| 摘要 |
<p>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 line (28) communicates between the outlet of the pump (24) and a fluid reservoir (21). A by-pass valve (25) is located in the by-pass line (28) 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 by a pre-load that requires a valve actuating steering input torque to cause displacement of the valve members from their neutral condition. In order to prevent cavitation in the chambers (18a) when, due to reduced friction between the road wheels and the road, a movement of the wheels takes place faster than the fluid can be supplied to the chamber (18a) in question, a pressure dependent control means (50) is provided controlled by a pressure differential in a flow or return conduit between the open centre power assistance valve (40) and the steering ram (18). This control means controls the flow in the by-pass line (28) in dependence on the sensed pressure.</p> |
