Power Steering Pump: Description and Operation

Design
The power steering pump is of the vane type and is mounted on a bracket on the engine. The pump is driven from the crankshaft (at the same speed) by a multiple V-belt.





1. To steering gear.
2. From steering gear.

Operation





The pump rotor is provided with ten radially movable vanes and rotates in an annular housing, in which the vanes are pressed against the chamber wall, partly by centrifugal force and partly by the fluid pressure. Since the pump chamber is elliptical in shape (see Fig. A), the volume of the space enclosed by the rotor body, the chamber wall and each pair of vanes varies as the rotor turns. As a pair of vanes passes from suction to discharge, the enclosed space first increases and fluid is drawn in. As the vanes approach the discharge port after the suction phase, the space between them begins to decrease, causing a rise in fluid pressure, and fluid is discharged to the delivery line. The pump capacity is doubled by providing it twin suction and discharge ports.

Control Valve





The pump incorporates a control valve which regulates the fluid flow as well as the maximum fluid pressure. When the pump starts, the valve (5 in Fig. A) is displaced to the left by the spring (7). The fluid from the pump flows through the inlet line (3), through the throttling orifice (2), into the delivery line (1) to the steering gear. Since the space to the right of the valve piston is interconnected with the steering gear delivery line (1) by the passage (8), both are at the same pressure. The throttling orifice (2) ensures that the pressure on the left-hand side of the valve piston is higher than in the delivery line (1) and to the right of the piston. Thus, the piston is displaced to the right when the spring force is overcome. When the pump speed is sufficiently high relative to the back pressure, the piston displacement will be sufficient to allow the excess fluid to return to the pump inlet (see Fig. A). Since the pump is intended to deliver less than its maximum capacity, this is defined as the normal valve position. If the fluid flow in the delivery line is interrupted, for example if turning of the wheels is blocked, the pressure in the delivery line (1) will rise and the pressure differential across the valve will be equalized. Under these conditions, the spring will displace the valve piston to the left, closing the return opening, and the pressure will rise further. At maximum pressure, the spring force on the relief valve (6) (the inner section of the control valve) will be overcome, permitting fluid to be discharged to the return passage (4) (see Fig. B). The pressure on the right-hand side of the valve piston will then fall, the complete valve will be displaced to the right and the return line will be reopened. When the pressure has returned to normal, the relief valve will close and the control valve will revert to its normal position.