[TowerTalk] mast strength

[W5LT]

Gregg:
The 87,000 is the stress in pounds per square inch required for the metal to
yield.  You need to know the cross sectional configuration of your mast, (OD
and wall thickness) and calculate what is known as the "section Modulus".
You then divide the bending force in inch-pounds by the section modulus to
determine the actual stress for your assumed conditions. Then divide the
answer into 87,000 to determine your safety factor.
You should have a 2:1 safety factor for good, safe design. 
If you want I can send you an Excel file with various size mast
configurations with section modulus figures and you can figure what is best
for you.

73s

Bob, W5LT

-----Original Message-----
From: Gregg Seidl [mailto:k9kl at centurytel.net] 
Sent: Saturday, November 18, 2006 4:48 PM
To: TowerTalk at contesting.com
Subject: [TowerTalk] mast strength

I'm considering stacking a Cushcraft XM-240 and 10 feet below that per 
Cushcrafts recommendations an X-7.I want to make sure that I get a 
strongenough mast so it doesn't go horizontal on me.The windload @ 80 MPH 
is142 pounds for the XM-240 times 10 feet is 1420 plus 202 for the X-7 comes

to 1622 pounds of bending force.Is this right?I see masts being rated at 
87,000,what does that mean,it takes 87 kpounds to bend it???Doesn't seem 
right.Thats a LOT of force,thats like driving our big John Deere on it and 
it wouldn't bend until the 4 foot level       20000 pound tractor times 4 
equals 80000.......I think I'm missing something here.    Gregg  K9KL