[Top] [All Lists]

[TowerTalk] Sources for 2 and 3 inch boom material

To: <>
Subject: [TowerTalk] Sources for 2 and 3 inch boom material
From: "Jim Thomson" <>
Date: Fri, 30 Jul 2010 01:26:19 -0700
List-post: <">>
Date: Thu, 29 Jul 2010 19:53:17 EDT
Subject: [TowerTalk] Sources for 2 and 3 inch boom material

Anyone know of any sources for aluminum boom material (2 and 3  inch) in 
the southeast US, specifically Florida?
Bill KH7XS

##  back in the 70's and 80's...  I used irrigation pipe.    It came in 
2"-3"-4" diam..
and all of em came in 20'-30'-40' length's.    Back then, I wanted a 36' x 3" 
boom.... so
they grabbed a 40' length ..and lopped 4'  off.. and tossed the 4' into the 
scrap pile. 
It was 74 cents per foot  x 36'  =  $26.64    Bothe 2"  and 3" were .050"  
wall.  The 4"
came in .100" wall.     NONE of it was 6061-T6... but a much inferior alloy.

##  to beef up the 36'  x 3" OD  x .050" wall  piece I had bought.... I also 
bought  a 
20'  length of 3" OD  x .050" wall, same material.    I cut the 20'  piece down 
the seam
[ this stuff is PIPE, not tubing].. and also cut it down opposite the seam... 
so split the
20' length into 2 x halves.    I wrapped the  2 x halves around the 36' boom... 
and slid  
several of those cast aluminum irrigation  clamps   over the 20' piece..and 
double walled the
inner 20' section of the 36' boom.   The truss line went  10' on either side of 
the tower.  The  last
8'  was un-supported,, and also single wall... and those 8' end sections STILL 
sagged ! 

## those cast AL  irrigation clamps are still available.  I see that DX eng 
sells em in 3" 
variety, but with a SS bolt.    The ones you get  from ur local farm supply 
used plated
bolts, and will eventually rust.    Where the 2"  +3"  cast irrigtaion pipe 
clamps will shine is..
for terminating  a truss line..superb.   The cast wrap around al clamps are 
very wide.. and dead 
flat surface, with loads of surface area... [ abt 50 x more than a u bolt] .   
They will  never slide
on a boom.. [ just don't use penatrox on em] . 

##  These days,  unless  ur on a budget, I would NOT reccomend  irrigation 
pipe, due to the lousy
alloy used.   In the single wall state [.050"]  if u use standard u bolts, with 
sheet metal saddles, it takes
no torque at all, to crush the boom. 

##  6061-T6  comes in .125"  wall.... but will NOT telescope.   If a inner 
splice is used [ 2.75" OD]  or
and outer splice [3.25" OD], then the  2.75" OD  will have to be turned down in 
a lathe..or swedged the entire
lenght .   IF  3.25" OD outer sleeve spliced used... then the ends of the inner 
3" OD boom sections will both 
have to be swedged.. or turned down in a lathe.  PITA both ways. 

## Plan B.... use 6061-T8... which now comes in .120"  wall.    Then everything 
telescopes perfectly. 
IE:   1.5-1.75-2.0-2.25-2.5-2.75-3.0    6061-T8 is a full  10 ksi stronger than 
 the T-6 version.   No swedging,
no lathe... just stuff 'A'  into 'B'..and use a pair of 1/4-20  SS  bolts and 
mating  SS nylocks. .... or a mess of
3/16" al  closed end rivets.  [ if u use rivets, they come in 2 x lenghts... 
designed for a total thickness of the 2 x joined 
pieces... short one is up to .250"  and 2nd one is  .250" to .375"      USE the 
longer rivet..desigined for .250" - .375" . 
If you reside in the middle of nowhere,  DX eng sells 6061-T8 in .120' wall... 
in 6' lengths.  You will have it within 3 days. 
Everything from 1.5-175"..... up to 3.0" .  

##  If u run any of this through mech  K7NV's   "YS 2.15"  or  
dx eng similar software..... you will see that
it's not required to use 3" heavy wall  boom sections for the entire length of 
the boom...waste of effort/money /weight. 

