Date: Tue, 04 May 2010 18:08:06 -0500
From: Dennis K5AVT <dariise1@gmail.com>
Subject: [TowerTalk] 2" Aluminum Tower Mast
My story about a 2" - 20 Foot Aluminum Mast = GOOD !
From day one, we used an aluminum 2" OD THICK WALL ALUMINUM CONDUIT Mast.
## what EXACT alloy ?? WHAT wall thickness ? How much KSI yield
strength ??
Once you know that, the rest is simple... just plug the number's into the arrl
mast spread sheet. [ which includes each ant at XXX height above top of tower]
## You can't just stuff 2" thick walled AL mast up... and cross your
fingers. I call that
hip pocket engineering. Your th6dx is up only 6'. Wind PRESSURE goes up to
the SQUARE
of the wind velocity.. regardless of who's formulae or standard you use. 70
mph is double
a 50 mph wind. 86 mph is TRIPLE a 50 mph wind. 100 mph is QUADRUPLE a 50
mph
wind. I'd say you are lucky. Now if you raised the th6 by just 1 foot, you
could easily be
in trbl.... or swapped the th6 for a bigger yagi... at the same 6' up the
mast.
## AL mast are great, esp the 2.875" to 3" OD thick wall variety. 6061-T6
is only 40 ksi
so you still need to run all of this through the spread sheet. When an AL
mast breaks, it
snaps off, it doesn't do a 90 deg, like steel.
We
engineered this tower with a Top mtg 2 meter beam @ 15 feet [Top], a 6
El 6 meter beam @ 11 feet [Middle] and a Hy Gain TH6DX @ 6 feet {Bottom]
with the balance of 5+ feet inside the tower 25G-A2 top section to the
HAM-M Rotor on a Rotor Shelf about 5 feet down.. No bearing , sleeve or
bushing used.
## OK, how how do you remove the rotor, if required, with now WAY less than
5' of mast inside the tower... and nothing to support it ??
The 2" mast did not show any wearing from the throat of
the top of the tower . A little scratching was evident however.
## careful here. I was sent pix, just a few months ago, where a fellow's yagi
had slipped
on the AL mast.. then slipped back, then forward, then back, never going more
than 15 degs
on either side. He only had one huge yagi.. and just 1 ' above top of tower,
so he never knew it
was out of alignment. One day a buddy of mine went up there... and the boom to
mast assy had
sawed its way through the soft walled AL.. just like a chainsaw.. over a 30 deg
arc !! It was
an accident waiting to happen. He had used mufffler clamps to mount the plate
to the mast, and the
saddles had sawed thriugh the al mast.
## I gave up on muffler clamps years ago... and started using the DX
engineering jumbo boom to mast clamps.
These things are superb, and have huge surface area on both sides of the mast,
and are a textured finish. They
come in 2"... 2.5" ... and 3" only. I like the F-12 method of using double
plates. The single plate concept is abt as
useless as it gets for installing yagis'. I recently made up a new batch of
plates for 4 x yagi's.... and made them
with 17" w x 8" H x 3/8" thick 6061-T6 plates for the horizontal
plates..... and 8" w x 14" H x 3/8" thick
6061-T6.. for the vertical plates. I used 2 x of those DX eng jumbo mast
clamps on the vertical plate..... and 6 x
SS U- clamps for the horizontal plates [ 3 x per side]. The SS U- clamps
are from DX eng, and all have super wide
1" SOLID AL saddles ! The dx eng jumbo mast clamps each use a pair of
3/8" long, G-8, armour plated bolts, to compress
the 2 x halves. I swear, one of those jumbo mast clamps = 12 x U /muffler
clamps.
## I mate the 2 x plates with 5 [five] 1/2" SS bolts, then one socket
wrench fits all of em. The 5th one in the middle
top... is tapped for a 1/2" thread. OK, now when raising the yagi [at a
cockeyed angle if you want]... you just mate the
2 x plates via the center bolt.. piece of cake... and eveybody can go home. I
use a block + tackle to get the yagi to top of tower,
then transfer [ via 2nd hook] to a cum-along. Cun-along is clmaped part way
up the mast. Fellow on tower has control
over lifting the yagi the last ting bit.. and mates the 2 x plates. Stick the
[cordelss] impact gun on there.. done.
## I got the 8" wide x 3/8" 6061-T6 plate from the local... 'metal
supermarkets' [ franchised across canada + usa].
The 8" wide x 3/8" plate comes in super long lengths.. and is called ..'econo
plate' . They just whack it off in their
water cooled giant band saw to any length you need. I have a roper whitney
bench punch, and use it to punch 3/8"
holes in the 3/8" thick plate. Then the holes are made bigger in the drill
press. [undersize hole for the tapped thread]
## this is the ultimate boom to mast plate assy I can come up with. You can
stand on it too. Where the plates overlap
[8" x 8" portion], it's 3/4" thick !
That is a mast that was up for over 30+ years,many wind storms and much aging
to
remain in good condition. I have many friends that said they were stupid
using steel pipe mast as most of them were bent from slight to about 90
degrees.
## yes.. and if your friends had used a 2" AL mast, it would still have
snapped off ! Put a big load on a AL mast.. and watch the
deflection, it can get pretty wicked.. VS steel... IF the AL is not thick
enough. That creates a 2nd problem. The WEIGHT is now off
balance.. and CG has shifted way out, which just compounds the problem..sorta
like a beam column under compression. Toss in
heavy wet snow + ice for good measure. When constantly stressed, AL will
only take so many load cycles.
The bottom line is: I will always use a 20 foot, 2" Aluminum mast and
not a steel one. Worked for me.
## worked for me too.... but not on my latest array. Even 2" SOLID AL won't
work, it snaps like a tooth pick. Run various configs
through any of the mast spread sheets.. you will get a real eye opener. 40
ksi 6061-T6 is appealing, since it's not quite 1/3 the
weight of steel... and stronger than 35 ksi steel water pipe. Once the wall
thickness is aprx 3/8" thick, you are wasting your time,
and better off to increase diam to 2.5" or 3". A 3" mast with a 1/4"
wall.... weighs the same as a 2" mast with a 3/8" wall...
but is way stronger. [ same material and ksi in both cases]
Later....... Jim VE7RF
The bigger beams are not in my
experience handbook..
Thanks for listening to my opinion,
Dennis, K5AVT
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