Power Steering Pump: Description and Operation

POWER STEERING (P/S) PUMP








The power steering pump is a vane type and is driven by the V-ribbed belt from the crankshaft.

FLOW CONTROL VALVE
As the discharge rate of the Power Steering (P/S) pump increases in proportion to the pump revolution speed, a flow control valve is added to control it so that the optimum amount of fluid for steering operation is supplied according to the engine speed (driving condition). Described below is its operation at different engine speed.





When Idling
The fluid discharged from the pump is supplied through the clearance around the rod in orifice A1 to the gear box.





When Running at Low Speed
As the engine speed rises, the pump discharge rate increases and causes a pressure difference to occur between both ends of the orifice (P1 - P2). Thus the pressure exceeding the flow control spring force pushes the flow control valve to the right in figure shown, making the opening in the orifice narrower through which only a necessary amount of fluid is fed to the gear box and the excess fluid is returned to the pump.





When Running at High Speed
As the engine speed rises higher, opening in the orifice is made narrower and fluid flow to the gear box reduces. As a result, hydraulic pressure application is slow at the start of the steering wheel turn. This provides straight-ahead stability to suit the driving condition with the steering wheel operated near its neutral position.

RELIEF VALVE





The relief valve located in the flow control valve controls the maximum hydraulic pressure.
The steel ball in the relief valve is under the hydraulic pressure in the circuit coming through orifice A2. When the steering wheel is turned and the hydraulic pressure increases higher than 8000 kPa (80 kg/cm2, 1137 psi), it compresses the relief spring to push the steel ball which then allows the fluid to flow to the P/S pump.
Such relief valve operation causes a pressure difference to occur between chambers A and B.
Then the flow control valve moves to the right to make opening in orifice A1 narrower, maintaining the hydraulic pressure constant.