Power Assisted Steering

                        Power Assisted Steering

Need for Power Assisted Steering

A reduction in input effort on the steering wheel rim in a manual steering system is possible by decreasing the steering box gear ratio. This increases the number of steering wheel turns from lock to lock due to which manoeuvring of the steering takes longer, and accordingly the vehicle's safe cornering speed has to be reduced. Due to the requirement of more weight on the front steering wheels of front wheel drive cars and the use of radial ply tyres with greater tyre width, larger static turning torques are necessary. Therefore, for faster driving and cornering, power assisted steering is desirable and in some cases essential if the vehicle is to be handled to match its performance.

Incorporation of power assisted steering on passenger cars reduces the drivers input to about 25 - 30% of the total effort needed otherwise to manoeuvre it. In heavy trucks the hydraulic power (servo) assistance can be in the order of 80 - 85% of the total steering. As a result, a more direct steering box gear reduction can be used to provide a more precise steering response. Consequently, the steering wheel movement from lock to lock can be reduced approximately from 3.5 to 4 turns for manual system to about 2.5 to 3 turns for power assisted steering arrangements.

The amount of power assistance provided to the steering linkage is normally restricted due to which the driver experiences the tyres interaction with the ground under the varying driving

conditions (Fig. 27.62). Consequently sufficient resistance is still transmitted back to the steering wheel from the road wheels so that the driver can sense the steering input requirements needed to steer the vehicle effectively.

Figure 27.63 illustrates the effects of reducing the input effort at the steering wheel with different steering gear overall ratios to overcome an output opposing resistance at the steering box drop arm. A typical power assisted steering input effort curve operating over a similar working load output range is also plotted with these manual steering gear ratios. From these plots it can be seen that for very low road wheel resistance roughly up to 1000 N at the drop arm, the input effort of 10 to 20 N is practically all manual.

Fig. 27.63. Comparison of manual steering with power assisted steering.

Fig. 27.64. Power assisted steering layout (control flow system).

the drop arm, the input effort of 10 to 20 N is practically all manual. This initial manual effort at the steering wheel enables the driver to feel the changes in resistance to steering under different road surface conditions, such as a slippery road.

The power assistance arrangements should satisfy certain requirements. It must be 'fail­safe', that means if the power system fails the driver must still be able to retain effective control. The amount of power assistance should be proportional to the effort put by the driver and the driver must be able to retain the 'feel' of the wheels. Hydraulic power is used on light vehicles to assist the driver to steer the vehicle.

27.8.2.