Re: Regarding friction factor

>why the value of friction factor varies from
>project to project and what are the main factors
>governs the value of fiction factor.

Friction varies with surface finish, surface cleanliness the presence or absence of lubrication or oxidation and to a degree with load (Unlike what we were all taught...) The actual mechanism (or so I was taught) is that friction results from microscopic pressure welds between surface asperities. The frictional force reflects the load required to break the welds. The more pressure the more asperities are brough into contact so the higher the friction force, so the friction coefficient is taken as a constant although it isn't. A tale I heard once (I never verified it) is that highly polished surfaces which are absolutely clean and flat will weld themselves in a vacuum. Galling in stainless steel is a form of pressure welding of this sort, so I'm inclined to believe the tale.

I've seen a study of friction coefficients for austenitic stainless against itself where the coefficients vary from 0.2 to 0.8. Actually when it galls it's a helluva lot higher than that. The high values reported for steel in some handbooks (around 0.7 I think) are for very clean and smooth surfaces, probably in a lab somehwere. The usual static values are around 0.2-0.3 for machined surfaces and 0.1-0.2 for kinetic friction. Your values of 4 or 5 aren't true friction coeficients--the highest I've ever seen reported are for rubber in concrete at around 1.3 under very special conditions.

A good way to estimate static friction is with a simple test. Find yourself samples of the materials in question mount one to a straight stiff plank and the other to a weight that will produce a contact pressure about what you expect in service Put the two sample surfaces in contact and raise one end of the plank until the weighted sample slips. The tangent of the angle of inclination will be the friction coefficient. You can try the same test with lubricants or water or finish the two surfaces and see the effect of surface condition. The results will probably be as good as you'll find in all but the most comprehensive handbooks.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw@skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)