Re: Re: Regarding friction factor

>ask your nearest
>precision machine shop to show you their gauge blocks.
I've seen this. My uncle, an ME (Cornell, 1930)to whom I owe more than I can possibly repay, showed me this with a set of jo-blocks once. He said it was atmospheric pressure holding them together once they'd been wrong. The surfaces were so flat that air was excluded from the interface when they were wrung together, so you had a force equal to atmospheric pressure times the area holding them together. The surface film on the blocks allowed them to slide so you could get them apart. I think if it were a weld they wouldn't slide and the block surface would be ruined once they were disassembled.

>I feel that the biggest problem with frictional coefficients is their
>unpredictability and the difficulty in arriving at a suitable safety
>factor to apply to an experimentally determined value.
Your rule of thumb is about right. The one thing I know for sure is that friction inevitably acts against the brilliant idea. Friction always opposes motion except when you need something to stay in one place.

>I do not think that single experiments are the right way to estimate
>design values for frictional coefficients.
Point taken. In fact a 'design value' doesn't exist, anyway. The best you can do is try to bound frictional effect with high and low values. Never depend on a particular friction coefficient to produce slippage or fixity--both friction coefficient and normal force are always changing.

>Reliance on accuracy is, in my opinion, dangerous in some
>circumstances.

I think I see your point, although I'd quibble a bit. Reliance on accuracy never endangered anyone, but the presumption of accuracy in the face of uncertainty is the chief ingredient of disaster.

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