[TowerTalk] TB3 Thrust Bearing

Bruce Ferratt uglyatc at msn.com
Wed Mar 15 16:10:22 EST 2006

My mast is 1 1/4" OD.   It is solid, round.  It states on the side ASTM 
6061-T6511.  It is 12ft long and will be mounted 4-5ft inside my Rohn 25g 
tower to the rotor plate with a thrust bearing at the top open section 
(Thrust bearing is the TB3).  I am looking to mount a Mosely TM-33A 
Tribander and stack a Diamond 2m/440 vertical. The aluminum mast weighs less 
than the old 6' steel mast that came with the tower!
I purchased this mast because there is a bushing inside the top of the 25g 
top section that limits me to 1 1/4" OD.  I welded a sleeve on the bottom of 
the mast so that my HD-73 rotor will clamp down on the mast.  Alliance 
HD-73's will tighten only down to 1 3/8" OD so the mast was too small.  I 
like the bushing in the top section to stay there as it will keep everything 
nice and snug but will be free enough to rotate the beam with some grease 
applied to it.  I think the bushing is plastic but there was some rust 
around the old 6 ft steel pipe/mast that I am replacing.  I can't believe 
that Rohn designed this tower to have a rotor mounted just below the top 
section!  It makes multiple accessory shelves worthless unless you don't use 
a thrust bearing!
Thanks for your email.

>From: W2RU - Bud Hippisley <W2RU at frontiernet.net>
>To: Bruce Ferratt <uglyatc at msn.com>
>CC: towertalk at contesting.com
>Subject: Re: TB3 Thrust Bearing
>Date: Wed, 15 Mar 2006 15:16:50 +0000
>Bruce Ferratt wrote:
>>Hello and thank you for letting me post a few questions.
>>Can someone tell me if a Rohn TB-3 thrust bearing will close/clamp down on 
>>a 1 1/4" aluminum mast pipe?
>Well, I would never use an aluminum mast for rotating ANYthing, but that's 
>your choice.
>I don't think the TB-3 will go down to 1-1/4" mast with the stock set 
>screws.  When the ends of my set screws are flush with the 2" interior wall 
>of the thrust bearing, they have no more than another 1/4" of possible 
>travel before the head of the set screw hits the exterior wall.  If  I drew 
>my picture of your mast in a TB-3 properly, you will need an absolute 
>minimum of 3/8" travel for all three set screws, which is 50% more than the 
>set screws on my TB-3 will allow.
>Now, it's possible that Rohn has changed the length of the set screws over 
>the years, but that's the size of the ones on my old TB-3.
>In addition, since you specified aluminum, you'll need even longer set 
>screws because your set screws will probably go INTO the mast a little 
>bit....and a little bit....and a little bit....
>Bud, W2RU

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