Operation of control valve and power piston

Neutral position (Fig. 9.14(a)) With the valve spool in the neutral position and no power assistance being used, fluid from the pump passes freely from the right hand supply port and annular groove in the valve housing, across the spool valve middle land to the return groove and port in the valve housing, finally returning to the reservoir. At the same time fluid passes from both the spool grooves to passages leading to the left and right hand power cylinder chambers which are sealed off from each other by the double acting piston. Thus whatever the position of the piston in the power cylinder when the spool is in the central or neutral position, there will be equal pressure on either side of the double acting piston. Therefore the piston will remain in the same relative position in the cylinder until steering corrections alter the position of the spool valve.

Right hand steering movement (Fig. 9.14(b)) If the drop arm pushes the ball pin to the right, the spool control edges 1 and 3 now overlap with the valve housing lands formed by the annular grooves. The fluid flows from the supply annular groove into the right hand spool groove where it then passes along passages to the right hand cylinder chamber where the pressure is built up to expand the chamber.

The tendency for the right hand cylinder chamber to expand forces fluid in the left hand contracting cylinder chamber to transfer through passages to the left hand spool groove. It then passes to the valve housing return annular groove and port back to the reservoir. Note that the ramrod itself remains stationary, whereas the power cylinder is the moving member which provides the steering correction.

Left hand steering movement (Fig. 9.14(c)) Movement of the drop arm to the left moves the spool with it so that control edges 2 and 4 now overlap with the adjacent valve housing lands formed by the annular grooves machined in the bore. Fluid flows from the supply annular groove in the valve housing to the axial passage in the spool and is then diverted radially to the valve body feed annular groove and the spool left hand groove. Fluid continues to flow along the passage leading to the left hand power cylinder chamber where it builds up pressure. As a result the left hand chamber expands, the right hand chamber contracts, fluid is thus displaced from the reducing space back to the right hand spool groove, it then flows out to the valve housing return groove and port where finally it is returned to the reservoir.

Progressive power assistance (Fig. 9.14(a)) While the engine is running and therefore driving the hydraulic power pump, fluid enters the reaction chamber via the axial spool passage.

Before any spool movement can take place relative to the valve housing to activate the power assistance, an input effort of sufficient magnitude must be applied to the drop arm ball pin to compress the reaction spring and at the same time overcome the opposing hydraulic pressure built up in the reaction chamber. Both the reaction spring and the fluid pressure are utilized to introduce a measure of resistance at the steering wheel in proportion to the tyre to ground reaction resistance when the steered road wheels are turned and power assistance is used.

Progressive resistance at the steering wheel due to the hydraulic pressure in the reaction chamber can be explained in the following ways:

Right hand spool reaction (Fig. 9.14(b)) Consider the drop arm ball pin initially moved to the right. The reaction ring will also move over and slightly compress the reaction spring. At the same time the hydraulic pressure in the reaction chamber will oppose this movement. This is because the pressure acts between the area formed by the annular shoulder in the valve chamber housing taking the reaction spring thrust, and an equal projected area acting on the reaction ring at the opposite end of the chamber. The greater the hydraulic pressure the larger the input effort must be to turn the steering wheel so that the driver experiences a degree of feel at the steering wheel in proportion to the resisting forces generated between the tyre and road.

Left hand spool reaction (Fig. 9.14(c)) If the drop arm and ball pin is moved to the left, the reaction washer will move over in the same direction to compress the reaction spring. Opposing this movement is the hydraulic pressure which acts between the reaction washer shoulder area formed by the reduced diameter of the spool spindle and an equal projected area of the reaction ring situated at the opposite end. If the steering wheel effort is removed, the hydraulic pressure in the reaction

, 9.14 External directly coupled power assisted steering

chamber will react between the reaction ring, the reaction washer housing and spool shoulders, and thereby attempt to move the spool back to its original central or neutral position.

Correction for the variation in cross-section areas on opposite side of the power piston (Fig. 9.14(b and c)) To counteract the reduction in effective area on the ramrod side of the double acting piston, the annular shoulder area of the spool (Fig. 9.14(b)) is made slightly larger than the reaction ring annular shoulder area (Fig. 9.14(c)) machined in the reaction chamber. Consequently, a greater opposing hydraulic reaction will be created when turning the steering to the left to oppose the full cross-sectional area of the power piston compared to the situation when the steering is turned to the right and a reduced power piston cross-sectional area due to the ram is exposed to the hydraulic pressure in the power cylinder. In this way a balanced self-centralizing response is obtained in whatever position the road steering wheels may be positioned.

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