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Case Study #8 — Fastener Failure Workshop

School Buses
 

Industry:

School System – Transportation

School Buses

Problem:

Every fastener used to bolt the exhaust systems of their school buses broke. This never happened in the history of their school transportation system.

Injuries:

None.

Damages:

Exhaust pipes were extensively damaged as well as some exhaust manifolds. Some undercarriages were also damaged by crumpled pipes. No estimates were given.

Observations:

All exhaust manifold stud-type fasteners had broken at a length consistent with the exhaust manifold thickness at the bolting holes. Chalky white residue was on the fracture surface of each stud, and some studs had a purple cast on the fracture surface along with the white.

Investigation:

According to the Supervisor of Vehicle Maintenance, mechanics had spent the summer break performing a major refurbishing of the buses. They replaced any visible fastener on the undercarriage or within the engine compartment. All hex nuts mounting the exhaust manifolds were replaced with new products. When refurbishing was complete, the vehicles were test driven. The fleet of 30 buses left in a continuous caravan from the maintenance garage on a predetermined route. After 15 minutes, one by one, starting with the first bus, the manifold studs broke, dropping the exhaust systems to the road. All 30 buses were disabled.

 

An investigation of the hex nuts showed they were not OEM, but an aftermarket all-metal lock nut that was gold in color. An independent lab determined that the base plating was cadmium with a yellow conversion treatment.

Conclusion:

Cadmium plating melts at a relatively low temperature and can cause liquid metal embrittlement. Here, it melted and diffused microscopically into the steel manifold studs, where it weakened the steel and caused the sudden failures. As the bus engines warmed up, heat was transferred to the nuts and the failure mechanism began.

Recommendations:

The OEM nuts for this type of application are usually made of brass. They can take the heat and you can remove them as needed. Plain steel nuts can be used in a pinch, but avoid common platings. Although more rare than cadmium, zinc plating has also been found to cause this type of failure.

 

As appeared in Fastener Technology International magazine.

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