[TowerTalk] Aluminum Tower Mast

Grant Saviers grants2 at pacbell.net
Wed May 5 16:47:39 PDT 2010


6063 is cheaper than 6061 so is the alloy of choice for electrical
conduit (sch 40) & irrigation pipe (sch 10 if it has gasketed
couplers).  Gas/water pipe uses NPT (national pipe thread) tapered
threads for sealing, conduit uses NPST (Nat pipe straight threads) which
have no taper, thus allowing more adjustment in joints.

6061 and 7075 are the common structural aluminum grades, usually in T6
or T651 temper and are the ones that should be used for a mast.  I had a
TH7DX at 15' above top of rohn 25, 2" od x 0.375 wall 6061 T651 and it
was up for 15 years and 3 hurricanes w/o a problem.  I used 6061 to save
weight because the tower was hinged at the base.  Weights: 94lb for 2" x
.25 wall 4140 vs 46lb for 6061 x .375 wall vs 70lb for rohn 45 x 10'.
So hoist of a 20' x 2" x .25 wall steel mast is more load than rohn45
for a gin pole.

A problem with aluminum not mentioned before is there is no defined
stress threshold to avoid fatigue, it just keeps going down with stress
cycling.  Alloy steel has a plateau in the fatigue curve which if not
exceeded, the structure will never have a fatigue failure  (about
35ksi).  But it takes 10 million cycles or so to get into trouble with
aluminum, so that may not be a concern for the typical ham mast, but
some airplanes did fall out of the sky until this was understood.

The Jorgensen handbook/catalog has a wealth of information, pdf at
http://www.emjmetals.com/pdf/bluebook-k.pdf

Grant
KZ1W

knormoyle at surfnetusa.com wrote:
> [Dennis] "From day one, we used an aluminum 2" OD THICK WALL ALUMINUM CONDUIT Mast."
>
> I was curious about aluminum conduit. A quick search says that modern aluminum conduit is 6063 temper T-1
> (our typical 6061 use is T6) 
>
> Wikipedia says ( http://en.wikipedia.org/wiki/6063_aluminium_alloy )6063 T-1 has a yield strength of only 9kpsi
>
> Regardless of the wall thickness, I would think the low yield strength would make many other options better for equivalent weight.
>
> also, conduit is specified as I.D. was this 1.5" I.D. -> 1.9" O.D. conduit? or 2" I.D. -> 2.375" O.D. ?
>
> ??
> -kevin
>
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