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Case Study #18 — Fastener Failure Workshop

SUVs
 

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.

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