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Section 8: The Fundamentals of Air Braking

Friction

Air brakes are mechanical devices that use friction to

slow or stop vehicles. An understanding of the laws

of friction will be a useful introduction to the concepts

behind brake design and maintenance.

Coefﬁ cient of Friction

Friction is the resistance to relative motion between

any two bodies in contact, and it varies not only with

different materials, but also with the condition of the

materials. The amount of friction developed by any two

bodies in contact is referred to as their coefﬁ cient of

friction, which is the amount of force required to move

the one body while it remains in contact with the other.

The coefﬁ cient of friction is expressed by the relation

of the amount of force divided by the weight of the

moving body. Let’s look at three examples:

In Figure 1, example (a), if the moving body weighs

100 pounds, and a force of 60 pounds is required to

keep it moving while it remains in contact with another

body, then the coefﬁ cient of friction between the two

bodies is 60% or 0.6. For (b), if 50 pounds force is

necessary to keep it moving, the coefﬁ cient of friction is

50% or 0.5. For (c), if only 35 pounds force is required,

the coefﬁ cient of friction is 35% or 0.35.

The coefﬁ cient of friction between any two surfaces

changes with any variation in the condition of one or

both surfaces. As an example, the introduction of oil or

grease between two dry, ﬂ at metal surfaces will greatly

reduce the friction between them, which demonstrates

that the condition of these surfaces plays a great

part in the actual friction they develop. This possible

variation in the coefﬁ cient of friction is always present

when any factor contributing to the frictional value of

any material is subject to change either permanently

or temporarily.

Heat is always present where friction is being

developed. For example, when a bearing is not

properly lubricated, the lack of lubrication causes a

rise in the coefﬁ cient of friction with a resultant rise in

the heat produced. The heat may reach a point where

the bearing fails.

Energy of Motion Becomes Heat Energy

Since friction is the resistance to relative motion

between two bodies in contact and since friction results

in heat, a more complete deﬁ nition of a brake would be

that it is a mechanical device for retarding the motion

of a vehicle by means of friction, thereby changing the

energy of motion into heat energy.

Stated another way, when the speed of a vehicle is

reduced by applying the brakes, the energy of motion

is actually changed into heat energy, and the brakes

must dissipate or absorb the heat developed.

Braking Force

It is difﬁ cult to appreciate the tremendous forces

involved in stopping a modern commercial vehicle,

particularly from the higher speeds.

A simple method of explaining this is to make a

comparison between the horsepower required to

accelerate a vehicle and the horsepower required to

stop it. A truck with an engine capable of developing

100 horsepower will require about one minute to

accelerate to 60 miles per hour. The same vehicle

should be capable of easily stopping from 60 miles

per hour in not more than six seconds. Ignoring the

unknown quantities, such as rolling friction and wind

FIGURE 2 - FORCES INVOLVED IN BRAKING

(a)

(b)

(c)

FIGURE 1 - COEFFICIENT OF FRICTION

Air Brake System Fundamentals

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