[TowerTalk] Heavy Duty Masting

[K8RI on TT]

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
> alloy.
>
> '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.

73

Roger (K8RI)


>    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.
>
> 73
> Dan