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Square-Taper Spindle Multiple Failures In Flats

Square-taper Neco-brand bottom bracket. Spindle failure in flats. From https://www.flickr.com/photos/63373992@N07/12288502766 as of 2014/08.

No reports of rider injuries. Service history unknown. No notes on how the failure was discovered.

Darkest area of failure is often the oldest part. It appears the upper-left part is darkest and furthest up the square taper, either at or near where the square taper emerges from from the square hole in the crank.

This is a common area for square taper spindle failure: inside the hole in the crank, some of the loads are carried by the crank, but where the spindle emerges, all must be carried by the spindle. Further, the spindle is typically weakest where the flats are narrowest, so is stronger away from the crank.

Note the spindle has several cracks where it has not failed through. These might have been caused by the main failure, but more likely the spindle was also starting to fail at these cracks.