Bicycle Mechanics - Bearing cage disintegrated in Shimano BB7500

So I recently replaced the original bottom bracket (shimano bb7500) on my fixed gear bike. When I opened it up the non drive side bearing cage was completely disintegrated, that is all the teeth were gone and only a flat ring remained, almost looked like a washer. The drive side however remained intact. The bb shell was full of black crud - probably dirt mixed with packing grease. The bottom bracket itself is old but was in a good condition and the ball bearings and cages were brand new and of good quality. I installed it about 4 - 5 months ago. I ride daily and sometimes in wet conditions.

My question is what would cause that to happen?
Are there any preventative measure I can take, since the original bb now has some pitting on the spindle?

That sounds like the bearing retainer (the "cage") was rubbing against something which wore it away. Assuming it was installed the correct way around, I've only seen retainer failures like that on BBs that have worked loose, thereby allowing the retainer to come into contact with something other than the balls. Was there any play in the BB before you took it to bits?

There are two likely causes of the cage failure.

The first is that it was installed upside down. This makes the cage drag so the balls rub on the fingers pushing it along.

The second is a loose BB adjustment. That cause the spindle to try to wedge down between the two lowest balls pushing the balls sideways, jamming them against each other and the cage fingers.

I lean toward the flipped cage theory because a loose adjustment would have caused similar wear on both sides.

Another possibility is corrosion, but I assume you've have seen some evidence of that, or at least a dry bearing.

BTW- in my experience left side bearings suffer more from corrosion because the right crank provides some protection against road spray. Howeer right bearings suffer more wear because the chain tension doubles (or more) the bearing load.

That sounds like the bearing retainer (the "cage") was rubbing against something which wore it away. Assuming it was installed the correct way around, I've only seen retainer failures like that on BBs that have worked loose, thereby allowing the retainer to come into contact with something other than the balls. Was there any play in the BB before you took it to bits?

There was no play when I installed the original bb. I am a bit paranoid about play in the bb so I check it every so often - I had a cartridge bb develop some play after about 3 months which really disappointed me because it was a dura ace bb as well.

There is a chance that I might have installed it upside down. And with the added accumulation of water and grease mixed with dirt it just ground through the metal.
Now if I did install it correctly - what would be the evidence of corrosion? I tossed the bearing cages, but I still have the cups and spindle. Cleaned them up and the metal seems to still be shiny and no visible rust.

Now if I did install it correctly - what would be the evidence of corrosion? I tossed the bearing cages, but I still have the cups and spindle. Cleaned them up and the metal seems to still be shiny and no visible rust.

The action of the balls keeps the tracks shiny but the surrounding area would show some signs of rust or at least darkening of the metal. If all is bright and shiny and there was still grease in the bearing, I think you can safely rule out corrosion as a factor. Based on the added info, a flipped cage is the most likely cause.

ok, thanks. I'll have a closer look at the spindle again. Interestingly enough the pitting on the spindle was on the drive side like you pointed out.

Assuming you either use the current bb despite the pitting or replace it with a new spindle, I'd assemble it with loose balls and not use retainers. That way you can add at least one more ball per side to distribute the load over more area and there is no ambiguity as to which way to install them. Retainers are primarily for ease and speed of assembly at the factory or shop level, they are not an advantage in any other way. Also, use PLENTY of good quality waterproof grease. The excess may leak out for the first ride or two but it will form an effective external dirt and water seal.

That way you can add at least one more ball per side to distribute the load over more area and there is no ambiguity as to which way to install them. Retainers are primarily for ease and speed of assembly at the factory or shop level, they are not an advantage in any other way. .

While there's no argument that retainers are an assembly convenience, it's a myth that removing them allows for an extra ball. Most retainers on better BBs do not take any space and the ball count is the same with or without them. Moreover, there's no evidence to support the conclusion that more malls improve anything. Conversely, an extra ball can lead to a rapid failure of the bearing.

The vast majority of commercial and industrial bearings, including those made for the most demanding conditions do not hold as many balls as might otherwise fit. I doubt that's a planned obsolescence scheme by the likes of SKF, Timkin, and other top bearing producers.

With retainers or no, do not think than an extra ball will magically do you any good. Remember that it's not the balls that wear out, but the races, and more balls doesn't mean more life.

Actually ended up replacing the bb altogether because I was going to start using the bike on the rollers for the winter months, it was part of general cleaning of the drivetrain, cleaned the cog, the hub, the chainring and got a new chain. I might install the old one for when I start riding outside again, or go to a cheapy cartridge which I will probably end up tossing every year, the way I am going through them currently.
I ended up reusing the bearings though. The cages were fitted with 11 1/4" bearings per side so I ended up with the same amount and it seemed tight enough.

While there's no argument that retainers are an assembly convenience, it's a myth that removing them allows for an extra ball. Most retainers on better BBs do not take any space and the ball count is the same with or without them.
Perhaps bb and hub bearings with their small overall race diameters and relatively large balls don't have room for an extra ball if the retainer is deleted but headsets certainly do and they benefit from changing the ball spacing if the race is "brinelled". That said, I seem to remember older SR and Suguino bb's with 9 caged 1/4" balls/side holding at least one more ball without the retainer and not overfilling the ball track.

BB retainer bearings can have 11 OR LESS balls.