The lady with whom I now share a set of bicycle tools heard
a loud crack from her bottom bracket - no, from her
bicycle's bottom bracket yesterday morning on the way into
work, and on finding the bb rather loose and clunky decided
not to ride the bike much before we could have a good look
at it. We stripped down the bb last night and found eleven
bright shiny balls in reasonably clean grease on the
accessible side.
However in the fixed cup there were what seemed to be ten
and two half balls in black gunge, with only the smaller
half ball at all shiny. There were also many sharp fragments
here and there in the grease, which puzzled me, because
there didn't seem to be enough missing balls to go around.
Fortunately the bearing surfaces seemed OK and we cleaned
everything up and put two new sets of bright and shiny balls
in with bright and shiny clean grease and all is fine.
Later closer inspection revealed that most of the apparently
whole balls in the ground-up bearing were distinctly
conical, which accounts both for the blackness of the gunge
and large number of fragments of ball. Since the only shiny
surface remaining in that set was that of what (appealing to
what I can remember of O-level geometry) I suppose you might
call a spherical cap that had apparently come off one of the
balls, I presume that that was the first failure and that
everything else followed.
I am inclined to suspect that this is "something that
happened" as an essentially random failure of a single badly
manufactured ball, however I wondered whether anyone had any
supportable theories to the contrary. What can cause this
sort of failure? Presumably if there was hard enough grit in
there when it was last assembled (by an LBS... mutter) it
wouldn't do it any good. Can you cause this sort of failure
by having assembled the bearing too tightly? Is it caused by
not wearing a helmet?
I've never seen such badly damaged ball bearings before. The
worst I'm used to is a bit of pitting in bearings that
someone has run dry for a while. No names, no pack-drill.
My O-level maths teacher would have a fit if he were still
alive to catch me saying "smaller half".