[TowerTalk] Fw: Rohn 45 Question

[Jim Lux]

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