GMC 4000 Series Service Manual page 111

Sec. 2
Page 104
FRAME
The straight crack will normally start from
the edge of a flange and progress across the flange
and then travel through the web section toward the
opposite flange of the same rail. This may be caus­
ed either by localized str ess concentration, ex­
cessiv e bending moment, or torsional loading. The
sunburst type cracks are caused by high loads ap­
plied locally at the mounting bracket or crossm em ­
ber whose attachment is not sufficiently adequate
or is not securely fastened to the side rail.
In either case both types of cracks may be
repaired using sim ilar methods. The procedure
for repairing frame rail cracks is as follows:
1. Remove any equipment that will interfere
with workable a ccess to the failure.
2. Locate the extreme end of the crack and
drill a quarter inch hole. (NOTE: It may be neces-
Figu re 6 — Scarfed (Tapered) Ends of Reinforcem ent
G M C S E R V I C E M A N U A L
sary to align the frame and level the rails before
repairing the frame.)
from the starting point to the quarter inch hole at
the extreme end. Using a hack-saw blade, the
crack should be opened (1/16") to allow complete
penetration of the weld.
the material of the basic rail.
outside of rail or reinforcement, being extremely
careful to eliminate weld build-up or notches on
the flange edges. (NOTE: Care should be taken
when grinding not to reduce thickness of base rail.)
not be overemphasized. After the repair is com ­
pleted, the repaired area should be painted and
should not be readily discernible from the unre­
paired area of side rail. Use a copper spacer be­
tween the flanges of cracked base rail flanges and
reinforcement flanges and repair each flange sep­
arately as the flanges must react independently to
prevent localized stress concentration.
using proper alignment procedures, then an ade­
quate reinforcement should be used (see frame
reinforcement) or offending equipment remounted
to obtain an improved transition of loaded to non­
loaded areas. DO NOT USE OXYACETYLENE FOR
WELDING FRAME RAILS.
Crossmember and Brackets
plished if the damage is not extensive. Crossm em ­
ber mounting flange cracks may be repaired in the
same manner as side rail cracks; however, the
weld bead should be built up to provide a good
smooth radius. If extensive damage is incurred to
a crossm em ber, the crossm em ber should be re­
placed, using bolts rather than rivets. All cast
mounting brackets that are damaged should be re­
placed as it is not practical to weld a cast bracket.
In the event that a frame crack appears in the area
of cast bracket, the bracket must be removed
while repair is made. Under no circum stances
should a cast bracket be welded to the frame side
rail.
failures before applying reinforcement. A common
misconception in the past was to patch a cracked
frame. This is incorrect; reinforce the failed area.
A reinforcement must be large enough (approx. 30
inches) to provide adequate stress relief from
rapid changes in section modulus. For this reason
it is extremely important that all reinforcement
ends be scarfed to change section modulus as grad­
3. Vee grind the entire length of the crack
4. Weld with proper electrode corresponding to
5. Grind the weld smooth on both inside and
6. Quality and neatness of workmanship can­
Buckled frame flanges should be straightened
The repair of crossm em bers may be accom­
REINFORCEMENTS
Review the discussion on analyzing causes of