Date: Thu, 7 Apr 2016 22:49:27 -0500
From: Fred Sanborn <cc-6569@comcast.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 2 inch Chrome Moly mast needed
First of all many thanks to all who responded here on the reflector and
also in private. Several options were offered.
Jim VE7RF has it right on for what happened here at this QTH. Besides
being hit by lightning, I also had a wind gust that bent the mast at
about 45 degrees. I hate to admit what I was using for mast at that
time - 2 inch sch 80 pipe. I knew at the time it was probably not a
good idea and learned the hard way. I have now paid many, many more
times what it would have cost for a proper mast in order to clean up the
mess by hiring a crane with a crew of 3 and pick the antennas off the
tower. So maybe I want a slight over kill this time.
73, Fred KG9X
## somewhere, buried in my files in basement, I have at least 35-40
old photos of bent mast pix. The last one included was just a few years
ago with dale, VE7SV bent mast horror show. His 2 el shorty 40m yagi
at top of the 10 ft mast, folded the mast 90 degs, 1.5 ft above the top of the
110 ft tower.
## You guys have no clue how damned dangerous it is to try and remove a
bent mast. In some cases, due to weight of yagis..think Telrex, the ice laden
yagis + ice laden folded mast, eventually broke off, with mast + yagis
crashing down,
and slicing some guy wires on the way, bringing the entire tower down..onto
a fellows roof ! That or the snapped off mast + yagis is hopelessly mangled
and mixed up in the top or next set of guys..+ parts of yagi eles and or booms
sticking through the tower. Its dangerous, even if a crane is used.
## hams are cheap, they are also stupid. 15-30 mins spent with the arrl or
dxe
mast calculator will tell you everything you ever wanted to know. Even
then,the
mast will deflect quite a bit, b4 it folds over. So if the mast calc sez it
breaks at 100 mph,
rest assured, the same mast will be deflecting like crazy, at just 90 mph.
The problem
with a mast deflection is... the center of gravity is now away from the tower.
That puts
a downward moment on the mast, further complicating things..esp with a heavy
yagi at
the very top of the mast. This is real bad news for stuff like 1.9 OD x
sched 80,
35 ksi masts. The boom is no longer parallel to the ground ! That puts even
more offset
weight onto the deflected mast. One side of the mast compresses, the other side
stretches.
( this is right where the mast folds). The engs call that... taking a set.
## 1.9 x sched 40 was popular here in town, back in the 70s. Since it could
be bought
already galvanized, was lighter than sched 80, and was relatively cheap. A
buddy
across town had a 4 el cubex quad on a 30 ft boom, and decides to mount the
quad
9 ft above the top of the rohn 25 guyed tower. 60 mph gusts..and it bent.
pita.
But they wouldn’t listen. Trust me on this, go through this gong show
once, and
you will not even blink an eye, when it comes to writing the cheque for the
DOM / CM mast. If its doable, use a 3.0 OD x .25 mast. 7 lbs per ft....
who cares. Use a 2nd thrust bearing, into the tower, but with no transverse
bolts tightened. Its there just to minimize mast deflection inside the tower...
and also to support the mast, when rotor has to be removed... after lifting the
mast + yagis up a little bit..to get rotor out.
Jim VE7RF
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