John,
The difference in yield strengths between 6061-T6XX and 6063-T832 tubes
are not very much!
The min yield strength for 6061 alloys are ~40ksi, I have seen certs
showing values of 42-44 ksi.
I have seen certs for 6063-T832 tubes that the late Force12 (in CO), who
I consulted for to mechanically design all their new antennas, showed 39
ksi.
This is not a problem if one has the tools, and knows how to design them
properly.
The difficulty I have seen is that it is hard to find sources for all
sizes of the 6061 tubes one might need that will fit together easily, as
many of them are made with 1/16" +/- thick walls.
While the 6063- T832 tubes are considered to be "architectural grade"
items, instead of "structural grade", they are available in .058" thick
walls that will fit together without any drama, and are quite suitable
for building antennas at a lower raw material cost point. This is what I
designed and built the new W7RN 80M elements with.
As you mentioned, the metric size tubes fit together more loosely, so
making adequate connections requires more fasteners....I doubled the
number of fasteners in the W7RN OB 80M antenna element connections to
correct the problem. None of those joints failed, only the lower
strength (larger) single wall tubes did, and much of that was due to the
fact that I couldn't find domestic sources for all the metric tube sizes
needed to properly reinforce the elements....we would have had to get
them from EU in the TBD alloys.
73, Kurt, K7NV
On 11/2/2017 4:08 AM, john@kk9a.com wrote:
I own several smaller 40m OptiBeam antennas which have held up fine and
from an RF viewpoint they are fantastic however I have heard the horror
stories about their 80m beams from other tower installers. OptiBeam also
makes a massive full sized 4el 40m which may have the same issues. I asked
Tom once what aluminum he uses and I seem recall a non specific answer
like aircraft aluminum or something like that. It is interesting that high
strength tubing may not be available in EU. Also metric tubing does not
telescope nicely, I wonder how that effects the strength of the joint.
Imperial sized tubing with a .058 wall fits together perfectly making it
easy to homebrew. For my homebrew antennas I only use 6061-t6, I see most
ham suppliers sell 6063-t8. What is the difference?
John KK9A
To: towertalk@contesting.com
Subject: [TowerTalk] Subject: Wind survival + load ratings... vs,
From: Kurt Andress <andresskurt@gmail.com>
Reply-to: kurt@k7nv.com
Date: Wed, 1 Nov 2017 21:47:45 -0700
Jim Thompson, I think your evaluation of the Optibeam 80 is lacking some
important knowledge. I learned more than I ever want to know about that,
by being the mechanical design engineer, and lead for all tower work for
the W7RN station. We tried twice to redesign, reinforce, and do everything
possible to make two of those antennas survive up there, with no success.
After a few years of effort, and constant search & inquiries, I learned
that many EU antenna builders do not have ready access to high strength
alloy tubes, like we do here. They can get ~40 ksi equivalent tubes in the
small tube sizes (maybe up to around 1" diameters), but not in the larger
ones.....the larger tubes are rolled and welded, with visible seams, not
drawn, with a yield strength of about 1/2 of what we can get here with
6063-T832 drawn tubing. I heard some of the tubes were made in Turkey.....
So, plug 23,200 psi yield into those analyses and see what it does. In the
UK, they can get real 6061 alloy tubes, but they are 1/16th wall and don't
telescope, so they have to grind tubes to get them to fit together. So, to
solve the W7RN problem, I designed from scratch, new elements for one
antenna with 6063-T832 American sourced tubing. The static YS analyses say
they are safe @ 130 mph. My Linear Finite Element analyses show that the
tips of the elements are deflected far enough to be parallel with the wind
at that speed, so they are capable of well over that wind speed. Emperical
experience says those elements survived the worst winter in over 10 years
up on the Comstock, with some significant icing....they are the only
elements that have survived up there for longer than a few weeks ;-)
YMMV......etc,...... back to net......
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|