Table of Contents
Steering System
The steering system allows the driver to guide the moving vehicle on the road and turn it right or left as the Desires.
During steering, the movement of the wheel must be positive and exact and no wheel should slide on the road. This aspect is influenced by the steering linkage mechanism, tyre and road conditions and vehicle suspension system.
Requirement of steering system
1) The steering mechanism should be very accurate and easy to handle.
2) It should multiply the turning effort applied on the steering wheel by the driver.
3) It should be a certain degree Irreversible so that the shocks on the road surface encountered by the wheel are not transmitted to the driver hands.
4) The steering mechanism should have self-rightening effect so that when the driver release the steering wheel after negotiating a turn, the wheel should try to achieve straight ahead position.
Steering linkages
In all type of steering systems, there is a cross shaft in the steering box. Which is operated by means of steering wheel through a rod in the hollow steering column and some type of steering gear.
The cross shaft is further connected to a drop arm also called pitman at its end. Thus the rotation of steering wheel results in the linear motion of the drop arm at it’s end which itself is connected through a ball joint to a link rod.
To meet different requirement of different vehicle different steering linkages having individual variations are used some important type steering linkage are direct cross type, Centre are steering, three piece steering etc.
A) Steering linkage for rigid axle suspension
Figure shows steering linkage for a conventional rigid axle suspension. The link rod causes the turning of the front wheel through the steering arms and the track rod.
The track rod and steering arms are connect by ball joint. The design of ball joint it such that the expanding spring compansates for wear and miss adjustment in the system.
A provision for change the length of track rod is also provided. In this steering linkage only one wheel is positively steered.
B) Steering linkage for independent suspension
In independent suspension, a provision has to be made regarding the motion of wheels relative to each other, to keep the steering geometry correctly at all time.
Figure shows to track rod are provided and a centrally idler arm actuate the two piece track rod. In this case both the wheel are positively steered.
Rack and pinion steering gear
In this type of steering gear, a pinion is mounted at the end of steering shaft. The pinion engages with the rack which is provided with ball joint at its each end in order to allow for the rise and fall of wheel.
The rotary motion of the steering wheel is transmitted to the pinion.
The circular motion of pinion is convert into the linear rack movement, which is further transmitted to tie rods to the stub axle through ball joint.
This type of steering gear provides sufficiently low gear reduction for car and it is quite suitable even for heavier motor vehicles if assisted with power. It occupies very small space and less number of linkage.
The rack and pinion gear design are of two type
1) Centre take off rack
In this tie rods are connected at the centre of the rack in instead of at the end. It has a large boot that covers the centre part of the rack and pinion housing. A slot in the housing permit the inner tie rode ends to move with the rack.
This type of design can be mounted high saving space and reduces the length of steering column.
2) End take off rack
In this tie rod is connected at the end of the rack this is less affected by bump steer. When the toe of wheel changes as they go over the bump, the vehicle is said to have bump steer.
Recirculating Ball type steering Gear
It is an improvement version of the worm and nut steering gear. It consist of a worm at the end of steering rod. A nut is mounted on worm with two set of balls in the grooves of the worm, in between the nut and worm.
The function of ball is to reduce the friction between the worm and nut. The nut has a number of teeth on the outside which mesh with the teeth on worm wheel sector.
The Drop arm is mounted on sector. Which transfer the motion to steering arms through linkages. When the steering wheel turn, the balls in the worm grove rolls, causes to nut slides on the worm. This linear motion of nut causes the wheel sector turn at an angle and actuate the link rod through drop arm, resulting in the desired steering of the wheels.
The end play of the worm can be adjusted with the help of adjuster nut provided. A screw is also provide by means of which the drop arm and hence the wheel sector can be positioned along its axis.
Introduction to Power Steering
With increasing demand for light control aligning tyre torque for keeping vehicle load acceptable, power steering has become necessity in most of the vehicle.
In order to reduce the steering wheel turning effort, turn sharp corner easily, negotiate winding roads and monocurve the vehicle in confirmed space as well as to offer some resistance to make the drive feel and retain some road feel or vehicle steering response power steering has been used.
Now, In heavy duty truck 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 the steering. This system called power steering has used compressed air, electrical mechanisms and Hydraulic pressure.
Hydraulic pressure is used on the vast majority of power steering mechanism today. A simplified sketch of hydraulic power steering with hydraulic booster shown in figure.
When the steering wheel is turned, the worm turns the sector of 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 to weak, then the worm, like a screw in a nut will be displayed 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 pipeline.
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 worm and will shift it together with the distribution slide valve to it 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 wheel.
Internal Link
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