Or slow it down by two or three to one with rotator outside of tower and chain
drive? Surely a prop pitch slowed down by a factor of 2 with Green Heron
controller ramping up and down would solve all problems?
Stan, K5GO
> On Dec 18, 2022, at 1:19 PM, Grant Saviers <grants2@pacbell.net> wrote:
>
> 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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