Vehicle speed responsive variable assist power steering system

Claim 1

A variable assist power steering system for vehicles including a hydraulic fluid source, a fluid reservoir, and a hydraulic pressure operated power cylinder adapted to be connected to a steering linkage, comprising: a control valve including valve elements relatively displaceable in response to a predetermined variable to define therebetween two parallel fluid flow paths connected between the fluid source and the fluid reservoir to produce pressure difference in the power cylinder in response to said predetermined variable, said two parallel fluid paths including a plurality of first variable flow orifices, each having an orifice area variable in response to said predetermined variable, wherein said control valve comprises a second variable flow orifice that has an orifice area variable in response to said predetermined variable, said second variable flow orifice being arranged in series with one of said first variable flow orifices, and a bypass path provided with an externally controlled variable flow orifice valve that has an orifice area variable in response to a second predetermined variable which is different from said predetermined variable, said bypass path having one end connected to one of said two parallel fluid flow paths at an intermediate portion between said second variable flow orifice and the adjacent one of said variable flow orifices.