Aim
To study and prepare report on the constructional details, working principles and operation of the following Automotive Steering Systems:
a) Manual Steering systems e.g. Pitman Arm Steering, Rack & Pinion Steering
b) Power Steering Systems e.g. Rack and Pinion Power Steering System
Purpose of Steering System
The steering system allows the driver to guide the vehicle along the road and turn left or right as desired. The system includes the steering wheel, which controls the steering gear. It changes the rotary motion of the wheel into straight line motion. Manual systems were popular but now power steering has become popular. It is now installed on about 90% of the vehicles being manufactured.
Types of Steering Systems
Figure shows a simplified pitman-arm type of steering system. The rack-and-pinion type is shown in fig. describes how the wheels are supported on steering knuckles. The steering knuckles are attached to the steering arms by ball joints. The bal joints at each wheel permit the steering knuckle to swing from side to side. This movement turns the front wheels left or right so that the car can be steered.
The recirculating ball steering gear is shown in fig. In these units, the worm gear on the end of the steering shaft has a special nut, running on it. The nut rides on rows of small recirculating balls. The recirculation balls move freely through grooves in the worm and inside the nut. As the steering shaft is rotated, the balls force the nut to move up and down the worm gear. A short rack of gear teeth on one side of the nut mesh with the sector gear. Therefore, as the nut moves up and down the worm , the sector gear turns in on direction or the other for steering.
The recirculating balls are the only contacts between the worm and the nut. This greatly reduces friction and the turning effort or force applied by the driver for steering.
The balls are called recirculating balls because they continuously recirculate from one end of the ball nut to the other end through a pair of ball return guides. For example, suppose the driver makes a right turn, then the worm gear rotated in a clockwise direction when viewed from the drivers seat. This causes the ball nut to move upward. The ball roll between the worm and the ball nut. As the balls reach the upper end of the nut, they enter the return guide and the roll back to the lower end. There they reenter the groove between the worm and the ball nut.
Rack and pinion steering gears
The rack and pinion steering gear has become increasing popular for today’s smaller cars. It is simpler, more dire acting and may be straight mechanical or power assisted operation. Figure shows a complete rack and pinion…
Rack and Pinion Power Steering
A power rack and pinion steering gear is another design of integral power steering The rack functions as the power piston. The tie rods are attached between the rack and the spindle steering arms. The control valve is connected to the pinion gear
Operation of the control valve is similar to that for the integral power steering gear When the steering wheel is turned, the resistance of wheels and the weight of vehicle cause the torsion bar to twist. This twisting causes rotary valve to move in its sleeve, aligning the fluid passages for the left, right, or neutral position. Oil pressure exerts force on the piston and helps move the rack to assist the turning effort. The piston is attached directly to the rack. The housing tube functions as the power cylinder.
The gear assembly is always filled with fluid, and all internal components are immersed in fluid. This makes periodic lubrication unnecessary, and also acts as a cushion to help absorb road shocks. On some rack and pinion power steering gears al fluid passages are internal except for the pressure and return hoses between the gear and pump.
Steering Linkages
Steering linkage depends upon the type of the vehicle, whether it is a car which has independent front suspension or a commercial vehicle having generally a rigid axle type front suspension. Each of these linkages will now be described.
Steering linkage for vehicle with rigid axle front suspension.
Figs show such a steering linkage. The drop arm (also called pitman arm ) is rigidly connected to the cross shaft of the steering gear at its upper end, while its lower end is connected to the link rod through a ball joint. To the other end of the link rod is connected the link rod arm through a ball joint. Attached rigidly to the other end of the link rod arm is the stub axle on which the road wheel is mounted. Each stub axle has a forge track rod arm rigidly bolted to the wheel axis. The other ends of the track rod arms are connected to the track rod by means of ball joints . The design of these ball joints is such that the expanding spring compensates for wear or mis-adjustments. An adjuster is also provided in the track rod to change its length for adjusting wheel alignment.
The steering gear provides mechanical advantage so that only a small effort is required at the steering wheel to apply a much larger force to the steering linkage. Moreover it also provides the desired velocity ratio so that much smaller movement of the stub axle is obtained with large angular movement of the steering wheel. When the steering wheel is turned, the swinging action of the drop arm imparts a near linear movement to the link rod. This movement is transmitted through the link rod arm to the stub axle so as to turn the later about its pivot, which may be a king pin or ball joints. The other wheel is steered through the track rod. Thus only one wheel is positively steered.
Rack and pinion steering gear
This type of steering gear is used on light vehicles like cars and in power steering. Maruti 800 cars employ this steering gear. It is simple, light and responsive. It occupies very small space and uses lesser number of linkage components compared to the worm and wheel type of gear.
Power Steering
Larger amount of torque is required to be applied by the driver for steering of medium and heavy vehicles. The power steering system provides automatic hydraulic assistance to the turning effort applied to the manual steering system. The power system is designed to become operative when the effort at wheel exceeds a predetermined value, say 10N. the system is always so designed that in the event of the failure of the power system, the driver is able to steer the vehicle manually although with increased effort. Fig shows a typical power steering system installed on a car.
The power steering systems are operated by fluid under pressure. The fluids usually used are oils of viscosity rating SAE 5 W or SAE 10 W or higher depending upon atmospheric conditions. The systems operate under fairly high pressures which may be as much as 7 MPa.
The principle of working of all the power steering systems is same. The slight movement of the steering wheel actuates a valve so that the fluid under pressure from the reservoir enters on the appropriate side of a cylinder, thereby applying pressure on the side of a piston to operate the steering linkage, which steers the wheel in the appropriate direction.
When the driver applies a force on the steering wheel to steer, the far end of the torsion bar, being connected to the spool of the rotary valve and the worm offers resistance. When the force at the wheel exceeds a predetermined value, the spool turns through a small angle, when the return line is closed and the fluid under pressure goes to one side of the rack piston and moves it to effect steering in the desired direction. The torsion bar is meant to give a feel of the steering to the driver. The rotation of the steering wheel in the opposite direction connects the other side of the steering gear to the pressure line. In the neutral steer position both sides of the piston (nut) are shut off to the pressure line and so they are at the same pressure but the return line is open due to which the fluid goes on circulating through the valve without causing any steering effect.
Reference Books
1 Automotive Mechanics – Crouse/Anglin
2 Automobile Engineering by Dr Kirpal Singh
3 The Motor Vehicle – Newton, Steeds, Garret, Butter Worths
Viva Questions
1 State the requirements of a steering system
2 What is the material used for front axle ? How it is manufactured?
3 What is stub axle?
4 What is the function of a king pin?
5 What is steering axis?
6 What is wheel alignment? Describe
7 Define Camber, SAI and Castor.
8 What is slip angle? Define
9 Define under steer and over steer.
10 What is the function of balls in recirculating ball type steering gear?
11 State the advantages of a rack and pinion type steering gear
12 What is power steering
13 Define cornering force and cornering power
14 Define the term ‘Toe in & ‘Toe out’
15 Define the term ‘camber’ & castor
16 State various factors of wheel alignment
17 Discuss the steering linkage for a vehicle with independent suspension
18 What are the effects of camber castor, toe out and steering axis inclination on the steering characteristics of a vehicle
19 Explain the necessity of power steering in automobiles
20 Discuss various types of steering gears with simple sketch and discuss advantages of rack and pinion type steering gear over other gears.