COOLING RIBS
The lining may be secured to the shoes by riveting
or by a bonding process in which the linings are glued
to the shoe. In the bonding process, pressure and heat
are applied to make a secure bond between the lining
CAST IRON
LINER
and the shoe. The bonding process allows the lining to
be worn comparatively thin without danger of cutting
or scarring the drum. When brass rivets are used to
STEEL
secure the lining to the shoe, the lining must be
replaced when it is worn to a specified amount to keep
the rivets from scarring the drum.
WARNING
Many types of linings contain asbestos, and dust
from normal wear may accumulate in the brake
drum assembly. For this reason, special procedures
ASf02031
are followed, including the wearing of a suitable
Figure 2-31.--Sectional view of a brake drum.
respirator, when working on asbestos brakes. Do
not use compressed air to blow out the drum or
When a brake is correctly adjusted, the lining
brake assembly, and use a suitable solvent for
attached to the brake shoes fits evenly against the brake
cleaning parts. Remember that asbestos dust is
drums when the brakes are applied. Also, the lining
treated as hazardous waste.
will be free of the drum when the brakes are released.
The linings must not drag against the drum, yet they
must be near enough to give the proper leverage
BRAKE DRUMS
between the operating mechanism and the friction
surfaces. The backing plate of some brake assemblies
Brake drums are made of pressed steel, cast iron, or
contains small slots for the purpose of checking these
a combination of the two metals. Cast iron drums
clearances with a feeler gauge.
dissipate the heat generated by friction more rapidly
than steel drums, and have a higher coefficient of
Most automobiles and some support equipment
friction with any particular brake lining. However, cast
have a mechanism that automatically adjusts the
iron drums are heavier and are not as strong as steel.
brakes. The adjustment takes place when the brakes
are applied as the vehicle is moving backward. As
Quite often, steel drums have a cast iron liner fused into
the brakes are applied, friction between the primary
them to provide the necessary strength and
shoe and the brake drum forces the primary shoe
heat-dissipating qualities (fig. 2-31). To further add to
against the anchor pin (fig. 2-32). Then, hydraulic
the strength and heat dissipation qualities, cooling ribs
pressure from the wheel cylinder forces the upper
are sometimes used on the outside of the drums.
end of the secondary shoe away from the anchor pin
and downward. This causes the adjuster lever to
BRAKE ADJUSTMENTS
pivot on the secondary shoe so that the lower end of
Due to normal wear of the brake linings, brake
the lever is forced against the sprocket on the
assemblies (except for disc brakes) require periodic
adjuster screw. If the brake shoes have worn enough,
the adjuster screw turns a full tooth. This spreads the
adjustment. Normal wear can result in excessive
lower ends of the brake shoes a few thousandths of an
clearance between the lining and drum and can lead to
inch, or enough to compensate for the shoe wear.
poor and uneven braking. One possible outcome of
poorly adjusted brakes is that during hard braking, one
Some brake assemblies and even autoadjusting
wheel may lock before the others are stopped, causing
brakes require first time adjustment in the same
the driver to lose steering control of the vehicle. It is
manner as nonself-adjusting brakes. Many have a slot
important, therefore, that brake adjustments be made
in the rear of the backing plate to allow a thin blade tool
to provide equal distribution of brake action to all
to turn the star wheel. Brakes with this type of
adjustment require you to turn the star wheel until the
wheels.
2-24
