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TM 9-8000
mate with a lip on the backing plate that provides the
minimize brake fade. For good braking action, the drum
rotating seal to help keep water and dirt from entering
should be perfectly round and have a uniform surface.
the brake assembly. Brakedrums may be made of
Brakedrums become out-of- round from pressure
pressed steel, cast iron, a combination of the two metals,
exerted by brakeshoes and from heat developed by
or aluminum. Cast iron drums dissipate the heat
application of the brakes. The brakedrum surface
generated by friction faster than steel drums and have a
becomes scored when it is worn by braking action. When
higher coefficient of friction with any particular brake
the surface is scored or the drum is out-of- round, it may
lining. However, cast iron drums of sufficient strength are
be necessary to machine the brakedrum until it is
heavier than steel drums. To provide lightweight and
smooth and true again. Care must be taken not to
sufficient strength, centrifuse brakedrums (fig. 34-7),
exceed the maximum allowable diameter according to
made of steel with a cast iron liner for the braking
manufacturer's specifications. If this caution is ignored,
surface, are used. A solid cast iron drum of the same
the drum can be distorted easily due to
total thickness as the centrifuse drum would be too
overheating, and result in brake failure. Each drum is
weak, while one of sufficient strength would be too heavy
stamped with the maximum diameter information and, if
for the average passenger car. Aluminum
exceeded, it should be discarded and replaced with a
brakedrums are constructed similar to the centrifuse
new one.
drums. They consist of an aluminum casting with a cast
Iron liner for a braking surface. This design allows heat
34-8. Hydraulic Actuation. The hydraulically operated
to be transferred to the surrounding atmosphere more
service brake uses fluid pressure to operate a hydraulic
readily and also reduces weight. Cooling fins or ribs (fig.
cylinder, which in turn operates the brakeshoes. The fluid
34-7) also are added to most brakedrums. The fins or
pressure is generated by a hydraulic system whose
ribs increase the surface area of the outside portion of
operation is discussed in paragraph 34-23.
the brakedrum, allowing the heat to be transferred to the
atmosphere more readily, thereby keeping the drum
349. Pneumatic Actuation. The pneumatically actuated
cooler and helping to
service brake uses a controlled, compressed air supply
from the air system as de-
Figure 34-7. Brakedrum Construction


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