Power Steering Pressure Control Valve: Description and Operation

CONTROL OF PRESSURE AND FLOW
The function of the control valve is to regulate the flow from the pump so that it remains constant, regardless of engine/pump rpm.
One side of the control valve is in direct communication with the delivery side of the pump (A). The pump outlet (B) incorporates a restriction from which a connecting passage (1) leads to the other side of the valve, which is fitted with a spring (2). In the rest position, the valve is pressed against the outlet side.
Inside the control valve is an overflow valve (a) which, when the pressure exceeds a certain limit, is actuated by the pressure on the spring loaded side of the control valve.
For the control valve to operate, a certain amount of fluid must circulate continuously through it, although not when the steering wheel is at full lock. The pump delivers a maximum pressure of about 90 bar and a maximum flow of about 6.0 liters per minute.





REGULATION AT LOW RPM
The pressure raised by the pump (shaded portion in illustration) is reduced slightly by the restriction in the pump outlet.
The reduced pressure is also communicated to the spring loaded side of the control valve, giving rise to a slight pressure difference between the two sides of the valve. Owing to the low speed of the pump, however, the difference in pressure is insufficient to overcome the force of the spring and actuate the valve.





REGULATION AT HIGH RPM (PUMP IN FLOW CONTROL MODE)
The flow of hydraulic fluid inside the pump increases with increasing rpm and owing to the restriction in the pump outlet the flow velocity also increases. This reduces the pressure in the connecting passage, with the result that the pressure on the spring loaded side of the control valve will be lower than that acting on the outlet side of the valve. The valve therefore overcomes the force of the spring, opening a port to the suction side of the pump and allowing a certain amount of internal recirculation of the fluid to take place so that the flow from the pump is maintained at a constant rate, regardless of engine/pump rpm.





STEERING WHEEL TURNED TO FULL LOCK
Pump speed in this case is often low. When the steering wheel is turned to full lock the control valve of the steering gear closes. The flow of fluid from the pump will then be zero.
The high pressure. thus arising is directed via the connecting passage to the spring loaded side of the control valve. The pressure opens the overflow valve and allows the fluid to pass to the inlet side of the pump. The pressure difference across the control valve forces it to move towards the spring and so open the port for recirculation of the full delivery flow from the pump.
The predetermined maximum pressure is maintained as long as the control valve remains closed.