[TowerTalk] Thrust bearing question

[Jon Pearl - W4ABC]

Hi John.

There's nothing wrong with the combination you cite.

At present, my rotator *is* inside the tower where it's been for more 
than twenty years but with no thrust bearing at the top of the tower.

If I wasn't clear, I'm going to continue with the rotator placed inside 
the top section of tower.  My question is simply this; since the rotator 
can take all the weight of the mast and antennas with the mast not being 
collared by the thrust bearing in an attempt to relieve the rotator of 
any of that downward force, what purpose does a thrust bearing serve at 
the very *top* of the tower?  And when I say 'collared', a Yaesu type 
bearing**(http://tinyurl.com/aa2g59n) has the ability to collar, 
utilizing four set screws (bolts) to limit downward movement by the 
mast.  US Towers calls for a TB-2 type bearing 
(http://tinyurl.com/bj9vykg) to be used which provides centering and 
limits side movement.

And "yes" I understand your argument against the use of a thrust bearing 
placed on a plate just above the rotator.  While it does mitigate side 
thrust applied to the rotator head, it also adds some complexity to the 
geometry.


Thanks for your reply.


73,


Jon Pearl - W4ABC


On 2/7/2013 8:00 AM, john at kk9a.com wrote:
> What is wrong with using a bearing at the top of the tower and mounting
> the rotator inside the tower? The rotator has an internal bearing so the
> mast is supported by two points. I have built all of my towers this way.
> Some people add an additional bearing just above the rotator but in my
> opinion it is just a place to bind and serves no purpose other than it
> allows you to remove the rotator for repair.
>
> John KK9A
>
>
>
> To:	towertalk at contesting.com
> Subject: [ TowerTalk] Thrust bearing question.
> From:	 Jon Pearl - W4ABC <jonpearl at tampabay.rr.com>
> Date:	 Wed, 06 Feb 2013 22:25:47 -0500
>
> Hello group.
>
>
>
> I've got a question about a future configuration and thought to ask your
> opinion.
>
> I've got a US Towers TX-455 that I'm about to purchase a new M2 OR2800
> for. I called US Towers and asked what they offer in the way of a
> bearing plate to be situated just above the rotator. Apparently, they
> /don't/ offer a bearing plate. Their plate that is designed for rotators
> isn't meant to mount a bearing due to the way it's manufactured. Their
> concern with using their rotator plate as a bearing plate centers around
> the fact that the plate doesn't permanently attach to the tower, but
> merely sits down inside of it, held in place by the three horizontal
> members (rungs) that it rests against - thus making for a situation
> where side loads placed on the plate could be transferred to one of the
> three (or all three) of the rungs - possibly bending them.
>
> Okay... I guess I can see that.
>
> Next, I called M2 and asked what I should do to promote a long life and
> prevent damage to their rotator with it mounted inside of TX-455 top
> section. Their answer, "Just use a thrust bearing."
>
> The TX-455 top section is flat. That's where the thrust bearing goes....
> on the very top of the tower. Several inches below it is another flat
> plate that's also welded into the tower. Between the top plate and the
> plate down below it is a center tube that's welded between them that the
> 2" mast will ride in.
>
> With a 21' long, 2" O.D. 1026 DOM tube projecting 16' out of the top of
> this tower - what possible protection could a thrust bearing at the very
> top of the tower offer the rotator more than five feet down, inside the
> tower? I'm concerned about with the side loads on the rotator head
> created by wind forces on the mast and antennas above the fulcrum.
>
> Is my thinking wrong on this? Shouldn't I try to mitigate the side
> thrust on the rotator head, just above it?
>
>
> What say you?
>
>
>
> 73,
>
>
> Jon Pearl - W4ABC
> www.w4abc.com
>
>
>