[TowerTalk] Lower Mast Question Again

[Steve Maki]

"Sylvan Katz" <jskatz@sk.sympatico.ca> wrote:

>TTians
>
>Thanks for all of the responses and ideas. However, no seems to have
>addressed question. I concede that the top section of the mast (i.e above
>the tower and a few feet into the tower) should be steel - preferrably
>chromolly. However, I can not locate a source for chromolly in Western
>Canada so I guess I will have to use standard steel pipe. BUT why can't the
>lower section of the mast - say the bottom 30-40'  - be a much lower
>strenght material than steel. Have I made an error in assuming that
>essentally the major stress on the lower section will be rotational with
>very small bending moment? Is there an ME in the group that can given me a
>rough idea of the materials that will handle the rotational stress?
> tnx  sylvan

Rotational stress will work harden aluminum more than you might
guess.

Many years ago I noticed my 3el 40 freewheeling. I climbed the
tower expecting to bring down the rotator, but when I got up
there noticed that the mast between the rotator and the bottom
of the tower support tube was still, even though the part of the
mast sticking out of the tower was rotating with the antenna.

After rubbing my eyes in disbelief a couple of times realized
the obvious - the mast had failed INSIDE of the support tube
purely from rotational fatigue.

The mast was 3/8 wall 6061 AL tube, 2" OD. Had been in service
for 3 or 4 years.

Replaced it quickly with 1-1/2" sch 40 steel pipe (1-7/8" OD),
and that's been in service now for 15 years or so.

Note - water pipe is not adequate for stacks, but has very
good long term rotational strength, as the above anecdote
illustrates.

73,
Steve K8LX

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