[TowerTalk] Thrust Bearing, etc: ALL the answers - from UST

[kr2q at optimum.net]

Gosh...just go to the US Tower site.  
See http://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