[TowerTalk] aluminum tubing strength with holes.

Sun Jun 9 10:33:17 EDT 2013

On 6/8/2013 11:32 PM, Matt wrote:
> 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.
+++++++++
So far so good, but Leeson, in Physical Design of Yagi Antennas covers 
the following and his analysis is that for round telescoping tubes, they 
act as a single tube of combined wall thickness and do not need to be 
fastened together periodically to share the shear. This is not true for 
other structural geometries, with the possible exception of square tubes.

see pages 4-19,20

Although my structural analysis capability is rudimentary, the Leeson 
analysis seems ok to me, and antennas I've reinforced without pinning 
the tubes haven't come apart.

Grant KZ1W

> 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 at contesting.com] On Behalf Of K1TTT
> Sent: Saturday, June 08, 2013 7:53 AM
> To: reflector -tower; antennas at 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 at arrl.net> mailto:k1ttt at 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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