Caterpillar Inc.

Gene R St Germain

Donald L Bianchetta

Elmer R Crabb

James Robert Bernhagen

James H Tunnell

John B Waggoner

Daniel G Roley

Marilyn S Murphy

Load sensing hydraulic systems are useful for distributing fluid from a variable displacement pump to the individual work circuits in accordance with the demand of each work circuit. Preventing load pressure from being drained from the motors of the work circuits during some operating functions heretofore required the use of a pair of pressure compensating valves and a pair of shuttle valves at each of the directional control valves. The subject load pressure duplicating circuit includes a single load pressure duplicating valve having an end subjected to the load pressure in a signal network and reduces pump pressure entering the valve to a pressure level at the output thereof equal to the load pressure. This duplicated pressure is then utilized as the control fluid for the pressure compensating valves and a displacement controller of the pump. The system is greatly simplified by the use of only a single duplicating valve for a hydraulic system having a plurality of work circuits.

Load pressure duplicating circuit

Pressure compensating valves located downstream of the directional control valves are useful in providing load independent, proportional flow control hydraulic systems and normally serve as load checks. The subject load check and pressure compensating valve is designed for use in a hydraulic system in which the pressure of a common pressure signal directed to all the pressure compensating valves of the system is limited to a predetermined maximum level. The pressure compensating valve includes a valve element disposed downstream of a metering orifice, a separate load piston disposed in end-to-end relationship with the valve element in a common bore and a pressure compensating spring disposed in a chamber behind the load piston and biasing the load piston and valve element to the load check position. The common pressure signal is communicated to the chamber containing the spring while the actual load pressure of an associated hydraulic motor is directed to a chamber between the valve element and load piston so that the valve element is held in the load check position by the actual load pressure when the actual load pressure is greater than the pump discharge pressure.

Load check and pressure compensating valve

Flow amplified control systems are useful in pilot operated steering systems. The load responsive flow amplified control system includes a directional control valve movable to an operating position to direct fluid from the operating fluid circuit through a variable orifice to a steering motor and to direct control fluid from a control fluid circuit through another variable orifice to be combined with the fluid going to the steering motor. The operating fluid circuit and the control fluid circuit are provided with fluid from a common variable displacement pump. The directional control valve is moved to an operating position by the control fluid acting thereon and is pressure compensated insofar as the flow of control fluid passing through the another variable orifice such that a substantially constant predetermined pressure differential exists across the another variable orifice. The rate of fluid flow in the pilot control circuit is determined by a steering control valve and is utilized for controlling the position of the directional control valve. The displacement of the variable displacement pump is responsive to the pressure condition in the pilot control circuit such that a substantially constant predetermined pressure differential exists between the variable displacement pump and the load pressure in the steering motor. The relative size of the variable orifices determines the amplification ratio between the pilot control circuit and the power operating circuit.

Load responsive flow amplified control system for power steering

An improved mechanical-over-hydraulic steering apparatus capable of providing automotive-type steering includes a steering wheel driven input control member, right and left turn control valves, and first and second actuators mounted for rotation along an axis common with the input control member. Each of the actuators has a cam operatively associated with a cam-following roller assembly that operates the respective control valve. The vehicle has a cross drive mechanism, and the steering apparatus includes an over-center variable displacement steering pump and associated displacement control controllably operated by first and second pilot lines connected to the right and left turn control valves. The steering pump controllably and bidirectionally communicates fluid to a steering motor for the steering input to the cross drive mechanism.

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Plainfield, IL, United States of America

Gene R St Germain