POWER STEERING

Introduction to Power Steering

                The purpose of power steering is to reduce the driver’s effort at the steering wheel

•             Almost all modern luxury cars are equipped with power steering system instead of manual steering

•             The power steering is increasingly becoming popular on light, medium and heavy vehicles also

Introduction to Power Steering

•             Power steering system is used in big cars and heavy commercial vehicles whose unladen weight is more than1 500 kg

•             It is useful while driving on rough roads at low speeds and while reversing the vehicle for parking purpose

•             The power steering mechanism employs electrical devices, compressed air and hydraulic pressure

•             The power steering system commonly take assistance of hydraulic power for their operation

•             They become operative when the manual effort exceeds about 1kgf(9.81 N)

•             The system is designed such that if power system fails, it can be operated manually

•             In this situation the driver’s effort will be more such as in manual steering

•             The ‘Power Brakes’ utilise the vacuum and hydraulic powers for their working, but the ‘Power Steering’ utilise hydraulic power only

•             Usually oils of viscosity ratings SAE 5W to 15W flow through the system at about 10 -11kgf/cm2 (1 MPa) pressure

Advantages of Power Steering

•             Lesser steering effort

•             Reduced driver’s fatigue

•             Efficient absorption of shocks

•             Better directional stability

Advantages of Power Steering

•             Higher performance

•             Higher maneuverability

•             Enhanced safety

Types of Power Steering Systems

1.            Integral type  

a.            Rotary spool valve type

b.            Reaction control valve type

2.            Semi-integral type

Ex: Trucks and other Heavy Vehicles

3.            Linkage booster type 

Ex: Rack and pinion type

4.            Speed responding type       

Principles of Power Steering

•             The Power steering is one type of hydraulic device for utilising engine power as steering effort

•             The Engine is used to drive a pump to develop fluid pressure

•             This pressure acts on the piston with in the power cylinder so that the piston assists the rack effort

•             The amount of this assistance depends on the extent of pressure acting on the piston.

•             If more steering force is required, the pressure must be raised.

•             Variation in the fluid pressure is accomplished by a control valve which is linked to the steering main shaft.

                The working principle of a power steering

(a)          Neutral (Straight-ahead ) position

(b)          Turning position can be understood in the next slide

Neutral (Straight-ahead position):

•             Fluid from the pump is sent to the control valve

•             If the control valve is in the neutral position, all the fluid will flow pass through the control valve into the relief port and back to the pump

•             At this time hardly any pressure is created and because the pressure on the cylinder piston is equal on both sides, the piston will not move in either direction

Turning  Position:

•             When the steering main shaft is turned in either direction, the control valve also moves, closing the one of the fluid passages

•             The other passage then opens wider, causing a change in fluid flow volume and at the same time, pressure is created

•             Consequently, a pressure difference occurs between both sides of the piston and piston moves in the direction of low pressure

•             Thus, the fluid in that cylinder is forced back to the pump through the control valve

Steering Gear Booster in a Typical Power Steering System

Parts:

1.Worm      2.Sector   3.Booster cylinder   4.Gear rack   5.Arm

6.Drag link  7.Distibutor slide valve   8. Pipe line

Type Power Steering System

•             A simplified diagram of a hydraulic booster is shown in the previous

•             When the steering wheel is turned, worm (1) turns sector (2) of the worm wheel and arm (5), which turns the wheel by means of drag link (6).

Type Power Steering System

•             If the resistance offered to the turn of the wheels is too high and the effort applied by the driver to the steering wheel is  too weak

•             The worm will be displaced axially (like a screw in a nut) together with distributor slide valve( 7 ) and will thus admit oil (or compressed air) into booster cylinder ( 3) through pipe line (8)

•             The piston will move in cylinder (3) and will turn the wheels via gear rack (4), a toothed sector, arm (5) and drag link (6)

•             At the same time, the worm sector will act upon the worm and will shift with together with the distributor slide valve to its initial position and stop the piston travel

•             When the steering wheel is turned in the other direction, the wheels will be turned appropriately in the same sequence

Various Power Steering Systems 

Various Power Steering Systems