Until a long term solution could be devised and implemented it was necessary to undertake risk mitigation actions to ensure the the fleet was running without any risk to passengers safety. This was possible by establishing a crack inspection regime. Ultra-sound Inspection trials were set up to determine the minimum detectable crack size.

Simultaneously, using samples of stress measured during the on-track testing and incorporating stress factor to account for cardan shaft ageing, it was possible to calculate an inspection interval to guarantee detection of a crack before a bracket failure.

The safety of the fleet was increased further by introducing a polished weld toe detail. This improved the situation in the following ways:

  • it removes weld toe material (this is inherently poor quality material with many defects and flaws that make it susceptible to fatigue failures).

  • it repositions the maximum stress away from poor quality weld material and into good quality polished parent material. Thus the initiation of crack growth was less likely.

  • it increases inspectability of the bracket (smaller cracks could be reliably detected thus cracks can be repaired sooner and the safety of the fleet is increased).

It was difficult explaining that removal of material from a critical section would result in a fatigue strength improvement but the method was endorsed by supporting finite element analysis and expert papers. The detail was easily and quickly introduced by a drilling and polishing operation.

Joints in the cardan shaft wear with time and increase the amount of out of balance force acting on the traction motor mounting brackets. Our analysis and calculations showed that we needed to decrease the inspection interval to accounted for an ever increasingly aggressive fatigue environment.
It was important to monitor the cardan shaft loading by continuous testing of cardan shafts on a balancing machine.

The problem was exacerbated when a new risk due to heat damaged rubber motor mounts was identified. Additional stress factors were introduced to account for the new risk and the inspection period was reduced accordingly.