Steering & Suspension: Steering Systems: Steering boxes & columns
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Flow-control valve

Summary
Power steering pumps have a flow-control valve to vary fluid flow and system pressures. A pressure relief valve prevents excessive pressures developing when the steering is on full-lock.
Flow-control valve

All power steering pumps have a flow-control valve to vary fluid flow and power steering system pressures. A pressure relief valve prevents excessive pressures developing when the steering is on full-lock, and held against its stops. The flow control valve is located at the outlet fitting of the pump.

During slow cornering, or when parking, pump speeds are normally low. There is less demand for fluid flow, but to provide the required assistance, high pressure is needed. Discharge ports direct the fluid to the outlet, and then to the steering gear. The outlet fluid pressure is slightly lower than the internal high pressure coming from the pump.

This drop in pressure occurs as the fluid flow passes the needle and orifice in the outlet fitting. This lower pressure is transmitted through a by-pass fluid passage to the spring end of the control valve. The pressure difference on the valve causes it to move away from the outlet fitting but the force of the spring prevents it moving far enough to uncover a return port, back to the pump inlet. Movement of the control valve controls the position of the needle valve in the outlet fitting. And this controls the fluid flow to the steering gear.

At higher speeds, with no steering manoeuvres, fluid flow is increased. This reduces pressure at the outlet. The lower pressure is transmitted to the spring end of the control valve. The valve moves, and opens the return port back to the pump inlet.

Movement of the control valve also controls the movement of the flow control needle in the outlet fitting. The needle closes in the orifice, and fluid flow to the steering gear reduces.

With the steering wheel held at full-lock, the steering rack power piston chamber becomes fully pressurised, and fluid flow stops.

This high pressure is transmitted back to the spring end of the control valve, opening the pressure relief valve. A small amount of fluid passes through the pressure relief orifice, providing a pressure drop. The valve moves, and uncovers the return port to the pump inlet. A pre-determined relief pressure is thus maintained.

The pump is normally a vane-type, with sufficient capacity for all operating conditions.