Dump Truck towing a Front-End Loader
The fasteners bolting on the trailer hitch of a dump truck broke.
The truck was moving a large front-end loader to a job
site on a flatbed trailer. When the failure occurred, the truck was
traveling at a considerable speed through a mountain tunnel. When
the trailer broke free, the instant loss of load sent the truck and
the trailer flipping and caused a spectacular multiple car collision.
This accident closed the only highway to a popular ocean resort destination
on the Friday afternoon of a three-day holiday weekend.
Fortunately, there were no deaths; there were many people that were
injured and received treatment and hospitalization.
About ten vehicles were involved in the accident, with damages ranging
from minor dents and scratches to severe damage. The highway was shut
down for more than eight hours; therefore, significant lost income was
sustained in the resort town.
The remains of the bolts used to hold the hitch to the truck were bent
and stretched out in the threaded area. The nuts were all-metal locknuts
with a plated appearance similar to the bolts. Markings on the hitch indicated
that flat washers were used. The fracture surfaces were about 45°,
typical for ductile failures.
The hex bolts were marked with six radial lines consistent with
SAE Grade 8 or ASTM A 354 Grade BD. The locknuts had
a dot at each chamfer corner, indicating a Grade C, and
the proper mating strength for the bolts. The flat washers
were also similarly plated, and were of the smaller SAE
diameter size. A metallurgical evaluation was conducted,
and it was found that the bolts and the nuts were of
normal material and hardness — matching the product markings.
The flat washers were through hardened, and of the proper
material. An analysis of platings found zinc with yellow
dichromate on the locknuts. Using a scanning electron
microscope, the bolt's fracture surfaces were evaluated.
"cups and cones"
normal for a ductile tensile failure. A trip to the company's shop
showed the hitch had been installed just that morning using a high-speed
impact wrench. The output torque was unknown.
The combination of slippery cadmium plating with the power and inertia
of an impact wrench was too much for the bolts. They were stretched to
nearly breaking when installed, and then pulled to failure in the tunnel.
This was proven on extra fasteners using this wrench.
Developing proper clamp-force demands proper methods. The use of
a calibrated torque wrench is one method, if precautions
are taken to prevent changes in the torque-tension relationship.
Direct Tension Indicators are another method, and account
for "real world" variables
typical of a shop. If these methods were used, it may
have been an enjoyable weekend.
As appeared in Fastener Technology