David,
First off, thanks for providing all of the great information resources over
the years. Since I don't see any reply to your question yet, I'll share
what I know in hopes that it may provide you some insight.
The drilled hole in the outer tubing creates a stress concentration which
reduces the strength of the member for certain types of loadings. If the
loading is purely axial in nature and uniformly distributed on the cross
sectional area of the tubing (not likely for an antenna element), the stress
concentration is theoretically about 225% more than for an undrilled tube.
The reinforced inner member would only strengthen the outer tubing under
this loading if it were attached to the outer tube on either side of the
drilled hole so that the load would be shared by both elements (which
creates additional stress concentrations).
If the loading is a bending moment, which is typical for antenna elements
(both from dead weight & wind friction), then the reduction in strength is
dependent on the radial angle of the hole with respect to the loading axis.
Bending loads translate to axial tension & compression stresses throughout
the cross section of the element. They are not uniformly distributed, but
rather they are maximum at the extreme outer surface that is perpendicular
to the direction of bending and theoretically neutral at the center of that
same section. One way to visualize this is to picture a purely horizontal
wind load that bends the element along its horizontal axis. A very small
hole that runs vertically through the element penetrates the neutral stress
axis and theoretically makes for little reduction in strength, whereas the
same hole running horizontally through the element penetrates the tubing
directly where the tensile and compressive stresses are peak. If the
loading is ice rather than a horizontal wind, then the opposite is true.
An inside reinforcing stiffener does not provide a whole lot of benefit for
bending strength unless its axial movement (slippage) is constrained along
its common axis with the outer element. This is true because the two
members slip at different rates when placed under bending load. This would
mean multiple screws along its length that work to lock the two elements
together - one can think of this as a way to make the outside load as it
were thicker material. Without being locked together, then the resulting
strength of the pair is approximately equal to the sum of the strengths of
the two individual members - which is considerably less than one thicker
member - especially if both the inside and outside elements have holes
drilled in them.
If you are not in a hurry and want a qualitative answer, I would be willing
to run a computer model on your element but might be a few days until I can
get some time to do it. I would need to know the wall thicknesses of the
inside and outside tubing.
73
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of K1TTT
Sent: Saturday, June 08, 2013 7:53 AM
To: reflector -tower; antennas@qth.net
Subject: [TowerTalk] aluminum tubing strength with holes.
I am assembling 5 of the 5 element 10m longjohns? I just got the first one
to the point that I put on the beta match and am not thrilled at the clamp
arrangement on the element. On these the driven element is split, the butt
end of it goes into a plastic insulator about 2? long that is then clamped
in the boom-element clamshell thingy. In their design an aluminum ring goes
around the element and is clamped with a #10 machine screw that also holds
the end of the rod and coax eye lug. I am considering replacing that ring
with a #10 bolt through the element. My concern is that the hole will be at
the point of highest stress for the driven element.
My question is this. What is the relative strength of an undrilled 7/8?
tube vs a 7/8? with ¾? piece inside it with a hole for a #10 bolt through
them.
David Robbins K1TTT
e-mail: <mailto:k1ttt@arrl.net> mailto:k1ttt@arrl.net
web: <http://wiki.k1ttt.net/> http://wiki.k1ttt.net AR-Cluster node:
145.69MHz or <telnet://k1ttt.net/> telnet://k1ttt.net
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