Rob I am forwarding a copy of this to Wayne for his information and any
others who read this. The "solution" Wayne proposes here is in error.
Please note he refers to "windloads". What he has done does not affect
the windload rating of his tower. What his solution *does* do is get the
weight of the antennas, rotating mast and other parts attached to the
rotating stuff off of the rotater.
*Windload* is the load imposed (impact force) by the wind blowing
against the *side* of the tower and all things mounted on the tower.
*Gravity* loads go straight down, not sideways (unless the tower is
laying horizontal ). So the methods he has used here, do not affect
the windload rating of the tower. Incidently, wind *is* a formidable
force. A wind of only 50 mph causes every square foot of tower,
antennas, coax, mast, etc to be pushed sideways with a force of about 13
lbs per sq foot. At 60 mph this becomes 18 psf; at 86.8 mph: 30 psf; at
100 mph: 40 psf; at 111.8 mph 50 psf; at 122.5 mph 60 psf; at 132.3
mph...and so on.
The next time you drive along an Interstate Highway take a look at the
sizable structure behind the information signs you see. The ones that
say, for example "LEWIS AVE NEXT EXIT". Those sort of signs are pretty
big. Maybe 4 feet high and 6 to 8 feet long. If they are 4 x 8 then they
contain 32 square feet that the wind can blow on. At a wind speed of
86.6 mph the force pushing the sign against its support when the wind
is coming straight along the highway is (30 x 32 = 960) 960 pounds! The
structure supporting these signs look suspiciously like a *tower* layed
sideways.
ROHN Tower company, among others recommend, for most of the US, to
assume you will commonly receive winds of 86.6 mph. Thus they recommend
when erecting a tower that you use 30 pounds per square foot as the
windload force in calculations for proper tower installations. In some
areas of the US they recommend 40 psf.
On (25 Dec 97) Rob Dennis wrote to All...
 RD>  Original article from: Wayne Sarosi 
 RD>  Reposted by Rob Dennis,Moderator HAM_TECH,for use of the FIDO
 RD> HAM_TECH echo.
 RD>   THIS POST/SERIES NOT TO BE USED IN ANY CDROM COMPILATIONS FOR
 RD> COMMERCIAL  SALE WITHOUT EXPRESS WRITTEN PERMISSION OF ORIGINAL
 RD> AUTHOR(S) OF THIS POST.                ALL AND ANY COPYRIGHTS RETAINED
 RD> BY AUTHOR(S).
 RD> SUBJECT:  Tower modifications for redistributing antenna windloads.
 RD>  Many Amateur Radio Operators are restricted to a maximum size antenna
 RD> because of the limitations their towers have.
 RD>  The windload rating can cut a dream antenna in half because the tower
 RD> is rated for something about the size of a ten meter beam. But,that's
 RD> all you can afford so you take your lumps and settle for something
 RD> less than you wanted.  I had that problem and decided to do something
 RD> about it.
 RD>  o I could purchase a larger tower.
 RD>   (BTW I finally had one given to me. It's still in the back yard).
 RD>  o Find a way to get the weight off the tower and on the ground.
 RD>   (Put the dynimite away!).
 RD>  Looking at the lastest ads,then looking in my wallet,then at my wife
 RD> and kids then again at my 9-in triangular crank up and 22-ft beam,then
 RD> back at my wife, I decided to to the latter of the two (gulp).
 RD>  How in the world was I going to relieve the weight from the tower and
 RD> still keep my beam up at the same height? That 9-in trangular crank-up
 RD> wasn't going to cut it in heavy winds. It was an accident waiting to
 RD> happen.
 RD>  At that point I was working on my "I need a better tower" speech to
 RD> my wife and remembered some of my statics and dynamics from college.
 RD>  I checked on a couple of examples in the book and found that my idea
 RD> would work. The solution? A drive shaft/bearing modification. With the
 RD> rotor moved to base of the tower and a vertical bearing set situated
 RD> just above the rotor.  The rotor would have no downward thrust upon
 RD> it. All it would have to do is turn the shaft. The bearings turned out
 RD> to be easier then planned.
 RD>  Edmond Scientific Catalog had Lazy-Susan bearing sets for sale.
 RD>  I purchased the 1000-lb table type for about $6.50 each. After
 RD> packing them in grease,I was ready to install them.
 RD>  The drive shaft can be a pair of push-up masts for crank-up towers or
 RD> water pipe for fixed towers. The shaft is guyed within the tower by
 RD> PVC tubing.  Each tube is about 2-3 feet in length and I used three
 RD> per forty feet.  The shaft pokes out the top for antenna mounting. In
 RD> my case,two sections of the push-up mast poked out the top allowing me
 RD> to work and mount a few VHF/UHF antennas.
 RD>  Here's the basic set-up:
 RD>               =================== 2m beam
 RD>                        ||
 RD>                        ||
 RD>      ====================================== HF beam
 RD>                        ||
 RD>                        ||
 RD>                       |||| - PVC tubing over shaft inside tower
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       ||||
 RD>                       |BB| - Bearing
 RD>                       ||||
 RD>                       |RR| - Rotor location
 RD>                -------|--|--------
 RD>  o The rotor only turns the shaft - no weight on the rotor.
 RD>  o The vertical weight is transformed to the bearing near the bottom.
 RD>  o The weight of the antennas are on the shaft instead of the tower.
 RD>  o The towers sole purpose is to guy the shaft and the forces directed
 RD> to    the tower are distributed along the tower's length.
 RD>  This system has been working here for over two years.
 RD> -WS
... Kamikaze Tutor:  "Watch carefully; I'll only do this once!"
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