----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
>
> > Rohn specifies fine threads on their tower bolts (at least for Rohn
> > 25), but no lockwashers. Presumeably this is because the nut on
> > a fine thread bolt has to turn farther to lose its preload and come
> > loose.
>
> Fine threads can tolerate a lower coefficient of friction without come
loose
> due to the axial load. In short: slope of threads has to be less than the
> friction coefficient.
>
It's helpful to visualize the physics if you unwind the screw
threads into an inclined plane. The fine thread looks like
a long gradual inclined plane, whereas the coarse thread
looks like a steep and short inclined plane. For the same
downward force (e.g. axial preload), the fine thread results
in a larger force component normal to the surface of the
inclined plane. The fine thread also provide greater
mechanical advantage for a given applied torque which
means the nut has to rotate farther before the assembly
loses its preload.
> I don't know what the torque specs are for Rohn 25, but the
> > steel tubing they use for the legs is soft enough that it will yied
> > slightly when you tighten up the bolts. In that sense, the tower leg
> > sort of becomes like a bellevue washer.
>
> And the bolt stretch, too...
>
Yes, there will be some of that, but I'd be surprised if there
was a lot in a tower leg bolt for Rohn 25. The legs are pretty
darned soft. I've never really pushed it, but I'd bet you'd
really deform the legs badly before the bolts twisted off.
> Probably... I ran across a spec for structural bolts in general and the
> recommended axial load (due to the nut) is supposed to be something like
50%
> of the ultimate strength (i.e. really, really tight).
>
> Fine threads require huge torques to get high axial loads.
>
Isn't it the other way around? Axial load is going to be proportional
to how far the nut displaces along the axis of the bolt. The nut
will displace farther (along the axis of the bolt) for a given amount
of wrench rotation with coarse thread than for a fine thread. If I
want to bust a bolt, I will have an easier time doing it (in terms
of applied force) cranking a breaker bar on a fine thread than with
a coarse thread. I will have to turn the breaker bar farther in the
former case, but the applied torque will be lower to get the same
amount stretch in the bolt. The sliding friction may be a bit
higher for the fine thread, but again the applied torque needed
to get the same axial load in the bolt will be smaller with the fine
thread due to its larger mechanical advantage over the coarse
thread.
> Indeed.. but bear in mind that the original design intent may be lost in
the
> mists of time, and materials (particularly things like threadlocking
> adhesives) have advanced in that time. Of course, do what the
manufacturer
> recommends "today" is sound advice (do what the manufacturer recommended
30
> years ago may not be such great advice).
Yes, I agree. Rohn still ships 25G sections without lock
washers, however (at least they did last time I bought tower
sections a few years ago) which I would take as a tacit
recommendation not to use lock washers.
73 de Mike, W4EF..............
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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