TYPES OF BRAKES
Efficient, reliable brakes are just as important to
powered support equipment as the engine. They are
Powered support equipment is equipped with
required not only to stop the vehicle, but to stop it in as
either two- or four-wheel brakes. In either case,
short a distance as possible, and then hold it in place
individual brake assemblies are provided for each
after it is stopped. Because brakes are expected to
braking wheel and are operated by a foot pedal.
decelerate a vehicle at a faster rate than the engine can
Powered equipment also has an emergency or parking
accelerate it, they must control a greater power than
brake, which is operated by a separate pedal or lever.
that developed by the engine.
This may be a separate brake assembly, such as a
transmission parking brake, or it may simply be a
The requirement for good brakes is not limited to
secondary method of controlling the wheel brake
powered equipment. Nonpowered equipment, such as
assemblies. The brake assemblies on nonpowered
oxygen- and nitrogen-servicing trailers,
equipment are similar to those on powered equipment,
air-conditioning units, bomb trailers, and work stands,
but are usually provided only on two wheels.
requires some type of braking system. While the
primary purpose of the brakes on powered equipment
Drum Brakes
is to stop the vehicle, their primary purpose on
nonpowered equipment is to secure the equipment
Drum-type brake assemblies may be classified
after it has been pushed or towed to the desired
into two general types--external contracting and
location.
internal expanding. There are different designs of
the internal expanding type, of which the
A brake absorbs mechanical energy by
conventional shoe is the most common type used on
transferring it into heat through friction. Friction is the
s u p p o r t e q u i p m e n t . T h e ex p a n d e r t u b e b r a ke ,
resistance to relative motion between two surfaces in
another design of the internal expanding type, is
contact with each other. Thus, when a stationary
used on some types of equipment. There are three
surface is forced into contact with a moving surface,
types of drum brake assemblies--external
the resistance to relative motion, or the rubbing action
contracting, internal expanding (shoes), and internal
between the two surfaces slows down the moving
expanding (expander tube).
surface.
EXTERNAL CONTRACTING BRAKES.--
In nearly all brake systems, the brake drums
External contracting brakes (fig. 2-20) are rarely
provide the moving surface, and the brake shoes are the
used for wheel brakes. However, they are often used
stationary surface. The friction between the brake
as parking brakes. Figure 2-21 shows the external
drum and the brake shoe slows the wheel, and the
contracting brake used as a transmission parking
brake. The brake drum is located at the point where
friction between the tires and the road surface brings
t h e d r ive s h a f t i s a t t a c h e d t o t h e r e a r o f t h e
the vehicle to a complete stop.
transmission.
This braking action is accomplished through rods
INTERNAL EXPANDING (SHOE).--Internal
and cables in a mechanical brake system, a liquid
expanding brakes are used almost exclusively as wheel
coupling (brake fluid) in a hydraulic brake system, and
brakes. This type permits a more compact and
air pressure in a pneumatic brake system. A
economical construction. The brake shoe and brake
combination of hydraulics and pneumatics (or
operating mechanism are supported on a backing plate
vacuum) is used to operate the brakes of some
or brake shield, which is attached to the axle flange in
equipment. Although electrical systems are used to
the case of nondriving axles or to the axle housing in
operate some brake systems, they are not commonly
the case of driving axles. The brake drum, attached to
used on support equipment.
the rotating wheel, acts as a cover for the shoe and
operating mechanism and furnishes a frictional surface
The AS is responsible for the maintenance of the
for the brake shoes. Figures 2-22 and 2-23 show the
brakes and brake systems on all support equipment.
arrangement of the brake shoe and operating
This includes servicing, inspecting, adjusting, and
mechanism of a typical wheel brake assembly.
repair. To accomplish these tasks, you must have a
thorough knowledge of the various types of brakes and
In operation, the brake shoe of an internal
expanding brake is forced outward against the drum to
brake systems and how they operate.
2-16
