Abstract: A fluid control circuit and method of operating equipment such as a blowout preventer having a two-way hydraulic piston and cylinder assembly. The circuit includes various features such as a dual pressure level independently controlled fluid supply, a dedicated secondary fluid supply reserved for supplemental use, a dedicated fluid supply reserved for exclusive use in one operating branch, automatic sensor for applying a reserve fluid supply on a sensed demand, a pressure intensifier for increasing available operating force, and control circuitry for preventing loss of enhanced secondary operating energy into a primary circuit.

Abstract: The invention relates to the control of a hydraulic jack which can be used, for example, for controlling the orientation of the rear wheels of a vehicle with front guiding wheels. In the event of a failure in the supply of fluid under pressure to the chambers (3, 4) of the jack, these chambers are isolated and the piston (2) is maintained in the position which it occupied when the failure occurred, by means of non-return valves (12). However, when certain conditions are satisfied, for example a sufficiently low speed, the electronic control member (20) activates an auxiliary circuit (30 to 39) in order to return the piston to its middle position and allow driving of the conventional kind with the rear wheels in line.

Abstract: An hydraulic steering system for a ground-engaging, self-powered vehicle, the system including an engine driven primary pump for pumping hydraulic fluid to the steering system when the vehicle engine is operating and a wheel-driven secondary pump for pumping hydraulic fluid to the steering system when the vehicle is in motion, even if the engine or the engine driven primary pump is inoperable, or if the output of the engine driven primary pump is inadequate. The wheel-driven secondary pump is an over-center, axial piston pump which is controlled so that it is unidirectional in its output regardless of any change in direction of its input power source and which, thus, avoids the need for check valves in the hydraulic lines of the secondary pumping system, regardless of whether the vehicle is moving forward or in reverse when the engine becomes inoperable.

Abstract: The steering system of some lightweight vehicles are electronically controlled and have a manually operated mechanical control as a backup. Such manual backup control would not be adequate for large earthmoving, construction and industrial type vehicles. The subject steering system includes an electrically controlled circuit actuated by rotation of a steering wheel for controlling a main pilot operated steering valve as the primary steering control. A hydraulic control circuit is also actuated by rotation of the steering wheel for controlling the same steering valve as a backup control when the electrically controlled circuit is not active. The hydraulic steering circuit is disabled when the electrically controlled circuit is activated. By using the same steering valve as an integral part of both circuits, the complexity and cost of the overall steering control system is reduced.

Abstract: In a system with an energy storing and releasing device the latter comprises a high pressure accumulator and switching over device for the charging and discharging of the latter from and to a low pressure accumulator using a hydraulic transducer which is connected with a device able to provide and take up power. The switching over device consists of four 2/1 way shut off valves of which two respective ones are placed in parallel and connected with a respective accumulator. The 2/1 way shut off valves are so connected with the hydraulic transducer that by paired switching of a respective one of the two 2/1 way shut off valves associated with one accumulator either the one or the other accumulator may be connected with the power input of the hydraulic transducer and switching over may take place from the power output of the latter to the other accumulator. By the use of simple 2/1 shut off valves the switching over device is generally simplified to a substantial extent.

Abstract: In the supply of priority and lower-ranking consumers the flow to the priority consumer must be maintained even when there is a leak in the hydraulic system of the lower-ranking consumers. For this purpose the priority consumer is connected via in each case a check valve to a pump and an auxiliary pump. If the pressure generated by the pump drops for example due to a leak a valve opening the pressure line to the discharge is shut-off and the priority consumer thereby supplied with fluid delivered by the auxiliary pump.

Abstract: A hydraulic system for an aircraft to provide improved reserve capability for the thrust reverser power system. There is a fluid reservoir having a vent tube with an inlet location located above the bottom wall of the reservoir. In the event that fluid in the reservoir reaches a predetermined low level, supply and pressure isolation valves isolate at least one pump from the rest of the hydraulic system, and also start that pump operating, with a reserve supply of fluid being available for supplying power for the thrust reverser.

Abstract: A hydraulic system for an aircraft to provide improved reserve capability for the steering and reserve brake power system. There is a fluid reservoir having a vent tube with an inlet location located above the bottom wall of the reservoir. In the event that fluid in the reservoir reaches a predetermined low level, supply and pressure isolation valves isolate at least one pump from the rest of the hydraulic system, with a reserve supply of fluid being available for supplying power for the steering wheel and reserve brakes.

