[TowerTalk] Re: Rohn TB-3 Warning

Tue, 28 Oct 1997 13:06:44 -0600

>>This may not be what you wanted to hear......but you were lucky.    Those
>>thrust bearing centering bolts were not intended to hold the vertical
>>weight
>>of a steel mast and maybe some beams mounted on the mast.    They are
>>simply meant to center the mast inside the "hole" of the thrust bearing.
>>
>>I have stacks of monobanders mounted on a 24' mast  (14' above the tower)
>>I use two muffler clamps above each of the two thrust bearings.   I mount
>>them
>>one on top of the other facing opposite directions.    The bottom muffler
>>clamp then puts the weight of the system onto the thrust bearing.    My
>>guess
>>is that the weight of the mast and etc could have been  ""released""  at
>>any
>>time and you could have had major damage or injury.    I 'm glad that did
>>not
>>happen to you.
>>
>>73,  Jerry Liley,  K0GUG,  Holts Summit,  Missouri

>This is an interesting observation.  As a Rohn Dealer, I have noticed that
>those bolts do not seem to be the right type to "bite" into mast and still
>keep the threads in good shape.  They are defineitely not "set screw" type
>bolts.  I have never seen any mention of any of this in any Rohn literature,
>however.  So, I was wondering how you came to the conclusion that the Rohn
>TB3 bolts should not be relied upon to support the mast and antenna weight?
>Just your own observation or did you read something from Rohn about this?  I
>don't disagree with your conclusion, I just wonder if Rohn ever said
>anything about using other clamps above and below the bearings to support
>the weight.
>
>Stan  w7ni@teleport.com
_____________________________________________________________________________
I guess ignorance is bliss and I've been most blissful in not realizing that
one shouldn't use a thrust bearing to bear thrust!  I thought that was what
it was made for and so when I put up my 20 foot Texas Towers 3/16" galvanized
mast on my 130 foot tower, I first drilled into the mast 120 degrees apart
3 indentations that would allow the bolts to "bite" into the mast and support
the weight of the mast and the TH7DX and 40-2CD that was installed thereon.

I was concerned that the bolts might back out and so put nuts and jam-nuts
on the bolts, screwed them into the indentations (not holes all the way
through the mast) tightly, and then tightened the nuts and jam-nuts up so that
they would not back out.  The indentations were made at some considerable
effort because the steel was so tough that I had to use a number of high
quality steel bits in order to get the indentations deep enough for the bolts
to "bite".  The indentations were also made such that the bottom of the mast
is held about 3/4" to 1" above the "resting spot" on the TT2 rotor.

I also worked wheel bearing grease into the indentations well so that time,
moisture, and oxygen would not be able to rust them.  I've been up on the
130 tower a number of times and have observed that there is no apparent rust
at those indentations that have been up there for 5 years now.  The TT2
rotor has had to be changed out 3 times, now, due to lightning.  It is a
pleasure to be able to change out the rotor and not have to worry about the
weight of the mast/antennas.

Am I dumb, lucky, ignorant, or correct in my approach?  Direct comments
welcome.

Regards,

Rod, N5HV
w5hvv@aeneas.net

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