Steering Rack Operation

STEERING

57-12

DESCRIPTION AND 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.