At 05:19 PM 1/24/2007, you wrote:
>The problem is that big antennas have a lot of wind load
>and a lot of momentum. With any backlash at all, they
>can bang back and forth using the element flexing to store
>energy. This violent jerking, which is hard to analyze
>using conventional engineering techniques, is what tears
>apart rotors.
>
>The prop pitch has a track record of not breaking under
>these conditions. Conventional gearmotors from the
>"Motorbook" may or may not be able to handle this. The
>torque ratings on them only tell you if they can turn
>the antenna. If I was going to experiment with those,
>I would want to get something with gears of comparable
>size to the prop-pitch. Look at the gears on K7NV's site.
>Pretty big, and there are three of them to share the load.
Most of the commercial gearboxes are similar in gear size. Either
the planetaries or the straight helical spur gears (it's going to
take at least 2 reductions, so keeping the shaft on the centerline
isn't a problem) can do this. To a certain extent, too, you'll be
able to backdrive the motor, which is a lot easier with smaller
reduction ratios.
They have very well defined failure loads in any case, so it's a
simple matter to put a shear pin in that will fail before the gear box does.
>Another thing you might look at are those ubiquitous 12VDC
>winches that you put on your SUV to pull it out of the mud.
>They mention 8000 lbs, although it isn't clear if that is
>the pulling force, or the vehicle weight (on a limited grade).
>Anyway, you could perhaps modify one of those winches to
>turn an antenna. Getting the 12VDC (at "too many" amps)
>up to the rotor is going to be another problem.
Having dismantled a number of those, I'd say that's the wrong
direction to head. They definitely are not designed for shock loads
(fairly small gears, and sometimes a worm/pinion style drive), a very
funky braking system, and they are designed for very small duty
cycles (a minute of steady running is about it) As for their
ratings... there's a fair amount of specsmanship and duty cycle
factored in. There are also 110V versions of these intended for use
as a hoist from Grainger, Harbor Freight, and the like.. they use
something like a 1/4HP or 1/2HP PSC reversible motor.
I think standard 90VDC motors might be the way to go. You can get an
inexpensive ($50) phase control speed control (looks like one of
those solid state relay bricks, just takes isolated DC voltage in and
puts out phase controlled AC) for them that could be driven by the
Green Heron style controller, and 90V at a couple amps is a heck of a
lot better than 12V at 40 Amps.
There are some 24VDC gearmotors in this sort of class (1/4 to 1/2 HP,
few RPM out) but I can't find them in the catalog right now. I seem
to recall they were designed for snowplow usage or something weird like that.
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