[TowerTalk] Thrust bearing question

Thu Feb 7 10:44:45 EST 2013

2 points with the first beingm John is correct, a 16' mast putting the 
antenna 16' above the top of the tower substantially derates the tower 
below the 18 sq ft original rating.

The other is, you already have a thrust bearing in the form of that 
sleeve.  It wont support vertical load, but that sleeve will 
substantially reduce any lateral/side load on the rotator produced by 
the leverage of the mast. the sleeve will serve as a pivot point with 
16' above it and 5' below it for a 16:5  ratio for lateral force on the 
rotator, or slightly less than 4:1 which is a big number.  Fortunately 
the sleeve limits the pivot ability with most of the load showing up as 
lateral load on the tower.  Thats good for the rotator, but for the 
tower? Not so much.

Assuming you install an antenna of 18 sq ft which is the tower rating, 
with 18' of antenna 16' above the top of the tower that is 18' 
multiplied by a 16' arm.  So the tower is going to see much more than 18 
sq ft of load..

The rotator was already designed to support a substantial vertical load 
so the bearing capable of supporting a vertical load is not necessary, 
but sure is handy if you need to work on the rotator without taking all 
the antennas down.  OTOH you can build a simple fixture to hold the mast 
in that case

IE "to me" 16' sounds like a bit much.

73 and good luck

Roger (K8RI)

On 2/7/2013 10:03 AM, Michael OBrien wrote:
> John,
>
> I don't think USTower anticipates 16 feet of mast sticking out the top of the tower. If you have much in the way of antennas attached to that mast, it may to exceed sideload potential foreseen by the tower designers.
>
> While I cannot cite official sources because my memory isn't that precise, I've always been under the impression that 4 or 5 feet of mast above the top is the recommended amount for a USTower. That's what I have on my HDX-555.
>
> Just a thought...take with several grains of salt.......
>
> 73,
> Mike, K0MYW
>
> --- On Thu, 2/7/13, Jon Pearl - W4ABC <jonpearl at tampabay.rr.com> wrote:
>
>> From: Jon Pearl - W4ABC <jonpearl at tampabay.rr.com>
>> Subject: Re: [TowerTalk] Thrust bearing question
>> To: john at kk9a.com
>> Cc: towertalk at contesting.com
>> Date: Thursday, February 7, 2013, 8:07 AM
>> 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
>>>
>>>
>>>
>>
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