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Bottom Bracket Bearing Failure

Bottom bracket bearing failure. From https://www.flickr.com/photos/63373992@N07/14181039184 as of 2014/08.

Make and model of bottom bracket unknown. No reports of rider injuries. Service history unknown. No notes on how the failures were discovered.

Bearing is marked "6002" indicating 15 mm ID, which would be unusually small for a bottom bracket. It is also marked "TH Industries", which has made bearings using 6002 seals but really 6002-16, indicating 16 mm ID. TH Industries makes bottom brackets, so might have originally made the bottom bracket. The 16 mm ID almost certainly means it is a square-taper bottom bracket.

Bicycle bearings often fail first due to grit intrusion, second due to a combination of overload and misalignment — even if the bearings are aligned perfectly at rest, bicycle parts tend to be lightweight and thus flexible, leading to mis-alignement under load. A mis-aligned bearing has lower load capacity than a loaded bearing, so "alignemnt" and "rating" are not easily separated.

Although bearings are nominally "sealed", a more accurate term might be "shielded", as bearing seals are not typically rated for bicycle service, which can include grit, which tends to abrade seals, making them leak more easily; and splashes or submersion, which create pressures that force grit and water past the light-weight sealing. Basic principles of good seal design date back at least 100 years, but many new/current products do not follow good practices. See HERE for more.

Bearings used in bicycles are mainly of two types: cup-and-cone angular-contact bearings and deep-groove cartridge bearings. Cup-and-cone bicycle bearings tend to have relatively low load ratings for their size, but are designed to be tolerant of misalignement. Deep-groove cartridge bearings often have higher load rating but derate quickly under misalignment.

A trade-off between cup-and-cone and cartridge bearings is that the cartridge bearings can be used to destruction then the unit replaced. Cup-and-cone bearings are typically more expensive, in that damaging the cup or cone (in the case of a bottom bracket, the spindle) requires replacing a more-expensive part, e.g., the spindle. For this reason it is typically adviasble to periodically clean and re-grease cup-and-cone bearings.

A note at the page above says "I'm happy with this style of bottom bracket and get around 10,000 miles [16,000 kms] from a set of bearings." For comparison, Jobst Brandt reported re-greasing his cup-and-cone square taper bottom bracket at about 16,000 km intervals; would get about 2-3 years (≈ 30-50,000 km) service from a spindle before the cone was damaged and needed to be replaced; and got over 150,000 km service from the cups. Replacing spindles periodically is notionally expensive, but as square taper spindles sometimes fail catastrophically in extended use, periodically replacing the spindle may be good practice anyway.