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My 2-cents worth on the T2X, HAM-4, and 25G

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Subject: My 2-cents worth on the T2X, HAM-4, and 25G
From: (jim henderson)
Date: Mon, 13 Jan 1997 03:45:11 +0000

        Regarding the current memos about T2X and HAM-4 rotors on 25G: I
chose the top bearing shelf (plate) with the Rohn thrust bearing. I have
used both of these rotors over the last 9 years to turn my modified KLM
7.2-10-30 LPDA (8 el on a 42' boom)at 75' with virtually no problems. 

        Since the rotor sits approx 18" below the bearing, I put the LPDA
boom no higher than that above the bearing. With only 1 bearing, it is too
easy for the loads to cause "coning" or yaw on the rotor, which is rapidly
fatal to it. A better arrangement would be to place a second bearing where
the rotor now sits, and move the rotor, a-la these discussions, to a movable
Rohn rotor plate a few feet below the second bearing.

        I replaced the KLM original overhead supports with my own version
consisting of 4 P'stran lines, 2 per each end of the boom. They attach to a
4' spreader 6' above the boom, and are tensioned equally to provide both
vertical and lateral support. This effectively moves some load up the 6'
mast and raises the C.G. well above the boom. This in turn increases the
"coning" effect, since the load is now above the thrust bearing by a longer
distance than the rotor is below it.

        All of this results in a loading exceeding the HAM-4's specs. In
spite of this, I used one for 3 years with out damage or break problems. I
balanced the LPDA carefully at the start, so I had no problems with starting
torque, sluggish starts, etc. When I replaced the original HAM-4 rotor after
3 years, the break notches were nearly gone, the expected result from the
constant desert winds. I replaced it with a new HAM-4.  Somewhere during
this time, I replaced the lines and re-tensioned them. I also didnt do it
quite right. In about 14 months I had to replace the HAM-4, and repair the
weld on 1 corner of the rotor mounting plate, a result of the unbalanced
forces caused by hasty tower work!

        I then re-balanced the entire load, and used a T2X. This arrangement
has proven very satisfactory. I inspected the rotor after 16 months, and see
no damage. During this time, we had a record-setting 100+ mph wind here,
which toppled the county communications tower. I am pleased to say mine
survived. Other than the usual "reluctant retraction" syndrome with the T2X
brake, it works well, and with this bearing shelf, has ease of access for
maintenance. (The tower has 9 guys, 2 sets torsion bars, and was engineered
for the Rohn "hurricane belt" wind data; this was the source of much local
laughter at the time!))

        Also, on my short tower used for satellite and 6m stuff, I use
either a T2X or HAM-4. I place one of the movable Rohn rotor mounting plates
(the one with bent-down corners and U-bolts) about 10' below the top bearing
plate, near where two sections join. I Place the plate in upside down, and
mount the rotor on it. The HAM-4 fits anywhere. The T2X will fit also;
depending on where you end up putting the plate, you may need to bend the
diagonal rungs a bit, but the movable plate give great flexibility in
installation. And for maintainance, the plate can be removed, carefully, and
the rotor extracted.

73 de Jim, KF7E
ex:     5X1XX, 7Q7JH, ZM7AH, ad nauseum. And, out of the tower biz, and
loving it!
Jim Henderson

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