Servolenkung für ein Fahrzeug.

摘要

A vehicle power assisted steering system comprises an hydraulic pump (24) having a constant volume output communicating with an open centre power assistance valve (40) which is adjustable from a neutral condition in response to a steering input torque during a steering manoeuvre to control fluid flow from the pump to opposed chambers of a ram (18b) providing power assistance to assist the steering manoeuvre. 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 inter-communication 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 manoeuvres 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.