[TowerTalk] Strongest mast material?

[Michael Tope]

Mike,

There are two factors which determine the ultimate strength
of a mast - (1) cross section geometry (e.g. section modulus),
and (2) material properties (e.g. yield strength). There is a
good discussion of both of these topics in Dave Leeson's
book, Physical Design of Yagi Antennas. In summary the
formula for determining ultimate strength is:

         BM =  ys*SM                                            [in*lbs]

Where BM is the maximum bending moment, YS is the yield
strength of the material measure in PSI, and SM is the section 
modulus of the mast geometry measured in inches^3.

The section modulus of a cylindrical tube can be calculated 
by the following formula

        SM = 3.14*(D^4-d^4)/(32*D)                  [inches^3]

Where  D is the outer diameter, and d is inner diameter of the 
tubing measured in inches. I have all this setup in a spreadsheet
which makes playing with the numbers very easy. 

I believe chrome moly is probably the strongest material, but 
it comes in different alloys so be careful - yield strengths can
vary anywhere from around 100,000 PSI to 180,000 PSI. 
The bigger this number, the stronger the material. 

Also, make sure you leave plenty of mast in the tower. With 
only 2' in the tower you are going to have huge sideloads on
your rotator. Also, consider that servicing may be tough unless
you allow for a second thrust bearing to capture the mast 
when the rotator is out. Chrome moly is available in 24' lengths,
getting 2 extra feet in the tower is probably worth your while. 

If you engineering a big stack, I highly recommend getting 
Leeson's book. He discusses things like stepped mast diameter 
for optimum strength to weight ratio (e.g. putting stiffening 
tubes inside the bottom most portion of a mast to add strength 
where its needed). The book is a must read for this sort of thing. 

Good luck,

Mike, W4EF..............

----- Original Message ----- 
From: michael watts <wy6k@yahoo.com>
To: List Antennas and Towers <towertalk@contesting.com>
Sent: Tuesday, April 18, 2000 10:21 PM
Subject: [TowerTalk] Strongest mast material?


> 
> So which mast is stronger:  Chromally or High Carbon
> Steel?  What is the general consensus on the strongest
> mast.  Double wall, triple wall?  
> 
> I need 22 feet, 20 feet of which will be above the
> tower.
> 
> Thanks!
> 
> Mike wy6k
> 
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