At 01:29 PM 8/25/2007, Ron Todd wrote:
>So why does a rotator want to have that second thrust bearing?
>
>I wanted two because when you take the rotor out and with only one thrust
>bearing you don't have control of the mast horizontally. With two TB you
>always have control.
I assume what we're talking about here is a bearing to take a radial
load as opposed to an axial load, or perhaps both (i.e. like a wheel
bearing on a car). I can see that having two bearings and having the
rotator floated so it can translate, but not rotate, relative to the
tower, could essentially remove all the non-torque loads from the
rotator. Heck, you could use a belt or chain drive to the mast in
that case, or even a friction drive, if you had a position encoder on
the mast. Maybe if you had a "too small" rotator (e.g. you bought it
many moons ago, and have been adding antennas over the years, and now
it's way overloaded) being able to take up the bending and axial
loads would help.
The latter scenario could explain why hams see reduced rotor failures
with doubled bearings. It's because they're operating the rotor
without sufficient margin (or the rotor mfr didn't really provide
much margin to begin with). Given the ham dictum of "if the antenna
didn't fall down last winter it wasn't big enough", so the size and
number of antennas tends to grow over time (without necessarily
reengineering the entire support system) and the equally
characteristic ham thing of never buy what you can scrounge in your
garage, overloaded rotators aren't all that surprising.
Jim, W6RMK
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