Hello,

The reason that different people use different values for steel on steel
friction is because the value varies so much in real life, based on the circumstances. the value of friction factor it depends upon number of factors one of these are the surface roughness (surrounding environment). For example, I took a look in Marks' Standard Handbook for
Mechanical Engineers in the chapter about friction, and found the following
coefficients of friction for steel on steel:
Static, "grease-free in a vacuum" - 0.78
Static, "grease-free in air" - 0.39
Static, "clean and coated with oleic acid" - 0.11
Static, "clean and coated with a solution of stearic acid" - 0.013

Dynamic coefficients for steel on steel seem to be based upon sliding
velocity:
Velocity (in/sec): 0.0001 0.001 0.01 0.1 1 10
100
Coefficient: 0.53 0.48 0.39 0.31 0.23 0.19
Now steel on steel " grease free in air " seems to be the most appropriate
condition which simulates the real site surface condition. Probably this is
the cause for this 0.3 value.
But allways remember it doesn't depends upon surface area because at micro level the actual contact surface area is one thousandth of the area seems with naked eye.

Cheers,

Saneh Gupta
Stress Analyst,
Fluor India, N.Delhi