[TowerTalk] Why a Prop Pitch?

[NPAlex at aol.com]

It appears that I may have started this thread with my response to "Why a  
prop pitch" I guess I need to make additional comment.   
 
Maybe standard components as has been described in the other emails will do  
the job, but that would still have to be evaluated.  
 
pitch assemblies modified for rotator service have a pretty good history of  
solid performance.  They do require some maintenance and K7NV's web site  
(_www.k7nv.com/)_ (http://www.k7nv.com/)) 
goes into  considerable detail on what can go wrong, and some detail on 
refurbishing, a  service he performs. 

Prop Pitch motors are still available, but getting more difficult to find.  I 
just sold one of my spares for $125, and I have seen them range from much 
less  (flea market deals) to more than double that.  
So cost is still one of the benefits.
 
Size is another - It will slip into a US HD 589 tower completely  assembled.  
I had to cut one of the upper struts on a TriEx LM 470 to  fit it in that 72 
ft tower upper section.  Rohn  45 and 55 I think are not a problem.  Others 
have commented  the size advantage. It also has a convenient mounting flange 
arrangement.   At one time TriEx use to offer a Prop Pitch mounting plate.
 
Connecting the Prop Pitch to your mast is pretty straight forward if you  
have the spline coupled Bevel gear with the prop pitch.  Again K7NV's site  shows 
a few mast clamping arrangements that have been welded to the bevel gear  
assembly and provide clamping devices for up to 3" dia. masts.  
 
My solution was a steel shaft welded to the gear, whose diameter resulted  in 
a slip fit into the heavy walled steel mast.  A pair of holes drilled  
through the mast and shaft provided a no slip, "pinned" coupling.  
 
Although "pinning" often results in a loose and floppy mast no matter how  
tight you try to get the bolts. There is always a difference in  diameters which 
results in virtually only two point  contact between mast and drive shaft. 
This can be overcome by using a  proper pinning technique.  Using Allen Socket 
head bolts instead of  conventional Hex head, and enlarging the "entrance" hole 
on the MAST to allow  the bolt head to engage the drive shaft makes a secure 
and reliable coupling  that for me has handled multi-band arrays including 
full size 40m beams. Note,  the "entrance" hole diameter should be a few 
thousands over the Socket head  diameter for best results.  I used two half inch 
bolts. 
 
Finally the problem of direction indication.  Selsyns where the  standard of 
the 50's, 60's and beyond.  Required gears, belts or friction  wheels to 
couple the mast movement to the selsyn shaft. Resolution was poor, and  typically 
required 115 vac to be routed up the tower. Oh and don't forget you  need 
rotation limit switches as well.  
 
There was even "point and turn" schemes using a third selsyn in the shack  
readout box.(Bill Orr's Handbook of circa 1950's.  But today there are more  
elegant solutions that result in reliable and accurate position  indication.  
Using either reed switch or Field effect transistor and a  magnet attached to the 
motor shaft.  The motor speed varies, but I think  ranges from 6000 to over 
9000 rpm. that also is the speed conversion ratio  of the planetary gear box 
(about 9000 to 1) Lots of torque, and resistance to  windmilling.  
 
The Green Heron Engineering controller box seems to offer the best method  of 
controlling a prop pitch system available today.  M2 2800 box does as  well, 
but has some lesser features.  It still is very possible to get the  job done 
with a good 24 to 30 vdc power supply capable of about 10 to 15 amps,  some 
relays and switches.  The read out could be as ordinary as a  potentiometer 
coupled to the mast, and conventional milliameter readout in the  shack.  
 
I like prop-pitches - note I have a couple of old time Radio  
Industries/Hallicrafter Loudenboomer rotators available.  Double worm gear,  1/8hp GE motors, 
high torque, continuous rotation with coaxial feed through the  rotator. See 
my web site - _www.w4qn.com_ (http://www.w4qn.com) 
 
Regards,
Norm W4QN