Galling/bearing materials choice?

I can use any two of the following for a medium-duty sliding bearing in an oxygen environment. Keeping cost and availability in mind, which would be
the best pair to use both as a low-friction bearing and to avoid galling (the cheapest and most available pair which would work reliably would probably be the best choice)?
I don't know much about choosing metals, there may be an obvious choice or two in there, and if such exists that's what I'd like to know - please don't omit the simple answers, I am a newbie at this!
Quantities involved are small, so availability of small quantities at reasonable rates is more important than raw cast.
Brass (any) Tin Bronze (but not Aluminium Bronze) Nickel 200 Copper 102 Beryllium Copper Monel 400 Monel K-500 Silver Inconel MA-754 Haynes 188 Haynes 242 Hastelloy C276
This selection is from ASTM manual 36 - I haven't heard of some of them before.
Thanks,
--
Peter Fairbrother


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don't
How about PTFE ?
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A few thoughts. Avoiding galling is a bit of a black art. Depends on load. Can sometimes be avoided by increasing clearances (e.g. in threaded joints). May depend on finishing process (e.g. rolled threads better than machined).
What temperature? Your superalloys (Haynes, inconel, etc) may be OK at red heat, the brass won't!
There are lots of brasses and tin bronzes. Leaded bronzes are traditionally considered to be very good bearing materials, but they are not as strong as unleaded. To avoid having to sleeve the shaft, I'd start by trying to identify a suitable shaft material. Is there a good reason why ferritic or martensitic stainless steels are not on your list? Although I don't have oxygen experience I would have thought they would be compatible.
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Newshound wrote:

It's a sliding bearing application. Load and speed are not high. I had planned to ream the inside of the bearing - can't really turn it as it is only 6 mm inside diameter, I just bought a new reamer for the job - and to polish the shaft quite highly.
Would those be suitable finishes to minimise galling/ rubbing?
One other possibility might be to "coin" the inside of the bearing - ie get a hard highly polished tapered rod and force it in to shape it - except that's not quite "coining" as there is a sliding action, I don't know the correct name?

A few hundred centigrade - hot, but not that hot.

I had already sort-of decided on leaded bronze as the bearing material. Strength isn't really that much a concern, I can make it as thick as needed, but it will have to hold oxygen at 3,000 psi.
The choice of shaft material would then be the remaining problem - I guess the harder the better? It can't be lubricated. Which would be hardest?

Nope, not under these conditions. 440C comes closest, but is only good to 1,000 psi or so. Above that it burns, which would be dangerous.
Designing for oxygen service is also a bit of a black art, but the basic principles are either 1) to avoid sources of ignition - sounds a bit iffy, but often you have little choice as almost _everything_ will react with high-pressure oxygen, especially when hot, even teflon will burn - or 2) use some of the very few materials like sapphire that won't react, or 3) use materials that won't sustain combustion in the sizes and shapes involved (or all three!).
It's not really possible to reliably avoid ignition sources in this application, I don't know of suitable unreactive materials, so I have to go with the last, materials that will not sustain combustion.
If galling occurs then fresh surfaces will be exposed and/or small particles will be produced, and the material will likely ignite and the parts will be ruined; but the following materials won't sustain combustion, the flame will go out, and the fire will be containable. There are probably some others, but I don't know for sure, these are the only ones on the NASA list:
Brass (any) Tin Bronze (but not Aluminium Bronze) Nickel 200 Copper 102 Beryllium Copper Monel 400 Monel K-500 Silver Inconel MA-754 Haynes 188 Haynes 242
--
Peter Fairbrother


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On Sat, 05 Mar 2005 11:52:24 +0000, Peter Fairbrother

Can't help on the material choice other than an uninformed guess that a Tin Bronze bearing with a polished superalloy shaft looks like reasonable choice.
A comment that may be more useful is to suggest the superfinishing trick of forcing a polished steel or carbide ball through a slightly undersize bore.
Jim
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Not *so* uninformed, I'd go with both of these. Not sure I'd like to say what would be a safe clearance.
Steve (ex tribologist)
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load.
joints).
machined).
get
Swaging. Sometimes used to finish thinwall valve guides and conrod little end bearings. You force a hardened ball through the guide after sizing to within a few tenths of finish size. Your easiest route would be to buy a 6mm ball bearing and press it through. Reamers tend to cut oversize though so this would probably be no use after a ream. An off the shelf 6mm reamer could well produce a hole the best part of 0.0005" oversize. Internal grinding and honing are normally used to get very accurate sizes rather than reaming.
Another way of getting fine surface finishes at accurate size is roller burnishing. This is how the gudgeon pin bores in pistons are finished. I suspect 6mm is too small for this to be done though.
A bit of fine wet and dry paper wrapped round a split rod is a good enough way to polish out any high spots though and easy to do yourself.
-- Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
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Ceramic might fit your criteria but was not on your initial list of options. Is it totally out of the question?
Cliff.
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Cliff Coggin wrote:

No, not at all. I don't know much about engineeering ceramics though.
From a chemical pov Macor might be okay for instance, and alumina or silica would certainly be okay, probably zirconia too, but would they be strong enough? Aren't they brittle?
Also, what about their use as the shaft in a bearing with eg leaded bronze? Should be hard enough.
--
Peter Fairbrother


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options.
I can't answer the specific questions in terms of strength etc. but I do know that ceramics are used industrially in low pressure pumps and is being investigated for use in IC engines. Maybe a Google search would yield some results.
Cliff.
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