RECONDITIONING BRAKE DRUMS AND SHOES
DRUM BRAKES
INSPECTION OF DRUMS
The friction surface of brake drums must be smooth, true
and concentric. Make certain with a visual check that
drums are not barrel shaped, bellmouthed, scored or
eccentric.
Hard or chill spots, Figure 1, in brake drum may produce
pedal pulsation and roughness or brake surge. If these
effects are present, drum should be replaced.
Figure 1 - Hard or Chill Spotted Drum
A
barrel
shaped
drum
(Figure
2)
results
from
overheating. If this barrel shaped condition is not
corrected, the braking surface is reduced and uneven
lining wear results.
Figure 2 - Barrel Shaped Drum
Extreme pressure which over a period of time will create
a bellmouthed drum as shown in Figure 3. Brake linings
on a bellmouthed brake drum will make contact only on
the inner surface of the drum. In addition to cutting the
braking surface to a minimum, it will also cause uneven
and rapid wear.
Figure 3 - Bellmouthed Drum
Scored drums are the result of worn linings to the point
where the drum-to-shoe contact is made or an
accumulation of small steel particles imbed themselves
in the brake lining (Figure 4). The steel particles form a
tough scale which is sometimes harder than the drum.
As a result, deep grooves are formed in friction surface
of drum.
Figure 4 - Scored Drum
Brake drum scoring never improves but continually gets
worse until both lining and brake drum are useless.
Attempting to reline brakes without turning scored brake
drum surface will quickly destroy new lining and make
effective braking impossible.
Check inner side of drum surface for tool marks to
determine if drum has been previously reconditioned,
Figure 5. If so, an oversize lining may be required.
Figure 5 - Check Brake Drum for Tool Marks
CTS-4082B - Page 3
