[TowerTalk] Mast Calculator ??

Jim Thomson jim.thom at telus.net
Mon Nov 9 23:31:53 EST 2015

Date: Mon, 9 Nov 2015 20:56:32 +0000 (UTC)
From: David Gallatin <kc9eev at yahoo.com>
To: Robert Harmon <k6uj at pacbell.net>, "towertalk at contesting.com"
<towertalk at contesting.com>
Subject: Re: [TowerTalk] Mast Calculator ??

The DX calculator is cool to use BUT it is only designed to be accurate with their chrome-moly pipe specs whereas the WD9P allowed you to plug in your own.?73, David,

##  The free mast calculator on the arrl website will factor in as many yagis as you want, and located anywhere..... on any length of mast.   It will also handle any OD, and ID,  and any wall thickness,
and any yield strength.   The dx eng mast calculator is only for their tubing.  I use 2.00 inch OD chromolly with a wall thickness of .375 inch....and a yield strength of 107 ksi.   Its fun to play with the
arrl mast calculator.... BEFORE you buy a mast.   You will soon see that a 3.00 OD mast with a .25 inch thick wall, is a helluva lot stronger than a 2.00 inch OD mast with a .375 inch thick wall...
yet they both weigh and cost the same. 

##  what it boils down to is..... yield strength  X  sectional modulus = bending moment.   Section modulus is determined by the  OD, ID, wall thickness.   Bigger OD and thinner walled masts have a higher
sectional modulus vs  smaller diam and thicker walled masts.   Once the wall thickness exceeds .25 inch,  its time to increase the OD instead. 

##  Think of the portion of the mast sticking out above the tower as a  giant Torque wrench.  The yagisS are loads being placed at various spots along the length of the tq wrench. 
## For XXX  wind velocity, pretty simple matter to calculate the windload for each yagi.    Windload  X  distance above the top of the tower = bending moment.   Total all the various bending moments
and see if they exceed the safe bending moment of the mast itself.   If the mast is gonna break, it will be right where it comes out of the top of the tower. 

##  the arrl mast software makes all these maths a lot simpler.  Just enter in the sq footage of each yagi..and where on the mast its located etc. 

Jim   VE7RF   

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