On Saturday 28 May 2011, intermodal freight train 5MP9 was travelling from Melbourne to Perth when one of its locomotives (SCT class) experienced a catastrophic wheel failure near Fisher, South Australia. The locomotive did not derail but sections of the broken wheel damaged a traction motor and associated components. The train travelled about 1976 m after the wheel failed and caused some sleeper damage and four breaks in the rail.
What the ATSB found
The investigation found that a fatigue crack had initiated at a small indentation on the inside of the wheel rim and then radiated towards the flange and tread regions before the wheel completely failed.
The rate of growth of the fatigue crack was influenced by high in-service mechanical loading of the wheel.
Inspection and measurement after the incident revealed that the locomotive wheel had sufficient rim, tread and flange thickness and was not worn beyond its service life, however it was likely the crack was present at a previous visual inspection and was not detected.
What’s been done as a result
The locomotive owner and maintenance provider has advised that they have implemented a revised program for more regular wheel re-profiling of its SCT class locomotive wheels to remove surface stressors in the wheel rim.
The locomotive maintenance program has also been enhanced to include a focus on visual inspections to detect impact damage to wheels and scheduled ultrasonic testing of locomotive wheels after mid-life is carried out when they are machined to detect cracks that may not be visible to the naked eye.
Operators of locomotives that are exposed to high in-service mechanical loadings should be aware of the potential for wheel failure due to fatigue cracking and ensure inspection and maintenance programs include techniques for detecting and assessing wheel defects with the potential to lead to fatigue cracks.