Variance of friction coefficient with surface finish 1

[pwduncan]

I am trying to find any information on how much the static and kinetic coefficient of friction vary with surface finish.

The dry static coefficient of friction for steel on steel is .8 and the lubricted value is .16, but I need to know the degree of variance when the surfaces of these materials are machined to a very smooth finish.

I am primarilly interested in steel on steel and steel on iron applications.

If anyone has any experience with this I would really appreciate your input.

 

[MikeMech]

I've heard that when steel is machined very very smooth, it can actually increase the static friction significantly due to suction or some such thing. Don't know if it's true or not...

Mark's Handbook says surface roughness influences friction coefficient. It gives sliding values for different finishes of hard steel on hard steel, lubed with mineral oil and oleic acid. Interestingly, there is not a directly proportional correlation: in some cases, u goes down when surface roughness goes up. The values given range from 0.095 to 0.378, finishes from 2 uin to 55 uin.

If it's critical, I'd run some tests. Coeff. of friction can vary widely due to many factors. From Mark's: "Coefficients of friction are sensitive to atmoshperic dust and humidity, oxide films, surface finish, velocity of sliding, temperature, vibration, and the extent of contamination."

Mark's also has values for rolling friction of steel wheels on iron rails. They vary nonlinearly according to speed from 0.242 at 0 mph to 0.027 at 60 mph.

 

[gjb1]

Lubricated Co-efficients of friction can be found from the Stribeck Curve (co-efficient of friction vs viscocity x velocity/pressure)and is dependant on difference between height of contacting surfaces and height of lubrication film. A very good book reference book is

Handbook of Tribology Materials, Coatings, and Surface Treatment Bharat Bhushan BK Gupta.

GB