On 12/27/12 1:49 PM, SPWoo wrote:
Hi Jorge,
It's easier to stick to a 2" OD. I went with 2" OD but with a 3/8" wall 4130 chromoly.
The cost difference between 1/4" and 3/8" is not significant. I want to make sure the mast
outlasts me! Attached below are the details of my mast and the cost. I had to pay another vender to
galvanize it for me for an additional $150. So a total cost of $650. Not cheap but when you look a the
big picture it's cheap insurance. Using the windload calculator this mast is good for 135mph for my two
yagis. We do get 120mph winds here once in a decade. GL and 73.
Description: 4130 CDS TUBING SR, ASTM A 519
2.000 OD X .375 W S/C 15'0" ( 180.0")
Heat: 001M59442 Item: 100549
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MECHANICAL PROPERTIES
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YLD STR ULT TEN %ELONG %RED HARDNESS
DESCRIPTION PSI PSI IN 2 IN IN AREA BHN
104628.0 122695.0 18.7 101
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EDDY CURRENT: OK
------------------------------------------------------------
With that yield (and hardness number!), it's definitely been heat treated..
Whether the hot-dip galvanizing changes the yield is hard to say. I've
been looking through my (very old) textbooks and steel company
application notes and most seem to say that the temperature of the
molten zinc is far enough below the temperature of the original heat
treat that it's not a big issue.
I did find one reference that says that hot dip galvanizing reduces the
fatigue resistance (i.e. the steel fails earlier from fatigue cracking).
In the ham antenna situation, I have no idea if fatigue is an issue.
Wind loads are repetitive, but generally quite small, so they may not
get up to where fatigue is an issue.
One would need to consult someone who actually knows this stuff to know
for sure.
Yes, a lot of folks invest in strong steel for their masts, and it works
just fine, but from an engineering standpoint, one wonders if it's worth
the extra expense. Putting up cold rolled 1020 at half the ultimate
yield might have survived just as well. Or, alternately, spending a few
hundred bucks to not worry about it might be worth it.
It's not like people instrument their masts to measure the actual loads.
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