Torque means a lot. It's the ultimate measure of turning power and the
ability to hold an antenna in the wind. OTOH, The torque it takes to
accelerate, stop, and hold an antenna an antenna in a given wind only
partially depends on the area. It also depends on how well the antenna
is torque balanced for lack of a better term.
At what angle to the wind does the antenna present the most resistance
to the wind as a lateral force on the tower? OTOH at what angle does the
wind present the most load for turning and holding?
These are the reasons I refuse to use a spur gear driven rotator on
large antennas. There are rotators with relatively thick, machined,
precision gears, not thin, stamped sheet metal. Regardless of the
weather, I want to be able to turn the antenna(s) even if the wind is 50
or 60 MPH and the band is open.. When I release the brake on a Ham
Series rotator, I can pretty much turn the bell and thus the motor by
hand and those spur gears just scream.. I've released the brake and
have had the rotator spin around to the stops before the motor could
stop the antenna and then move it to the desired heading. That kind of
stop is hard on everything in the whole antenna system.
IOW the rotator turning torque and holding (brake) torque are the only
fixed values, or measurable values. The "flat plate equivalent wind
resistance of the antenna is only the starting point. There is also the
mass and lever arm. The longer the boom and the heavier the antenna the
more torque it takes to start and stop it on a calm day. Wind
complicates things as it may reduce the starting torque required, or
increase it. Hence the ramping up and down for starting and stopping
the rotation.
I can't imagine these gears failing unless they were severely over
loaded, or abused.( http://www.rogerhalstead.com/Gears.htm ) Look at
the size of the worm gear wheel in photo 6. The worm gear is shown in
photo 3. You can see some brass spalling on the worm gear caused by
rust from the failed bearings in the gear lube.
Unlike some others I do blame the rotators. Having to add devices to
cushion the start, stop, and absorb wind oscillation points out
weaknesses in the rotator. Those devices let the ham get away with using
under rated rotators. Of course most of us at one time or another have
been guilty of over loading a rotator, tower, or other "things". I
certainly have and knew I was doing so at the time. In addition, those
devices by their very nature allow the antenna to swing more (farther)
in the wind, adding to the stress the boom and elements must absorb.
They remove the shock loading, but by allowing the antenna to swing
farther in wind induced oscillation they actually increase stress. BUT
at this point we must differentiate between shock loading and overall
stress. Shock loading is like tapping with a steel hammer. It takes far
less force to do damage with the hammer than to sum the forces of 4 or 5
"hits" (or even more) compared to a steady pressure, or force of that
total magnitude.
73
Roger (K8RI)
On 5/3/2016 Tuesday 8:41 PM, Jim Thomson wrote:
Date: Tue, 3 May 2016 16:47:28 -0400
From: "john@kk9a.com" <john@kk9a.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Rotator Choice for Larger Yagi
I am not even sure that torque means much. All of the rotator issues that
I have had (broken gears, worn keyways, failed potentiometers, etc)
occured while the station was QRT.
I currently use a small and a medium prop pitch. The small has a 9576:1
gear ratio and the medium has a 7063:1 ratio. It is possible that the
small one has the same or more torque than the medium. Which one is less
likely to break, I would guess the medium. I think you would be happy
switching to prop pitch rotator and Green Heron controller.
John KK9A
## A buddy had the monsterstepIR, turned by an OR-2800....and it lasted
exactly 9 days ! 2 other folks with the exact same ant / rotor combo had
similar results.
## If I remember correctly, the small prop pitch had its final gear driven in
3 x places, every 120 degs, by small gears. The OR-2800 has its final gear
driven by only one small gear. By using 3 x small gears, the back lash is
distributed over 3 gears.....and not just one.
## Back in the late 70s, early 80s, a japanese rotor maker had a rotor out that
used from 1-4 external, bolted on motors, every 90 degs around the perimeter
of the motor. You could buy it initially with 1 motor, then add more, if you
required
more tq..and less back lash.
## I currently have 1 OR-2800 left..and its final small gear is chewed up /
trashed.
Back in the late 70s, early 80s, I sold my ham-4 to a buddy, who used it to
turn a
hb 6-el 10m yagi on a 36 ft boom. He always kept it pointed in one direction
99%
of the time. Eventually, due to constant wind buffeting, it literally ate 1-2
teeth, completely
gone.
## perhaps what needs to be done is to use a worm gear to drive the final gear,
that would
eliminate the back lash, + provide more surface area. That and /or use a
better grade of steel.
and /or thicker gear teeth. Or provide some form of isolation between mast
+ rotor.
Jim VE7RF
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73
Roger (K8RI)
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