grease retainer. All parts of the self-adjusting brake
much material from the drum, leaving it too thin and
systems must be free of rust and corrosion for them to
weak. (All drums are stamped with the maximum
work properly. While checking springs, clevis and
allowable area to be removed.) When this occurs, the
cotter pins, you should clean the star adjustor threads
drum must be replaced. Also, a drum that is cracked or
with a hand-held wire brush and apply a thin film of oil
badly distorted should be replaced.
to ensure proper operation.
BRAKE SYSTEMS
In the case of hydraulic brake systems, the
hydraulic brake cylinder should be checked at this
Although brake assemblies and brake drums are
time. The condition of the brake linings and brake
similar on all equipment, the operating mechanisms
drum must be checked thoroughly at this time.
may differ greatly. The two most common types of
Brake linings that are riveted to the brake shoe
brake systems used on support equipment are
should be replaced if worn to less than 40 percent of
mechanical and hydraulic.
their original thickness. Also, the shoes must be
Mechanical Brake Systems
replaced if a rivet head is riding or is about to make
contact with the brake drum. If the lining is bonded to
Brakes operated entirely by mechanical linkages
the shoes, the remaining lining must be at least
from the brake pedal or lever are called mechanical
1/16-inch thick.
Brake linings that pass inspection for wear, must
rods, cables, or any combination of these.
be securely fastened to the brake shoes and free from
APPLICATION.--Equalizing brakes, so that all
grease and oil. Small grease or oil spots can be
brakes will be applied at the same time, has always
removed from the lining with an approved cleaning
been a problem of mechanical brake systems,
fluid. Do not use a petroleum-based cleaner, as it leaves
particularly on vehicles with brakes on all four wheels.
an oily residue on linings. Most shops recommend dry
For this reason, mechanical brake systems are seldom
cleaner solvent. If the lining is saturated with grease or
used for wheel brakes on powered support equipment.
oil, it must be replaced. The source of grease and oil on
They are used, however, for transmission or parking
the lining must be located and remedied.
brakes on these vehicles. In fact, most external
contracting brakes are operated by mechanical
WARNING
systems. Mechanical brakes will stay set better than
When using cleaners, you must wear safety
hydraulic brakes, so they are very good for holding a
glasses for eye protection.
parked vehicle.
In regard to support equipment, the major use of
m e c h a n i c a l b r a k e s y s t e m s i s o n n o n p ow e r e d
If the brake lining needs replacing, the brake shoes
e q u i p m e n t . A l m o s t a l l i t e m s o f n o n p ow e r e d
must be removed. Before the shoes are removed, the
equipment that require brakes are equipped with some
front and rear shoes should be marked so that they will
form of mechanical brake system.
be replaced in their original position. The brake shoes
may be removed by first removing the brake shoe
COMPONENTS AND OPERATION.--Figure
return spring and the anchor pin locks. (See fig. 2-23.)
2-33 illustrates a typical mechanical system for the
operation of parking or emergency brakes. This is
For good braking action, brake drums should be
simply a secondary means of controlling the rear wheel
perfectly round and have a uniform surface. Excessive
brakes. Variations of this arrangement are used to
pressure exerted by the brake shoes and heat developed
operate the parking brakes on some powered support
by their application often cause the brake drums to
equipment.
become out of round. Drums should be inspected for
distortion, cracks, scores, roughness, and excessive
A cable assembly is the main working element of
glaze, which lowers braking efficiency.
this system. A single cable connects the control lever to
the intermediate lever and spreader. From here, two
Light score marks can be removed with emery
cables complete the connection to the two rear wheel
cloth. When the surface becomes badly scored or out of
brake assemblies. Large portions of the cable slide
round, it may be reground in a lathe to a true and
smooth surface. Excessive grinding will remove too
within a flexible conduit. This conduit not only serves
2-26
