There is only about a 2% difference between the two sizes of mast you
listed, with the 2" x 0.25" being the stronger. Assuming 87,000 psi yield
strength, they would handle a max of about 3900 ft-lbs of moment. But make
sure you get your vendor's spec for yield strength -- 4130N can vary quite a
bit. And it really should be galvanized.
Steve, W3AHL
> Date: Sun, 16 Aug 2009 22:15:22 -0400
> From: <john@kk9a.com>
> Subject: Re: [TowerTalk] For Chrome Moly and tower/wind load experts
> out there - Mountain top tower installation
> To: <TOWERTALK@contesting.com>
> Cc: , trappaport@austin.rr.com
> Message-ID: <9FE2FFFFDC4740B88C362B40B2325B5A@Office>
> Content-Type: text/plain; charset="iso-8859-1"
>
> I doubt there's much difference in strength between the 2dia x 1/4 wall
> and
> the 2.5dia x 1/8 wall mast, however you can calculate this yourself using
> any mast software. K7LXC sells a product for this and there are also a
> few
> free programs on the internet. If a reasonable sized mast does not
> provide the strength that you need, you could always use a tower ring
> rotator and mount one of the antennas directly on the tower or use a
> single
> beam that will cover all of the bands that you desire.
>
> I'm not sure what protection adding a lightning rod on top of your tower
> will provide as all of the energy will still go through your tower will be
> induced into anything nearby.
>
> John KK9A
>
>
>
> To: towertalk@contesting.com
> Subject: [TowerTalk] For Chrome Moly and tower/wind load experts out
> there -
> Mountain top tower installation
> From: <Date: Sat, 15 Aug 2009 10:20:11 -0500
>
> <snip>
>
> HERE is the question: I have both 2.0" and 2.5" Chrome Moly Mast tubing
> available, each about 18' long. The 2.0" Mast pipe (I have learned that
> the
> word "tube" is the preferred term by experts, instead of "pipe") has a
> wall
> thickness of 1/4", and the 2.5" mast tube has a smaller 0.12" wall
> thickness. I
> suppose there is a book with a formula for yield strength of tubes of
> different
> inside and outside radii, made of the same material, its probably an
> undergrad
> ME question, but I was hoping some of you out there might know off hand
> the
> answer to this question:
>
> Which is better/stronger/wiser to use for ice/wind strength on a mountain
> top
> tower, and why?
>
> a. The 2.0" OD 0.25" wall thickness Chrome Moly 4130N tube, weighing 100
> lbs
> for 18'
> b. The 2.5" OD 0.120" wall thickness Chome Moly 4130N tube, weighing 75
> lbs
> for 18'
>
> And a second question: would it be wise to put a little 4' or 5' lightning
> rod
> above the mast pipe to attract electrons, and if so, is there a cheap/easy
> way
> to affix the lightning rod to the top of the mast, and what should it be
> made
> of...I am thinking of a sacraficial metal element that might offer
> protection,
> etc. and would be very hard to see ("invisible") so as not to exceed
> zoning
> height requirements.
>
> Thanks to all... 73 Ted N9NB trappaport at austin dot rr dot com
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