Power Steering Pump: Description and Operation

OIL PUMP & TANK
The oil pump is belt-driven from the engine. The oil flow is controlled in response to engine speed so that an adequately "heavy" steering effort is maintained during high-speed operation.
The oil pump is a vane type. It is integrated with an oil reservoir and houses the flow control and relief valves.





1) Vane pump
The vane pump consists of a rotor, cam rings, and ten vanes.
When the rotor rotates, the vane located in each groove of the rotor is radially swung out by centrifugal force and pressed against the cam ring. The tip of the vane slides along the inner oval wall of the, cam ring so that oil is delivered to the chamber formed by the rotor, cam ring and vane by way of a pea-shaped groove. Oil from the chamber is discharged into the oil circuit via the discharge port.





2) Flow control valve
The flow control valve adequately regulates the discharge flow of oil which increases in proportion to pump speed and delivers it to the gearbox. It consists of orifices 1 and 2, valve spool, return port and flow control spring. When a pressure differential occurs between the front and rear of orifice 2 in response to increases in discharge flow, the valve spool moves against the tension of the flow control spring so that the oil flow is controlled by the open and close operation of the return port and orifice 2.





(1) When the pump begins to rotate, P increases, and the valve spool is moved to the right by this pressure. In the pump speed range A to B B, the total amount of oil delivered by the pump is sent to the gear box through orifice 1.
- Pump speed range A to B





(2) As the pump speed increases, P1 increases further, and the valve spool is pushed further to the right. As a result, orifice 2 opens. Accordingly, a constant flow of oil is maintained.
- Pump speed range B to C





(3) When the pump speed increases further, the valve spool is pushed to the far right position. At this spool position, orifice 1 is restricted while orifice 2 opens wide. Accordingly, the oil flow decreases. When the spool moves further, orifice 1 is restricted by the straight portion of the needle, and the oil flow is kept low and constant.
- Pump speed range C to E