I looked at a used XStream 7mm ruby under a jeweller's loupe... a flawless surface though I guess you would expect that with a Moh's Scale of 9 (a diamond is 10).Upon disassembling it, we saw that the bearing had worn unevenly -- was even pitted, apparently through some grit, but we couldn't really be sure and found nothing that would cause that. It was replaced by the ruby and has been quiet ever since . . .
Since incoming air to the HP seat has just been through the sintered filter, maybe the pitting in the original stainless ball was due to point loading by the actuating pin vibrating and causing minute fretting or spalled areas which then flake off?
What's the difference between brinelling, spalling, and fretting?
When discussing bearing damage — especially in the context of linear bearings — three terms that come up quite often are brinelling, spalling, and fretting.
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