[TowerTalk] Adding Expanding Foam inside boom

Jim Thomson jim.thom at telus.net
Thu Apr 26 11:37:34 EDT 2018

Date: Thu, 26 Apr 2018 02:49:21 +0000
From: Charles Morrison <junkcmp at gmail.com>
To: "towertalk at contesting.com" <towertalk at 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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