Hydraulic System, Brakes

Hydraulic System Operating Principle





The rotary motion of the steering wheel is converted, via the steering rack pinion, to the lateral motion of the rack. Steering assistance is provided by pressurized fluid being directed against a piston operating within the rack cylinder. Rack movement is achieved by differential pressures being applied on either side of the rack piston, caused by unequal restrictions within the hydraulic control valve.

The main components of the hydraulic control valve are the rotary distributor, control sleeve and hydraulic reaction piston. The rotary distributor and control sleeve turn relative to each other, forming the unequal restrictions which create the differential pressures to operate the rack piston; the displacement of the distributor and sleeve being controlled by the elastic deformation of a torsion bar which is concentric with the pinion and valve. Refer to Positive Center-Feel Torsion Bar.

The hydraulic reaction piston moves axially, relative to the rotary distributor, and is connected to the control sleeve by a three-bearing helical screw. Pressure applied either side of the hydraulic reaction piston is translated into a rotational force which increases steering effort. For details of Servotronic speed-sensitive steering control, refer to Servotronic Control.

When the vehicle is traveling straight ahead, the valve restrictions are balanced, thus providing equal pressures on either side of the rack piston. When load is applied at the steering wheel, the two halves of the control valve (rotary distributor and control sleeve) are displaced making the restrictions unequal. The resulting differential pressures on either side of the rack piston, assist the steering rack to move to left or right. As the turning load is removed, the pressures equalize again and the steering returns to the straight ahead position, aided by suspension geometry.