[TowerTalk] Friction (was Mast Clamps)
art.boyars at verizon.net
Fri Mar 3 17:13:52 EST 2006
In the long and useful thread about mast clamps that slip under torque, several contributors have discussed the friction between the clamps and the mast. I'll point out that the linear relationship for friction force F=mu*N is only a model (an approximation) that works pretty well in most cases -- but not in all cases. Indeed, we commonly hear about "static friction" and "sliding friction", because the simple linear model neededd a little modification. I've heard some Mech E's mention "sticktion" (sp?), and it sounded like they meant something other than static friction.
I recall reading about race cars that cornered so well because they achieved mu>1, which was said to violate the linear model.
And, for sure, if the mast clamps have those groovy little cuts to dig into the mast, the "friction" is not going to be linear with the clamping force.
We use linear models lots of places because: a) they are accurate enough; b) we can handle the math.
And if you'll indulge me, here's a parallel example from the Real World (electricity). Perhaps you have seen Ohm's Law in "point form": is J=sigma*E. (It's the same thing as I = (1/R)*(V): current density = conductivity*Electric field.) Well, in actuality (as I read somewhere many years ago), this Law is not true for all materials. It does holdsfor a broad class of conducting materials, and they are called ... Ohmic conductors!
Back to my towerless lurking.
73, Art K3KU
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