[TowerTalk] Erections (TWR)

[Tom Rauch]

>  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...

Bolt stretch and thread distortion is what keeps the
headbolts tight in your car engine (unless you own a Caddy
with the very long bolts) and the lugnuts tight. As a matter
of fact the critical bolts in my racing engine are tightened
to a specific amount of stretch, rather than torque.

The bolts won't stretch or distort in a Rohn 25 enough to
lock threads, however. The metal in the legs is far too
soft. You still don't need a lock washer. Nor do you need to
excessively tighten the bolts, which mostly serve as pins.
There really isn't anything making the nuts back off. They
won't come loose unless the metal wears away from the joint
rocking.

> Fine threads require huge torques to get high axial loads.

Coarse threads require huge torques to get high axial loads.
Fine threads require less torque.

In almost any case, the cause of a bolt loosening is loss of
material under the fastener heads, excessive torque, or
under torque. A lock won't fix that. Once you overload the
bolt or material under the bolt, you often have a bigger
problem than having it a bit on the loose side.

A soft thin round material normally requires some type of
load spreading washer like a thick hard flat washer commonly
used on aluminum like cylinder heads. The washer would have
to be a tensioning type, like a "split ring" lock, if you
could not reach the point of thread distortion or bolt
stretch.

All of that, however, is totally unnecessary in Rohn 25. The
legs are weak and they are round, so the bolt and nut have
huge pressures in a small area with only modest torque. Even
fairly low torque flattens the leg in a small area and locks
the joint. If you tighten so much you distort the leg
outside of the bolt head and nut, you are grossly overdoing
it.

If you needed locking hardware or chemicals, I'm sure Rohn
would have supplied it with the bolt kits.

73 Tom