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This paper deals with energy-savings in a hydraulic power steering systems by the reduction of the driving torque of a hydraulic pump in non-steering operation. In normal driving, steering operation is infrequent although the pump is continuously driven by the engine. Therefore, for energy-savings in power steering, it is effective to reduce the pump driving torque during non-steering operation. A reduction of the pump driving torque in non-steering operation is possible by decreasing the pump outlet flow rate regulated by a flow control valve. The authors have investigated a prototype pump with an additional device to minimize the regulated flow rate under a non-steering condition and to restore the flow rate during steering operation. The mechanism is a kind of pressure reducing valve that changes the operation of the flow control valve in response to the pump outlet port pressure which is an indication of steering operation.

This paper deals with a center-closed rotary servo valve in a hydraulic power steering (PS) system having an improved straight line stability during driving. Conventional rotary servo valves are of a center-open type and are poor in straight line stability because the assist pressure to steer occurs with even slight steering inputs. In the vicinity of neutral position the center-closed rotary servo valve does not produce assist pressure and leaves room for manual steering effort. The center-closed rotary servo valve is composed of a center-closed circuit and a center-open circuit in parallel. The center-closed circuit precisely controls the direction of oil flow to the cylinder chamber A or B in the vicinity of neutral position. The chamfers formed at center-closed metering edges control a lower level of assist pressure. Also the change of assist pressure from manual- to power-steering is smooth.