Variation in wear of SST ball on Sapphire (non standard jewel bearing)
Variation in wear of SST ball on Sapphire (non standard jewel bearing)
(OP)
I have a small motorized microscope stage design I'm working on.
In it we have a couple of different types of hardened stainless steel (440C or 440F) balls/ball ends on sapphire bearing.
One type in horizontal plane is a simple linear plain bearing where the ball end slides across the sapphire pad. An initial proof of concept (PoC) had a little trouble where one sapphire pad seemed to wear the matching ball end, but we believe this was due to some roughness on the sapphire - and the action of the ball wearing makes logical sense.
The other type is in vertical plain where ball tips of ball screws push against sapphire pad. These balls rotate when the screws are turning, they also slide when the stage is moving in the other axis or potentially could be turning and sliding at the same time if moving in both axis. On the PoC this worked fairly well over thousands of cycles. Some of the running was done dry and some was done with a little grease.
We've now built a full prototype of the stage and are having trouble with the vertical bearings. The balls/ball ends are chewing up the sapphires after just a small number of cycles - maybe a few hundred. This is with just the sliding motion - no simultaneous rotation element. These bearings are running dry as we can't really seal the stage and our customer base aren't good with preventive maintenance so we don't really want to have to use lubricant.
Initially when using custom ball ends we noticed they were rougher than we'd specified, they were replaced with off the shelf balls of smaller radii but we still see the problem.
My research has taught me that Sapphire is anisotropic in terms of hardness, but even in the 'soft' direction it should be twice as hard as the SST on Knoop scale. Surface finish may also be a contributor but we call out a 2 micro inch finish on the Sapphires and are buying from a company that makes jewel bearings as well as optical windows.
We did do a crude test of scratching sapphires from the original PoC and the new prototype with a carbide scribe. The new sapphires scratched fairly easily and cleanly while the old ones just kind of grazed slightly & required more force to do so.
Any thoughts on what might be going on and/or how or what we need to specify to correct it?
Thanks,
(I'm posting here as the bearing forum didn't look particularly active.)
In it we have a couple of different types of hardened stainless steel (440C or 440F) balls/ball ends on sapphire bearing.
One type in horizontal plane is a simple linear plain bearing where the ball end slides across the sapphire pad. An initial proof of concept (PoC) had a little trouble where one sapphire pad seemed to wear the matching ball end, but we believe this was due to some roughness on the sapphire - and the action of the ball wearing makes logical sense.
The other type is in vertical plain where ball tips of ball screws push against sapphire pad. These balls rotate when the screws are turning, they also slide when the stage is moving in the other axis or potentially could be turning and sliding at the same time if moving in both axis. On the PoC this worked fairly well over thousands of cycles. Some of the running was done dry and some was done with a little grease.
We've now built a full prototype of the stage and are having trouble with the vertical bearings. The balls/ball ends are chewing up the sapphires after just a small number of cycles - maybe a few hundred. This is with just the sliding motion - no simultaneous rotation element. These bearings are running dry as we can't really seal the stage and our customer base aren't good with preventive maintenance so we don't really want to have to use lubricant.
Initially when using custom ball ends we noticed they were rougher than we'd specified, they were replaced with off the shelf balls of smaller radii but we still see the problem.
My research has taught me that Sapphire is anisotropic in terms of hardness, but even in the 'soft' direction it should be twice as hard as the SST on Knoop scale. Surface finish may also be a contributor but we call out a 2 micro inch finish on the Sapphires and are buying from a company that makes jewel bearings as well as optical windows.
We did do a crude test of scratching sapphires from the original PoC and the new prototype with a carbide scribe. The new sapphires scratched fairly easily and cleanly while the old ones just kind of grazed slightly & required more force to do so.
Any thoughts on what might be going on and/or how or what we need to specify to correct it?
Thanks,
(I'm posting here as the bearing forum didn't look particularly active.)
Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?





RE: Variation in wear of SST ball on Sapphire (non standard jewel bearing)
These guys pull sapphire from partial? melts, and can give you fully crystalline material, there may be others:
http://americas.kyocera.com/kicc/pdf/Kyocera%20Sap...
They are spendy, though.
RE: Variation in wear of SST ball on Sapphire (non standard jewel bearing)
RE: Variation in wear of SST ball on Sapphire (non standard jewel bearing)
On the balls, the original PoC used relatively small balls but the herzian stress was OK and it had OK life as far as we took it.
On the prototype originally we had larger effective dia ball ends (to further reduce herzian contact stress for some reliability concerns during shipping etc.) and they caused a wear problem.
We switched back to the same size balls as on the PoC, switched to fresh sapphires, and still saw the problem.
In both cases the balls showed little wear even under magnification.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Variation in wear of SST ball on Sapphire (non standard jewel bearing)
Translation of a steel ball end on a planar jewel would appear to be sparsely charted territory.
How do you feel about having the steel ball end rotate in a cup jewel, and having the planar backside of the jewel translate on a steel surface? That's the least worst thing I can think of right now.
... but you should be talking to application engineers at your sapphire supplier or their competitors, e.g. ITI, Bird Precision, Swiss Jewel, etc. They must have done something like your application already. If you are lucky, you will find someone who has failed enough to map out the portion of the solution space that's likely to bring success.
Mike Halloran
Pembroke Pines, FL, USA
RE: Variation in wear of SST ball on Sapphire (non standard jewel bearing)
We thought about some variation of the ball in sliding cup idea a while back, seems to have some issues but maybe we'll think about it again.
Thanks Mike & btrue.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?