Steering Rack Operation

Pump Pressure Relief Valve Operation

1 Inlet port 2 Discharge port

3 Metering orifice 4 Pressure relief valve

The pressure relief valve is located in the centre of the flow control valve. If the pressure on the discharge side of the metering orifice reaches a predetermined level, a spring loaded ball in the centre of the valve will lift from its seat and allow pressurized fluid to recirculate within the pump.

The pressure relief valve will operate if the discharge from the pump is restricted, steering held at full lock. If the discharge from the pump is completely blocked, all fluid discharged will be recirculated through the pump. As no fresh fluid is drawn into the pump from the reservoir, the fluid temperature inside the pump will increase quickly. Consequentially, periods of operation of full lock should be kept to a minimum to avoid overheating the pump and the fluid within it.

Steering Rack Operation

Rotary movement of the steering wheel is transferred via the steering column to the input shaft of the valve unit on the steering rack. The rotary movement of the input shaft is converted into linear movement of the steering rack through the rack and pinion. With the engine running and the PAS pump operating, pressurised fluid is available to the steering rack for power assistance.

Neutral Position With no movement of the steering wheel being applied, the slots in the outer sleeve and the input shaft are so aligned that the fluid flows across the valve unit with minimal restriction. Some pressure is applied to the full area port and the annulus port which in turn is felt at either side of the piston in the hydraulic cylinder. With the forces approximately equal on each side of the cylinder, the steering remains in the neutral position. Fluid delivered from the PAS pump is returned from the valve unit via the fluid cooler back to the reservoir. With minimal restriction across the valve unit and through the return hose, the pressure applied to each side of the piston is very low.

Right Steering (RHD Models; Left Steering on LHD Models) Clockwise (on RHD, anti-clockwise on LHD) rotation of the steering wheel rotates the input shaft and the torsion bar in the same direction. The slots in the input shaft move out of their neutral alignment with the slots in the outer sleeve, as the torsion bar twists, and restricts the flow back to the reservoir. This restriction causes the supply pressure from the PAS pump to increase. The increased PAS pump pressure is directed to the annulus side of the hydraulic cylinder. Due to the relative displacement of the slots, the full area side of the cylinder is opened to return, producing a pressure difference across the piston. This results in a linear output force along the rack which provides the power assistance to turn the road wheels in the appropriate direction.