REPAIR OF SEMI- TRAILER
Section I. UNDER CARRIAGE ASSEMBLY
1 / 16 inch ). To avoid sticking cam or cam roll over and
obtain maximum lining wear, oversize roller cam followers
The undercarriage is one complete unit that may be
should be installed when linings become approximately
removed from the trailer bed for air shipment or a unit
50% worn. Roller cam followers are available in four
replacement. It contains all of the running gear wheels,
sizes, each having been designed for a specific purpose,
brakes, brake air chamber, axle and springs.
7-2. Brake Drum Repair
Generally there is no repair to a brake drum. Reboring of
a. Standard-1.250 Inches. Standard rollers are used
brake drums it not encouraged due to the reduced
when installing standard lining with standard drums, "X"
strength of refaced drums. Under circumstances where
lining with 1/ 16 inch oversize drums and "XX" lining
refaced drums and oversize linings are used, precautions
with1/8 oversize drums.
regarding cam travel should be observed to present
sticking cams or cam "roll over". This condition prevails
when the linings become worn. The instructions relative
If the drums were previously refaced and
to the use of oversize roller cam followers should be
oversize rollers installed, the oversize rollers
7-3. Brake Shoe Replacement
a. The brakes are a type of two shoe brake signed
for use with power. The shoes are lubricated steel and
b. 1/8 Inch Oversize-1.375 Inches.
are mounted on individual ''anchor pins on an open
oversize rollers are to be used with the installation of
spider. To replace brake shoes, perform the following
standard liners with worn or trued drums which are 1/32
inch oversize. One-eighth inch oversize rollers should be
b. Remove wheel as described in TM 5-6120-250-
installed and the liners circle ground a few thousandths
less than the drum diameter.
c. Remove shoe return spring (1, fig. 7-l) and shoe
c. 1/4 Inch Oversize -1.500 Inches. Where "X"
return spring pin (2).
liners have been installed with 1/16 inch oversize drums
d. Remove lock ring (3), washer (4) and bushing (5)
and standard roller, these rollers should be removed and
from anchor pin (6).
1/4 inch oversize installed before the lining becomes worn
e. Tap out anchor pin (6) bushing (5) washer (4) and
to a point where the brake cam is no longer effective with
remove shoe assembly (7).
the standard rollers.
f. Unhook shoe web retainer spring (8)and remove
d. 1/2 Inch Oversize-1.750 Inches. Where "XX"
roller cam follower (9).
liners and 1/8 inch oversize drums have been used, the
g. Install new brake shoes in reverse order.
standard rollers should be removed when the liners
h. With the cam in the full release position, the lining
become approximately 50% worn and 1/2 inch oversize
should be circle ground 0.070 inches less than the drum
diameter. If 80% of the lining has not cleared up, the cam
7-5. Brake Disassembly
should be adjusted and the linings ground until 80% To
a. During an overhaul the following parts should be
contact of the drum diameter is obtained. The 80%
carefully checked and replaced as required.
contact must be continuous and in the middle of the
(1) Inspect spiders for looseness or sheared rivets.
(2) Inspect anchor pins for wear or misalignment.
(3) Inspect brake shoes for wear at anchor pin holes.
7-4. Use of Oversize Linings
(4) Inspect cam shafts and cam shaft bearings or
To compensate for material removed when refacing brake
bushings for w ear.
drums X (1 /16) oversize and XX (1/8" oversize) linings
(5) Replace shoe return springs at the time of
should be used. When oversize rigs are used the drums
overhaul or when lining is replaced.
should also be machined in increments of 1/16 inch radius