Industry:
Automobile Manufacturing
Problem:
Wheel lug stud failures for a limited time were prevalent on one
specific
imported vehicle. The wheel studs would break off when the nuts were being
loosened or retightened during routine tire rotations, and some studs
broke while the vehicles were being driven. In this incident, the right
rear wheel came off at approximately 55mph (88.5kph). These failures were
the direct cause of death.
Injuries or Fatalities:
A young, female surgeon lost her life when the studs failed. The
vehicle
flipped end-over-end 5 times before coming to a stop. The driver was tossed
violently within the vehicle, although the seat belt was fastened. She
was pronounced dead at the scene.
Damages:
The vehicle was destroyed during this accident; however, as traffic
was
very light at that time of day, no other vehicles were involved.
Observations:
On the wheel that came off, four of the six studs were broken, and
the
nuts stripped on the other two. The products that fractured appeared to
be fatigue failures. The wheel assembly has the wheel first placed over
the wheel studs, and then a plastic decorative wheel cover is installed.
Lastly, chrome-plated wheel nuts are installed. These nuts are domed style
top design, and with a conical bearing surface beneath the hex. This conical
surface contacts the plastic cover, and then clamps the wheel in
place.
Investigation:
In coordination with the law firm retained by the husband of the
deceased
driver, an extensive investigation was conducted. The National Highway
Traffic Safety Administration became involved, as this was the first death
after a number of complaints. The vehicle's manufacturer cooperated
by supplying answers to an Official Government Inquiry. Of note, there
were 893 warrantee claims for wheel stud replacements out of an estimated
vehicle population of 33,000, about 2.7%. Complicating the situation was
that the design characteristics of the studs and nuts changed several
times during the failure period. Rather than keep the same part number
and issue a revision, the products changed part numbers each time. Further,
with each change to the fasteners, the torque values stayed the same,
whether it was a steel or aluminum wheel. Lab testing showed mostly fatigue
cracking. Hardness varies on the studs and nuts depending on what part
number/engineering change was being checked. The specific stud and nuts
that led to the death were nearly identical in hardness.
Conclusion:
There were too many variables for one torque value, and the plastic
cover
eliminated a solid joint. Connections like this require friction at the
threads and bearing surface. Softer internal threads than the studs could
have accomplished this; without the plastic cover.
Recommendations:
None were requested or offered. A pre-trial, out-of-court settlement
ended
discussions. The manufacturer has changed its ways and remains
in competition.
As appeared in Fastener Technology
International magazine. |
