GMC 4000 Series Service Manual page 108

Listed under "Specifications" at the end of
this section are selected sizes of welding elec­
trodes for use in repairing frame side rails. Rec­
ommended current ranges for various electrodes
are given when performing either flat or overhead
welding. When welding SAE-1023 steel, type E-7011
electrodes are recommended with type E-7016
suggested as an alternate. To ensure permanent
frame repairs, correct material identification,
proper electrode selection, and professional weld­
ing techniques are required.
A N A LY ZIN G FAILURE CAUSES
This analysis is not intended to cover the
causes of all possible frame problems; however,
it should be of valuable assistance in preparing
complete, concise reports.
The purpose of this discussion is to emphasize
the fact that frame failure don't just happen, there
must be a cause or reason. An example of this
would be a vehicle involved in a collision. The rea­
son for failure in this case is apparent; however,
other failures can be encountered where the rea­
sons are not so apparent.
Frame failures can be classified by three types:
(1) Collisions
(2) E xcessive bending moment
(3) Localized str ess concentration
Failures caused by collision should be repair­
ed, using proper methods and reinforcements,
where necessary.
G M C S E R V I C E M A N U A L
Figure 3 — Fram e B e n d in g M o m e n t (Typical Straight Truck)
E xcessive bending moment failures are caus­
ed by overload, improper weight distribution, or
misapplication of the vehicle. E xcessive bending
moment failure will occur at different areas on
various types of vehicles; therefore, for easier
understanding, the effects of excessive bending mo­
ments will be discussed by type of vehicle.
TRACTORS
The maximum bending moment of vehicles
used in tractor service is in the area of the leading
edge of fifth wheel (fig. 2). Failures maybe caused
by overload, excessive fifth wheel setting, ex c es­
sive fifth wheel heights, poor fifth wheel install­
ations, severe operating conditions and severe
braking operations (inertia of certain loads), which
induce excessive bending moments in the frame.
These failures will start at edge of lower flanges
and progress across the frame flange and up the
web section of the frame rail. Instances may occur
where upper or lower fram e flanges buckle.
STRAIGHT TRUCKS
The maximum bending moment occurs in the
area near the rear of the cab on vehicles having
van or platform bodies (fig. 3). Failures may be
caused by overload or can occur when loads are
dispersed in diminishing quantities allowing the
balance of a load to remain in the extreme front of
the body.
In both tractor and straight truck operation
the highest tensile str ess is applied to the bottom
side of the lower frame flange. However, it must
Sec. 2
Page 101
FRAME