Aluminum brake drums are constructed similar to
the centrifuse drums. They consist of an aluminum
casting with a cast-iron liner for a braking surface. This
design allows heat to be transferred to the surrounding
atmosphere more readily and also reduces weight.
Cooling fins or ribs are added to most brake drums.
The fins or ribs increase the surface area of the outside
portion of the brake drum, allowing the heat to be
transferred to the atmosphere more readily, which
keeps the drum cooler and helps minimize brake fade.
For good braking action, the brake drum should be
perfectly round and have a uniform surface. Brake
drums become out-of-round from pressure exerted by
brake shoes and from heat developed by application of
the brakes. The brake drum surface becomes scored
when it is worn by braking action. When the braking
surface is scored or the brake drum is out-of-round, it
may be necessary to machine the brake drum until it is
smooth and true again. Care must be taken not to
exceed the maximum allowable diameter according to
the manufacturer's specification. Each drum is
stamped with the maximum diameter information and.
Figure 7-12.Brake drum construction.
if exceeded, it should be discarded and replaced with a
BRAKE SHOE ADJUSTERS.Brake
adjusters maintain correct drum-to-lining clearance, as
the brake linings wear. Automatic brake shoe adjusters
normally function when the brakes are applied with the
vehicle moving in reverse. If there is too much lining
clearance, the brake shoes move outward and rotate
with the drum enough to operate the adjusting lever.
This lengthens the adjusting mechanism, and the
linings are moved closer to the brake drum, thereby
maintaining the correct lining-to-drum clearance.
Many vehicles use a star wheel (screw) type brake
shoe adjusting mechanism. This type consists of a star
wheel (adjusting screw assembly), adjuster lever,
adjuster spring, and an adjusting mechanism. The
adjustment system may grouped as follows (fig. 7-13):
Cable typeThe cable type self-adjusting
system (fig. 7-13) uses a braided steel cable and the
expanding action of both brake shoes to accomplish the
self-adjusting action in forward and reverse directions.
A one-piece cable is attached to the adjusting lever and
passes through a cable guide on the primary shoe. The