To avoid (or minimize the likelihood of) binding with a thrust bearing,
you might want to look at a self-centering flange mount bearing assembly
instead. Last year I purchased one of these monsters from a local bearing
supply place. The bearing itself is somehow pressed into place in a cast
iron flange. The cross section of the opening in the flange allows the
bearing to pivot so that minor deviations from coaxial installation of the
mast in the bearing (up to about 1.5 degrees, I think the specs say) can be
accommodated. That is, this type of bearing is more tolerant of slight
misalignments of the mast/rotator/shelf below it than a strict TB3, TB4 or
This is the gizmo that I said I relubed last week during an annual
inspection of the tower. I used marine grease which is water resistant (It
takes quite a bit of rubbing with a cloth to remove it from your hands. The
grease just laughs at soap and water.)
The cast iron base for mine is 6-1/4 inches square and slightly over an
inch thick. It's a Fafnir SCJ series flange cartridges; the bearing
assembly is a GYA-RRB (that's what the spec sheet says.) I bought it from
Applied Industrial Technologies (www.appliedindustrial.com ). Sorry, but
their spec sheet is faxed and I can't read their model number.
The best thing about this device is it was a few $ less than a TB3 -
around $72 in DC area. I had to get the model next in size above a nominal
2 inch ID. The 2 inch OD CM mast I bought was too large for the first
bearing I had. I shimmed out the small difference with a piece of stainless
IMHO, if you're planning on using a Rohn flat top, this would very
easily fit and work quite well.
Gene Smar AD3F
From: TexasRF@aol.com <TexasRF@aol.com>
To: email@example.com <firstname.lastname@example.org>; email@example.com
Cc: firstname.lastname@example.org <email@example.com>
Date: Thursday, December 05, 2002 5:26 PM
Subject: Re: [Towertalk] New Rotator In Place; Mast Now Binds
>Here is the straight skinny on 25G top sections:
>Several years ago (ten or more) Rohn redesigned the 25AG2 and 25AG3 top
>sections to use a 2.25 inch o.d. 14 gauge wall top support tube. After
>galvanizing these top sections will fit with a 2 inch o.d. mast (also
>galvanized) and have a clearance of about .050 inch left over.
>Before the redesign a 1.9 inch o.d. mast would fit ok but a 2 inch o.d.
>would bind in the top tube.
>As a side note, Rohn also redesigned the TB3 thrust bearing to fit the 2
>o.d. masts instead of the older 1.9 inch o.d. masts.
>I just tested a new 2 inch o.d. mast in a 25AG2 top section and the fit is
>just right for free rotation of the mast. The clearance is close enough
>though that without shims in a Ham 4 or T2X rotator there will be binding
>the mast. Also, the associated AS25G rotator plate is not exactly a
>built piece of equipment and some "adjustment" of the mounting ears might
>required to center the rotator mast clamp with the support tube.
>the 25AG2 or 25AG3 top section suport tube can be out of plumb just enough
>cause binding; they are not exactly precise either. Adjustments of the
>rotator plate can accomodate all of these errors but some patience might be
>73 de Gerald Williamson/K5GW/Texas Towers
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