Lesson 1/Learning Event 1
The brake operating linkage alone does not provide enough mechanical
advantage for good braking. Some way of increasing the pressure of
the brake shoes is needed. A selfenergizing action can be used to
do this, once the setting of the shoes is started by the movement of
the linkage. There are several variations of this selfenergizing
action, but it is always done by the shoes themselves as they tend to
turn with the turning drum.
When the brake shoe is anchored and the drum turns in the direction
shown, the shoe will tend to turn with the drum when it is forced
against the drum. Friction is trying to cause the shoe to turn with
the drum. When this happens, the shoe pushes against the anchor pin.
Since the pin is fixed to the brake shield, this pressure tends to
wedge the shoe between the pin and drum. As the cam increases the
pressure on the shoes, the wedging action increases and the shoe is
forced still more tightly against the drum to increase the friction.
This selfenergizing action results in more braking action than could
be obtained by the pressure of the cam against the shoes alone.
Brakes making use of this principle to increase pressure on the
braking surfaces are known as selfenergizing brakes.
It is very important that the operator control the total braking
action at all times, which means the selfenergizing action should
increase only upon the application of more pressure on the brake
pedal. The amount of selfenergizing action available depends mainly
on the location of the anchor pin. As the pin is moved toward the
center of the drum, the wedging action increases until a point is
reached where the shoe will automatically lock. The pin must be
located outside this point so that the operator can control the
braking.
When two shoes are anchored on the bottom of the brake shield, self
energizing action is effective on only one shoe. The other shoe
tends to turn away from its pivot. This reduces its braking action.
When the wheel is turning in the opposite direction, the self
energizing action is produced on the opposite shoe.
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