BRAKE
BAND
BRAKE
LEVER
FULCRUM
BRAKE
DRUM
LINING
ASf02020
Figure 2-20.--External contracting brake.
action is very helpful in accomplishing this, once
setting of the shoes is started by physical effort. While
there are variations of this action, it is always obtained
CONTROL LEVER
by the shoes themselves, which tend to revolve with the
revolving drum.
BRAKE
BAND
Figure 2-24 illustrates the self-energizing action of
a brake shoe. As shown, the drum is revolving
LINKAGE
counterclockwise. When the shoe is forced against the
drum, it tends to rotate with the drum. As the initial
braking pressure is increased on the cam, the wedging
action increases and the shoe is forced more tightly
against the drum to increase friction. This
self-energizing action results in more braking action
than could be obtained with the actuating force alone.
Brakes making use of this self-energizing principle to
increase pressure on the braking surface are known as
BRAKE DRUM
servo brakes. (Servo is the action or device used to
convert a small movement into a greater movement or
ASf02021
force.)
Figure 2-21.--External contracting transmission
The operator controls the self-energizing action;
parking brake.
that is, as the operator adds pressure to the brake pedal,
produce the braking action. One end of the shoe is
the self-energizing action increases. Thus, it is most
hinged to the backing plate by an anchor pin, while the
important that the operator control the total braking
other end is unattached and can be moved in its support
action at all times. The amount of self-energizing
with the operating mechanism. When force from the
action available depends mainly upon the location of
operating mechanism is applied to the unattached end
the anchor pin. This is determined by the manufacturer
of the shoe, the shoe is forced (expanded) against the
and is located at the point outward of the braking
drum and brakes the wheel. A retracting spring returns
surface where the operator has the maximum control of
the shoe to the original position when braking action is
the braking action. If the anchor pin is located too near
no longer required.
the center of the drum, the shoe will automatically lock
when the brakes are initially applied.
The brake-operating linkage alone does not
provide sufficient mechanical advantage for positive
When two shoes are anchored on the lower part of
braking. Some means of supplementing the physical
the brake shield, self-energizing action is effective on
application of the braking system must be used to
only one shoe. The other shoe tends to revolve away
from its pivot, which reduces its braking action. When
increase pressure on the brake shoes. A self-energizing
2-17
