Rack and Pinion Steering

Rack and Pinion Steering

The steering rack housing generally contains a helically toothed pinion, which meshes with the rack. By turning the steering wheel and hence the pinion, the rack is displaced transversely to the direction Linear steering rack of travel. A spring-loaded pressure pad presses the rack against the pinion. For this reason the steering gear always functions without backlash. At the same time, the sliding friction between pressure pad and rack acts as a damper to absorb road shocks. Advantages of rack and pinion steering include the shallow construction, the very direct steering, the good steering return and the low cost of manufacture.

Variable Pitch Rack The basic construction and the advantages are similar to those of a rack and pinion steering gear with constant pitch. In a rack and pinion steering gear with

Progressive steering rack

variable pitch, a rack is used which has teeth that diminish in size towards the ends. This makes it possible to increase the transmission ratio constantly. This means, in practice, that more steering wheel turns but less effort is required in order to turn the wheels. As a result, the steering moves more easily when applying lock, than when moving in a straight line. This makes parking considerably easier.

Summary ฀฀ There is a wide range of steering boxes and steering layouts. On light vehicles, the most common by far, is the steering rack. This is because it has a shallow construction, is very direct, has good steering return and the cost of manufacture is low.

 State the main advantages of rack and pinion steering.

 Look back over the previous section and write out a list of the key bullet points here:

INTRODUCTION TO POWER STEERING

Introduction The effort required to steer the front wheels depends primarily on the axle load. This is particularly apparent in the following situations:

• Low speed • Low tyre pressures • Large tyre contact area

• Tight cornering.

Steering Ratio ฀฀ Steering ratio cannot be increased too much, because a large number of steering wheel turns would be necessary for the steering movement. Generally, a steering force of 250N should not be exceeded. Therefore, the need arises for power steering in heavy cars, trucks and buses. The power assistance is generally produced by hydraulic pressures. However, electric systems are now becoming popular.

Requirements of Power Steering The requirements of a power steering system are:

Precise onset of power assistance Maintenance of driver feel Continued ability to steer should the power system fail.

Hydraulic Power Steering

Hydraulic power assisted steering (PAS) systems use an engine driven pump to supply PAS rack

PAS pump

pressurised fluid. A control valve directs the fluid to a ram that assists with movement of the steering. If the fluid supply or ram fail, the steering works like a manual system.

Electric Power Steering Early electric power assisted steering systems used a motor to drive a

hydraulic pump. It is now becoming common for the Electric PAS electric motor to act directly on to the steering rack, or the steering shaft.

Four-Wheel Steering ฀฀฀To understand four- wheel steering it is first useful to recall or imagine the effects of rear wheel steering. If you have ever driven, or watched the movement of

a forklift truck, you will realise the different effect moving the rear wheels has on vehicle position. This is the same effect on a normal car when reversing - it is why some drivers have trouble reversing into a parking slot or out of a garage! The key point is that the trailing end of the vehicle tends to slew in the direction that the wheels are turned.

Direction of Movement When all four wheels are turned, the overall effect on the vehicle changes again. The effect varies depending on which way the rear wheels are moved. The effects could be described as a turn or a drift. At low speeds, the wheels are turned in opposite directions to improve the drag or slip on the tyres as well as reducing the turning circle. At high speeds, the wheels are turned in the same direction such as for when changing lanes on a motorway. The amount of turn on the rear wheels is much less than the front.