[TowerTalk] 30 foot Rohn 25G calculations

[Roger (K8RI) on TT]

On 2/13/2015 10:25 AM, TexasRF--- via TowerTalk wrote:

Gerald, you hit the key, "Tight fitting bolts".  Having worked in 
industry for 26 years before going back to school, I was an "instrument 
tech" which essentially is a multi-tradesman without the dues.  When 
aiming for a long lasting joint we would use adjustable reamers, so the 
bolt was a snug fit, but could easily be tapped into place. The hole was 
reamed and the bolt had a fine finish.  The tightness of the bolt causes 
the tower legs to grip each other tightly.  For lack of a better term, 
there is a "sweet spot" at some torque.  Tightening beyond that point 
deforms the legs, reducing the contact area.  The contact area may be 
tighter with a stronger grip, but it is smaller and non uniform.  The 
grip is such that it allows the joint to move, or rotate perpendicular 
to the bolt leading to accelerated egging out. Also, as the leg bolts 
are all parallel the weakest direction is perpendicular to the bolts.
Using bolts that have to be, screwed in breaks the galvanizing as well 
as leaving an irregular shaped hole. ROHN specifically states not to do 
this, but to enlarge the hole to a snug fit using a taper pin punch, yet 
thy use galvanized bolts which by their nature is an irregular shape.

I use the taper pin in one bolt hole while easily inserting the bolt in 
the other.  Because it is tapered the holes need to be "adjusted" from 
both directions.  It's easier to do this on the ground so the bolt fit 
can be closely matched to both sections. It might take a few hours of 
additional prep, but do all the holes in the tower before starting the 
installation.  It will really speed and ease the tower erection.

Over tightening to the point of deforming can reduce the leg strength.
ROHN does not want the galvanizing inside the bolt holes removed or 
broken.  Getting the fit "just right" with a taper pin punch takes some 
skill.

Again, for maximum strength and resistance to "egging out", the holes 
must match and be a snug  fir to a smooth bolt.  This is a critical 
problem with Aluminum towers as there is a very fine line between too 
loose and too tight. Most tower alloys are not "hard". If the 
unlubricated bolt is too tight, it will gall.  Lubricating will also 
accelerate the egging out process

In some cases, the only way to salvage a joint is reaming the holes.  
The galvanizing is already gone, so reaming shouldn't hurt.    The 
sections should be fitted together, aligned, and then reamed to get 
maximum strength and alignment.  Reaming the egg shaped holes in one 
section and then the other almost guarantees the holes will not align well.

73

Roger  (K8RI)


> Joe, the over tightened leg bolts you saw were very likely someone's
> attempt to recover from an egged out leg hole situation. It won't work; only
> tight fitting joint bolts will keep the joints properly joined.
>   
> The cure for preventing egged bolt holes is proper tightness in the first
> place as John and others have reported.
>   
> To be fair, I have also seen Rohn 25G, 45G and 55G tower sections with
> egged out bolt holes, caused by the same fault, under tightened joint  bolts.
>   
> I have recovered a few of these sections by reaming the hole back to round
> and using an oversized tight fitting bolt. I doubt that Rohn would approve
> this  but a frugal Ham (like myself) would do it anyway.
>   
> I don't see any reason this "fix" can't be used on an aluminum tower
> section as well.
>   
> 73,
> Gerald K5GW
>   

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