Chevrolet 70 1969 Series, Shop Manual

Page: 115 / 913

FRAME 2-6
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 maybe caus­
ed either by localized stress concentration, ex­
cessive bending moment, or torsional loading. The
sunburst type cracks are caused by high loads ap­
plied locally at-the mounting bracket or crossmem­
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 similar methods. The procedure
for repairing frame rail cracks is as follows:
1. Remove any equipment that will interfere
with workable access to the failure.
2. Locate the extreme end of the crack and
drill a quarter inch hole. (N O T E : It may be neces-
Figure 6— Scarfed (Tapered) Ends of Reinforcement
sary to align the frame and level the rails before
repairing the frame.)
3. Vee grind the entire length of the crack
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.
4. Weld with proper electrode corresponding to
the material of the basic rail.
5. Grind the weld smooth on both inside and
outside of rail or reinforcement, being extremely
careful to eliminate weld build-up or notches on
the flange edges. (N O T E : Care should be taken
when grinding not to reduce thickness of base rail.)
6. Quality and neatness of workmanship can­
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.
Buckled frame flanges should be straightened
using proper alignment procedures, then an ade­
quate reinforcement should be used (see frame re­
inforcement) or offending equipment remounted to
obtain an improved transition of loaded to non­
loaded areas. D O N O T W E L D H E A T - T R E A T E D
RAILS E X C E P T T O REPA IR A C R A C K .
Crossmember and Brackets
The repair of crossmembers may be accom­
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 crossmember, the crossmember 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 circumstances
should a cast bracket be welded to the frame side
rail.
REINFORCEMENTS
Review the discussion on analyzing causes of
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
CHEVROLET SERIES 70 -80 H EAVY DUTY TRUCK SH O P M A N U A L