Power Steering System Operation

Fig. 1:

Fig. 2:

NOTE: The illustrations show a sectional view of the steering box with a section through the valve spools directly above to demonstrate valve spool movement and fluid flow.

Fig. 1 shows steering wheel in the straight ahead position, valve spools in the neutral position.
Fig. 2 shows hydraulic flow when steering wheel is turned clockwise.
The steering housing (1) contains a complete mechanical steering gear box and the steering control valve. It also forms the power cylinder.
Steering wheel rotation is converted into axial movement of the piston (2) by a chain of balls (5) running in the worm (4). When the worm is rotated, the balls enter the recirculating tube (6) and pass to the other end to form an endless chain. The sector shaft (3), which is at right angles to the piston axis, is rotated by meshing teeth. The steering drop arm, fitted to the sector shaft, transmits steering movement via the steering linkage to the road wheels.
The worm is connected to the input shaft (9) by the torsion bar (10). The worm head contains two valve spools (11 and 12), which are at right angles to the axis of the worm. Two pins on the input shaft engage, without play, in the valve spools. When the steering wheel is turned, there is relative rotation between the input shaft and the worm, which is made possible by the torsion bar. The valve spools follow the movement of the steering column, when the torsion bar twists the pistons are moved from the neutral position into their working position. When the steering wheel is released, the torsion bar and the valve pistons return to the neutral position. The force required at the steering wheel to overcome the torsion bar and therefore move the valves from the neutral position provides the driver with good steering feel.


Hydraulic fluid flow
Fluid from the hydraulic pump (18) flows into an annular chamber and surrounds the part of the valve body, which houses the two valve spools. When the valve is in the neutral position, see illustration, the fluid flows through the inlet grooves (13) and (14) to the radial grooves (7) and (8). The radial grooves are connected via oil ways to the right and left hand cylinder chambers. Fluid also flows back to the fluid reservoir (17) through the open return grooves (15) and (16). When the steering wheel is turned clockwise, see illustration, the piston (2) moves to the right in the power cylinder. At the same time the valve spools are moved into their working position. Valve spool (11) moves to the right, inlet groove (13) is opened. Valve spool (12) moves to the left, closing inlet groove (14). Pressure fluid now flows via radial groove (8) to the left hand side of the cylinder, assisting movement of the steering wheel. The fluid in the right hand cylinder is pushed out by the piston, and flows back to the fluid reservoir via radial groove (7) and return groove (15). When the steering wheel is turned anti-clockwise the valve operation is reversed, pressure fluid flowing to the right hand side of the cylinder.
The quantity of fluid required for the system is adjusted by the pressure and flow limiting valve (19) in the PAS pump. The flow limiting valve ensures that maximum demand for pressure fluid is met regardless of engine speed.

Steering limit valve
A steering limit valve is incorporated in the piston head (2) as a safeguard against overloading the steering linkage, lock stops and hydraulic pump. This ball valve is always shut by pressure in the left or right hand cylinder. However shortly before the piston reaches full travel in either direction, the valve is opened by a pin, resulting in pressure drop. Hydraulic assistance is greatly reduced, and full lock can only be achieved with increased manual effort by the driver.

Adjustment
The shape of the sector shaft gearing makes axial adjustment of the shaft possible. This allows any play between the two gears, which might occur after a long period in service, to be eliminated using the sector shaft adjuster screw.