[TowerTalk] Thrust bearing question
Gene Fuller
w2lu at rochester.rr.com
Fri Feb 8 09:22:16 EST 2013
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 at pacbell.net>
To: "Gene Fuller" <w2lu at rochester.rr.com>
Cc: <towertalk at contesting.com>; "K8RI" <K8RI-on-TowerTalk at tm.net>; "Jon
Pearl - W4ABC" <jonpearl at 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 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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>>
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