[TowerTalk] Big rotor question

Sun Dec 18 14:19:34 EST 2022

After observations of my JKantennas 2L 80m beam (32 sq ft projected) in 
high wind, the questions I have about rotators for large antennas are:

1. Is brake torque if exceeded, that will cause the set azimuth to 
change?  ie freewheel.

2. Is brake torque if exceeded, that which will damage the gear system? 
If not, what is that limit?

3. Is motor torque the output torque with rated input voltage, but for 
how long? Does the motor burn out?

4. What is the angular deadband (free motion) of the rotator gear system?

Antenna wind load is an important lateral force of tower loading.  What 
does antenna wind load have to do with torques? (ans: they are vaguely 
related).

The reality of the rotational inertia and kinetic energy of the antenna 
as it traverses the deadband of the rotator gearing system is that it 
generates high peak torques as the antenna and mast come to a stop.  I 
think this is the large antenna rotator killer force.  Also, perhaps the 
antenna killer from peak stress in the elements and fatigue as they 
oscillate.  Also, perhaps why some towers fail from twist.  AFAIK, 
nobody specifies angular backlash/deadband for their gearing system.

I say this after observing my 2L 80 driven with an Orion 2800 and small 
K7NV Prop Pitch in turbulent winds inducing significant free motion 
around the set azimuth.  The PP was moving much more than Kurt expected 
and he advised that since it has the highest gear reduction of all PP's, 
a larger PP would not reduce the motion.  (note that a PP in aircraft 
use does not regularly experience oscillating torques) The 2800 was an 
emergency replacement for the failed PP (water in motor but unknown re 
the gearing).  However, the 2800 was used and had excessive free play in 
the output splines, rust of both M & F parts.  As a result of antenna 
motion from this backlash, a main drive gear tooth (hardened gear) 
broke.  A newer 2800 with very little backlash was installed and so far 
is ok.  I asked Ken of JK about using a 2800 and he said several of his 
80m 2L/3L are using them with success.

Yaesu makes an rough estimate of rotational inertia for antennas and 
sets a limit on that value as part of the rotator spec (antenna turning 
radius x antenna weight = K factor). So at least one rotator 
manufacturer has specified this as a limit.  Plus, Yaesu has a maximum 
continuous duty spec, presumably motor temperature limited.  Controllers 
such as the RT21 offer speed ramp up/down to minimize the inertial force 
from +/- acceleration.

Since rotational inertia is really I = mass x radius squared, comparing 
an 80m beam with a stack of shorter antennas of the same total "wind 
load" could be misleading.  Perhaps the 48 sq ft stack load in question 
has less (or more) rotational inertia than my 32 sq ft 80m beam with 
100ft long elements (that radius squared term). Or estimate using 
Yaesu's K calculation.  My 80m beam K = 177kg x 15m = 2618 kg-m or 
19,500ft-lbs.  For a correct "I" rotational inertia calculation the mass 
and center of mass of every 1/2 element (and boom) need to be found and 
then used in the formula I=2*M*R^2.  Note that R in this formula is the 
distance from the mast to the 1/2 element center of mass, not how far 
out the it is on the element.

Back to trying to answer the original question:  Is the Prosistel a 
better choice for 42sq ft of stacked antennas.  If I understand the 
Prosistel design correctly it directly drives the mast thru a worm gear. 
  Worm gears can be designed to lock against reverse force if the worm 
to wheel gear angle is sufficiently small.  They can also have 
adjustable engagement to reduce backlash to a very small value, which is 
done in precision machining fixtures.  Perhaps backlash is a small value 
in the Prosistel design, I have no experience with them.  However, worm 
gear lubrication is tricky see 
https://www.machinerylubrication.com/Read/1080/worm-gears

A program like YagiMech (DXeng) could be improved to calculate the 
correct inertia for each antenna element.  Some CAD programs can also 
calculate these values.  (Such an analysis assumes the antenna is a 
rigid structure which of course it is not.  The dynamic behavior is well 
above my pay grade).

Another solution to the backlash problem is to add some sort of brake 
system to the mast.

Grant KZ1W

On 12/18/2022 07:56, Jon Zaimes, AA1K via TowerTalk wrote:
> Looking for recommendations for rotor suitable for 42 sq ft of wind load -- three JK beams on 3" mast.
> Just a bit too much for Orion 2800 or the new DXE RT4500HD (I think).
> Prop pitch not so available since K7NV is SK.
> Prosistel? Others??
> 
> 73/Jon
> Jon P. Zaimes, AA1KTower climber for hireFelton, Delawarehttp://www.aa1k.us/Cell: 302-632-2353 Reviews of AA1K tower work on eham website: http://www.eham.net/reviews/detail/12922 Hug your favorite tower every day, and always stay connected to it.
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