Abstract: An hydraulic supply circuit for a vehicle power steering system of the type having a steering actuator and control valve therefor with hydraulic input, sump and overflow (by-pass) connections and in which any fluid not actually consumed by the actuator is diverted to the overflow connection. A positive displacement pump is provided having an output line connected to the input connection of the steering system. An accumulator having a charging line and a discharging line is used to supply fluid pressure in lieu of the pump. To control flow of fluid to and from the accumulator a charge-discharge valve is provided having charge and discharge modes. In the charge mode the accumulator is charged by the overflow and in the discharge mode pressure fluid is supplied from the accumulator to the input connection for maintaining the steering system in operation while blocking the overflow.

Abstract: The control mechanism is for pneumatic apparatus of the kind comprising a piston-and-cylinder motor coupled between two relatively-movable parts (such as door-leaves) for driving them, when energized, to a predetermined relative position. A pilot valve is actuated when required to switch the apparatus to an emergency mode by isolating the motor from its power source and discharging it into an emergency reservoir having a volume large compared with the maximum volume of gas in the motor. While the pilot valve is actuated, the relatively-movable parts are urged towards their predetermined relative positions by a force small compared with the normal operating force. The pilot valve may be arranged to operate operation of an emergency release bolt, on obstruction detected by a detector valve (in which case a timing reservoir provides time delay before full power is restored), on power failure, or as required.

Abstract: The invention relates to a steering control system for a bidirectional servomotor connectable to steerable wheels. The servomotor may be of the differential type in which a rod extending from one side of the piston thereof causes the effective areas on opposite sides of the piston to be unequal. A steering control unit controlled by a steering wheel has two motor ports connected to opposite sides of the servomotor and pump and tank ports connected to pump and tank units. The steering control unit has a metering motor controlled by the steering wheel which meters fluid to and away from only one side of the servomotor. The steering control unit has oppositely acting valves which may be throttle valves for establishing flow paths for opposite steering directions and the neutral condition.

Abstract: A working fluid supply system for a hydraulic power booster and a power steering system includes first and second fluid supply pumps, a hydraulic power boosting apparatus, a power steering unit, various fluid passages connecting thereto and a control apparatus. The first pump supplies pressurized working fluid to a first inlet port of the hydraulic power boosting apparatus. The second pump, controlled by the control apparatus, selectively supplies pressurized working fluid to a second inlet port of the power boosting apparatus to render the fluid supply flow rate applied to the power boosting apparatus substantially constant at a predetermined level when the fluid discharge rate of the first pump is less than the predetermined level. By this, a sufficient amount of pressurized working fluid is supplied to the hydraulic power boosting apparatus to fully operate it under all actual engine speeds.

Abstract: Implement and steering actuators are selectively actuated through steering and implement control valves respectively by fluid power output of respective steering and implement pumps. A pressure compensator is provided for at times diverting excess fluid from the steering pump to the implement control valve, and the implement pump applies fluid pressure to the implement valve through an unloading valve that gives the charging of an accumulator by the implement pump preference over operation of the implement actuator. The sensing of loss of fluid power in the steering pump activates an emergency control valve to connect the accumulator to the steering valve and to disconnect the pressure compensator from the implement control valve.

Abstract: An emergency hydraulic system for a vehicle having a hydraulic steering mechanism normally supplied by an engine driven pump. An electric motor driven pump supplies the steering mechanism under emergency conditions. The electric motor driven pump is inoperative during normal operation of the engine but if the engine ceases to operate it comes on responsively to the occurrence of a predetermined condition in the hydraulic circuit. In order to prevent the electric motor driven pump from operating during engine starting a relay is provided having a normally open switch in circuit with the motor. The relay is operated by a normally open engine condition switch which is arranged to close when the engine is running. The relay holds itself in, thus preparing the electric motor for operation should the predetermined condition occur in the hydraulic circuit of the steering mechanism.

Abstract: A hydraulic system having a single acting, mult-stage, synchronous hydraulic jack having a trapped oil space which provides the required synchronism. The hydraulic system additionally includes a first external pump in fluid flow communication with an oil space disposed between the outer cylinder and an intermediate section, and a second external pump disposed in fluid flow communication with the trapped oil space between the intermediate section and an inner plunger section. Oil leakage from the trapped oil space is detected when the sections are retracted. The next time the sections are to be extended following such detection, both pumps are automatically started, with the second pump being deactivated when the trapped oil space has been replenished.

