K1TTT Technical Reference

Prop Pitch rotors topics: mast adapters general testing and maintenance  
Subject: 
        Re: Prop Pitch
   Date: 
        Mon, 14 Oct 96 10:16 EST
  From: 
        David & Barbara Leeson <0005543629@mcimail.com>
    To: 
        towertalk 


I've seen a number of ways to interface masts to the bevel gear of a
prop pitch, but none of them seem to be really clean and precise.  I
haven't got around to doing it yet, but I intend to use a high density
polyethelene (HDPE) bearing in place of the impossible to get bearing
that sits under the bevel gear and rusts, and plan to press or thermal
fit an adapter shaft into the center of the bevel gear.  About 0.002
in. interference should handle the full torque of the prop pitch
(1320 ft-lb or almost 16,000 in-lb).

The top of the adapter will be a flange with tapped holes like the
end of an automotive crankshaft, so any mast adapter can be bolted
on.  A rubber wheel will run on the edge of the adapter for direction
indication, and the internal limit switches are modified by machining
the cam to permit 350 degrees rotation.  I need to put a weatherproof
can over the adapter and pot, and plan to use an inverted paint can
with a seal at the top (as installed) for the mast.

I have found that all of the bearings are available in modern bearing
catalogs (except the two that are used on the inside and outside at
the bevel gear), and there are modern seals available that can seal up
the gearbox better.  The bearings are available in plain and sealed forms
(interesting that the WWII prop pitch used metric size bearings, but
almost all bearings have been sized that way since they were invented).

I have a dozen prop pitches in various states, and some day I will get
started on this project.  I'd be happy to share ideas and drawings with
anyone who has the energy for this.  BTW, for the seals, a bit of machining
is required on the gearbox case, but this shouldn't be a big deal.  This
should make it possible to prevent the typical prop pitch failure, which is
that the gearbox oil leaks out the bottom seal and ruins the motor, while
the now unlubricated bottom bearing rusts and siezes.  This wasn't a 
problem on the electric propellers because the prop pitch is horizontal
and spinning (guys who are familiar with B17's tell me that the electric
props had a habit of freezing in the wrong pitch on takeoff, and they
much preferred the hydraulic props).

I pay $100 for a good, unmodified prop pitch in reasonable condition (all
parts there including bevel gear).  They run fine from 24 VDC with no
filtering made from standard diode bridge and small transformer.

Sorry this is still all theoretical, but I just haven't had the time this
year to get going on this project.

73 de Dave, W6QHS

David & Barbara Leeson wrote:
> 
> I've seen a number of ways to interface masts to the bevel gear of a
> prop pitch, but none of them seem to be really clean and precise.  I

when i was using one i had a stainless steel piece of bushing material
machined to fit snuggly inside the output shaft.  the inside of it was
machined to fit over the outside of the mast and drilled for 2 cross
bolts.  the bottom of the bushing was drilled with 4 holes that were
centered in the access holes in the bottom of the shaft, these holes
were tapped for 1/4"-20.  i cut a slot in 4 stainless 1/4-20 bolts and
rounded the corners a bit and screwed them into the 4 bottom holes 
through one of the side access ports.  i never had a problem with the 
bushing, but did regularly sheer off the 1/4-20 ss bolts thru the mast.
at the time i had a 4 ele 20m telrex on it.  if i hadn't switched to 
a commercial rotor (yaesu g-2700sdx) when i put up the 6 ele telrex
i would have redrilled those holes for probably 5/16 or maybe 3/8
hardware.  If i had it to do all over i would probably make up the
shock absorber with one of those drive shaft rubber things.



-- 
k1ttt@arrl.net   or   robbins@arrl.net
http://www.k1ttt.net/k1ttt.html

Subject: 
            Re: Prop-pitch rotor help needed
       Date: 
            Thu, 01 Aug 1996 21:43:43 +0000
      From: 
            David Robbins 
Organization: 
            k1ttt
        To: 
            Chuck Claver 
 References: 
            1


Chuck Claver wrote:
> 
> I hope this is not repetitive.  I sent this request ealier this morning
> but since it han't shown up on the reflect yet I suspect something went
> wrong.
> 
> I am in the process of putting up a beam for 40M and have recent
> acquired a prop-pitch rotor that I would like to use as the antenna
> rotator.  I have one small problem - I know nothing about the electrical
> requirements of a prop-pitch.  The unit had been previously used as
> an antenna rotator so most of the hardware is there, antenna clamp,
> tower plate, and AC syncronous tX/rx for the direction indicator.
> 
> My main questions at this time are:
> 
> 1) What voltage does the prop-pitch opperate at? is it AC or DC?

the one i have used is 12v ac or dc, many are 24v ac/dc.  the 12v
one draws about 20a starting and a bit less running, so have a good
power supply and heavy cable.  i used 10ga solid copper for a 350' run,
but had to put 24v in it at the shack to get 12v at the motor.


