----- Original Message -----
From: "Michael Tope" <W4EF@dellroy.com>
To: <K8fu@aol.com>; <towertalk@contesting.com>
Sent: Wednesday, July 14, 2004 7:08 PM
Subject: Re: [TowerTalk] Erections (TWR)
> 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.
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...
Combine that springy
> preload from the slightly deformed leg with a fine thread bolt and
> you may not need a lockwasher. If you add a lockwasher, and
> use the same torque as without the lockwasher, you probably end
> up with less "squeeze" on the tower leg. Perhaps that "squeeze"
> is intentional and desired.
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.
>
> This is all idle speculation of my part of course :) Best to ask the
> manufacturer if you are not sure.
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).
>
> 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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