surface roughness & friction 1

[191178]

Hallo everybody,
I have a little question: if I know the value of surface roughness (i.e value of Ra)can I derive the friction coefficient (like coulomb friction). I have to use some tables or formulations? Please show me how to find it(or the links to similar problems).
Thanks in advance.

 

[CoryPad]

Friction coefficient is not a direct function of surface roughness. For example, say you have a piece of polytetrafluoroethylene (Teflon) and a piece of rusty steel, both with the same R_a surface roughness. The PTFE piece has a MUCH lower friction coefficient because of its chemical properties.

Regards,

Cory

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

 

[EdStainless]

It gets even worse, different material pairs will give different cf, and the values don't 'balance'.

Rz, load, speed and the specific material are all factors.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[CdotS]

I agree with EdStainless on the factors that contribute to friction.
Rough surfaces generally offer higher coefficient of friction due to mechanical locking and need for the asperities (peaks) of one surface to climb over the other. This is true for the initial sliding conditions and may be different once the surface gets "smoothened out" due to wear.

 

[Tunalover]

Here's something to stir the pot. Does the value of the friction force vary with the contact area?


Tunalover

 

[eng1234]

Tunalover,

Interesting point to bring up. However I seem to recall way back in engineering basics (block sliding on a wedge) that it's only the weight of the block (or the normal force component) that is important and not the amount of surface are in contact.

 

[Tunalover]

eng1234-
Yes, I remember those simple mechanics problems. Then why aren't car tires as skinny as bicycle tires? Intuitively, a car tire that is twice as wide will stop a car faster will it not?




Tunalover