> 
> 2) What can I do to give it a good check out before putting it up on the
>    tower?  Short of tearing the whole thing apart and rebuilding it.

plug it in and try it.  if it sounds like a mix-master and you can't
stop it from turning its probably ok.  the bottom shell usually comes
off with a couple small screws which allows you to inspect the motor and
make sure its not full of water.  this is the (usually dented) sheet
metal looking cover, check for a couple small screws near the top of it.

> 
> 
> 4) I noticed that the output shaft of the prop-pitch has essentially no
>    backlash.  Should I be woried that the unit is ceased and will need
>    some serious overhauling? or is this normal for a prop-pitch?

they are normally very tight, if there is lots of play i would suspect
bad gears or bearings.  the gearing is about 1000:1 with multiple
planetary gears.  if you are careful you can disassemble the cases and
grease them, its not really a critical assembly, just be sure to mark
the cases so you realign them the right way.

> 
> Any help would be greatly apreciated.
> 
> Cheers and hope to see you on 40M soon,
> Chuck Claver de NJ6D
> claver@noao.edu

On Fri, 17 Jan 1997, Phil Duff NA4M wrote:
> 
> How do I remove the aluminum shell that covers the motor?  I dont see 
> any obvious fasteners for it on the outside other than what appears to
> be a widely notched ring around the shell where it enters the gear case.
> I tapped on it thinking it was perhaps a retaining ring holding the
> shell but it didn't seem to want to unscrew and I didn't want to force
> it unless I know thats whats holding the cover.

The aluminum shell is held in place by screws around the periphery of the
shell.  The shell may be a little snug, but it will come off!  The notched
ring is a retainer that attaches the motor to the gearbox.  It screws out
counter-clockwise and you can remove it by tapping firmly with a cold
chisel.  A machinist friend fabricated a special tool for me that that
slips over the motor and matches the notches perfectly.  It makes the job
a little easier, but I used only a cold chisel and hammer for many years
before that!  You may need to loosen this ring if the aluminum shell is
binding against it, but the ring is not intended to retain the shell, only
the motor.  After you have removed the retaining ring, the motor will pull
straight out of the gearbox.

73!
Frank
W3LPL
donovanf@sgate.com

I too used the chisel method until one time I had a strange problem and I
kept having to take the aluminum shell off and on, and with the chisel, it
deformed the shell so much that I could barely get it back on. This
procedure was occurring on a tower at 32 degrees.  So I decided to find the
tool for the "spanner" nut.  It is called a "spanner wrench".  There are
several types (adjustable hook, adjustable face and adjustable pin, spanner
wrenches) the one that seems to work best on the prop pitch is the
"adjustable hook spanner".  The one I bought was for circle capacity 2" to
4-3/4".  It is available at Wholesale Tool 1-800-521-3420 for about $35.  

By the way the strange problem was a lead on a capacitor that after 15 years
in the air decided to short out to the aluminum shell,  took me several
trips to figure it out.

73-Mike W9RE

>I recently obtained a used prop-pitch motor. I believe it to be a >"small" prop-pitch based on the attached metal tag which states the >gear reduction to be 9576:1 > >I've partially disassembled it and my question is how to determine >if this prop-pitch has the dreaded "modification" that changes the >rotation speed at the expense of the units braking torque?? I'm >assuming this to be a change of some type to the gear mechanism.  > >73 de Phil NA4M  na4m@texas.net I'd put about 10 volts AC on it and see how long it takes to make one revolution.  If it has been modified, it will turn about 1 rpm with 10 volts AC on it.  If it is not modified, it will take about 35 volts AC to make it go 1 rpm. Stan  w7ni@teleport.com
Hi Phil!

Yes, the small prop-pitch has a 9576:1 reduction ratio.  The most
desirable version uses a plug-in motor easily removed by unscrewing a
retaining ring.  A less desirable version also uses a plug-in motor held
in place with a bolt-on retaining ring.  An undesirable version uses
a motor that is mechanically and electrically very difficult to remove
from the gearbox (I do not use these!).

The modification you refer to involves a modification to the output
gear/shaft and the final planetary gear mechanism.  If your prop-pitch is
"stock," the output shaft will include a gear that couples into the
planetary gears.  If it has been modified, the gear teeth have been cut
away from the output shaft and the shaft has been bolted to the body of
the planetary gear mechanism.  This is an awful modification that
ruins the usefulness of the prop-pitch motor.

A very common problem with prop-pitch motors is failure of the rubber seal
for the output shaft.  After fifty year or more, the rubber seal will have
dried out!  As a minimum you should replace the seal, but much better
is to use an additional box to prevent water entry entirely.  K6NA sells
the required hardware to enclose your prop-pitch and prevent water entry
that will surely destroy your precious prop-pitch.

You can reach Glenn at rattmann@cts.com

73!
Frank
W3LPL
donovanf@sgate.com