Abstract: An emergency or backup steering system includes a backup pump which is driven by an electric motor. A control circuit for the motor automatically energizes it upon the main supply of hydraulic power steering fluid becoming inadequate for normal steering operation. The control circuit provides for checkout of the portions thereof governing actuation of the backup system and provides for indicating failure in the backup system of both the electrical and hydraulic portions thereof.

Abstract: An emergency steering system includes a backup pump which is driven by an electric motor, a control circuit for the motor includes a flow sensing switch and a pressure-sensing switch connected in series and operative to automatically energize the motor when a main pump is outputting no flow and the pressure at the pump outlet is below a predetermined minimum value for adequate steering operation.

Abstract: A heating and/or cooling system comprising a windmill for driving a first refrigerant compressor in parallel with a second refrigerant compressor driven by an electric motor or other power means. The second compressor operates the system when the wind velocity is low. When the wind velocity becomes high enough, the refrigerant pressure created by the wind driven compressor, is used to automatically shut off power to the second compressor. The wind driven compressor then takes over the function of the second compressor.

Abstract: A power steering device for vehicles having a main hydraulic pump and a hydraulic motor type vehicle speed sensor, the number of rotation of which is regulated to be proportional to vehicle speed. In order to make a power steering system safe even when the main pump breaks down, and to warn the driver of trouble in the power steering system by the increase of resistance to the steering operation, an emergency switchover device is provided to alter a hydraulic motor functioning as the vehicle speed sensor to a substitute hydraulic power source when a main power source breaks down.

Abstract: The standby supply for delivering pressure fluid to a user component includes a fluid pump as a primary fluid source coupled to the user component by a first check valve, an accumulator as a secondary fluid source coupled to the pump by a second check valve for charging the accumulator and a valve arrangement coupled to the pump, the accumulator and the user component responsive to pressure from the pump to control a connection from the accumulator to the user component.

Abstract: A transfer valve assembly controls the change-over in the supply of pressurized fluid to a steering servomotor from engine driven and vehicle driven pumps. The outflow rate of fluid from the engine driven pump is metered to control shift of the transfer valve assembly between its two operating positions preventing any delay or overlap in the change-over between pumps because of low or residual output pressure from the engine driven pump.

Abstract: A dual vehicular steering system with a primary system for normal operation and a secondary system for emergency operation triggered by pressure sensitive switching and valving means. Both systems have in common a pair of single rod hydraulic steering cylinders with each cylinder being connected to a steerable wheel of the vehicle, a vehicular engine and hydraulic fluid stored in a tank. Cylinder shuttle valves connected to each steering cylinder function as gate devices to separate the primary steering system from the secondary system. The primary system includes a main pump driven by the engine, a high pressure filter, a priority flow divider to divert fluid in excess of an acceptable flow level, a control valve, an integrated valve package to proportionately regulate fluid flow between the cylinders, and means to direct fluid from the tank to the pump, through the filter, through the priority flow divider, through the control valve, through the integrated valve package and to the steering cylinders.

Abstract: A dual hydraulic circuit arrangement is provided each comprising essential elements of an independent booster steering system, each having valving mechanisms for steering control wherein the valving mechanisms pressurize and exhaust the power cylinder of the respective circuit. The valving mechanisms are operable in a sequential order so that initially and ordinarily one power cylinder effects steering, but in the event of failure or very heavy requirement the second power cylinder is capable of operating the steering mechanism or of assisting in such operation. It is contemplated that the pump of the initially operative circuit is engine driven and the pump of the other or emergency circuit is driven by the vehicle wheels. Thus, in the event of vehicle engine failure, the emergency power circuit can effect steering as long as the vehicle is moving. On the other hand, when heavy steering load is encountered, as in parking, both circuits are operative.

Abstract: Two fluid circuits respectively supply pressurized fluid from engine driven and vehicle pumps to separate servomotors in a vehicle steering system for steering operation during vehicle travel. Both fluid circuits are connected to the engine driven pump alone during vehicle standstill by a reversing valve which is shifted to a travel position by pressure output of the vehicle driven pump.

Abstract: A hydraulic brake booster mechanism for use with an open or circulating fluid pressure system includes a divider valve for the fluid flow interposed between a source of continuously circulating fluid pressure and a power steering mechanism, a control valve operatively associated with a brake pedal for applying the brakes, and a change-over valve actuated by means of the displacement of the divider valve upon the failure of the source to deliver the fluid pressure from an accumulator to the control valve for assuring an emergency application of the brakes.

