It's an interesting question about the strength of the top section in a
crank up, so I looked at the UST calcs for the HDX589, 85mph 3 sec gust,
71mph fastest mile (EIA-222-F). I'm not a PE so this not a definitive
look and mostly an educated guesstimate.
A number of factors limit the crankup tower capacity - lift cable
strength, leg compressive strength, section max bending moment, etc.
There are five sections in an HDX589, UST numbers 9 (base) to 5 (top).
cable safety factor: top section 14.3 next to bottom section 2.7 (max
antenna + rotator + coax = 277lbs)
leg compressive strength safety factor: top section 7.8; bottom section
1.58 (max antenna 12.1 sq ft)
outside overlap strength: top 11; bottom 5 (max antenna 74 sq ft)
overlap web strength: top 13; bottom 1.5 (max antenna 12.3 sq ft)
foundation moment safety: 1.4 (although the calculation appears to be
very conservative).
When multiple loading factors are calculated I used the worst case ratio
safety factor. The lowest safety factor governs the maximum load.
This tower is rated 13.7 sq ft round members at 1 ft above the top plate
for the specified EIA conditions.
So, it appears that the limiting factors are in order: the foundation
(1.4), then the bottom section web strength in the overlap (1.5) and the
bottom section leg compressive strength (1.58). Since the top section
safety factors are so large, it seems unlikely that it would fail first
in an overload situation. i.e it's unlikely the rotator and mast will
rip out first.
Since I'm out on a limb, I thought I keep sawing re a conventional
wisdom re towers. It has been said "freestanding towers are designed to
fail somewhere near the middle". Logic seems to contradict that since
unguyed towers are made at different heights and capacities out of a
family of identical sections. UST uses the same series of sections for
all HDX crankup heights and loads. Trylon does the same for the fixed
T200/300/400 etc series of tapered free standing. So it would take some
magic to make a section #6 too strong at the top and too weak in the
middle and then too strong at as the base section of a tower. The calcs
show the bottom section is the most likely to fail for an HDX589 and I
think that is most likely for many free standing towers. There are
probably many contradictions given the uncertainties of winds and
maintenance history etc etc.
However, falling towers and other objects often fail (buckle) at a
distance up their height AS THEY FALL. This leads the observer of the
failed (or those observing the fall) structure to conclude the failure
started at that point. google "falling chimney problem" or go to
http://myweb.lmu.edu/gvarieschi/chimney/chimney.html
Grant KZ1W
On 2/8/2013 3:23 PM, kr2q@optimum.net wrote:
Gosh...just go to the US Tower site.
Seehttp://www.ustower.com/#!__product-pages/ham-towers
and click on the DOWNLOAD FAQ
Here are some excerpts from their FAQ
QUESTION : Why do I need a thrust bearing?
ANSWER : The thrust bearing is designed to support the weight of your
antenna(s) and mast
off of your rotor and extending its life.
Translation: As Ward said, "you don't need one with the OR2800...save your
money"
QUESTION : How tall of a mast is recommend for a tower?
ANSWER : This is dependent on the tower. TMM SERIES: up to a 15’ mast. MA SERIES: up
to a 15’ mast.
TX SERIES: up to a 20’ mast. HDX SERIES: up to a 20’ mast.
Translation: "We say 20 feet, but we don't say how much wind load - so YMMV. IE,
BE CAREFUL."
I'm no engineer, but one calculation I would be concerned about is not the
bending moment at the base,
but rather at the top of the tower (via the sleeve) and down to the rotor
plate. Yes, it is giant fulcrum
and you could possibly rip the hell out of the top 4 feet of the top
section...if the wind is high enough.
If you keep the tower cranked down in a big wind, well, now you have all those
other sections helping
to "support" the inner section (and the winds are usually less at lower levels
above ground). YMMV
Finally, in the real world, thrust bearings can serve two purposes.
1. As used on a ham tower, they redistribute the vertical weight at the level
of the bearing
2. More traditionally, the are used to hold "an axle." Think of the old time
printing presses:
big, heavy steel drums spinning horizontally at high speed.
Way back when, when I designed a rotating tower (in the late 1970's),
See
https://picasaweb.google.com/dougzzz/K2GLFullHistory#5246737104987133522
https://picasaweb.google.com/dougzzz/K2GLFullHistory#5246736666452767074
BTW, that is 10 over 10 on 10m (Telrex)
I didn't use a collar that wrapped around the tower; I used a big, fat beacon
mount (Rohn) and
put a bearing on that (inverted). The bearing had four places to anchor it - I
used those as the tie points
for the 4 guy wires. When I went to Bobker Bearing to buy one (I was clueless), I
asked "how much
weight can it hold." It was in the multi-ton range. They were surprised that
I asked that question.
They asked me, "No, you want about how many rpms." They said the one I was
looking at was good
for 3000 rpm. When they asked me, "How many RPM," we all laughed when I said
ONE!
de Doug KR2Q
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|