Coefficient of friction aluminium on aluminium

[Cody87]

Hello I'm new here and to be quite honest not sure if I'm posting this in the right section, so apologies up front if I have this wrong.
Basically I was wondering if anyone has any idea on why the dynamic coefficient of friction for aluminium on aluminum is greater than the static coefficient of friction?
Values given here http://www.engineershandbook.com/Tables/frictioncoefficients.htm are 1.05-1.3 for static and 1.4 for dynamic.

 

[Simon Bridge]

I'd imagine that different test methods were used.
Imagine how you would go about verifying these figures.

Note:
Al is a soft reactive metal
in a static test, the two samples would be cold. In a dynamic test the friction would heat the two samples, making them stickier.
It's similar for some other substances on that list - something makes the surfaces sticker in dynamic friction tests than in static friction tests.

 

[Cody87]

Thank you for answering my question, this is very helpful, but my thinking on the subject was that surely once you overcome the initial static friction it should be easier to keep an object moving, not become harder. But as you have mentioned I don't know what test methods were used to obtain this value and after a couple of days of searching this is the most logical reason I've come across so again thank you very much.

 

[Simon Bridge]

The coefficient does not have to be constant... it may be initially easier, but become harder (or easier) as the situation changes.

Probably this will be easy to test out for yourself - Al is pretty cheap.

 

[Cody87]

Thank you for the reply.

 

[AlephZero]

My bet would be that "1.4" is a typo for "0.4".

As you say, for the same conditions it is logically impossible for the dynamic coefficient to be bigger than the static.

On the other hand, all the data in tables like this is more or less worthless, since it doesn't state the conditions that it applies to.

 

[Cody87]

I thought that might be the case so i checked multiple sites and they all gave the same value, but I have since spoken to one of my engineering lecturers and at first he was surprised that that the value was even above 1 for the coefficient of static friction and more surprised by the higher dynamic value of coefficient of friction. But he informed me that these values were probably most likely taken from experiments performed under ideal/vacuum conditions and this is what resulted in the higher than normal values for both as friction in a vacuum is greatly increased. He then pointed me towards another site that gave practical values - http://blog.mechguru.com/machine-design/typical-coefficient-of-friction-values-for-common-materials/