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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