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".