POWER STEERING

Power steering systems have become more and more widely used in the last few years as the time passed because of the increasing front axle loads of vehicles on the one hand and also because of the trend towards
vehicles with more agile steering properties and hence direct transmission steering systems on the other. With the exception of some members of the ‘sub-compact’ class, power steering systems are optionally or automatically included as one of the standard features in automobiles.

The reduction of steering wheel torque, for comfortable to human being, to levels that are ergonomically acceptable has been obtained by applying power assistance to the existing mechanical systems. So power steering has spread from heavy luxury cars and industrial vehicles to virtually all cars.

Manual steering systems are used as a basis for the power steering systems because of the advantage that the mechanical connection between the steering wheel and the wheel and all the components continues to be maintained with or without the help of the auxiliary power. The steering-wheel torque applied by the driver is detected by a measurement system that is located in the region of the input shaft of the steering gear or may be in the steering tube. Additional forces or moments are introduced into the system. This process follows a characteristic curve (valve characteristic) or group of curves depending on the height of the steering wheel torque and if another quantity is to be shown like driving speed, is entered as a signal. The steering boost is thereby reduced because of the aim of achieving better road contact at higher speeds.

Principle of Power Steering

Power steering is a type of hydraulic device for utilizing or using engine power to reduce steering effort by the driver. Similarly the engine is used to drive a pump to develop fluid pressure and this developed pressure acts on a piston within the gear box so that the piston can assist the sector shaft effort. The amount of this assistance to sector shaft effort, depends on the extent of pressure acting on the piston. So if more steering force is required then the pressure must be raised. The variation needed in the fluid pressure is accomplished by a control valve which is linked to the intermediate shaft and the steering main shaft.

Brief History of Power Steering System

The first power steering unit was invented by Francis W. Davis in the mid 1920’s, but was not introduced in passenger cars until 1951. A figure of the system can be seen in Figure given below. This system was of the type: ball and nut, and still this is in use in vehicles with higher steering forces, like typically larger trucks.
The predominant system which are in use today is of the type: rack and pinion, which was introduced in the late 1960’s in medium performance sports cars. There are several different power assisted steering are available, PAS, solutions for passenger cars on the market today. But the most common is the rack and pinion solution with a constant flow controlled pump like Hydraulic Power Assisted Steering – HPAS system. More recently an Electric Power Assisted Steering, EPAS system, was also introduced in smaller cars.

Types of Power Steering System

• Hydraulic Power Steering Systems

Hydraulic power steering systems are still the most widely used systems. The method of using oil under pressure to boost the servo is sophisticated or altered. And the advantageous are in terms of cost, space and weight. Sensitivity to movements is caused by the road surface and so the effect of torsional impacts and torsional vibrations passing into the steering wheel is also noticeably (according to the need) 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 become possible to apply with an additional steering shock absorber in most vehicles with hydraulically rack and pinion steering, whereas it is also required for the same vehicles with manual steering.

• Electrical Power Steering Systems

In small and medium size cars electric power steering (EPS) systems have become widespread these days. They are applied to rack and pinion steering boxes and operate. Also through an electric motor to add torque to the steering column, or to the pinion directly. In case of electro-hydraulic power steering systems, the power-steering pump driven by the engine of the vehicle via V-belts is also replaced by an electrically operated pump. The electrically operated power pack supplies the hydraulic, torsion-bar controlled steering valve with oil. The pump is electronically controlled. At the time when servo boost is not required, the oil supply is reduced.

Advantages of Power Steering

• Reduces steering effort
• Very comfort and reliable at high as well as at low speed
• Reduces the complexity of steering mechanism
• Easy cornering