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 TM 9-8000
scribed in section VI. The brakes are actuated by a
b. Single Anchor, Self-Centering. In this configuration
rotating cam, which is connected to a camshaft and in
(B, fig. 34-9), only the primary brakeshoe is self-
turn operates by the air system.
energizing in the forward direction and therefore provides
the majority of the brake force. This system is self-
centering, in that the lower shoe anchor does not fix the
34-10. Operation. Brake systems, as discussed in
position of the brakeshoes in relation to the drum. The
paragraph 34-6, require a rotating and non- rotating
shoes are allowed to move up and down as needed.
member. In the drum brake assembly, whether
Some configurations provide eccentric cams for front to
hydraulically or pneumatically actuated, the drum
rear brakeshoe adjustments. One brake cylinder is
provides the rotating member and the brakeshoe the
provided in this system.
stationary member. The primary function in the drum
brake assembly is to force the brakeshoes against the
c. Double Anchor, Single Cylinder. In this
rotating drum to provide the braking action. Most drum
arrangement (C, fig. 34-9), each brakeshoe is anchored
brake assemblies use what is called self-energizing
at the bottom by rotating eccentric- shaped anchor pins.
action. This self-energizing action is produced as the
Only the primary shoe is self-energizing, and the system
brake- shoe engages the rotating brake-drum. As the
does not develop servo action. Spring clips are used at
brake actuating mechanism forces the shoes outward (A,
the middle of the shoe to hold the shoes against the
backing plate. Brakeshoes are adjusted manually by
wedge to the rotating brakedrum and rotate with it. This
rotating the anchor pins. One wheel brake cylinder is
effect on brakeshoes greatly reduces the amount of
provided in this arrangement.
effort required to achieve a given amount of retardation.
If two brakeshoes were linked together, as shown in B,
d. Double Anchor, Double Cylinder. In this system
(D, fig. 34-9) the brakeshoes are provided with an anchor
self-energizing effect and also a servo effect. The servo
at each heel. The anchors are eccentric-shaped to allow
effect is a result of the primary shoe, or shoe towards the
for adjustment and centering. Each shoe has a single
front of the vehicle, attempting to rotate with the
piston cylinder mounted at the toe of the brakeshoe,
brakedrum. Due to the fact that both shoes are linked
which allows both shoes to be self-energizing in the
together, the rotating force of the primary shoe applies
forward direction only. Eccentrics mounted in the middle
the secondary shoe. This effect is termed servo action
of the shoe also allow for brake adjustment.
(B, fig. 34-8). In the forward position, the anchor point for
both brakeshoes is at the heel of the secondary shoe. As
the vehicle changes direction, the toe of the primary shoe
34-11. Disadvantages. The drum brake assembly,
becomes the anchor point, and the direction of self-
although well suited for the wheeled vehicle, has some
energizing and servo action changes (C, fig. 34-8). The
disadvantages. One problem that might occur during
most popular configurations of the drum brake assembly
heavy braking is what is known as brake fade. During
are discussed below.
panic stops or repeated harsh stops, brake lining and
brakedrums develop large amounts of heat that reduce
a. Single Anchor, Self-Energizing Servo Action. In
the coefficient of friction between brakeshoe and drum.
this configuration (A, fig. 34-9) both brakeshoes are self-
This reduction in friction greatly decreases the vehi- cle's
energizing in both forward and reverse directions. The
stopping ability, and in most cases, additional pressure
shoes are self- centering and provide servo action during
directed on the brake pedal will not increase the vehicle's
brake application. This system is provided with one
stopping performance. The enclosed design of the drum
anchor pin, which is rigidly mounted to the backing plate
brake assembly does not allow for cooling air to enter the
and is nonadjustable. Both the forward and reverse
brake assembly and therefore heat developed during
brake torque is transmitted to the backing plate through
braking must be dissipated through the brake- drum and
the anchor pin. One brake cylinder with dual pistons is
backing plate. As brakes heat up because of repeated
used in this configuration. This system is used on many
application, cooling air flowing past the drum and
modern
vehicles.
backing plates is limited. This condition causes the
radius
of
the
34-7
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