Hydraulic power steering system is still the most widely used. The method of using oil under pressure to boost the servo is sophisticated and advantageous in terms of cost, space and weight. Sensitivity to movements
caused by the road surface and hence the effect of torsional impacts and torsional vibrations passing into the steering wheel is also noticeably reduced, particularly with rack and pinion steering. This can be attributed to the hydraulic self-damping. It might also be the reason why it is possible to dispense with an additional steering shock absorber in most vehicles with hydraulical rack and pinion steering, whereas it is required for the same vehicles with manual steering
The oil pump in hydraulic power steering system is directly driven by the engine and constantly generates hydraulic power. As hydraulic power steering systems have to be designed in such a way that a sufficient supply volume is available for fast steering movements even at a low engine speed, supply flow limiting valves are required. These limit the supply flow to about 8 l per minute in order to prevent the hydraulic losses which would otherwise occur at higher engine speeds. Depending on the driving assembly and pump design, the additional consumption of fuel can lie between 0.2 and 0.7 l per 100 km.
Assemblies which are added to provide auxiliary power are shown in Fig. 9.1-16, taking the example of the rack and pinion steering used by Opel in the Vectra (1997). The pressure oil required for steering boost is supplied direct to the steering valve 6 located in the pinion housing from vane pump 1 via the high-pressure line 2 and the cooling circuit 3. From here, depending on the direction of rotation of the steering wheel and the corresponding counterforce on the wheels, distribution to the right or left cylinder line takes place (items 7 and 8). Both lead to the working cylinder which is integrated in the steering-gear housing 5. A disc located on the gear rack divides the pressure chamber. Differences in pressure generate the required additional axial force in the gear rack FPi via the active areas of the disc