Friday stumper: is the coefficient of friction between threads in this situation? 2

[cbrf23]

I'm running myself in circles trying to solve my own problem. Looking for some smarter minds than mind to slap me straight.


I have a nut made of stainless steel.
I have a bolt made of alloy steel which has been completely coated in a CVD coating which has a .05 CoF.

If I apply moly/graphite lube to the threads, which has a .075 CoF, what then is the CoF between the threads?

Originally, I was thinking it would just be the minimum value (.05), but I've gone and over-thought it and no I'm not so sure that is a safe assumption.

I think this is much easier to think of in another way:
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The CoF numbers provided are determined from an ASME standard that measures each material's CoF against plain dry steel.
In this standardized test, the lubricant (e.g. moly-lube or PVD coating) is applied to some object which is set on a plain dry steel plate, the plate is then slowly tilted until the object starts to slide.
The angle at which the object begins to slide is measured, and the CoF is derived from this angle.

Performing this test on a plain steel object with no lubricant, tells us that the steel has a .80 static CoF.

If I were to replace the plain steel plate in this test, with one that had the CVD coating (static CoF=.05) and I were to run the Moly-lube test again, at what angle would the moly-lube slide?

Is it going to hold on to the same angle, corresponding to .075 CoF?
Or will it slide at an angle corresponding to the lowest static CoF, meaning .05 is the result?
Or, will slide at something less than that, some product of .05 and .075?

Is there anyway to calculate this other than experimental testing? (I need to use a number for theoretical modeling and I can't do experimental testing)

 

[RolMec]

from a purely practical p.o.v.:
I'd not propose to duplicate on lubricants.
If the CVD's doing the job, so be it, nice & square.
If you add another lubricant, rule of thumb is Last on is First engaged. The last added layer of coating defines the ruling CoF.
You may not want to compromise on correct tightening torque due to incertainty of CoF.

You might want to contact "Boellhoff" for support, I like them (but am not related) for their openminded knowledgesharing..at least over here.

http://www.bollhoff-usa.com/ae/us.php

Regards

RSVP