Abstract: An economical and effective auxiliary steering system for a vehicle having a relatively low maximum speed in reverse direction. A ground driven pump having an output when the vehicle is going forward is directed to a diverter valve, which valve is responsive to loss of hydraulic pressure in the main steering circuit to direct the said output to the main steering valve. A single spring loaded check valve isolates the main steering valve under auxiliary steering conditions.

Abstract: A dual steering system for vehicles, particularly for large and heavy off-highway vehicles, wherein a back-up steering system is provided for safety purposes. Dual hydraulic systems are provided and coordinated whereby in the event the primary steering system fails for any reason, the back-up steering system automatically assumes the entire steering operation, thus providing a safety feature for the vehicle.

Abstract: A hydraulic pumping arrangement for pumping a liquid to a hydraulic system, which comprises pumping means for pumping the liquid, swash means including a variable swash member which enables the liquid output of the pumping means to be infinitely varied between minimum and maximum, a first piston-and-cylinder arrangement for increasing the position of the swash member to increase the liquid output of the pumping means, a second piston-and-cylinder arrangement for decreasing the position of the swash member to decrease the liquid output of the pumping means, control means for controlling the flow of liquid for the second piston-and-cylinder arrangement, switching means operative to pass liquid from the system to the control means at a first condition in the system and operative to stop liquid from the system passing to the control means at a second condition in the system, and valve means for isolating the pumping arrangement from the system and from a main pump normally supplying liquid to the system, the cont

Abstract: A valve for use in a system having a source of fluid, two pumps and means responsive to a change in the flow condition of the fluid caused by a lack of proper performance of one of the pumps to effect a utilization of the fluid pumped by the other pump. The valve includes a body having a pair of inlet ports and an outlet port spaced from the inlet ports. One of the pumps is connected to a first inlet port with the other pump being connected to a second inlet port. A passageway is provided in the valve body and extends between and is in fluid communication with the pair of inlet ports and the outlet port. A first normally closed valve is provided in the valve body for controlling the main flow of fluid from one of the pumps and is openable in response to a predetermined pressure differential in the fluid at the first inlet port and the outlet port. The first normally closed valve is closed, in addition to the normal closing thereof, in response to a fluid flow from the other pump.

Abstract: A dual hydraulic steering system for a vehicle has two steering circuits each having a pump and a steering valve unit. A first circuit is connected to the main working spaces of the double-acting steering cylinders while the second circuit is connected to the piston rod spaces of the cylinders. The first circuit has an engine driven pump while the second circuit has a ground wheel driven pump and an accumulator. Both circuits are connected to a common tank which is divided below a lowest normal oil level. Pressure switches and electromagnetic valves feed flow from the first circuit to the second in case of low pressure therein. This cross-over is prevented when a level switch detects low tank oil level.

Abstract: Control apparatus is provided for an apparatus which has first and second mechanisms. The control apparatus automatically, controllably energizes the second mechanism in response to a preselected condition of the first mechanism and selectively, controllably energizes the second mechanism independently of the condition of the first mechanism.

Abstract: A valve device having a pressure operable valve piston, particularly for booster steering system, to selectively control pressure flow to a consumer device such as a hydraulic servomotor in order to permit use of a reserve pump in event of failure of a main pump. The valve in a neutral position combines the output of both pumps but is responsive to a first increase in the flow rate beyond a predetermined limit to shunt flow from the reserve pump to a sump. Upon further increase in the flow rate the valve piston moves to a position to also shunt part of the output from the main pump to a sump. Signal means is actuated by movement of the valve piston to apprise a vehicle driver that the steering system is operative.

Abstract: A hydraulic actuator for a gate or like valve such as the main steam isolation valve in an electric power plant characterized in that the actuator is a compact self-contained unit secured to the valve housing and comprises a hydraulic cylinder having its piston rod connected to the movable valve member to open or close the valve, an air-driven hydraulic pump for pressurizing dual hydraulic circuits (active and standby) each of which includes an accumulator, an air-piloted accumulator control valve, an air-piloted directional control valve for the hydraulic cylinder, and solenoid-operated pilot valves for said control valves in a pneumatic circuit including an air pressure reservoir. The hydraulic actuator is further characterized in that fast closing or opening of the valve is achieved by combined accumulator and pump discharge through the directional control valve and slow opening of the valve is achieved by pump discharge alone.

