[TowerTalk] Thrust bearing question

[Gene Fuller]

Sorry for the way that got formatted. I'll try for a narrower format for the 
table.-

item             sq ft      lbs/sq ft    ft to fulcrum   moment
----             ----      --------    -------------  --------
mast(2"cyl surface) 2      x            6                  12x
HF tribander      8          x            1                    8x
6 meter              4          x            7                  28x
2 meter              1          x          12                  12x
                                                                      -----
Calculated  Total                                              60x
                                                                      ====
Allowed Total   18        x             "4"                64x
                                                                      ====



----- Original Message ----- 
From: "Jon Pearl - W4ABC" <jonpearl at tampabay.rr.com>
To: "K8RI" <K8RI-on-TowerTalk at tm.net>
Cc: <towertalk at contesting.com>
Sent: Thursday, February 07, 2013 12:10 PM
Subject: Re: [TowerTalk] Thrust bearing question


> Hi Roger and Mike.
>
>
> On 2/7/2013 10:44 AM, K8RI wrote:
>>
>>
>> 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.
>
> I've yet to finalize the spacing between the HF, six and two meter 
> antennas but I may well find some economy of space, allowing me to drop 
> the rotator further down into the tower.  I can't make it lighter but I 
> can make it stiffer by shortening it's effective length above the top of 
> the tower.
>
>>
>> The other is, you already have a thrust bearing in the form of that 
>> sleeve.
>
> I guess that's the point I was trying to rationalize between my own two 
> ears.  I received another email privately that essentially said the same 
> thing.
>
>> 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 M2 OR2800 is rated at 1800# vertical load and it incorporates a Center 
> Guide (cone) for the mast to rest on.  It's been a while since I've looked 
> at the slop between the present mast that's on the tower and the sleeve. 
> If it's substantial then a thrust bearing might be of service, if only to 
> cut down on the lateral movement.
>
>> 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.
>
> Yes, I've looked at the same ratio and it may change.
>
>>
>> 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 actual numbers from bottom to top are 9.25 sq. ft., 2.5 sq. ft., & 2.7 
> sq.ft.
>
>>
>> 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
>
> When it's all horizontal at waist level, it's all much more manageable.
>
>>
>> IE "to me" 16' sounds like a bit much.
>>
>> 73 and good luck
>>
>> Roger (K8RI)
>>
>>
>
> Thanks again and 73,
>
>
> Jon Pearl - W4ABC
>
>
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