4130 masts

[Stan Griffiths]

>Has anyone had a failure with 2" x .25" 4130 chrome moly. If so, what 
>did you have on your mast and what were the weather condz at the time of 
>failure. I would also like to hear from those who are using 2" x .25" or 
>bigger who have not had failure. I would like to know what you have on 
>your mast (sqft, boom length, how much out of the tower, etc). If there 
>have been any failures, what will you use next time?
>
>Also, are there any opinions, pro and con, to using either a larger 
>diameter and/or thicker wall? One would think that either one would 
>increase the PSI rating. But I am finding that not to be true. In some 
>cases the PSI decreases. Can anyone explain the the math on this? Or is 
>it a function of the percentage of elements in the alloy? This would 
>seem to be the logical explantion. I have found 2" x .25" 4130 with PSI 
>ratings from 110,000 up to 168,000.
>
>A number of years ago Stan, W7NI, had an excellent article in NCJ on 
>masts. Does anyone remember what year and/or issues?
>
>73
>Bob, K6OMB

Hi Bob,

I can't even remember what issue it was in!  Actually, it was in two issues.
NCJ reprinted it in the 10th Aniversary issue as one of the best articles
fromt he past.  I was extremely flattered!  In any case, if you can't find a
copy of it, I can print you one and mail it to you.  Let me know.

As I understand it, and I am not a metalurgist or a mechanical engineer
(however I consulted some when I wrote the article), what you call the "PSI
rating" I called the Yield Strength of the material.  It is not a function
of the shape of the material, rather it is a function of the alloy of the
material itself and perhaps the way it has been heat treated or hardened.  I
can't explain why you are seeing more than one value for 4130 unless there
are actually different grades of 4130.

I bought a couple of steel masts in 1971 long before I did any real research
or wrote the article you mentioned.  I do not remember what the alloy is but
I told the steel company what I was doing and asked them to select an
appropriate alloy for me.  I guess they did since both of those masts are
STILL in use after 26 years.

What I actually bought were two lengths of 2" OD 0.219 wall steel tubing,
not galvanized.  I paid about $50 each for them.  You could not specify an
exact length so I told them at least 20 feet.  I got two pieces 22 feet
long.  They were very heavy and I did not think I could handle them on top
of they tower at that length so I cut them down to 18.5 feet and I have
installed them both two times so they can be handled.

They are both mounted in top sections of Rohn 25 and in both cases, the
rotators are mounted at the junction of the top of the section just below
the top section and the bottom of the top section which means about 8 feet
of the mast is inside the tower and about 10 feet is above the tower.

One tower uses a flat top (25AG4) with a thrust bearing on the top plate and
a Rohn wooden block mounted on a rotator shelf where it will fit about 2
feet below the top.  This arrangement allows me to remove the rotator for
serevice and not worry about the mast tipping over.  This tower is a total
of 115 feet high with 4 set of guy wires on it and has a 3 element Telrex 20
meter beam at the 125 foot level.  At the top of the tower is a HyGain 3
element shorty forty.

The other tower uses a pipe top (25AG3) and a thrust bearing mounted on a
rotator shelf where the rotator normally would be.  I can remove the rotator
from this tower for service as well with no worry about tipping masts.  The
total height of this tower is 102 feet with a 5 element Wilson 10 meter beam
(20 foot boom) at the 112 foot level and a 6 element Wilson 15 meter beam
(32 foot boom) at the 102 foot level.

I have never had any problems with either of these masts.  I painted them
with Rustoleum and I have been happy with that as well.  Both towers have
other beams sidemounted at lower levels and most of them rotate.

I have a third tower (110 feet of AB105) with a homebrew 5 element 20 on a
48 foot boom and nothing stacked above it.  The mast I use there (believe it
or not) is the two 3.5 foot pieces I cut off of the other two masts and
welded them together with a piece slipped inside for strength at the weld.
It has been up without problems since 1975.

I am sorry my story does not match exactly what you wanted to know.  I can't
swear the alloy I am using is 4130 nor do my masts have 0.25" wall but they
are fairly close.

Incidentally, I had two separate failures of water pipe as masts earlier in
my ham career and that was enough of THAT.  What led to me writing the
article was my good friend W7WHB wanted to put up a stack of three
monobanders on 140 feet of Rohn 45 and he pressed me to help him calculate
the mast so it would not bend.  In his case, the mast is 14 feet out of the
tower and is composed of three separate pieces.  At the bottom is a 2" OD
0.25" wall piece about a foot long that fits standard rotators. That piece
slips inside of and is welded to a 2.5" OD 2.5 wall piece that extends up
for about 10 feet.  Here another 2" OD 0.25" wall piece slips inside and is
welded and extends up another 7 feet or so.  The top of the tower had to be
modified to accommodate the 2.5" OD mast.  Gordy has three monobanders on
that mast.  At the top of the tower is his 5 element 20 with a 40 foot boom.
7 feet above that is his 6 element 15 on a 40 foot boom, and 6 feet above
that is his (I think) 6 element 10 on a 32 foot or so boom.  This stuff has
all been up since about 1976 and is still there with no problems.  We
determined that a 2" mast just was not big enough for the load he wanted to
put on it which is why we had to go to 2.5" where the stress was greatest.
Sorry but I do not remember what alloy he selected either.

Stan  w7ni@teleport.com


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