Date: Thu, 26 Apr 2018 02:49:21 +0000
From: Charles Morrison <junkcmp@gmail.com>
To: "towertalk@contesting.com" <towertalk@contesting.com>
Subject: [TowerTalk] Adding Expanding Foam inside boom
<Has anyone tried this ?
<Can it improve survivability of very large yagis?
<It won?t prevent a boom failure, but could it help last a little longer?
<-Charlie N1RR
## waste of time as others have pointed out. Instead use dx engineerings
..yagi mechanical software to evaluate the boom weak areas..which is typ
right where the boom meets the mast. Think of the boom as 2 x cantelivered
objects... that are back to back...on a common.... mast... mount. The
software
is dirt cheap to buy, runs on windows 8 or 10, etc. They can either email it
to you,
or you can get a disc version. Or download the free trial version, then
upgrade
via email to the full blown version. Aprx $40.00 and can be used for
everything
from elements to boom, to mast, verts etc, etc,
## Just a few mins with yagi mechanical will tell you all you need to know.
Typ fixes
are to just re-enforce the center of the boom with an inner sleeve. Yagi
mechanical
will depict how thick the inner sleeve has to be..and also how long the
sleeve has to be.
That way you dont needlessly add excess weight to the center of the boom. In
most cases,
the inner sleeve wont add a lot of weight to the array... but will increase the
booms .. V max
by a bunch.
## 2nd method is to use both a top truss...and also a pair of side trusses.
Simplest way to add
the pair of side trusses, is to install a 2nd plate on the mast, a few
inches below the boom. Then
install a 4-6 ft long, length of 1 inch to 1.25 OD tubing on either side
of 2nd plate. Inner ends
of each tube is held to 2nd plate with a pair of u bolts, or better yet, use
the DX eng types, with the
solid Al saddles, etc.. 4 used in total, 2 at top of 2nd plate, 2 at bottom of
2nd plate. 2nd plate to mast connection
done with another pair of u bolts..in center of 2nd plate.
.25 inch or .315 inch dacron rope.... or philly, from ends of each tube... to
way out towards the ends
of the boom. Four separate ropes in all. Once correct lengths are figured
out,
the final tension can be done by extending, or shortening each tube. By
using 2 x separate tubes, each tube
can easily be fully retracted, so you can easily get at the tip of each tube,
if required. M2 uses the above
method I just described.
## To get some more tension on all 4 x ropes, the entire 2nd plate can be
moved down the mast just a tiny bit, like a
fraction of an inch.. Once tweaked, the boom deflection is greatly reduced.
## I know a few fellows who used that expanding foam inside verticals, and
tapered verticals. The idea was to
reduce the excess mechanical vibration. The concept works, tops are capped
off to prevent moisture + water
ingress etc. But to pull this stunt off, a long flexible plastic tube has to
be run the length of the vertical, or at
least to the half way point. Tube is slowly retracted as the foam is pumped
in. If done in two halves, once one half
is done, the tube is inserted into the other half..and process repeated. The
foam does not add any mech strength,
only anti vibration qualities.
Jim VE7RF
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