Strangely crunchy Sells Z-axis

Recently I fitted 608-bearing constraints near the top both of the Z-axis threaded rods on my modified Sells Mendel, in case I could gain any small additional increase in accuracy by stabilising them, and thereafter a strange knocking/crunching noise has been coming from the Z-motor bracket whenever it's in motion, seemingly centred on the pulley 608 bearing, yet as far as I can see from outside, nothing has come loose, there are no metal filings appearing around there and the Z-axis continues to function normally noisily.

Video on Vimeo (16 seconds long)

The noise continues even if I remove the Z-rod constraints, and I've tried adding a few drops of oil on top of the bearing hoping they would seep through the metal shield, though I can't see that any has been lost anyway.

I'm glad that it still seems to be printing OK, but a bit puzzled and worried about the noises. This printer has gone through about 6 or 7 kg of plastic over the last 1.5 years and is still on its original Z-belt, which looks just fine, so I'll be very surprised if the 608 bearing has worn out in some way, but perhaps it had a rare manufacturing defect (I do seem to be jinxed for that sort of thing).
I'd rather not have to spend hours unnecessarily disassembling my whole X-Z axis complex, so wondered if anyone has a better guess than me as to what's going on here.

tl;dr does anyone recognise the noise in that video?

Edited 1 time(s). Last edit at 09/18/2012 01:49PM by 4ndy.

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[engineeringourfreedom.blogspot.co.uk]

Sounds like a bum bearing to me, although I can only speculate as to why it would have failed. Spilled liquid and rust? normal wear and tear (prolonged side loading for a bearing meant to be loaded against the axis of rotation)? mechanical trauma (over torqued nuts, dropped tool, etc.?)

The bearings most folks use around here aren't what might be called expensive, so perhaps its time to consider doing a bit of replacement maintenance?

A few people on IRC, including ScribbleJ who designed the part, said that z-rod constraints are just A Bad Idea, and don't actually have any benefit. I think I might have slightly damaged a bearing by putting those on.
While removing them doesn't stop the noise entirely, it does reduce it, so I think I'll leave them off from now on and try my best to replace that bearing without losing my x-levelling whenever I have time to.

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[engineeringourfreedom.blogspot.co.uk]

I've had a similar problem before, and I think it's got to do with the printed pulley and belt not meshing correctly. Perhaps the pulley is slightly off center.
I think the 'thunk' is from when the tensioned belt finally slips into place and it's somewhat like strumming a guitar.

My pulleys were always centred fine, but after my printer was making unbearably loud squeaks on the last print, I just tried changing out the Z-bearings and they seemed fine. The Z-axis seems to present some extra resistance at a few distinct heights just now, so I'm thinking maybe one or more of the Z-nuts and studding have had their threads worn down.
Time for even more dis-assembly.

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[engineeringourfreedom.blogspot.co.uk]