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Folks,
Any root cause analysis of this problem must start with the location of the break. These axles have several shoulders machined onto them to allow for bearing locations, etc. As anyone familiar with stress concentrations can tell us, square corners at these shoulder locations are stress risers which invite crack development and further propagation (growth). I would assume the axle manufacturers have a designed radius at these locations but maybe some aren't using a generous-enough radius as to minimize the concentrated stress. Another possibility is that the axle radius is proper but the steel spacer used between the bearing and shoulder may not have a radius in the interface location that is more generous than the axle. This would result in a cutting action into the softest member of the interface, the axle, which would further result in crack initiation. Compounding this, the cantilever forces acting on the axle at the birdcage interfaces along with any heating/cooling cycle embrittlement just add to the possibility of a crack. And, just to help things along here, we reduce the axle wall thickness so that the critical flaw size necessary for crack initiation becomes smaller. Any additional high forces resulting from contact pretty much guarantee issues.
Inspection and proper birdcage maintenance along with insuring that stress riser points are minimized are a few of the things I do in an effort to maximize axle integrity. I am also not a fan of the ultralight axles for the reasons mentioned above but I understand the physics of keeping rotating weight as low as possible. I just have not been convinced that the reward is worth the risk.
Tim Simmons
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