[TowerTalk] aluminum tubing strength with holes.

Sun Jun 9 12:44:01 EDT 2013

Hi Grant,

Thanks for sharing.  My understanding is that shear stress is not dependent
on slippage since it is acting perpendicular to axial tension/compression
which results from bending, so I agree with Leeson.  I am very curious if he
found that the same is true for bending stress in telescoping elements?

73
Matt
KM5VI

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Grant
Saviers
Sent: Sunday, June 09, 2013 9:33 AM
To: Matt
Cc: 'reflector -tower'; antennas at qth.net; 'K1TTT'
Subject: Re: [TowerTalk] aluminum tubing strength with holes.

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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