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[TowerTalk] Rohn TB-4 bearings, sheared off transverse bolts !!

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Subject: [TowerTalk] Rohn TB-4 bearings, sheared off transverse bolts !!
From: "Jim Thomson" <>
Date: Fri, 2 Jul 2010 00:15:39 -0700
List-post: <">>
OK... the 3 x 3/8"   G-5 bolts  [ that appear to be cold galvanized]  on BOTH 
my  Rohn TB-4 bearings
are rusted really badly.    I soaked all 6 of em with WD-40.. from both sides.  
 I got 4 of em out with no problems
and of course 2 x of em sheared right off flush to the aprx 3/4" thick AL 
collar!   [ one sheared on each bearing]

Of the 4 that came out ok.... a  new  3/8"  SS bolt fits like a glove.   There 
is NO way the original rohn bolts  were
ever hot dipped galvanized.   If they were, the tapped threads in the solid AL 
collar  would be tapped slightly oversized.  
[and a 3/8"  SS  bolt would then make a sloopy fit.. and they don't]     I 
suspect the stock bolts  were just cold galvanized. 
Cold galvanizing adds  just .001"  or less..zinc.    Hot dipped [per the ASTM 
spec] adds  .0025" of zinc on all sides... IE; bolt diam
then increases by .005".... therefore the mating female nuts /threads are 
always slightly oversized to compensate. 

My game plan was to replace the rusted bolts  with SS bolts and SS nuts.  I 
can't use real, hot diped galvanized bolts.. since the
threads in the solid AL collar's are not big enough in diam.   I also don't 
have a slightly  oversized tap either.  We are not using the
top bearing to take any weight at all   [ the rotor takes all the weight] .  
The lower bearing doesn't require the 3 x transverse bolts 
anyway. it's virtually impossible to align two bearings + a rotor.    So just 
the top bearing + rotor are aligned.   The lower bearing
just provides some lateral support in high winds.   This is on my Trylon  T-500 
tower.   I custom made my own bearing and rotor
plates with  3/8' thick  6061-T6  AL plate. 

Funny thing is... the  4 x 1/2"  bolts that hold each bearing to the AL plate 
came out very easily.   They are real G-5 bolts that are
hot dipped galvanized.. and ditto with the mating nuts.   One would think that 
Rohn would have figured out this problem
with the transverse junk bolt/nuts on their   TB-3/4  bearings..after 50 years 
in production. 

OK.. what's the... 'easy'  way to get the 2 x sheared off bolts out.. without 
destroying the AL threads ??    I was thinking of drilling em
out, starting with a small drill bit, and progressively using bigger bits.   
Even then,  that all hinges on  initially drilling, dead center
in the sheared off bolt.   In retrospect, I shoulda inverted the bearings into 
a dish.. full of wd-40.. and soaked em for 2 x days. 
Just high enough level of WD-40.. to encompass the 3 x bolts.. and not get into 
the main bearing assy. 

Plan B was to just  drill, and tap a new 3/8"  hole.. adjacent to the sheared 
off bolt. 

Fit to  be tied.. and just pissed.

later......... Jim  VE7RF 

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