Date: Thu, 29 Jul 2010 19:53:17 EDT
From: Cqtestk4xs@aol.com
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 software..like 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
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