|Explore Topics of Steering System
Automobile is fragmentary without the steering system as it gives the vehicle a directional stability. Steering enables the driver to easily maneuver the vehicle according to the path. It doesn’t mean that automotive, when developed, included steering mechanism. So, how the steering wheel which we see on today’s car was invented? The automotive freaks, during those days, got the idea of steer the vehicle from boats which used tilters that helped them to turn left or right. The idea was implemented on automobiles too, but didn’t last longer as it required more effort and the steering ability was almost poor. The ultimate discovery of the steering wheel was seen in the Panhard model which was tested in the Paris-Rouen race. This innovation was then seen in every car model that followed and as such the round steering wheel became important and compulsory element of the car.
The steering system is however not as easy as it seems, where you simply turn the steering wheel that turns the road wheels. It is composed of complex mechanisms, linkages, joints and gears that actually make the wheels to turn.
Let us have a sneak peek on how the steering system actually works-We know that the basic function of steering system is to turn the wheels in the desired direction. However, it is interesting to note that when the vehicle is turned, the front wheels do not point in the same direction. Let’s see how…
For a proper steering system, each of the wheels must follow different turning circle and when a perpendicular is drawn from centre of each wheel; all the perpendiculars will meet at one single point, which is known as instantaneous centre.
But then, the question still goes unanswered, that how these wheels actually turn?
Looking closely to the figure, we can see the complex mechanism of the steering system which makes the car wheels to turn. The system works on four bar chain mechanism where four links are flexibly connected with each other and imparting motion, to one of the link, will cause other links to move accordingly.
The steering wheel is directly connected to the steering gear, whose main function is to convert the rotary motion of the steering wheel into oscillatory motion of the pitman arm. This motion is transferred to the tie rod through the drag line, which moves the tie rod left or right, depending on the steering wheel input. The tie rod ends are connected to the pivots that actually turn the wheels.
The mechanism of steering remains same in all the steering systems, but the difference can be seen in the type of gearing arrangement used in the steering gearbox. The type of steering gear mechanism used in automobile power steering is listed below:
- Rack and Pinion Steering gear
- Worm and Sector steering gear
- Worm and Roller steering gear
- Cam and Lever steering gear
- Worm and Nut steering gear
Most common types include rack and pinion and re-circulating ball (Worm and nut) type arrangement.
1.Rack and Pinion Steering
The system consists of a pinion attached to the steering shaft, which meshes with the flat rack on the drag line. Pinion imparts left or right motion to the rack, which causes the tie rods to move the road wheels consequently. This mechanism is usually adopted universally in all the cars that are available today with some major improvements.
Earlier, steering system was robust, though it was quite difficult and heavy to use. The need of more flexible and smooth system was desired and as such some of the vital experiments in 1920 gave birth to the power steering system. The entire credit goes to Francis W. Davis who was an engineer in automotive company. With his efforts, power steering system began to be used commercially in all the cars.
2. Worm and Sector steering gear
In this type of steering mechanism, worm is connected at the end of steering shaft and sector is mounted on a sector shaft. The sector looks like a ball and worm looks like a gear. The teeth of the worm mesh with the teeth of the sector. To provide free play, worm is mounted on a bearing. It is also known as “Pitman arm shaft”
3.Worm and Roller steering gear
Worm and roller is similar to the worm and sector type of steering mechanism. A toothed roller is mounted on a roller shaft and worm gear is mounted on a steering shaft. Gear tooth of worm gear meshes with that of the roller and motion is transmitted. The roller is mounted on a ball bearing. This mechanism has low friction compared to Worm and Sector mechanism and is widely used on American passenger cars.
4.Cam and Lever steering gear
Cam and lever is similar to the worm and sector gear mechanism. The worm is replaced with cam and sector is replaced with a lever. Cam is engaged by a bearing and lever, carries two studs that are mounted on bearing. As the driver turns the steering wheel, studs move up and down on the cam so lever and pitman arm shaft rotates.
5.Worm and Nut steering gear
A nut is meshed with the worm and screws up and down relative to the movement of the worm. This is also known as “re-circulating ball type” mechanism. Nut is used with ball that reduces the friction and power loss.
Without increasing further impatience, let us look the power steering system implemented on a normal rack and pinion steering system.
Power steering refers to the use of hydraulic power in assisting the steering motion. When coupled with any of the steering system, the power steering makes use of pump which forces the fluid under pressure, to help the steering gearbox, to overcome the friction and easily turn the gears and impart movement to the road wheels. Power steering makes steering wheel to turn easily and also a small effort, on steering wheel, will give sharp turning of road wheels.
Power steering got a new dimension with the use of electric motor and sensors that do not use engine power but run separately using battery. The sensors are used to sense the torque and motion is applied to the steering wheel. Based on this, the control unit, powers the motor to assist in steering. At this juncture, it is also important to know the term steering ratio which helps to determine the steering ability of particular steering system. Steering ratio is defined as the ratio of number of degrees that the steering wheel turns to the number degrees of wheel turn.
Steering ratio for cars vary from 12:1 to 20:1. E.g. If we want to turn the wheels to 20 degree by one complete revolution of steering wheel, the ratio will be 360:20 or 18:1.
A higher steering ratio requires more turns of steering wheel but effort required will be less.
New Technologies and Innovations in the Steering System
Many new innovations in steering system are available today that gives best possible directional stability to the cars with extreme comfort levels. Some of the improvements include; 4-wheel steering which assists in steering all the four wheels of the car. This helps in maneuvering the car at high speeds. Some of the cars today, also include a system known as speed sensitive steering, which uses a concept that during high speed steering requirements are less and as such the power assistance is reduced. It is increased during low speeds as more steering effort is needed. Future developments in the steering system are being done and experts have come up with an idea of steer-by-wire mechanism in which the steering will be achieved completely using electronics. The main aim of the system, is to eliminate the complex system of linkages and joints giving more space for engine and interior designing aspects.