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A couple of days ago my girlfriend's bicycle broke. The rear wheel was stuck. Tonight I had finally some spare time to have a look. The bearings had failed, so much so that the wheel must have spun for some time piecewise with the axle.

As a consequence, if you are picturing this right, the left nuts had tighten as that very hot place, whereas the nuts and cup on the drive side were completely loose (the axle had screwed itself to the left).

I don't know what caused this. I had replaced the bearings in the past, something I've done many many times with no problems. What I'm asking here is whether this level of damage is normal or the bearings are cheap. I mean: the rightmost bearing in the picture can be separated into a smaller (about 3/16) inner ball and an outer 1/4" shell!

This is the first time I used bearings from ebay, and would like to have your opinions.

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I've never seen bearings fail in that fashion. –  Daniel R Hicks Nov 17 '12 at 4:11
    
yeah very unusual, I've only ever sheered bearings in half in my experience –  jamiethepiper Nov 17 '12 at 10:43
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This covers roller and sleeve bearings, and raceway damage; it has a lot of pertinent information: skf.com/files/099926.pdf –  WTHarper Nov 17 '12 at 14:26
    
How long ago did you install them? It seems like you've been had. Maybe stay away from those eBay bearings. –  hillsons Nov 17 '12 at 19:36
    
@WTHarper I quickly skimmed through it, without finding an answer. It's very interesting though, thanks for sharing, +1 –  astabada Nov 17 '12 at 21:42
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3 Answers

Unfortunately I have seen this all too many times working as a workshop manager in a performance bike shop, Cheaper wheels and in certain cases some expensive japanese made ones use a cup and cone system which when correctly adjusted and maintained will give the same minimal rolling resistance and smoothness as ceramic catridge bearing wheel sets.

Unfortunately if the wheel is not maintained or correctly adjusted the cones work loose allowing dirt ingress and pitting of the bearings. This in turn if the wheel is continually used will produce the kind of bearing failure that you see in the image.

Whether this has been increased by the bearing quality I can't advise but I would say the only way to truly have a low maintenance wheelset is to move to catridge bearing hubs. My personal favourite are a british company called "hope" but these are performance wheels and therefore you will pay a higher price.

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But in this case the bearings aren't pitted -- they "peeled" apart in layers, like Russian nested dolls. This probably results from improper heat treating of the bearings, resulting in fairly distinct layers with different properties. –  Daniel R Hicks Nov 20 '12 at 13:06
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You want quality graded bearings. Settle for no less than grade 25. Lower number = better/more precise. Skip generic grade 300.

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Very useful comment, +1 –  astabada Nov 20 '12 at 14:09
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I've also never seen a bearing ball fail like that. In Jahaziel's answer to this question:

When do bearings need to be replaced?

he notes that in Honduras where he lives they sometimes could only get cheap Chinese bearings that "used to 'peel off' like an onion". Perhaps you got some of those? See also the response of "Mike" in this thread [1], who had a similar (albeit worse) experience with cheap bearings.

Good properly tightened bearings should last for many thousands of miles/kms, and usually races tend to show the damage more clearly before the balls do. I've never actually over-tightened a hub and tried to ride on it to see how it fails when it seizes (and I'm not saying you did that, but it's the only other explanation I can think of other than cheap bearings), but based on both the SKF paper linked above and this paper [2] and this analysis [3] it seems like ball bearings delaminating or fragmenting is not the most common failure mode, because none of them discuss it. Particularly in [3] you can see that the balls in seized bearings polished off a contact patch against the race rather than disintegrating.

[1] http://www.bikeforums.net/archive/index.php/t-23340.html

[2] http://www.elongo.com/pdfs/BearingFailures990519.pdf

[3] http://met-tech.com/failure-analysis-of-ball-bearings.html

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