Coefficient

[Reciprotator]

First post with this user name - longtime beneficiary of this blog.

I am looking for subtle (or, perchance, significant) differences in coefficient of friction between (obviously) non-galling alloys of steels running in dry friction.

Specifically, I am looking for any combination pair of alloys that have a high C of F.

Is there any data available, look-up tables, reference sources, SAE articles, etc.

Thanks,

AJ

 

[tbuelna]

What is your definition of a "high C of F"?

Also, your requirement for "non-galling alloys of steels running in dry friction" is unrealistic. Any combination of clean, dry steel sliding against clean, dry steel will naturally have a high Mu. But this combination of materials will also produce significant galling/smearing.

 

[racookpe1978]

Light pressure and high (initial) speed?

Or braking type high pressure down towards zero speed?

How many times in a cycle?

Does it need to repeat? Or is this a one-time emergency stop feature?

 

[Reciprotator]

Thanks for the responses. I agree, going in blind (with no testing) would always risk galling but there are pairs of alloys that are compatible and do not gall when running under light load dry friction - we have that track record.

We are looking for any pair of (assumed) different steel alloys that has a relatively high coefficient of friction (mu, C of F) than what we are using now. This is a dry clutch that is electromagnetically engaged from a slow relative speed to a, more or less, lock-up between the two steels. There is not much torque to hold and we have a relatively large radius of engagement, two surfaces that are perpendicular to the axis of rotation.

I hope that helps. I understand that there are dedicated clutch materials that would work better than steel-on-steel and a spray-on plasma coating or plating that could be used but usually those are formulated for a "slipperier", lower mu or to avoid wear.

I knew going in that this was an unusual request. Again, thanks in advance for any help to better optimize our "lock-up" mode of operation (some sliding permitted).

AJ

 

[Tmoose]

Could you provide an example or 2 of the non-galling pairs?

Do you feel you are on the edge of galling now, so cannot increase clamp force a bit to gain the increased holding power?

If the required torque is low, why do you need more friction?
Is the requirement for static or dynamic friction?
Would adding another pair of known disks get the extra capacity I think you are looking for?

 

[tbuelna]

Reciprocator-

I don't have any references for the specific CoF data you ask about, but here's a couple suggestions.

First, if you want a combination of materials for dry-steel-on-dry-steel sliding contact that will have the least likelihood of galling, relatively speaking, then I would suggest a free-machining steel (like 12L15) against a nitrided/ground/lapped alloy steel (like 4130/4140). The free-machining steel surface is resistant to galling and the nitrided/ground/lapped steel surface is very hard and smooth.

Second, if consistent static and dynamic friction characteristics are of concern then a good combination of materials is dry steel on Vespel SP-21. This combination would provide a very consistent Mu of around 0.30 with low wear and no galling.

Third, not all "spray-on" coatings produce "slippier" characteristics. A combination of steel and a tungsten carbide coated surface would provide a high CoF with reduced galling/wear.

Hope that helps.

 

[Reciprotator]

tbuelna,

I think these recommendations are worth checking out / pursuing. This is the exact type of information / recommendation I was hoping would turn up. If these are materials / coatings are a practical option, we will try them out.

Thanks,

Reciprotator (that is the intended spelling)

 

[mrfailure]

You may want to consider the following anecdote as a word of caution in dealing with such an application: I once had a customer who used a non-galling "self-lubricating" alloy as a piston in a fill-cylinder used in a consumed product application. However, that did not prevent galling from taking place with cylinder contact that generated metallic particulate which got into the product. Our subsequent friction testing generated a fair amount of particulate and an increasing CoF as testing progressed and the component wore.