Friction: Static vs Dynamic

[dshaffer1001]

I'm doing an analysis of two mating parts, in which friction plays a prominent role. Both parts are aluminum.

All the data sources I can find show that the static friction coefficent for aluminum-on-aluminum ranges from 1.05 to 1.35. However, the dynamic coefficent is given as 1.40.

Here's a typical source:

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

How can this be ? I always thought that the static coefficent had to be greater than the dynamic, i.e., it requires less force to move an object once it's already in motion. If the dynamic coefficent is higher, wouldn't it effectively BE the static coefficent ? Somebody please explain how this works !

 

[MotorVib]

I think that there is something is worng. I would think that something is worng. Dynamic coefficent is always less static friction coefficent.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[BobM3]

You're (or the link - I didn't check it) is off by one decimal point. Static values would range closer to .10-.135.

 

[BobM3]

and, continuing my last last post, the dynamic friction coefficient is less than the static coefficient.

 

[CdotS]

Friction coefficient (FC) can vary as a function of applied load, speed and the environment. I have measured FC for aluminium against steel and also against itself. For Al-steel couple, FC varied from 0.4 to 0.6 whereas Al-Al 0.4 to 0.8, depending on the operating conditions.

The static FC may be lower if it slides on the oxide layer that exists on all aluminium surfaces under low load and speed conditions. In other words, the contact is essentially between oxides of aluminium. Once the rubbing starts, the oxide layer is broken leading to metal to metal contact and thus the dynamic FC is higher.

In any case, it seems odd for FC higher than unity. FC at 0.1 and 1.0 seem to be out of range for dry sliding of Al under ambient conditions.