I think I"m going to reexplain what I want to do there.
It's a simple concept, in that I want to mount the rotator outside the
25G tower and drive the 2" mast inside the tower.
Remember there's enough torque with a good sized antenna to turn the
mast inside the rotator clamp under gusty high wind conditions.
Any chain or belt and sprocket has to be capable of holding and not
stretching or suffering from rapid wearing under these loads which can
I based the following calculations on a rotator capable of 1000inch #
and 1500 inch #. My rotator in the 45G runs about 12,000 inch #. I
calculated that with a 4" OD sprocket on a 2" mast the force would be
between 500 and 750 linear # on the belt or chain. with the 5" OD it
drops to 400 or 600#. This comes in pulses and they may be alternating
in direction, so it's a non trivial force for both the chain. and gear
I am not tied to any specific power transfer system, be it chain, timing
belt, or gear train. However it must handle the load with room to spare,
have no slack, and be rugged enough to withstand the weather and hold
the antennas on a heading with out noticeable give. I can build a
cover to protect it from snow and ice.
Remember, my goal is to mount the rotator outside the tower and have it
work efficiently and to spec. This creates several forces that the
mast, rotator, and tower do not normally have to contend with. Any load
transfers as a "pull" between the rotator and mast. This puts a lateral
force on the tower legs "Unless" the thrust bearings for the mast and
the rotator mount are on the same metal plate, or plates bolted
together. Generally this force can be ignored except when experiencing
The mast requires the sprocket or sprockets be between two bearings that
are relatively close so there will be no lateral flexing of the mast.
Conversely the rotator is not designed for a lateral force of much
magnitude, so it needs a well centered mast that also has a top bearing,
or thrust bearing and the load is strictly sideways(lateral)
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