Abstract: An accumulator for use in a brake booster having a housing with a piston located in a bore. The piston has a plug attached thereto. The piston moves in response to a predetermined thermal expansion of a fluid in the accumulator. A predetermined movement shears the plug from the piston to prevent damage to the housing by venting the bore to a reservoir.

Abstract: A hydraulic system having a plurality of motor driven pumps in parallel legs, at least one being active and one in reserve, so combined with sensors to sense pump efficiency and filter efficiency that a condition deemed pump failure or filter failure results in activating the reserve leg and deactivating the inefficient leg. Other sensors detect flow meter failure and establish warnings of conditions deemed near-failure. The system may be duplicated both on the pressure side and return side of the pumps. The system reservoir is characterized by a tank for return fluid to be cleaned and a separate tank for the fluid to be delivered under pressure; so constructed as to (a) remove turbulence, (b) prevent vortex overlap, (c) remove silt fines and (d) maintain a proper pumping level.

Abstract: Hydraulically operated steering systems normally supplied with fluid from an engine driven pump rely on an auxiliary pump upon failure of the primary pump. A ground driven pump incorporating a pressure sensing device automatically provides fluid to the power steering system in the event of an engine or primary pump failure.

Abstract: An emergency hydraulic system for a hydraulically powered device such as the power steering mechanism used on large earth-moving vehicles and the like is disclosed. The emergency system includes an auxiliary pump connected to the wheels or some other part of the vehicle so that the pump is operated despite engine failure whenever the vehicle is in motion. An inlet valve for the pump is held in the closed position whenever the vehicle prime mover is in operation so that the pump is deprived of operating fluid when an emergency condition does not exist. If the prime mover stops running so that the main hydraulic system is not operating and the power steering system is deprived of its source of power, the inlet to the emergency pump is opened so that the pump supplies operating fluid to the steering system so long as the vehicle is in motion. Since the pump is deprived of operating fluid when an emergency does not exist very little power is consumed.

Abstract: A motor vehicle fluid power circuit is disclosed which includes a primary pump, a closed center brake booster, and an open center steering valve. These components are hydraulically connected in series with the fluid flowing from the pump to the closed center brake booster and then to the open center steering valve. The brake booster includes valve means throttling fluid flow from the pump to the steering valve and maintaining the outlet pressure of the pump at at least a predetermined pressure differential above the working pressure of the brake booster under normal operating conditions in order to conserve output load requirements of the pump while assuring adequate pump pressure for brake booster actuation. In a second embodiment, a back-up pump is also provided in the circuit, and a pilot valve hydraulically disconnects the primary pump and hydraulically connects the back-up pump to the brake booster in response to a fluid pressure failure in the primary pump.

Abstract: Disclosed is a vehicle power steering system including an electro-hydraulic safety backup system for supplying pressurized fluid to the steering motor of the vehicle when the pressure between the main pump and the power steering system drops below a predetermined pressure level. The safety backup system includes an electrical circuit which is added on to the ignition and starting circuits of a vehicle such that the backup system will not operate in response to intermittent short duration pressure drops. The electrical circuit includes an arming feature to prevent the backup system from operating during initial start-up of the vehicle and further provides for operation of the vehicle's starting motor to have priority over the backup system operation.

Abstract: A hydraulic fluid supply system includes a single motor driven pump which supplies a dual pressure level output for providing both a high pressure oil output and a low pressure oil output. In combination with a turbomachine, the dual pressure level output of the oil pump obviates the requirement of pressure reducing valves in the low pressure supply lines and losses associated therewith. Moreover, since only a portion of the entire oil requirement is raised to the higher pressure level, the pump motor horsepower requirements are reduced to save energy and thereby improve overall system efficiency. A further improvement is a system of pressure differential valves whereby under certain conditions, high pressure oil flow is conserved even though the low pressure oil flow momentarily drops below a preset value.

Abstract: A hydrostatic servomechanism adapted to control the direction of vehicles and self-propelled machines. The mechanism has a power circuit with at least one servopump, at least one servomotor and at least one distributor, a control circuit with a measuring pump actuated by a command element as for example a steering wheel connected by means of control conduit branches to reset mechanisms and at least one feedback simulator with a flow meter in the power circuit and a controllable throttle means incorporated between the control branches of the control circuit. The feedback simulator forms a combined feedback simulator consisting of the flow meter and the throttle means controllable in steps, the throttle means being formed as a combination of several connected controllable throttle means.

