Troubleshooting guide

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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. Understanding the laws of friction
serves as 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. Friction 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 the coeffi cient of
friction, which is the amount of force required to move
one body while it remains in contact with the other.
The coeffi cient of friction is expressed as 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 of force is necessary to
keep the body 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 changes with any variation in
the condition of one or both surfaces. As an example,
introducing oil or grease between two dry, fl at metal
surfaces will greatly reduce the friction between them.
This potential for variation in the coeffi cient of friction
is always present. Variation can occur when any factor
contributing to a material’s frictional value is subject to
change, either permanently or temporarily.
Heat is always present where friction is developed. For
example, an improperly lubricated bearing increases the
coeffi cient of friction, with a resultant increase 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,
let’s consider a more complete defi nition of a brake: A
brake 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
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 higher speeds.
A simple way to understand 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.
Traveling at 60 miles per hour, the same vehicle should
be capable of stopping with ease in six seconds or less.
Ignoring the unknown quantities, such as rolling friction
and wind resistance (which play a part in all stops), the
brakes must develop the same energy in six seconds as
the engine develops in 60 seconds. In other words, the
brakes do the same amount of work as the engine in one-
tenth the time and must develop approximately 1,000
horsepower during the stop.
FIGURE 2 - FORCES INVOLVED IN BRAKING
(a)
(b)
(c)
FIGURE 1 - COEFFICIENT OF FRICTION
Air Brake System Fundamentals