Here are my first thoughts on the subject.
IF, I wanted to keep the forces contained as much as possible and allow the
torque to be transferred to all surfaces pretty equally I would make a
combination guying plates/rotor plates/mast bearing plates and make it so it
actually bolts in as a mini tower section (maybe 12" total length, 2 plates
spaced enough to contain the bearing and gears with a small margin) in
between the 25G sections.
The new plates would be set up with guy arms and then star guyed. That takes
care of your rotational torque problem.
The rotor mounted on the outside of the plates would bolt up to a shaft
supported between two bearings which has one gear. The inside end of the
plates have two more bearings which bolt up to the antenna shaft and a gear
with matching chain to transmit the rotational torque when needed. Half way
between I would install a small idler gear with a screw tensioning system so
the slack can be trimmed out of the chain as it wears or removed so the
chain can just be slipped off the gears for replacement if necessary.
By putting it in the joint between the two sections you get to take some
advantage of the ease of installation and replacement and the ability to
reinforce the bracing between the legs at the joint in the new shorty
I hope I explained my ideas enough to be understood.
It still seems like a lot of work and manufacturing just to enable the
external mounting of a rotor.
Mike Baker K7DD
All QSO's are uploaded to LOTW and EQSL.
[mailto:firstname.lastname@example.org] On Behalf Of K8RI
Sent: Sunday, April 29, 2012 8:52 AM
Subject: Re: [TowerTalk] Sprockets with 2" bore needed
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 be
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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