Vehicle power assisted steering system

Abstract

A vehicle power assisted steering system comprises a hydraulic pump 24 having a constant volume output communicating with an open center power assistance valve 40 which is adjustable from a neutral condition in response to a steering input torque during a steering maneuver to control fluid flow from the pump to opposed chambers of a ram 18b providing power assistance to assist the steering maneuver. A by-pass valve 25 communicates between the outlet of the pump 24 and a fluid reservoir 21 and is responsive to vehicle speed to increasingly open communication between the pump 24 and the reservoir 21 as vehicle speed increases and conversely. The valve 40 has valve members displaceable from a neutral condition in response to a steering input torque to direct hydraulic fluid to actuate the ram 18b for power assistance. In the neutral condition of the valve members, they provide intercommunication between the pump 24, the chambers 18a of the ram and the reservoir 21. The valve members are biassed to the neutral condition. In order to prevent large fluid flows occurring during severe steering maneuvers at high speed, a flow sensitive valve 100 is provided in the line 28,29 connecting the pump 24 to the reservoir 21 by way of the by-pass valve 25 whereby the flow sensitive valve 100 acts to reduce the flow of fluid through the by-pass valve 25 when the flow of fluid reaches a predetermined maximum flow.