[TowerTalk] Friction (was Mast Clamps)

[K8RI on Tower talk]

> 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 just dug out my old Calc based physics book trying to find the names and 
formulas.  When it comes to sliding somehting it is a two stage problem as 
it takes more force to start the movement than it does to continue the 
movement. Of course you can use calculus . It's amazing. I don't think I 
could now answer enough of those problems to even pass the course and I have 
a math minor.  I never had to use any of the math since I graduated.

>
> 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.

Just think of it as a lathe with a very wide cutting bit.

>
> 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!

And if you are working with DC.

One of our exercises was to calculate the average speed of the free 
electrons in a conductor of a given size for a given current.

I couldn't do a derivative or intergral today if my life depended on it.

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com



>
> Back to my towerless lurking.
>
> 73, Art K3KU
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