[TowerTalk] Tower/mast question

[n4kg@juno.com]

N4KG response follows question.

On Tue, 23 May 2000 11:32:38 -0400 Ted Sarah <w8tts@gwis.com> writes:
> 
> I currently have a Glen-Martin hazer on a tower with a Mosley
> TA33-JR-N-WARC mounted just above a thrust bearing, which 
> is about  3'  above the G-8000S rotor.  What I would like to do 
> is add a mast to this, so  that  there is  an additional 10 feet 
> above the top of the hazer thrust bearing, and mount the 
> TA-33 at the top of the mast.  This would do a number of things 
> for  me  including being able to adjust the height of the antenna 
> a little, getting the antenna 10' higher without adding tower, and 
> as I do now being able  to lower the antenna if there is a high wind 
> warning.  Yes I know, the  wind  load on the antenna 
> (96lbs at 80MPH) isn't much, but better safe  than  sorry.
> 
> Now my questions.  Is it safe to go that high above the thrust 
> bearing, which is a Glen-Martin, with a mast and antenna?  
> And, I'm looking  at using 1-1/4" Schedule 80 pipe for the mast,
> is this over kill?
> 
> TNX in advanced,
> 
> 73 - Ted - W8TTS
> 

You need a LOT more information to come to an exact calculation,
but my guess is that you are asking for trouble.

Your antenna windload (96 lbs at 80 MPH) times a 10 ft mast length
means that the 1-1/4 inch dia. mast must withstand 960 ft-lbs of
bending moment at the thrust bearing.  Do you really think a 1-1/4
inch pipe can stand up to that?  I often bend 1 inch dia pipe turning 
in screw anchors with a 3 ft lever arm.

The bottom end of the mast must also support 960 ft-lbs of
bending moment.  How far is that end below the bearing?
If it is only one ft below, then there would be a lateral force
of 960 lbs on the rotor / mounting hardware.

The maximum bending moment for a solid rod goes up roughly 
as the 4th power of the diameter.  This means that a 2 inch dia
solid rod is 6.5 times as strong as a 1-1/4 inch diameter rod.
For pipe, the formula is a function of the 4th power of the 
outside diameter minus the 4th power of the inside diameter.

I wouldn't put anything more than a foot or so up on a 1-1/4 
inch diameter pipe.

You really need to understand all the forces involved and 
yield strength of your materials before extending your mast.

de  Tom  N4KG


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