Study of Ackerman’s Steering Gear Mechanism

Aim

Study of Ackerman’s Steering Gear Mechanism

Apparatus used

Steering Mechanism Apparatus

Theory

Definition of Steering mechanisms, Classification of steering mechanisms, Diagrams of steering mechanisms, Working & Construction, Advantages & Disadvantages, Applications, Examples of steering mechanisms

Steering Gear:When an automobile takes turn on a road all the wheels should make concentric circle to ensure that they roll on the road smoothly and there is line contact between the tyre and the surface of the path, preventing the excess wear of the tyre. This is achieved by mounting the two front wheels on two short axles, known as stub axles. The stub axles are pin-jointed with the main front axle which is rigidly attached to the rear axle. Thus the steering is affected by the use of front wheels only.

Types of Steering Gears:

There are two main types of steering gears:

1. The Davis steering Gear
2. The Ackermann steering gear

Davis Steering Gear:A Davis steering gear has sliding pairs which means more friction and easy wearing. The gear fulfils the fundamental equation of gearing in all the positions. However, due to easy wearing it becomes inaccurate after some time

Ackermann Steering Gear: An Ackermann steering gear has only turning pairs and thus is preferred. Its drawback is that it fulfils the fundamental equation of correct gearing at the middle and the two extreme position and not in all positions. This steering gear consists of a four link mechanism having four-turning pairs.

Three positions of the correct gearing are:

1. When the vehicle moves straight.
2. When the vehicle moves at a correct angle to the right
3. When the vehicle moves at a correct angle to the left.

In the Ackerman steering gear, the mechanism ABCD is a four bar crank chain, as shown in figure below. The shorter links BC and AD are of equal length and are connected by hinge joint with front wheel axles. The longer links AB and CD are of unequal length.

The following are the only three positions for correct steering.

1. When the vehicle moves along a straight path, the longer links AB and CD are parallel and the shorter links BC and AD are equally inclined to the longitudinal axis of the vehicle, as shown by firm lines in the figure.
2. When the vehicle is steering to the left, the position of the gear is shown by dotted lines in the figure. In this position, the lines of the front wheel axle intersect on the back wheel axle at I, for correct steering
3. When the vehicle is steering to the right, a similar position may be obtained again.

Sometimes incorrect steering condition with a combination of high speeds and bad road conditions can lead to over and under steer. These are the two major drawback of incorrect steering.

1. Under steer occurs during a turn of a corner, when you try to turn the vehicle but it continues in a straight line, heading off the road. This occurs because the front tyres have reached their adhesion limit and cannot grip the road adequately enough to turn. The best way to correct under steer is to brake, release break, steer in the opposite desired direction and quickly change back to turning in the correct direction. However, you should not brake into a corner as this will cause further under steer, rather try and brake on the straightest line possible. This will ensure the cars slow down enough to regain enough grip to turn.
2. Over steer occurs when turning a corner and the vehicles back slides out as the rear wheels do not have enough grip to complete the turn. To counter such an even do not accelerate further through the corner, but rather very carefully break gently in the straightest manner possible. Full force braking can become more dangerous than helpful. However, braking gently can correct your steering in situations such as over steer.

Conclusion

Ackerman steering gear mechanism was studied and is preferred one. The Ackerman steering gear mechanism is much simpler than Davis steering gear and hence quite popular.