Power-assisted steering system

Claim 1

A power-assisted steering system in an automotive vehicle, comprising: a hydraulic pump driven by a prime mover of the vehicle for supply of fluid under pressure; a hydraulic power cylinder operatively connected to the dirigible road wheels of the vehicle; a changeover valve unit including an input member arranged to be moved by the driver's steering effort applied thereto, an output member operatively connected to said power cylinder, and valve means for controlling the supply of fluid under pressure from said pump to said power cylinder in response to relative movement between said input and output members; a hydraulic reaction mechanism associated with said changeover valve unit to apply a reaction force to said input member in accordance with a hydraulic reaction pressure applied thereto, said reaction mechanism including a reaction fluid chamber to be applied with the hydraulic reaction pressure; first flow control valve means disposed within a first fluid circuit connecting said pump to said valve means of said changeover valve unit and within a second fluid circuit connecting said reaction fluid chamber to a fluid reservoir to permit a predetermined small quantity of fluid under pressure supplied therethrough from said pump to said reaction fluid chamber; a first throttle disposed within a bypass circuit connecting said first fluid circuit to said second fluid circuit between said first flow control valve means and said reaction fluid chamber; a second throttle disposed within said second fluid circuit between said first flow control valve means and said fluid reservoir; and second flow control valve means disposed within said second fluid circuit downstream of said second throttle to decrease the quantity of fluid discharged therethrough into said fluid reservoir in accordance with an increase of travel speed of the vehicle thereby increasing the hydraulic reaction pressure in said reaction fluid chamber.