Description of power assisted steering system

(Figs 9.12, 9.13 and 9.14) This directly coupled power assisted system is hydraulic in operation. The power assisted steering layout (Fig. 9.14) consists of a moving power cylinder. Inside this cylinder is a double acting piston which is attached to a ramrod anchored to the chassis by either rubber bushes or a ball joint. One end of the power cylinder is joined to a spool control valve which is supported by the steering box drop arm and the other end of the power cylinder slides over the stationary ramrod. When the system is used on a commercial vehicle with a rigid front axle beam (Fig. 9.12), the steering drag link is coupled to the power cylinder and control valve by a ball joint. If a car or van independent front suspension layout is used (Fig. 9.13), the power cylinder forms a middle

Fig. 9.12 Steering box with external directly coupled power assisted steering utilized with rigid axle front suspension

External Power Assisted Steering

Fig. 9.13 Steering box with external directly coupled power assisted steering utilized with independent front suspension

Fig. 9.12 Steering box with external directly coupled power assisted steering utilized with rigid axle front suspension

Fig. 9.13 Steering box with external directly coupled power assisted steering utilized with independent front suspension moveable steering member with each end of the split track rods attached by ball joints at either end. The power source comes from a hydraulic pump mounted on the engine, and driven by it a pair of flexible hydraulic pipes connect the pump and a fluid reservoir to the spool control valve which is mounted at one end of the power cylinder housing. A conventional steering box is used in the system so that if the hydraulic power should fail the steering can be manually operated.

With the removal of any steering wheel effort a pre-compressed reaction spring built into the control valve (Fig. 9.14) holds the spool in the neutral position in addition to a hydraulic pressure which is directed onto reaction areas within the control valve unit. Provided the steering effort is less than that required to overcome the preload of the reaction spring, the spool remains central and the fluid is permitted to circulate from the pump through the valve and back to the reservoir. Under these conditions there will be no rise in hydraulic pressure and the steering will be manually operated. Consequently, the pump will be running light and therefore will consume very little power.

When the steering effort at the driver's wheel is greater than the preload stiffness of the reaction spring, the spool valve will move slightly to one side. This action partially traps fluid and prevents it returning to the reservoir so that it now pressurizes one side or the other of the double acting piston, thereby providing the power assistance necessary to move the steering linkage. The more the spool valve misaligns itself from the central position the greater the restriction will be for the fluid to return to the reservoir and the larger the pressure build up will be on one side or the other of the double acting piston to apply the extra steering thrust to turn the steering road wheels.

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