Abstract: A system for providing auxiliary power comprises a primary circuit including a primary pump, an output line for providing primary power, and a return line. An auxiliary circuit includes an auxiliary pump, an output line for providing the auxiliary power and a return line. The auxiliary circuit also includes a valve for controlling fluid flow in the output and return line of the auxiliary circuit. A reservoir is connected to the primary return line and to the auxiliary return line. A controlled leakage apparatus for cooling the auxiliary pump is connected to the auxiliary circuit, the reservoir and the primary circuit, and in response to a predetermined output in the primary circuit directs a portion of auxiliary output fluid to the reservoir. Approximately all of the auxiliary output fluid is directed to the auxiliary output line when the primary circuit provides less than the predetermined output.

Abstract: An Emergency or back-up Hydraulic System for use with a primary hydraulic system in motor operated equipment is disclosed. The system illustrated is a steering system for a vehicle and includes a motor driven pump and a wheel driven pump for supplying hydraulic fluid to the steering system of the vehicle. The motor driven pump and the wheel driven pump are connected in series so that the fluid discharged from the motor driven pump is supplied to the inlet of the wheel driven pump. The inlet of the wheel driven pump also communicates with the reservoir as for example through the anticavitation check for the engine driven pump. By-pass connections are provided around the wheel driven pump so that fluid in excess of the demands of that pump are channelled to the steering circuit when the engine driven pump is in operation.

Abstract: A power steering system for a motor vehicle. The power steering system includes a power steering pump which is belt driven by the motor vehicle crank shaft. An auxiliary electric motor is provided for driving the power steering pump when the motor vehicle engine is not running (as when stalled) and the ignition system is in the "ignition on" condition.

Abstract: Flow sensing and control apparatus is provided to selectively combine the fluid output of an auxiliary pump with a hydraulic system. The apparatus includes a valve body having a primary pump passage therein through which fluid from one of a pair of primary pumps is conducted to its respective inlet of a control valve, an auxiliary pump passage interconnected between the auxiliary pump and the primary pump passage and a fluid receiving passage which is connected to the auxiliary pump passage. A pilot operated valve is disposed within the fluid receiving passage and operative in response to fluid flow through the primary pump passage for selectively controlling fluid to the fluid receiving passage so as to permit the combining of any available fluid from the auxiliary pump with that of the primary pump up to a predetermined minimum flow rate through the primary pump passage and for diverting any excess fluid from the auxiliary pump to the receiving passage above that necessary to maintain the minimum flow rate.

Abstract: An emergency hydraulic system for use in a vehicle having an engine which not only powers the vehicle but also drives a main hydraulic pump to supply hydraulic fluid under pressure to a hydraulic control means is disclosed. The emergency hydraulic system includes an auxiliary hydraulic pump driven by an electric motor to provide an auxiliary source of hydraulic fluid under pressure to the hydraulic control means under emergency conditions. An electrical control circuit for the electric motor is described which control circuit will automatically activate the electric motor whenever the engine is operating normally but the flow of hydraulic fluid from the main hydraulic pump falls below a given level and will automatically inactivate the electric motor whenever the engine is operating normally and the flow of hydraulic fluid from the main hydraulic pump is above a given level.

Abstract: The disclosure relates to a control system for hydraulic pumps driven by the engine of a vehicle or the like and used for operating an auxiliary mechanism such as a vehicle mounted implement, a power steering system or the like. The disclosed system includes means for interrupting the flow of operating fluid to the pump during periods of time when the output of the pump is not required. With the pump inlet closed, all of the hydraulic fluid on the suction side is immediately pumped out the pump discharge, leaving only a small amount of residual oil on the surface of the working parts to provide lubrication. A valve is provided to prevent fluid from returning to the pump because of downstream pressure in the circuit. Control means, utilizing pressure downstream from the pump, for operating the valve in the inlet of the pump are disclosed.

Abstract: A method and plural embodiments of apparatus for delivering a pressure-compensated supply of fluid to a hydraulic steering circuit, wherein first and second pumps are respectively operated by a vehicle engine and by a ground wheel when the vehicle is in motion, output fluid from the two pumps being simultaneously directed to the steering circuit by a control valve with inlet relief valve means modulating output pressure of the sources in response to operation of the control valve. One embodiment contemplates dual inlet relief valves for separately modulating output from the two pumps while another embodiment contemplates one of the pumps having a variable output.