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[TowerTalk] rebuilding Rohn TB-3 thrust bearings

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Subject: [TowerTalk] rebuilding Rohn TB-3 thrust bearings
From: "Dubovsky, George" <>
Date: Thu, 17 Apr 2008 13:46:08 -0400
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I recently was given a TB-3 that had been on a tower, where the mast had
not been rotated in perhaps two years. The slight rocking, back and
forth, in the wind had "fretted" pockets in the aluminum bearing race,
over perhaps 60 degrees. If you've ever rotated a similarly damaged TB-3
by hand, you know that it feels "lumpy" , as though it has sand in it. I
decided to see if the damage could be repaired.


First, I tried cleaning up the race with a form-ground tool on the
lathe. (All really good projects require dangerous tools - the more, the
better!) The damage proved to be too extensive, so I decided to try
replacing the entire bearing assembly with a bushing. I have been
successfully using a slab of molybdenum disulfide loaded Nylon as a
tower bushing under my 40 meter beam, so I decided to try it in this
application. Note: this description works much better if you have a
disassembled TB-3 in front of you.


I machined the OD of the rotating part of the TB-3 down to a depth just
about to the bottom of the inner bearing race. That left a
straight-sided cylinder that rides on the inside of the Nylatron
bushing, taking side thrust loads. You do have to square up the
horizontal surface under the top lip of the TB-3 - that's the surface
that will carry the vertical loads. Then I removed just enough of the ID
of the stationary part of the bearing to achieve "roundness". You don't
want to make this too thin, because the top surface of this "cylinder"
is the other vertical load-carrying surface. Then I turned a piece of
Nylatron GS with the OD sized for a light press fit into the stationary,
bolted flange part of the TB-3, with a shoulder that bottomed on the
"cylinder" referred to earlier (the shoulder is maybe 0.3" thick), and
with an ID that is a slip fit with the rotating part of the TB-3. When
you put it all together, it looks just like an unmodified unit, until
you flip it upside down and the rotating aluminum part slips out. This
modification removes the resistance to upward axial loads, so maybe it's
not such a hot idea for a tilt-over ;-)


I think what I have is a better bearing that is immune to
micro-fretting, uv, corrosion, carries acceptable loads, and never needs
lubrication. It's going up on my next tower install this summer.




geo - n4ua

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