friction coefficient

[Manini]

Hi!
Could someone suggest me some formula to evaluate friction coefficient
- between two rough surface
- without lubrication
- with high pressure
Thank you
Manini

 

[Bbird]

Despite not a mechanical engineer myself I know any decent mechanical handbook will have tables on the friction coefficients for various materials surfaces.

I also seem to recall in school that the horizontal friction equal to vertical load times the friction coefficient. The high pressure therefore doesn't change the frictional characteristic but can alter only the magnitude of the horizontal pull.

Have the the technology advanced so much that the formulas are available and there is no need to know what material is involved?

 

[Manini]

Thank you Bbird for your answer
Yes, it’s true. For low force value (low pressure) friction coefficient is independent to pressure, but when the pressure is high, friction coefficient depend also to pressure. (I don’t know this correlation).
About material. I expect that correlation contain some material property and also some geometrical property of the surfaces (roughness). So I’d like to find some correlation like
Friction coefficient=function(load, material properties, geometrical properties)
Thank you again
By
Manini

 

[profengmen]

Not sure exactly what you are looking for. You can measure the coefficient of friction using the inclined plane method. The friction is determined by placing a piece on material on a surface. Raise the surface untill the piece slides, then measure the angle of the surface. The static friction between the two pieces is the Tangent of the angle.

W sin È = µN

where

W= weight
È= inclined angle
µ= static friction
N= Normal force (equal to the weight for equilibrium, therefore dividing both sides by "W cos È" gives us "tan È = µ" )

 

[Sparweb]

These coefficients are determined by test. Surface conditions of two materials can vary widely, too, and a test is about the only way you'll get good numbers for the paticular materials, finishes, and lubricant combinations you're considering.


STF

 

[Speedy]

Having said all that, and I agree about doing experiments........Is there any good source of coefficients friction values out there....even just 'ball park' figures?

It is one of those things I always get stumped on when doing calculations. e.g. does one use 0.05, 0.1 or 0.2, it has a huge effect on results.

 

[GregLocock]

Easy.

0.05 is real slippery for sliding surfaces, 0.2-0.4 is normal, and 1 is decidedly sticky.

The problem is that there are so many gotchas that if your design is at all expensive or critical then you HAVE to do the test - the one I particularly liked in another thread was the difference that the normal oxide layer on steel makes. In my own experience we discovered interesting things about the effect of water, oil, and a combination of the two, on the friction of polyurethane.





Cheers

Greg Locock

 

[ivymike]

if your design is at all expensive or critical then you HAVE to do the test

...or find someone familiar with your application who has "done it all before" and can tell you what value to use. Preferably someone (or something) within your organization, like a design guidelines database.

 

[Manini]

Thank you friends
The problem is more clear
By
Manini