On 12/28/2012 7:15 AM, Jim Lux wrote:
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
----------------------------------------------------------------
MECHANICAL PROPERTIES
----------------------------------------------------------------
YLD STR ULT TEN %ELONG %RED HARDNESS
DESCRIPTION PSI PSI IN 2 IN IN AREA BHN
104628.0 122695.0 18.7 101
----------------------------------------------------------------
EDDY CURRENT: OK
------------------------------------------------------------
With that yield (and hardness number!), it's definitely been heat
treated..
I see some suppliers offer HRC45 heat treating of 4130 which takes the
yield strength to over 200,000 psi - that is about 6x low carbon!
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).
Interesting, I was aware that electroplated zinc might cause hydrogen
embrittlement which accelerates fatigue. Normalizing for 4130 is done
at 1600d F and molten Zn is probably 860d F, so the reduction in
hardness/strength is probably minor for the amount of time in the bath.
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.
I think the rule of thumb is that fatigue life is unlimited if the
cyclic stress doesn't exceed 25% or so of the tensile stress. At 50%
stress, life is about 10,000 cycles from a reference I found, which
could happen in a mast. One time high stress substantially reduces the
fatigue life. So designing with a big safety factor is a good idea.
One would need to consult someone who actually knows this stuff to
know for sure.
Agree, I'm not that person, so YMMV.
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.
Previous posts have emphasized the benefit of increased diameter for a
mast. Generally, going thicker than 0.25" wall is not a good investment
and has excess weight. A 2.5" od mast with 0.25" wall has more than 2x
resistance to bending and increase in strength with a 29% weight
increase vs 2.0"od and .25" wall . A 2.0" od by .375" wall only
increases strength by 27% with 50% more weight.
Also, heat treating and 4130 costs money. Normalized 4130 has a yield
strength of 63,000psi and is tough to machine, yet is available heat
treated to more than 3x that, so if cost is no object or weight/diameter
is a constrained, then heat treating 4130 is a good choice. OTOH, A513
steel has a yield of 72,000psi and is about 40% the cost of normalized
4130, so for me it is my choice for high strength DOM (drawn over
mandrel) tube.
It's not like people instrument their masts to measure the actual loads.
I think the mast calculators are pretty decent estimates of loading.
There are also numerous web applets for calculating stresses and
deflections in beams with almost any load configuration that a mast will
have. It is instructive to see how much a mast can bend without damage.
Grant KZ1W
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