[TowerTalk] Thrust Bearing, etc: more answers from UST calcs

[Grant Saviers]

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 at 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
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