Coefficient of friction

[vscid]

I have a chucking mechanism inside a horizontal pipe.
The chuck has 2 fixed and 1 moving feet. The fixed and moving feet have different materials. If I am trying to find the frictional forces between the feet and the pipe, what coefficient of friction do I consider? Is there a way I can find the average CoF of the 3 materials?

Thanks!

 

[gr2vessels]

Try this link from google;- this is the first one of thousands of hits...

http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm

Cheers,
gr2vessels

 

[MintJulep]

The link given by gr2vessels is the nice, clean and pretty story of friction that they taught us in high school physics class.

Unfortunately, that model for friction is far too idealized for real-world applications.

Dig a little deeper on the roymech site and read the "disclaimers" towards the bottom.

As is often the case, if you really need to know the holding power of your chuck then you'll need to test across the range of conditions that it will really be used in.

 

[cbrn]

As MintJulep says, where i work we experience similar problems when introducing friction in assemblies with moving parts. The most critical example is with the slide bearings of a valve's trunnions.
So, our Company has performed, through the years, lots of laboratory tests under controlled conditions, which of course are NOT "real working conditions".
But now we at least know the "lower bound" and the "upper bound" the coefficient of friction can reach depending upon the application.
Similar job can be done by yourself, with some more approximation, by choosing upper- and lowe-bounds coming from known material pairs tested by manufacturers or independent laboratories. YOUR particular pair of materials may not be listed anywhere, but you will always be able to find "neighbours".
If you are calculating a braking device, assume a lower friction; if you are calculating an actuating device, asume an higher friction...

Just my 2-cents...

Regards

 

[tbuelna]

vscid,

Establishing an accurate (and consistent) value for Mu is like try to catch a greased pig- It's not easy and the results may leave you embarrassed!

Coefficient of friction (Mu) values can vary greatly, depending upon things like surface finish, materials, temperatures and lube conditions. And static (stiction or breakaway) friction values will always be much higher than sliding friction values.

Since you have a 3-point (expanding?) chuck constrained inside a pipe, I would propose that the fixture would not experience slippage until the static friction force at the contact point interface with the highest Mu is exceeded. All three contact point normal forces should be in equilibrium, and friction is simply Fn x Mu. To be safe, I would apply a FoS of at least 2.0 to any published Mu values you might find.

Good luck.
Terry

 

[PeterCharles]

Years in engineering has taught me that friction is anything but simple.
I generally prefer to think not of F = mu x N, but F = ff x N where ff is the "friction factor" for the application being considered, and is best found by carrying out practical tests the mimic the application.

 

[GregLocock]

...particularly in the presence of liquids or dirt. You'll probably end up with quite a large range of values, your design will need to work at both extremes.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.