The new PPS(Progressive Power Steering) varies, the steering operation effort in accordance with the vehicle speed. The steering effort is lighter at low speed and heavier at high speed.
The hydraulic reaction type progressive power steering uses a thinner torsion bar than in ordinary power steering to reduce the steering effort required during stationary steering or when traveling at a low speed. However, this causes the steering required effort to become too small (that is, the steering wheel feels too “light”) when the vehicle speed increases. To prevent this, the required steering effort is increased as when there is a thicker torsion bar, by the provision of a hydraulic reaction chamber for suppressing the rotation of the control valve shaft (in the control valve housing) by means of four hydraulically operated plungers. The hydraulic pressure applied to the hydraulic reaction chamber is low at low vehicle speeds and high at high vehicle speeds. It also increases as the hydraulic pressure in the power cylinder rises as a result of steering operation. In the hydraulic reaction type progressive power steering, the required steering effort varies according to the vehicle speed and steering operation.