GB962968A - Improvements in or relating to a servo system - Google Patents

Abstract

962,968. Power steering. ROSS GEAR & TOOL CO. Inc. March 24, 1961 [April 4, 1960], No. 10930/61. Heading B7H. [Also in Division F1] A servo system comprises a reversible hydraulic motor 10, a reversible control pump 11, a power-driven pump 12 and a control valve 13 which permits or prevents the pump 12 to deliver to the motor 10 and also includes a by-pass responsive to the flow of liquid from the pump 12 to by-pass some of the liquid from the pump 12 around the pump 11. The system is applicable to a power steering mechanism for vehicles in which the pump 11 is normally operated by the steering wheel and the motor 10 is connected by a rack-and-pinion 17 to the steering wheels. Valve 13 has a bore 21 provided with grooves 23, 24, 25, in which slides a spool 22 having a central land 26, intermediate lands 27, 28, and end lands 29, 29. An axially-extending passage 62 in the spool connects the space between lands 27 and 29 with the space between lands 28 and 29 and in the neutral position of the valve, passage 62 communicates with groove 23 through one or more radial passages 62 in land 26. The valve 13 is provided with springs 77, 78 which bias the spool 22 towards the neutral position, in which position it will remain until the differential pressure in chambers 68, 69 created by operation of control pump 11, and transferred through conduits 70, 71 is sufficient to overcome the stress in springs 77, 78. Motor ports 35, 36 are connected by conduits 37, 38 with opposite ends of motor 10, control ports 30, 31 are connected by conduits 32, 33 with opposite sides of control pump 11. A pressure port 40 receives liquid discharged from power-driven pump 12 and a return port discharges through conduit 42 into a sump 43 from which pump 12 draws liquid. Control ports 30, 31 have portions 45, 46 of reduced diameter upon which are seated by springs 47<SP>1</SP>, 48<SP>1</SP> the valve members 47, 48 through each of which extends an orifice 50. A passage 52 intersects both of ports 30, 31 outwardly from the shoulders on which valves 47, 48 are seated, and its ends are connected through check-valves 53, 55 respectively with the reduced-diameter portions 45, 46 of ports 30, 31 respectively. Pressure port 40 is connected by a passage 60 with groove 23, and return port 41 is connected through a valve 63 to a passage 61 which communicates with the bore 21 and through a check valve 65, with passage 60. When the pump 12 is operating, if pump 11 is also operated to an extent such that the pressure differential in chambers 68, 69 is sufficient to overcome the action of springs 77, 78, spool 22 will be displaced, say to the left and will open communication between groove 23 and port 31 and block communication of port 31 and groove 25. Accordingly, liquid delivered by pump 12 enters portportion 46 from which it can escape only through orifice 50 in valve 48, since this valve 48 is initially seated and check valve 55 prevents flow into passage 52. The liquid then passes via conduit 33, pump 11, conduit 32, port 30, orifice 50 in valve 47, motor port 35 and conduit 37 to left end of cylinder 15. However the pressure drop across orifice 50 in valve 48 will raise this valve to provide communication between portion 46 and passage 52, allowing some of the liquid supplied to port 46 to pass through this passage, past valve 53 to port 45 where it combines with the liquid delivered by pump 11 to conduit 32. Thus only part of the liquid delivered by pump 12 will flow by via pump 11 to cylinder 15. Liquid displaced from right-hand end of motor cylinder 15 flows through conduit 38, or 36, passages 62, 61, port 41 to sump 13. Specification 911,401 is referred to.