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. |
