Hydraulic reaction force apparatus for power steering system

Claim 1

A hydraulic reaction force apparatus for a power steering system, characterized by comprising: a pump (P); a power cylinder (2) for supplying an auxiliary steering force to a steering wheel; a control valve (1) for switching a hydraulic path from said pump (P) to said power cylinder (2) in accordance with a steering direction of said steering wheel; a first variable orifice (12; lla, 12a) arranged between said pump (P) and said control valve (1), said first variable orifice (12; lla, 12a) being arranged such that an opening thereof is controlled by a solenoid (11) driven in response to a current supplied to said solenoid (11) in accordance with operating conditions of a vehicle; a hydraulic reaction force chamber (4), formed between an input member (5, 33) rotated by said steering wheel and an output member (6, 31) interlocked with vehicle wheels, for supplying as a reaction force a pressure corresponding to a hydraulic pressure applied to said power cylinder (2) so as to restrict rotation of said input member (5, 33); and a spool valve arranged between said pump (P) and said hydraulic reaction force chamber (4) and having a spool (13) moved in accordance with a pressure difference caused by said first variable orifice (12; lla, 12a), said spool valve being provided with a supply port (14) connected to said pump (P), a reaction port (15) connected to said hydraulic reaction force chamber (4), a tank port (16) connected to a tank (T) and a second variable orifice (17) controlled by said spool (13), said second variable orifice (17) being adapted such that an opening thereof is changed by a displacement of said spool (13) caused by the pressure difference between upstream and downstream pressures of said first variable orifice (12; lla, 12a), thereby connecting said reaction port (15) to said supply port (14) and said tank port (16) and controlling a magnitude of the hydraulic reaction force.