I'm a little confused by all the stress numbers being quoted. I think
the number that matters is yield strength so the mast doesn't
permanently bend. Tensile or ultimate has value only to insure it won't
break. Here are the specs I found from Jorgensen. These are the usual
"chrome-molly" alloys used for aircraft, roll cages, etc.
4130 cold drawn tensile 110ksi yield 95 ksi
4130 hot drawn 90ksi 70ksi
4140 cold tensile 120ksi yield 100ksi
4140 hot 120ksi 90ksi
heat treat P110 yield 110 to 140ksi (do the material certs give an
actual value??) one reference says the spec is "110ksi minimum" so that
is a nice guarantee, but not much over the as drawn 4140 value (which is
probably a nominal, not minimum value).
As I thought, 4140 is the strongest. VE7RF quotes a very wide range for
not heat treated yield values, what is the source? I am pretty sure
that hot dip galvanizing (at 860deg F) after heat treat is a no-no, that
temp is way above any normalizing temperature used after heat treating.
This leads to a question - what is the safety factor that should be used
for masts? If I accept the EIA or local wind max 3 second gust
standards and plug these into the various mast stress calculators, what
should the allowable calculated stress be vs the mast yield
specification? 100% - not so good I think. 75% - maybe. 50% - getting
pretty heavy. 33% - probably if life safety is involved and also
insures no fatigue failure. What allowances for wear or corrosion?
These alloys could be magnafluxed and probably would be (or xray'd) for
critical high pressure or oil well drill piping but I won't be doing that.
If I buy from Jorgensen etc., I can get the mill certification
paperwork. Has anybody gotten these from one of the ham radio vendors?
Any lab tests of masts?
Also, since strength gains from increased mast diameter are bigger than
gains from thicker walls, where do I find 2.5", 2.75", and 3" thrust
bearings?
(more) Advice?
Grant
KZ1W
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