When it come to towers I am still very much in a learning process. I have
never owned a tower and I joined this group to learn everything I can so that
some day I could get one installed. Please keep this in mind as you read my
below question.
This thread is centered around thrust bearings for a US Tower TX-455. Early in
the thread it was stated that this tower has a wind load rating of 18 sq. ft.
and this rating has be used in a few of the follow-on messages. However, when
I checked the US Tower website it seems to me that it is stating that the
rating is 7.4 sq. ft. at 76 mph with 90 mph 3 second gust. There is a big
difference between 7.4 and 18. If I was putting up a TX-455 I would much
prefer to be able to install a 18 sq. ft. antenna vs. a 7.4 sq. ft. antenna,
but I have read many times in this group that you should always follow the
manufacture's recommendations.
So, what am I missing here? I need to know before I can move forward in
designing my dream setup and maybe some day getting it installed.
Thanks to those that can set me straight on this.
Larry, W0NQW
On Feb 8, 2013, at 8:22 AM, Gene Fuller wrote:
> Hi Grant -
> It's been a few decades since I took my ME courses also. There are obviously
> many refinements to my suggested quick look, such as conditions on the mfg's
> spec's, guying, column loading, material, aging, etc., that's why I prefaced
> my comments with "simple minded" and "approximate". I really think that any
> serious discussion of this should start, and perhaps end, with the tower
> manufacturer, who hopefully would be best informed on the assumptions and
> conditions that have gone into his spec's.
> Gene / W2LU
>
> ----- Original Message ----- From: "Grant Saviers" <grants2@pacbell.net>
> To: "Gene Fuller" <w2lu@rochester.rr.com>
> Cc: <towertalk@contesting.com>; "K8RI" <K8RI-on-TowerTalk@tm.net>; "Jon Pearl
> - W4ABC" <jonpearl@tampabay.rr.com>
> Sent: Friday, February 08, 2013 1:35 AM
> Subject: Re: [TowerTalk] Thrust bearing question
>
>
>> It been a while since my mechanics course, but I think the accurate way to
>> calculate the loads is to translate the individual antenna wind loads to
>> moments at the tower base. Then sum these and see if they are less than
>> the rated base moment for antennas. UST rates their towers at 1 foot above
>> the top plate, so an 18 sq ft rating on a 55' tower is 56 x 18 = 990 ft-sq
>> ft at the base times the EIA class wind load per sq ft. The old rule of
>> thumb was 70mph was 20# load per square foot. So the permitted load for
>> antennas in addition to the tower and cable load (some cable allowance is
>> included the tower itself) is 19,800 ft-lbs. Unless the site has extreme
>> exposure, assuming the wind load is the same for all antennas on the mast of
>> a 55 footer is probably ok.
>>
>> Then each load is calculated the same way, height above ground x area x wind
>> load, add them up, and as long as the sum is less than 19,800 ft-lbs the
>> tower is not overloaded. The mast can be assumed to be a point load at
>> one-half its height above the top plate.
>>
>> Of course, this assumes that the mast can handle the loads. The various
>> mast calculators can answer that question.
>>
>> One caution is to make sure all specs and calculations are done consistently
>> for flat or round sections. Often, this is confusing (or specsmanship
>> confused) in the documents.
>>
>> YMMV etc.
>>
>> Grant KZ1W
>>
>>
>> On 2/7/2013 11:27 AM, Gene Fuller wrote:
>>> 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@tampabay.rr.com>
>>> To: "K8RI" <K8RI-on-TowerTalk@tm.net>
>>> Cc: <towertalk@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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