814,618. Fluid-pressure servomotor control systems. NEW YORK AIR BRAKE CO. Feb. 21, 1956 [April 29, 1955], No. 5316/56. Class 135. An hydraulic system comprises a, doubleacting motor 11 controlled by a relay valve 13 supplied by a continuously running pump 7 from a sump S, hydraulic pressure-operated means 88, 99 for shifting the valve in opposite directions from a neutral position in which the motor is hydraulically locked, a combined relief and unloading valve 8, and a pilot or control valve 9 comprising two relatively movable ported members 26, 35, one 26 being movable by the motor 11 and the other 35 by a hand lever 38 and the members 26, 35 being arranged so that in a neutral position a supply port 17a, a discharge port 22a, and a port 16a leading to the hydraulically operated means are all connected and that on relative movement of the members the supply and discharge ports are regulated in opposite senses. A float control valve 10 adapted to permit floating movements of the motor 11 is arranged between the pump 7 and the control valve 9. The discharge from the relay valve 13 is controlled by a flow control valve 119, 121, 122 to prevent cavitation within the motor 11. The control valve 9 comprises a plunger 35 operable by the hand lever 38 which is linked to a fixed part 25. The plunger 35 has a recessed part 36 and is slidable in a bore 34 in a housing 26 itself slidable in the fixed part. The housing has a pressure equalizing passage 43 and is returned to a neutral position relative to the plunger 35 by the motor 11 through a linkage 27, 31. The unloading valve comprises a piston with lands 67, 68 controlling ports 14b, 14c and a valve nose 72 controlling a port in a seat member 65 leading through a restricted passage 66 to a sump connection 22. Pump pressure in the connection 14 normally maintains the valve in the open or " unloaded " position shown against the action of a spring 71 and the reduced pressure in a chamber 78. The latter chamber is fed through a restricted passage 77, further restricted by an interchangeable pin 79, from the pump connection 14. The valve therefore only closes to load the pump when the pressure in the chamber 78 backs up to that in the pump connection and this happens only when the valve 9 is moved from its neutral position or the valve 10 is in an end position. If the pressure in the chamber 78 rises beyond a predetermined value, a pilot valve 73 loaded by an adjustable spring 75 opens to relieve the pressure and the valve 8 functions as a relief valve. The float control valve 10 comprises a piston operated by a knob 49 and having lands 51, 54, 56 separated by necks 53, 55. The valve may be held in either of two positions, a " run " position or a " float " position by spring- loaded detents 61 engageable with a groove 57 or 58. The valve has lost-motion corresponding to the distance between the grooves 57, 58 with respect to a spring seat 48 and is movable beyond the " float " position to a " start to float position against a spring 59. In the " run " position a connection 15 from the unloading valve 8 is connected via a connection 17 with the control valve 9 and with a chamber 86 in the relay valve. Simultaneously the valve 9 is connected through connections 16, 18 with a chamber 104 in the relay valve. In the " float " position, the connections 15 and 17 are both in communication with the sump and the connections 16 and 18 are blanked off. In the " start to float " position the conduits 15, 18 are connected, conduit 16 is blanked off and conduit 17 is open to the sump. The relay valve comprises a valve stem with pistons 98, 102, 103, 99 of equal diameter and a further piston 88 of reduced diameter. The pistons 102, 103 control conduits 21, 19 leading to the motor. The piston 99 is open to the pressure in the conduit 18 and the piston 88 to that in the conduit 17, the spaces between the pistons 88, 98 being connected to the sump. The valve is maintained in the neutral position by a centering spring 89 mounted or a reduced end of the stem 101 between seats 92, 93. The movements of the seats are restrained respectively by the inner end of a slidable cap-shaped member 87 and a snap ring 94 mounted therein. The member 87 is retained in one or other of its end positions by spring-loaded detents 95 engaging a groove 96 or a groove 97. In the " run " position liquid under the pressure in conduit 17 is supplied constantly to the small area piston 88 and to the cup-shaped member 87 and is simultaneously supplied to the chamber 104 under the control of the valve 9 which causes the pressure in conduits 16, 18 to rise or fall depending on the direction of its movement. In the " start to float " position, the chamber 86 is vented and the pressure in chamber 104 overpowers the spring 89 and forces the member 87 full stroke to the left so that supply ports 14d, motor ports 19d, 21d and exhaust ports 105, 106 are all in communication and the motor piston is free to float. In the " float " position the conduit 18 is blanked off and liquid is trapped in the chamber 104. At the same time, the pump is unloaded. The flow control valve has lands 121, 122 controlling exhaust ports 109, 111 and a valve nose 119 controlling a port in a plug 112. A plunger 116 mounted in the plug 112 is subject to the supply pressure to the relay valve through a passage 117. The flow control valve is loaded by a spring 125 and its left-hand end is connected with the exhaust chamber 107 through a restriction 126 with a metering pin. The flow control valve thus tends to open as the supply pressure increases and to close when the exhaust pressure rises.