Hydraulic pressure control valve for power-assisted steering system

Claim 1

A hydraulic pressure control valve for use in a hydraulicpower control device of a power-assisted steering system of a motor vehicle, which control device includes a source of hydraulic pressure and a power-steering control valve communicating with said source of hydraulic pressure for controlling hydraulic pressure delivered therefrom in dependence on the steering resistance of the motor vehicle and having a reaction chamber, said hydraulic pressure control valve comprising: a valve body having formed therein an inlet passageway communicating with said source of hydraulic pressure and said power-steering control valve, an outlet passageway communicating with said reaction chamber of said power-steering control valve, a discharge passageway communicating with said outlet passageway, a flow restriction of constant effective cross sectional area located between said outlet passageway, and said discharge passageway, and a bore which provides fluid communication between said inlet and outlet passageways; a primary valve spool slidably accommodated in said bore of said valve body and having opposed, larger and smaller working surfaces, the larger working surface being subjected to output hydraulic pressure in said outlet passageway and the smaller working surface being subjected to input hydraulic pressure in said inlet passageway, said primary valve spool also having an inlet port communicating with said inlet passageway, an outlet port communicating with said outlet passageway, and a bore for providing fluid communication between said inlet and outlet ports; a first pair of spring means disposed in the bore of said valve body for biasing said primary valve spool in its balanced position; a secondary valve spool slidably accommodated in the bore of said primary valve spool; a second pair of spring means disposed in the bore of said valve body for biasing said secondary valve spool in its balanced position; a variable flow restriction provided between said inlet and outlet ports of said primary valve spool, the effective cross sectional area of said variable flow restriction being controlled by a force resulting from said output hydraulic pressure acting upon said larger working surface of said primary valve spool opposing a force resulting from said input hydraulic pressure acting upon said smaller working surface of said primary valve spool; and actuating means having a plunger cooperating with one end of said secondary valve spool, said actuating means being arranged to cause said plunger to move said secondary valve spool in dependence on the driving speed of the motor vehicle, whereby the effective cross sectional area of said variable flow restriction is further controlled in dependence on the driving speed of the motor vehicle.