Chevrolet 70 Series 1970, Service Manual

Page: 179 / 841

FRAME 2-6
Figure 5 —Types of Cracks
The straigh t cra ck w ill norm ally sta rt .from
the edge of a flange and p r o g r e ss a c r o ss the flange
and then travel through the web section toward the
opposite flange of the sam e r a il. T his m aybe ca u s­
ed either by lo ca lized s t r e s s concentration, e x ­
c e s s iv e bending m oment, or torsion al loading. The
sunburst type cra c k s are caused by high load s ap­
plied lo ca lly at the mounting bracket or c r o s s m e m ­
ber w hose attachm ent is not sufficiently adequate
or is not se c u r e ly fasten ed to the sid e r a il.
In either c a s e both typ es of crack s may be
repaired using sim ila r m ethods. The procedure
for repairing fram e rail cra ck s is as follow s:
1. R em ove any equipment that w ill in terfere
with workable a c c e s s to the fa ilu re.
2. Locate the extrem e end of the cra ck and
d rill a quarter inch hole. (NOTE: It may be n e c e s -
Figure 6 —Scarfed (Tapered) Ends of Reinforcement
sary to align the fram e and le v e l the r a ils b efore
rep airin g the fram e.)
3. Vee grind the entire length of the crack
from the startin g point to the quarter inch hole at
the extrem e end. U sing a h ack -saw blade, the
cra ck should be opened (1/16") to allow com plete
penetration of the weld.
4. Weld w ith p rop er electrod e corresponding to
the m aterial of the b asic ra il.
5. Grind the weld sm ooth on both in sid e and
outside of r a il or reinforcem ent, being extrem ely
carefu l to elim in ate w eld build-up or notches on
the flange ed ges. (NOTE: C are should be taken
when grinding not to reduce th ick n ess of b a se ra il.)
6
. Quality and neatn ess of workm anship can ­
not be overem p h asized . A fter the r ep a ir i s co m ­
pleted, the rep aired a rea should be painted and
should not be readily d isc ern ib le from the u n re­
paired a rea of sid e r a il. U se a copper sp a cer b e ­
tween the flan ges of cracked b a se ra il fla n g es and
rein forcem en t fla n g es and rep air each flange s e p ­
arately a s the fla n g es m ust rea ct independently to
p revent lo c a lized s t r e s s concentration.
B uckled fram e flan ges should be straightened
using proper alignm ent p roced u res, then an ad e­
quate reinforcem ent should be u sed (se e fram e
reinforcem ent) o r offending equipment remounted
to obtain an im proved tran sition of loaded to non­
loaded a r e a s. DO NOT WELD HEAT-TREATED
RAILS EXCEPT TO REPAIR A CRACK.
C rossm em b er and Brackets
The repair of c r o ssm e m b e r s may be a cco m ­
p lish ed if the dam age is not ex ten siv e. C ro ssm e m ­
ber mounting flange cra ck s may be repaired in the
sam e manner a s sid e rail crack s; how ever, the
weld bead should be built up to provide a good
sm ooth radius. If exten sive dam age is incurred to
a c ro ssm e m b er, the c r o ssm em b er should b e r e ­
p laced, using b olts rather than r iv e ts. All ca st
mounting b rack ets that are damaged should be r e ­
placed a s it is not p ra ctica l to w eld a c a s t bracket.
In the event that a fram e cra ck appears in the area
of c a st bracket, the b racket must be rem oved
w h ile rep air i s made. Under no circu m sta n ces
should a c a st bracket b e w elded to the fram e sid e
r a il.
REINFORCEMENTS
R eview the d isc u ssio n on analyzing c a u se s of
fa ilu r e s b efore applying rein forcem en t. A com m on
m isconception in the past w as to patch a cracked
fram e. T his is in correct; rein fo rce the fa iled area.
A rein forcem en t m ust be large enough (approx. 30
inches) to provide adequate s t r e s s r e lie f from
rapid changes in sectio n m odulus. F or th is reason
it is extrem ely im portant that a ll rein forcem en t
ends be sc a rfed to change sectio n m odulus as grad­
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL