a double lever 36 pivoted on the casing 137. The underside of the V block engages a second knife edge 37 on the piston-rod 38 to which is pivoted a second lever 40 loaded by a spring 49 and pivoted at 43 to the pistonrod 44 of the relay piston 45 and at 41 to the pilot valve 42 controlling the relay piston 45. The pilot valve 42 is in the form of a piston with lands controlling a pressure port 47 and an exhaust port 50, the intermediate waisted portion being permanently in communication by a pipe 52 with the relay piston 45. The lever 40 acts as a floating lever to restore the pilot valve 42 to its normal position after movement of the relay piston following a controlling impulse. The piston-rod 44 is pivoted at 56 to a third lever 57 having a pin-and-slot connection 58, 59 with the rod 60 of piston-type control valve 55. The connection 58, 59 serves as an adjustment for varying the response of the hydraulic jack to changes in signal line pressure. The valve 55 is loaded by a spring 54 and has lands which in its neutral position block ports 70, 71 connected by conduits 17, 18 to the ends of a jack 19, Fig. 1, operating a control surface 25 through an arm 24. The two outer necks on the control valve are in free communication with ports 66, 67 leading back through a conduit 21 to the reservoir 10. An intermediate neck opposite a port 69 communicates with a highpressure line 68 direct from the pump 11. The follow-up in this case is provided by a roller 61 on the lever 57 engaging a cam surface 62 on an arm 63 linked to the arm 24. The cam 62 may be designed so that in the central range of the travel of the pilot's lever, large movements of the latter produce only small variations in the position of the control surface whilst near the end of its travel a small movement of the lever produces a large movement of the control surface. The pipes 17, 18 are also connected by ports 77, 78 with a piston valve 72 loaded by a spring 74 and normally maintained in the position shown by the high pressure in the conduit 68 acting on a piston 73. In the event of the pump pressure failing the ends of the jack are connected through the port 77. Fig. 3 shows a spring-loaded piston type of pressurereducing valve 48 having inlet ports 82 connected by a line 83 to the high-pressure channel 68, outlet port 84 connected by ports 85, 186 and a pipe 86 to the reservoir 10, and an intermediate port 88 connected to the pressure port 47 of the pilot valve and also through passages 87, 89 to the chamber 90 above the valve 48. This valve operates in normal manner to regulate the pressure passing out at the port 88. The latter does not lead directly to the pressure port 47 but through an intermediate piston valve 91 urged by a spring 92 into light contact with a roller 102 on the lever 36. In the position shown reduced pressure passes via ports 88, 94, 98 to the pressure port 47 whilst the exhaust port 50 is connected with the exhaust pipe 86 via ports and passages 99, 97, 100, 84, 85, 186. Should the signal line pressure below the piston 28 fail, the roller 102 depresses the valve 91 and reverses the connections to the ports 47, 50 so that the relay piston is now depressed by pressure from the port 50 until the pilot valve 42 contacts an adjustable stop. The further movement of the piston 45 raises the piston '28 and hence the roller 102 so that the connections to the pilot valve 42 are again reversed and the pressure on the piston 45 is relieved. The latter and hence the jack stays in its adjusted position determined by the stop 104. A passage 188 connects the upper side of the piston 28 with the passage 100. Specifications 483,049, 536,612, 542,682 and 551,258 also are referred to.