Troubleshooting guide

54
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Applying This Concept To An Air Brake
System
The most easily recognized lever in an air brake
system is the slack adjuster. The length of the lever
arm of a slack adjuster is always the perpendicular
distance between the center line of the brake camshaft
opening and the center line of the clevis pin.
Another form of lever – not always recognized – is
the brake cam. All brake cams are levers and are used
to transmit and modify the torque and turning motion
of the brake camshaft. Cams do this by spreading and
forcing the brake shoes against the brake drum, not only
in the proper direction but also with the proper force.
Spreading the shoes in the proper direction depends on
correctly locating the cam with respect to the location of
the brake shoes. The transmission of the proper force is
partially determined by the effective lever length of the
cam. If the effective lever length of the cam is too long or
too short, the brake shoe force will be correspondingly
too little or too much.
The effective lever length of the cam must remain
constant as the lining wears and the shoes have to be
spread further; otherwise, the brake performance would
vary as the lining wears.
The brake shoe is a third form of lever found in drum
brake forms of braking systems. The shoe is one of
the simpler forms because it is easily recognized as a
beam, fulcrumed at one end on the hinge pin, which
forces the brake lining against the drum when the brake
cam force is applied to the other end.
Perhaps the least easily recognized lever in a
drum brake system is the relation of the brake
drum diameter to the tire diameter. To understand,
remember that while the brakes stop the brake drums
and wheels, it is always the tires and road surface that
stop the vehicle. This is clearly demonstrated when quick
stops are attempted on wet or icy roads. Under these
conditions, the brake equipment may still be as effi cient
as ever in stopping the wheels. But the system’s ability
to stop the vehicle quickly diminishes because there is
insuffi cient friction between the tire and road to develop
the necessary retarding force.
Let’s return to the principles of leverage involved in the
relation of the tire and brake drum size. The retarding
force developed by the brake shoes acting against the
drum is operating on an effective lever length equal to
the brake drum radius. Counteracting this force is the
retarding force developed between the tire and the road,
operating on an effective lever length equal to the rolling
radius of the tire.
Since it is not practical to have brake drums as large as
the tires, the principles of leverage require development
of a greater retarding force between the brake shoes and
the drums than between the tire and the road. Also, a
rubber tire on a smooth, dry road surface has a higher
coeffi cient of friction than brake lining against a brake
drum. As a result, it is necessary to develop additional
retarding force between the brake shoes and brake drum
to overcome the difference in friction.
FIGURE 6 - DECELERATION
Applying This Concept To An Air Brake System