Power-assisted steering system

Claim 1

A power-assisted steering system in an automotive vehicle, comprising: a primary hydraulic pump driven by a prime mover of the vehicle for supply of fluid under pressure; an auxiliary hydraulic pump driven by the prime mover for supply of a predetermined small quantity 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 primary hydraulic 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 connected to said auxiliary hydraulic pump to be supplied with the predetermined small quantity of fluid under pressure; a first throttle disposed within a bypass circuit connecting a first fluid circuit between said primary hydraulic pump and said valve means of said changeover valve unit to a second fluid circuit between said auxiliary hydraulic pump and said reaction fluid chamber; a second throttle disposed within a third fluid circuit connecting said second fluid circuit to a fluid reservoir; and flow control valve means disposed within said third 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 increase the hydraulic pressure in said reaction fluid chamber.