A simple minded, approximate approach to this would be to look at it as a
bending moment problem. Making an assumption that the manufacturer's 18 sq
ft allowed load is 4 ft above the sleeve, which is also assumed to be the
fulcrum and that the wind pressure is "x" lbs. /sq ft. This would give you
an allowable 64x ft lbs of "overturning" moment at the fulcrum. Spread this
out as you will on the mast, the total would want to be held to 64x ft lbs.
e.g. -
item sq ft lbs/sq ft
ft to fulcrum bending 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
====
Allowable Total 18 x
"4" 64x
====
This crude analysis would indicate that even modest antennas at 1/7/12 feet
above the fulcrum would nearly max out the 18 sq ft spec. without accounting
for any additional ice loading.
Similar calculations could be a starting point for mast evaluation, when max
allowable mast bending moment is given or calculated and the wind
speed/pressure is determined.
Last resort, consult the tower manufacturer. If you shared the above type
calculations with them it would at least indicate that you had given some
consideration to the problem.
Gene / W2LU
----- 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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