In a message dated 97-11-11 02:48:44 EST, CRB@knersus.nanoteq.co.za writes:
> 1. Any idea what would have happened under these conditions if the
> mast was not pinned? I would think that the kind of torque
> that could cause this much play would have allowed the mast
> to turn in the rotator, necessitating several trips up the tower
> during the lifetime of the system. Maybe those same trips could
> have been used to do some routine maintenance instead.
I also pin the rotator to the mast ONLY if the station owner insists on
it and despite my advice not to. Besides the galling that potentially can
occur to the bolt hole, the torque of the antenna system justs winds up at
the next weak point of the system - usually the gear train or brake of the
rotator.
Allowing the antenna system to harmlessly turn in the rotator
occasionally in high wind situations (with subsequent re-alignment required)
to me is MUCH more preferable to replacing a busted rotator.
>
> 2. Following healthy engineering practice and using a pair of thrust
> bearings above the rotator would remove anything but torque from
> the rotator. That being the case, I cannot see how the mast
> could damage the rotator if there is play in the mast.
If there is "excessive" play in the mast/rotator junction, the wind can
get a running start at the mechanical weak point and slam against it over and
over with increasing force and potential damage.
Chris, I'm not sure I get your point. It's not the mast, of course, that
damages the rotator. It's the wind pressure on the antennas and mast that
translates into torque at the rotator (and then the rotator plate and then
the tower legs and then the guy bracket and then the guy wires).
One thing you can do to reduce wind induced torque on your mast/rotator
with multiple antennas is to install the antennas on opposite sides of the
mast. If you have 2 identical antennas mounted on the same side of a mast,
the torque total is 2X (torque X of one antenna + torque X of the second
antenna). If you mount one antenna on one side of the mast and the second
antenna on the opposite side, the torque forces cancel each other out - a
MUCH more desirable situation. The same principle can be applied
approximately to a stack of 10, 15 and 20M yagis. Mount the 10 and 15 on one
side and the 20 on the other. It won't be an exact cancellation but the net
result is still enough to make it HIGHLY recommended.
73, Steve K7LXC
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