Actually I think the old, cheap, TV towers had it right. They had a sleeve
at the top and that served as a "side" thrust bearing.
This is a lot more than the original question, but I thought this would be a
good time and place to address the thrust bearing issue.
To directly answer your question: It depends. It depends on the ratings and
design of your rotator. It also depends on the actual layout of the system.
If the weight of the mast and antennas is within the specifications of the
rotator and IF the rotator is designed to run with weight then the thrust
bearings should not support any weight. IF I am not mistaken the Tail
Twister is designed to support a substantial weight. You do not want to have
it end up with that weight eliminated or worse yet, have a pull against the
top of the rotator.
Whether you run structural steel, or aluminum mast there is the risk, or
rather likelihood of the mast and tower expanding at different rates due to
temperature. IF the tower expands faster than the mast you can end up with
substantial *pull* on the top of the rotator.
I don't know, but I have an idea that this has probably destroyed a lot of
rotators. The thrust bearing is probably one of the most misused items when
it comes to antenna installations.
Were it me, and it's not, IF there is enough room I'd install a short sleeve
over the mast and let the TB-3 clamp to that so the mast would be free to
move vertically inside the TB-3.
My antenna system, which I still don't have back together (although it's
still "up there" swinging in the breeze held in place by nothing more than a
come along) is going to be done differently this time. Instead of a thrust
bearing on top I am installing a sleeve much like the taper top sections
available for many towers. The OD of the mast going up is 2 1/2". I am going
to use a two to three foot long piece of pipe that will be a slip fit over
this for a bushing/thrust bearing. If need be I'll turn some bushings to
get a proper fit, but I don't think that will be necessary. This pipe will
fit between the bottom side of the tower (45-G) top plate and a rotator
mounting plate. This will make the entire thing adjustable. The pipe will
also have a number of zerks and lots of white lithium grease.
There are several reasons for this. The two major ones being it is a very
effective thrust bearing for side forces and it will allow the full weight
of the system to set on the rotator which is designed to support well over
twice this much. Another reason for the two to three foot sleeve is the
antenna system extends to 30 feet above the top of the tower. This has
completely destroyed one TB-3 in five years. Also with that kind of height
and the rotator being well down in the tower it serves as a pivot point
which allows for a lot of mast movement. The sleeve will tend to prevent
this pivoting and should reduce stresses substantially for the whole system.
Yes there is a TB-3 mounted between two and three feet above the rotator to
aid in rotator removal. The mast clamping bolts are backed out. There is
also a saddle clamp above that with a clevis to which I can easily and
quickly attach a come along to lift the weight of the whole system which in
this case is substantial.
I should be up there right now redoing the boom to mast clamps on the
6-meter and 2-meter antennas getting ready for next week's work, but I'm
suffering from a severe case of "Lack of ambition". That means I'll
probably be up there in the snow next week when the LMR-600 and connectors
arrive still working on the antennas.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
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