Power-assisted steering system for automotive vehicle

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, and valve means for controlling the supply of fluid under pressure from said pump to said power cylinder in response to movement of the input member;
• 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;
• a fixed throttle disposed within a first shunt circuit between a first fluid circuit connecting said pump to said valve means of said changeover valve unit and a second fluid circuit connecting said reaction fluid chamber to a fluid reservoir;
• first flow control valve means disposed within a second shunt circuit between said first and second fluid circuits, said first flow control valve means being arranged to act as a fixed throttle when the pressure in said first fluid circuit is lower than a predetermined value and to act as a constant flow control valve when the pressure in said first fluid circuit becomes higher than the predetermined value;
• and
• second flow control valve means disposed within said second fluid circuit to decrease the flow 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.