##  My F-12  6-el  15m yagi is on a 36'  boom.. and consists  of  6 x 6' boom 
sections, all 2"  OD... with 6" long x 2.25" outer
sleeves for the 5 x splices.   The wall thickness  in the middle starts at  
.125", then .058"  wall, then .049" wall at the boom tip. 
Then repeat for the other 1/2 of the boom.  It's all made from  6061-T6.  IF 
the boom to mast plate is in the center of the boom,
it works out that the boom is good for 125 mph.  In that condition, the 1st 6' 
on either side of the boom to mast plate is maxed
out at 40 ksi.  the next 6' section beyond that [ on either side] is at aprx  
28-30 ksi.   The tip sections are at 10.3 ksi.   IOW, the
last 6' section, at each end of the boom, is operating at only 1/4  of it's 
ultimate yield strength.   If I wanted to increase the windspeed
rating, [and stick with a 2" boom from end to end]... then the weak link  [ the 
center 12']    has to be beefed up... either with a longer
inner liner.. or a longer outer sleeve.  There is no point in making the boom 
end sections thicker, since [A] they are only operating at  1/4
of their ultimate strength.. and [B]  the weak link is in the middle. Unless 
you improve the weak link, you are wasting your time, and putting
weight where you least need it. 

##  Nobody uses uniform ele diam + wall thickness.. when  building a  20m 
Reflector,  nor a self support tower.   When the boom
is  broadside to the wind, the ele's  are  pointed into the wind....and vice 
versa.  Don't confuse strength  with flexing either. 
When you design, say a 20m REF, you can get away with 1.125"  x .116" wall in 
the middle 9'.... and taper down in 1/8" increments,
to just 3/8"  x .035"  wall at the tips... and it will survive a 100 mph wind. 

##  using the software, you can really get an eye opener.  A 2.5"  x .125"  
wall  boom.. [with a splice in the middle] ... OR  just one
piece of 2.5" OD  x .125"  wall  section in the middle, then tapering down to 
2.25" x  .125"  wall, then 2" x .058"   on the tips makes for
a very strong boom.    Similar combo's can also be used.... like 3" - 2.75" - 
2.5"- 2.25"- 2" .    My F-12  340N  uses 3"-2.5" - 2.25"  
[  they used a short 2.75" inner  sleeve to jump from 3"  down to 2.5"].  

## The point here is, you can carve a LOT of weight/wind-load out of both a 
boom and also els's.... and still have it survive high winds. 
No need for 175 lb style Telrex  20m yagi's.   Double the weight is = double 
the money.  Bigger OD  boom + ele sections = more windload. 

##  sure, a  3"  boom is stronger than a 2"  boom.   The 3"  boom also has 50%  
more windload.   Ditto with bigger diam ele's.  So the 1st
50%  increase in strength  from the 3" boom  vs 2" boom,  just got negated by 
the 50% increase in windload.    It's a series of tradeoff's.
EG:  a  3" x  .25"  wall  MAST is a helluva lot stronger than a 2" x .375" wall 
mast....yet they both weigh the same 7 lbs per foot. [same yield].
That's fine for a mast... since the slight increase in mast windload [ 3.33 sq 
ft  VS 2.22 sq ft]   is miniscule compared to the 50 sq ft rating of my
tower.  However, for a bunch of long booms, plus loads of long els... a  50% 
increase in average diam  really eats into the tower's max rating.
Or conversely, with a smaller windload yagi, the tower's max windspeed survival 
will be greater.  OR, you can add more ant's, before you get
to the max tower rating for a given wind speed.  

## here's a tip for the software.  If in a pinch, you can also use the  free 
ARRL  mast  software.   The mast software is only interested in the 
portion of the mast above the tower.  You simply enter zero ants/zero windload 
for the ants... and focus on the windload of the mast itself. 
You can then enter a  "mast height"    equal to 1/2 of your proposed boom 
length.... and then enter the boom diam at the center [boom
to mast plate], plus OD + wall thickness + yield strength.   Now if your boom 
to mast plate is offset [ it usually is].. then  simply enter the
LONGER of the 2 x halves.    IE:  40'  boom, with plate at 18'.    One half of 
boom is 18'.. the other half is 22'.   Enter 22' .  Also  notice that
long end of boom is 4' longer than the short end..... but the plate offset is 
only 2' !! 

later... Jim  VE7RF         


TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>