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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