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Re: [TowerTalk] Why a Prop Pitch?

Subject: Re: [TowerTalk] Why a Prop Pitch?
From: Jim Lux <>
Date: Wed, 24 Jan 2007 09:34:00 -0800
List-post: <>
At 07:44 PM 1/23/2007, wrote:
>I can only guess from your comment that you have not had any experience  with
>a prop pitch motor used as a rotator.  There are several reasons that  one
>would use a prop pitch motor.
>One was cost, as surplus motors were available at very reasonable prices.

And, of course, with a lot fewer propellor planes using electrical 
adjust (small planes use hydraulics, and so do many large ones), 
they're not available in the surplus market.

>They bring some premium now do to limited availability.

Does someone have a typical spec for a prop pitch motor (torque/speed 
curve, for instance).
I found a couple references to 1320 ft-lb, and at 1 RPM, that's about 1/4 HP.
I also found a reference that the current was 20A at 12V, which is 
240W, or about 1/3 electrical HP, so it's in that ballpark.  I 
suspect that there's a fair amount of gear train inertia and 
friction, so the no-load current isn't a lot different than the 
under-load current.

>Another reason was power, or Torque. They can handle very heavy  loads.
>Another reason is size and shape.  They fit inside relatively small  tower
>cross sections, with no projections beyond the tower sides as some 
>of the  new
>models do.  This makes them very ideal for crank-up towers as all is  within
>the tower envelope.

It should be noted that you can buy brand new DC motors with 
planetary gear sets (or multi stage spur gears) in almost any 
configuration you can imagine.  They're just not all that 
cheap.  But, having not ever actually bought a prop pitch motor(I 
have been given a few over the years or inherited them), I don't know 
what the going price is.  I'd guestimate that a DC gearmotor with a 
planetary reduction set to, say, 4 RPM would run about $250-$400, 
depending on the HP.  A 1/40HP RPM gear motor is about $200.  A 1/6HP 
AC gearmotor from Grainger runs about $380.  A DC motor is going to 
be about 20-30% more (for a 90VDC version.. maybe 50-60% more for 
24VDC, because of the increased copper size for the lower voltage and 
higher current)

That would get you a totally enclosed motor (i.e. it can operate out 
in the great outdoors and work with rain/snow/etc.) with standard 
NEMA flanges, etc.  For what it's worth, it would make hooking it up 
a lot easier, because all the shafts, brackets, etc are all standard 
sizes.  The gear boxes just slide onto the end of the motor shaft and 
bolt to the motor housing. The output shaft is usually a standard keyed shaft.

>The electric motors (nom. 24vdc) are held in place with a large castle ring
>nut, spin that off and the motor unplugs for easy maintenance.  The gear  box
>removal is on a par with most rotator types.
>One of the drawbacks however is direction sensing.  That has to be  added.
>Historically that was done by use of Selsyn tx/rx coupled to 
>the  mast.  Today
>both M2 and Green Heron Engineering provide controllers with  pulse counting
>direction indication.(K7NV design used with GHE, check out his  web 
>site).  The
>GHE controller has many features and provides ramp up and  down which is
>useful with large arrays.

On motors designed for "motion control" applications (which might be 
overkill for an antenna), if you're buying new, they often have a 
rotary encoder mounted on the back of the motor. It will be a 
quadrature encoder (sometimes with an index pulse) with anywhere from 
32 to 256 pulses/rev.  You can also get an encoder for the output, 
but they tend to be expensive (because people want a lot of 
pulses/rev for most applications).

>I have had the same prop pitch motor in service for nearly 40 years,  trouble

I think one could expect the same from any reasonably sized 
industrial gear motor today. As the supply of prop pitch motors 
decreases, you'll probably see more antenna rotator designs using 
standard commmercial parts. Sort of like the gradual decrease in 
Merlin engine availability as they break, wear out, etc.  OTOH, with 
it getting increasingly hard to put up towers in many places, maybe 
the demand for high power rotators will fall exactly in sync with 
their availability.

Just as a question, since I have my Baldor catalog handy, what is the 
inside diameter of one of the telescoping tubular towers?  (i.e. how 
small does the motor have to be?).   It might well be that a standard 
56C frame motor won't fit, which does make the motor/gearbox 
selection a bit trickier.  There are skinny motors made (for use in 
well pumps, typically) for such applications.  Franklin Electric is 
one supplier.

Jim, W6RMK
>summarizing - Power (torque) and Size.


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