[TowerTalk] Rotator Choice for Larger Yagi

Wed May 4 19:27:59 EDT 2016

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 at kk9a.com" <john at kk9a.com>
> To: towertalk at 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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