One of the reasons is to increase the safety factor when raising (and
lowering) the mast as you put antennas on top of it and servicing.
A good example of a poor installation is one that we took down recently
where a 23' mast was only 2' inside the 50' bracketed Rohn 45 tower.
Picture on the 2" steel mast:
Cush-Craft X9 at the top
Cush-Craft 2/6 meter beam in the middle
Cush-Craft 2-El 40 about 4' above the top section.
Rohn TB3 thrust bearing on a flat top-plate.
Without a crane, this can become a very dangerous situation. In order to
remove the antennas, the rotor had to be removed first. Luckily, there
was enough room to do this without having to lift the mast. With a
lift-rope attached to the mast with a pulley system, the thrust-bearing
bolts were loosened a little -- no movement! Loosen some more -- no
downward movement! The array begins to lean and that's when things get
really exciting. Just to the math and you can imagine what was beginning
to happen.
Fortunately, there was no wind and a bit more loosening of the TB bolts
finally let the mast go free to descend. Once it was about 5-6 feet
inside the tower we were able to breathe normally again.
Further inspection of the top section and plate revealed that one of the
legs had begun to crush right at the top plate attach weld as the hole
in the leg at this point was severely elongated and the leg pipe was
crimped. I can't really say if this was an existing condition, or if it
happened during the disassebly. I think I would rather not know!
As a result of this experience, if long masts are planned above the
tower, there will either be 1) A sleeved top section 2) Two thrust
bearings
or 3) A flat-top/thrust bearing and enough mast inside the tower to keep
the bend moment manageable.
In Rohn 45, you can pretty much put the rotor plate anywhere, but in
Rohn 25, you are somewhat limited to the top window or the bottom joint
between sections unless you wish to modify the rotor plate.
Good Luck!
--
Thomas M. Delker - K1KY CFO#395 - Smyrna / Gallatin / Short Mtn. TN
A.R.R.L. District Emergency Coordinator - Tennessee District #4
C.A.T.S. (W4CAT) - 147.30/145.23/145.49/145.17/223.98/444.45/444.65
Stones River A.R.C. (K4FUN) - http://www.members.home.com/srarc
Tennessee Contest Group (K4TCG) - http://www.k4ro.net/tcg.html
CODE WARRIOR © #2 - Mobile CW Operator
"Putting code on the road using the primary mode"
--
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