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Re: [TowerTalk] Need the holes spacing on a Rohn TB-3 thrust bearing

Subject: Re: [TowerTalk] Need the holes spacing on a Rohn TB-3 thrust bearing
From: "Roger (K8RI)" <>
Date: Tue, 03 Aug 2010 23:21:39 -0400
List-post: <">>

On 8/3/2010 10:33 PM, Kevin Elliott wrote:
> I am trying to see if I can mount a TB-3 thrust bearing to a Rohn 25 rotor
> platform, I realize I may need to re-drill the holes.
Not redrill, it's drill new holes as the ones in the TB-3 and TB-4 which 
are identical do not match any holes in the 25G accessory shelf
>   That is why I need the
> center to center measurement for one set of holes.
I measure 3 13/16", but that was just with a ruler so give or take 
>   Also if someone can
> recommend another bearing to mount to a Rohn 25 rotor plate please let me 
> know.
Yaesu GS-605. It won't handle the load the TB-3 will, but it looks a lot 
nicer and is a lot smoother.    Does that mean anything? Probably not 
particularly if you are only putting up a tribander, even a large 
tribander. I'd not hesitate to use one with the Force 12 C-19 XR with 
the new WARC-7 stacked 6 feet above it.  ("They", meaning force 12" say 
you can stack the new WARC-7 as close as 4' to the C-19 XR. Ao I'm 
qualifying that statement.   No, those holes won't match either, but 
it's a smaller bolt circle with smaller bolts.  However due to the 
smaller bolt circle and the "cut-out" for mounting the "Ham series" of 
rotators so you can get at the terminal strips, positioning the bolt 
holes in the accessory shelf will take a bit of creativity.  IF you are 
careful in alignment, you end up with "half a bolt hole" on each side of 
that cut out.

AS I am going to be installing a 25G with shelf, C-19 XR and possibly 
the WARC 7 on an Aluminum 6061-T6  2" X 3/16" (or 1/4") wall mast with 
the GS605 AND a Ham IV I am going to take a 1/4" thick aluminum plate, 
drill the matching holes (the box comes with the bolt circle template on 
the bottom) in the plate AND the accessory shelf with the bolts aligned 
to one of them lines up in the center of the cut-out. The plate will be 
bolted in place not only with the three thrust bearing bolts that go 
through the accessory shelf, but with 4 bolts (One near each corner of 
the Aluminum plate). Those 4 bolts will likely be 5/16" but 1/4" or 3/8" 
would also be fine.   Normally there is not a great deal of stress at 
this location so the size of the bolts is not critical, nor is the 
placement of the 4 bolts for the thrust bearing...that is as long as 
they line up with the holes in the bearing.

The reason for going with 1/4" plate is two fold. The first is even 
support for the bearing due to the cut-out in the accessory shelf. The 
second is I have a dozen 6" X 8" pieces of 1/4" on hand <:-)) as they 
make good boom to mast clamp mounts for smaller antennas and are just 
plain all around handy pieces.


Roger (K8RI)
> Thanks,
> Kevin
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