involves the replacement of pistons, seals, and dust
The expander tube brake assembly is hydraulically
operated and may be used with any of the conventional
boots. To perform this type of service, it is necessary to
hydraulic brake systems. When the brake pedal is
remove the caliper assembly from the vehicle. The
applied, the fluid is forced into the expander tube. The
procedures for this type of service will be listed in the
spider and frames prevent expansion inward or to the
manufacturer's maintenance manual.
sides. Thus, the pressure of the fluid in the tube
Internal Expanding (Expander Tube) Brakes
overcomes spring tension and forces the blocks
radially outward against the brake drum, creating
The expander tube brake is used on some types of
friction. The tube shields prevent the expander tube
support equipment. For example, the A/S32K-1C/1D
from extruding between the blocks, and the torque bars
weapon loaders are equipped with expander tube
prevent the blocks from rotating with the drum.
brakes. The technical manual frequently refers to this
Friction created by the brake is directly proportional to
brake as the aircraft-type expander tube brake because
brake line pressure.
the expander tube brake was designed originally for
When the brake pedal is released, the return
aircraft. Several models of naval aircraft are equipped
springs return the blocks inward, compressing the
with brakes of this type.
expander tube and forcing the fluid back to the brake
An exploded view of an expander tube brake is
control unit.
shown in view A of figure 2-30. View B shows the
brake completely assembled, and view C shows a
BRAKE LININGS
cross-sectional view of the assembled brake. The main
There are several important qualities that are
parts of the brake are the spider, frames, expander tube,
desired of the material for brake linings. First, of
brake blocks, and return springs.
course, it must have high frictional qualities. Second,
The spider, sometimes referred to as a flange or
the material must have the ability to withstand high
torque flange, is the basic unit around which the brake
temperatures. Since there is a lack of cooling facilities
is built. The main part of this spider is secured to the
around the brake assembly, it is necessary that the
wheel support. The detachable metal frames form a
brake lining be able to dissipate the heat rapidly. In
groove around the outer circumference into which the
addition, the material must be durable and
expander tube, brake blocks, springs, and so on, are
moisture-resistant.
fitted.
The two main types of brake linings are organic
The expander tube is made of neoprene reinforced
and metallic. One form of organic lining is woven
with fabric, and has a metal nozzle through which
material composed of asbestos fiber, cotton fibers, and
hydraulic fluid enters and leaves the tube.
copper or bronze wire. It is treated with a
The brake blocks are made of materials quite
mineral-based chemical to resist the effects of oil and
similar to that used in the linings of shoe-type brakes.
water. It is then pressed and undergoes a baking
The actual braking surface is strengthened by a metal
process that compresses the fibers into a dense
backing plate. The blocks are held in place around the
material, helping the lining to resist the effects of heat
spider and are prevented from rotating by the torque
from friction. This material has a very high frictional
bars, which are secured to the frames. The size of the
quality but a low rate of heat transfer or dissipation.
b r a ke a s s e m b l y va r i e s w i t h d i ff e r e n t t y p e s o f
Also, it is severely affected by oil, even after treatment.
equipment. As the size of the assembly changes, the
Therefore, the woven material is used mostly on
number of blocks per assembly changes.
transmission brakes.
The brake return springs are semi-elliptical or
Another form of the organic type is the molded
half-moon in shape. One is fitted between each
lining. This lining is made of a dense, hard, compact
separation in the brake blocks. The ends of the springs
material, and is cut into various forms to fit different
are designed to fit into slots in the brake frames. The
types of shoes. Generally, the compact materials are
bowed center section of the spring pushes inward,
resins and mineral fibers, mixed as a semi-liquid,
holding the blocks firmly against the expander tube
molded, pressed, and baked. The result is an extremely
(fig. 2-30, view C). This prevents the blocks from
dense material. Very often copper or bronze wires are
added to the mixture.
dragging against the drum when the brake is released.
2-22
