On 4/8/2011 9:06 PM, Dan Zimmerman N3OX wrote:
>> Next batch of tube I order is going to be stronger and harder than 6061-T6.
> But it's not going to be [i]stiffer[/i] which is what you want if you're
> worried about the sag when raising it. The modulus of elasticity varies
> very little across types of aluminum... different alloys have SLIGHT
> differences but a quick sampling on Matweb suggests there's something like a
> 5% elastic modulus difference between, say, pure aluminum and expensive 7075
> 'Course if you tried to put up a pure soft aluminum mast the size of the one
> you did, it would probably fold over in ten places... but that's because you
> would have exceeded the yield stress.
> Probably makes more sense to go BIGGER next time. Tube stiffness goes like
> the fourth power of the diameter.
Agreed, but there are physical limitations involved with most structures
and this one is no different.
2/1 or 3" with only an 1/8" wall would have been far more rigid than the
2" with 1/4 inch wall, But unfortunately I don't have room for 3" and
many antennas (like the AV640) will not fit even 2 1/2" mast. True I
could modify the mount in this case but the 2" 6061-T6 is sufficient for
this application, even if it does look like a fly rod trying to life a
very heavy weight. OTOH there is no where near the flex in the 2" as
there is in the AV640 itself.
In the case of a 24' mast for a temporary back up antenna the extra cost
of the larger diameter mast would hardly be justified.
Typically in towers I use DOM for mast and schedule 40 structural steel
tube for structure which is far stronger and much more rigid. (Lot
heavier too) <:-))
Also for mounting larger antennas I can change out the boom to mast
clamps with any "reasonable size" I wish to use.
> 2.5 inch tubing would be nearly 2.5 times
> as stiff as 2 inch tubing... and even if it had a thinner wall it would
> tend to be considerably stiffer.
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