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Re: [TowerTalk] Thrust bearing question

To: Gene Fuller <w2lu@rochester.rr.com>
Subject: Re: [TowerTalk] Thrust bearing question
From: Grant Saviers <grants2@pacbell.net>
Date: Thu, 07 Feb 2013 22:35:16 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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