Obviously, you've done a controlled lowering and removal of the antennas,
perhaps without much trouble. This may be irrelevant as you may have
decided to cut it anyway. This is just another variation over using a
come-along.
On my tower (not Rohn 45 - though I work on 6 at my friend's station) the
tower top is larger across than 45. I had a machinist copy the US Tower
MRF (mast raising fixture). It is easily seen in the section of an HRO
catalog for US Tower. In my case a heavy piece of C-channel aluminum with
some pillow blocks for the thin wire-rope that coils up onto the small
Fulton winch at the end of the channel as it sits offset off the side of
the tower. The whole thing is bolted, temporarily, onto the flat plate of
the tower top. I'm not sure there is enough room on the 45 but perhaps it
is doable, some way.
I used two Yaesu thrust bearings that may not be as HD as the TB-3 but
served me well. My mast is around 2.5 in. OD. With the two levels of
bearing the whole mast is held above the rotator plate below and locked
between the bearings. The wire-rope feeds down through the two bearings
between the mast and the bearing wall. With it attached to the bottom of
the mast and with the rotator removed (so the mast could stick down below
its plate) I built my stack of 6m yagi on top, 2 el shorty 40 yagi next and
a Force 12 5BA (large 5 band yagi) about a foot above the top plate. The
mast stuck around 16 feet above the tower.
I'd attach an antenna, crank the mast up and secure the coax as it went.
Attach the second and then third antenna. Except for a little help with the
5BA I could handle the stack by myself. I could turn the whole array by
hand. I then installed the rotator with zero weight on it from the
antennas. Taking it down, a simple reversal of the process. Then you remove
the MRF. Granted, I think most of the reason for thrust bearings was for
lateral thrust (so I believe from reading what little is out there I found)
I found that the side bolts held the mast in place pretty effectively and
allowed slight adjustments in centering it all.
If this was a poor engineering design, I will stand corrected.
Unfortunately, it didn't stand the test of time to see... as it all came
down when the tower failed in a wind storm. The tower was under-engineered
though I followed the structural engineer's recommendations. Luckily,
insurance repaid me for it all. I've written about this episode, here,
years ago but I'm not sure if I wrote about the use of the MRF. FWIW, it
worked well and easily in my application. YMMV. I could have easily placed
the rotator plate lower down to use up mast length to build a shorter stack.
Aloha, 73
Kimo Chun, KH7U
Subject: [TowerTalk] FW: Cutting 2" Heavy Wall Cr-Mo Mast Up on Tower
Message-ID: <00b801cfa91a$45aa8280$d0ff8780$@net>
Content-Type: text/plain; charset="us-ascii"
I want to clarify the current situation of the mast relative to my
methodology and safety issues.
The 22 ft mast has been lowered in order to remove all the antennas. The
top of the mast is currently 4 ft above the top of the tower.
The weight of the mast is supported at three support plates and a steel
support under the bottom of the mast. My chain hoist is also supporting the
mast. My intention is to use my heavy duty gin pole to lift the 10 ft
section from the top of the tower once it has been cut.
Thanks for all the input that I have received so far.
Vic, N2PP
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