Steering: Description and Operation

SYSTEM DESCRIPTION
This power steering system is speed-sensitive. Its main components are a mechanical speed sensor, a rotary valve-type gearbox, and a constant volume, vane-type pump. The system regulates the power assist according to vehicle speed, road conditions, and driving style. The amount of power assist changes linearly from high assist, when driving at low speeds or parking, to low assist, when driving at high speeds.





FLUID FLOW DIAGRAM
The fluid volume from the pump, which is driven by the crank pulley, is regulated by the flow control valve built into the pump. The regulated fluid is fed to the valve body unit located outside the pinion shaft of the steering gearbox. The fluid is regulated to the proper pressure by the valves in the valve body unit and by the power steering speed sensor before it flows to the power cylinder. The direction of the flow is deermined by the rotary valve. The fluid then returns to the reservoir through the return passage and is fed to the pump again.









WHEN STOPPED:
When the vehicle is stopped with the engine idling, the fluid flow is shut off because the power steering speed sensor rotor is not rotating. The standard pressure generated by the pressure control valve is applied to the assist control valve to move the valve to the led. When the assist control valve is in this position, the orifice valve is closed. Therefore, the fluid discharged from the pump is fed to the low speed valve of the valve body unit. Because the low speed valve has the small chamfer and thereby the small opening angle, the large hydraulic pressure (steering rack thrusts results in the smallest steering force (high assist).





MIDDLE SPEED RANGE:
When the vehicle starts to move, the power steering speed sensor rotor starts to rotate as well, reducing the standard pressure and generating the power steering speed sensor signal pressure. The valve returns to the right point where the pressure of the power steering speed sensor signal equals the spring force of the assist control valve. When the assist control valve is in this position, the orifice of the valve slightly opens. Therefore, the fluid pressure from the pump is applied to the high speed valve of the valve body unit. Because the high speed valve has a larger chamfer and the compound opening area with the low speed valve is larger than that when the vehicle is stopped, the fluid pressure (steering rack thrust) is not generated unless the displacement angle is large (large steering torque), resulting in moderately heavy steering (medium assist).





HIGH SPEED RANGE:
When the vehicle is driven at high speed, the power steering speed sensor rotor rotates at a high speed, greatly reducing the standard pressure. The pressure of the power steering speed sensor signal is at the lowest value, and the assist control valve is at the right end position. When the assist control valve is in this position, the orifice of the valve opens fully. Therefore, the fluid pressure applied to the high speed valve is larger than when the engine speed is at the middle speed range. The compound opening area with the low speed valve is the largest this time, and the fluid pressure (steering rack thrust) is not generated unless the displacement angle is very large (low assist).