Steering (Operation): Description and operation

Operation

Power assistance is integral with the steering gear. Hydraulic pressure to operate the system is generated by a roller vane pump driven by a belt from the crankshaft pulley. Fluid direction is controlled by the steering gear power valve, the position of which is governed by a drive pin attached to the pinion shaft.

Between the pinion and shaft, drive is transmitted through a pre-loaded spring ring driven by a peg on the pinion shaft and riding on a similar peg on the pinion. When the steering wheel is turned at high steering loads the reaction force created by the rack against the pinion causes the spring ring to open out allowing the pinion to be left slightly behind the pinion shaft.

This change in alignment cause the power valve inner spool (attached to the pinion shaft) and the outer spool (attached to the pinion) to move in relation to each other, so opening ports and directing fluid.

Power steering fluid flows from the reservoir to the steering pump, from where it is pumped to the power valve on the steering rack. Depending upon which directional port is open, pressurised fluid is diverted to left or right chambers in the steering rack to provide power assisted steering.

The amount of assistance provided is dependent upon the steering effort required.

When driving at high speeds where the steering wheel movements are small and forces are low, such as lane changing, the reaction forces of the rack are insufficient to cause deflection of the spring ring and as a result the power valve does not operate. The steering gear is operating in its positive centre phase and responds as a manual steering system.