In heavy duty (dump) trucks and power tractors the effort applied by the driver is inadequate to turn the wheels. In this case a booster arrangement is incorporated in the steering system. The booster is set into operation when the steering wheel is turned. The booster then takes over and does most of the work for steering. This system called power steering uses compressed air, electrical mechanisms, and hydraulic pressure. Hydraulic pressure is used on a vast majority of power steering mechanism today.
When the steering wheel is turned, the worm turns the sector of the worm wheel and the arm. The arm turns the road wheel by means of the drag link. If the resistance offered to turn the wheels is too high and the effort applied by the driver to the steering wheel is too weak, then the worm, like a screw in a nut will be displaced axially together with the distributor slide valve. The axial movement of the distributor slide valve in the cylinder will admit oil into the booster cylinder through the pipe line. The piston in the booster cylinder will turn the road wheels via the gear rack, the toothed worm sector, arm and drag link. At the same time, the worm sector will act upon the work and will shift it together with the distribution slide valve to its initial position and stop the piston travel in the boost cylinder. When the steering wheel is turned in the other direction, the wheels will be turned appropriately in the same sequence.
The more the steering mechanism and wheels resist turning, the more the control valve is displaced. Hence, power assistance is always supplied in proportion to the effort needed to turn the wheels.
Electronic power steering
Electrically assisted power steering is used in some cars. The assistance can be applied directly by an electric stepper motor integrated with the steering column, or the steering mechanism, or it can be applied indirectly with hydraulic assistance pressurized by electric pump. EPS attached to the rack and pinion-steering-exists in Honda-City vehicles.
Electronic power steering Improves steering feel and power saving effectiveness and increases steering performance. It does so with control mechanisms that reduce steering effort. Nissan’s Blue Bird passenger car series use an electronically controlled three way power steering. This power steering is responsive to vehicle speed, providing maximum assistance as the speed rises. The driver can also select his or her own performance from three levels of assistance that make the steering effort heavy, normal or light.