Steering (Pressure control): Description and operation

Pressure control

Low assist at higher speeds:

When steering resistance is low, the input shaft is in or near the neutral (central) position, there is little or no fluid flow to either power cylinder orifice. Most of the feed pressure from the pump is directed by the valve to the reservoir. Because of this, the pressure is equal on both sides of the power cylinder, resulting in low or no power assistance. The steering gear is operating in its positive centre phase and responds as a manual steering system.

High assist at lower speeds:

When steering resistance is high, car stationary or moving slowly, the torsion bar twists and rotates the valve to open fluid passages on one side and close the fluid passages on the other. The fluid pressure increases in the side of the power cylinder fed by the open side of the valve. The increased pressure pushes on the rack piston, reducing the effort required to turn the steering wheel. On the other side of the power cylinder, the return passage in the valve is open and the displaced fluid is returned to the reservoir.

Because fluid passages in the valve change in size in proportion to steering resistance, when steering resistance is high the fluid passages become larger and power assistance increases. When steering effort is low, the fluid passages become smaller and power assistance is reduced.