My MA770-MDP has only a single set screw to hold the mast. For this reason,
I always rest the boom-to-mast clamp on the rim of the top section. That
way, the mast can't slip into the tube. In addition, I think it's a very
good idea to keep the antenna low on the mast to minimize the moment arm at
the top of the tower. I believe these tubular jobs are rated with the load
no more than a foot or so above the top section.
73, Dick WC1M
> -----Original Message-----
> From: JC Smith [mailto:jc-smith@comcast.net]
> Sent: Friday, June 10, 2005 1:54 PM
> To: 'Michael Keane K1MK'; towertalk@contesting.com
> Subject: Re: [TowerTalk] installing monstermasts intowers(and
> ?twothrustbearings?)
>
>
> Mike,
>
> There are obviously different bearing sold to amateurs for
> the top of their towers. Although I have not disassembled it
> to look at the location of the races, my TX-472MDP's TB-2US
> is obviously not designed to support any axial weight. It's
> obvious because the only means of clamping the mast to the
> bearing is a single, allen/socket-head set screw. No way is
> anyone ever going to tighten that enough to support the axial
> load of even a moderately sized mast and antenna. I also
> have a MA-550MDP with rotor base. Now there's a thrust
> bearing that's designed to support some weight. Incidentally,
> even though the top of this tubular tower does have fairly
> heavy-duty mast clamping arrangement, my mast still slipped
> down inside the tower until the antenna (a little 3L
> tribander) rested on the top of the tower. Fortunately, no
> coax got pinched and since the whole tower rotates, no real
> harm was done. The mast does weigh about 70# but still a
> relatively light load considering the small antenna. The
> antenna was only 10' above the tower top. What that taught
> me is that when you get that mast whipping around a little
> (and we don't have much wind here) you really can't rely on
> any of these collar clamps to support an axial load over a
> long period of time. You'd have to run a hardened pin
> through the collar and mast to be guaranteed no slippage over
> time, and I don't think that's recommended either.
>
> 73 - JC, k0hps@amsat.org
>
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com]On Behalf Of Michael
> Keane K1MK
> Sent: Friday, June 10, 2005 9:16 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] installing monster masts intowers(and
> ?twothrustbearings?)
>
> At 12:33 PM 6/8/05, Roger K8RI on Tower wrote:
> >All of the Thrust bearings I've had here had three, 3/8" bolts, or
> >5/16ths bolts to center and hold the mast. They are
> designed to hold
> >weight in
> both
> >the vertical and horizontal directions.
>
> Well, some of the generalizations about about what's typical
> have left me confused. I do recognize the above description
> as something more familiar, the Thrust Bearings that Rohn
> sells, the TB3 & TB4. For me, those are the sorts of bearings
> that first come to mind when we start talking about thrust bearings.
>
> All that's been said has left me confused enough to ask: does
> anyone know definitively whether a Rohn TB3 is designed as a
> radial bearing for taking lateral loads or a thrust bearing
> for taking axial loads?
>
> What I recall from having disassembled several TB3's for
> refurbishing many moons ago, is that the balls are contacted
> between races in the top and bottom faces of the bearing and
> not between the inner and outer walls.
>
> To me, that makes a TB3 a thrust ball bearing by design and
> implies that a TB3's intended primary function is to carry
> axial loads. It's comes as something a bonus that a TB3 also
> transfers radial (wind) loads to a degree; a TB3's ability to
> perform that task would be limited by the bearing's radial load limit.
>
> If one's primary concern was transferring wind loads rather
> than carrying weight, then a radial bearing is what should be
> used and not a thrust bearing. In that case the bearing
> functions as a very well-lubricated bushing.
>
> It would seem that all my tower experiences have been with
> bearings that were designed to function as thrust bearings. I
> can't remember ever encountering a radial bearing being used
> at the top of the towers I've worked on.
>
> 73,
> Mike K1MK
>
> Michael Keane K1MK
> k1mk@alum.mit.edu
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
> "Wireless Weather Stations", and lot's more. Call Toll Free,
> 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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