[TowerTalk] Mast to rotor coupling for antennamart prop pitch

[Kurt Andress]

W1km@aol.com wrote:
> 
> I have an old antennamart prop pitch which has a 1 7/16" dia shaft --anybody
> have any ideas on how to couple this to a rohn mast--I'm afraid just sliding
> it over the shaft and pinning might not be the best way to go.
> 73
> Greg W1KM
> 

Hi Greg and All,

Longer than usual, but offered for those who chose to follow such
stuff...considered saying nothing, but opted for saying just enough to
mean something to those interested.

Lot's of ways to connect a mast to a prop pitch.
Pinning is just one of them, and should not be ruled out too quickly.

Humans have been successfully executing pinned connections for longer
than I have been alive. There seems to be just a few important features
that are required to make them work, also understood before I was born.

Pinned connections will work if properly sized for bearing area and
material yield strength and installed to obtain tight fits. They need to
be press fit to have any useful service life.
The normal hacker methods usually result in sloppy messes! 
Just drilling a hole and sticking a bolt in it is not good practice,
that's the last thing I'd ever do! 
Any clearance between the bolt and the holes will eventually become lots
of clearance.
There are probably 100 guys around here that will testify that bolts in
loose fitting holes don't work very well. I know there are hundreds who
have proved it.
It doesn't necessarily mean pinning doesn't work, just that common bad 
practice doesn't work, as we should expect.


Here are a couple of suggestions:

Option #1: Proper straight pinned, harder to get right

If predisposed to using threaded fasteners, get some AN type fasteners.
These are high quality close tolerance aircraft fasteners. The solid
shank lengths come in 1/16" intervals so you can get the correct length
to prevent any threads from bearing on the connection. The shank
diameters are also closely held, making it easier to get a reliable fit.
You can get a variety of SS alloys, if you want a real strong one get
one made from A286 steel. These are heat treated and strong!! But not
corrosion resistant, nor cheap, by Home Depot standards. I seem to
remember the AN line has some austenitic SS alloys that should be fine.

Overkilling the fastener strength doesn't hurt (until you get some
really brittle fastener alloy) but it also doesn't really help. The
fastener shear strength only needs to be as strong as the size and
material 
yield strengths dictate.
The size of the pins is selected to produce enough bearing
area for the weakest material in the connection.

Next go to the industrial supply and buy a straight reamer that is
.0001-.0003 In. under the nominal size of your fastener. If you get the
fasteners first and measure them with a micrometer you'll know which
reamer to ask for. Drill a hole 1/16 smaller than nominal size. 

Side note about drilling holes:
Also relates to comments about problematic drilling of 4130 thread.
Accurate drilling of holes in almost any material is not difficult! It
requires a sound procedure and can benefit from the proper use of a
pilot hole.
When drilling a pilot hole, select a drill size that is just slightly 
(.010 - .015) larger than the center web of the final size drill. Most
garden variety twist drills do not cut in the region of the center web,
so in this area the drill just rubs and work hardens the material. By
drilling the proper size pilot hole, we have removed the material in the
non-cutting region of the final size bit and provided a guide hole that
will allow the desired size drill web to register in providing accurate
location reproduction.
The best type of drill bit to use for the pilot is a cobalt drill. All
of these drills have a secondary grind on the center web to allow them
to actually cut material in the drill center web region. This allows the
bit to cleanly cut its way thru, on location, without wandering.
The absolutely worst thing to do when hand drilling is to drill a rather
large hole, then try to finish it with a drill that removes very little
material. The final bit, removing little material, readily grabs big
chunks of material and jerks and binds, causing the intended alignment
to go all over the planet. This method almost always guarantees an
irregular, offsize hole.
Any machinist will tell you that annealed 4130, what we use for masts,
is a piece of cake to machine with the right cutters and techniques.
Tip: Dull drills that don't cut in the center web region are your enemy!
They work harden the material making it almost impossible to remove, and
causes the drill to wander.
The same thing happens with most stainless alloys. Use sharp, center
cutting bits at lower speeds with high feed rates (lot's of muscle). If
the bit rotates just a few times without curling a chip out of the hole,
you have just work hardened the material and are on your way to becoming
dead meat.

Back to our straight pin installation...
Then carefully drill 1/32" undersize being careful to keep hole
straight.
Then ream the hole and install fastener. It will be a reasonable press
fit so you will have to tap it in with a hammer.
When doing this with a hand drill (especially on the tower) use a larger
interference fit as you are likely to screwup the fit by not drilling
and reaming in a perfectly straight line.

If the hole size is chosen to provide ample bearing area for the mating
material yield strengths, this type of pin will not get loose or
woggle out the holes.

A good rule of thumb here is to size the mast connection at >=2 x
the rotator torque capability. When I build prop pitch systems for
folks,
I size the connection components for around 20000 In-Lbs. Seems to be
ok.


Option #2: Taper pinned, easier to get right.

There is another pinning method which I prefer, if pinning is the thing
to do. Use standard Morse taper pins instead of bolts. This requires a
morse taper reamer. You should be able to get the pins and reamer at a
bearing house, like King Bearing. These have been used in industrial
machinery connections for eons. 
The Morse taper is just one really long locking taper. The pin sizes are 
identified by number based on where along the length they are taken. 
Each pin size is a short portion of the big picture. The guys at the 
supplier can tell you what the sizes are.
Drill undersize and ream the tapered hole to a size that allows the
pin to extend beyond the connection at both ends. Then you tap the pin
into place and end up with a tight fitting connection. The
fact that the taper is below the critical angle to create a locking
taper keeps it from getting loose. 
If you're squeamish about this, run a thread on the small end of the
pins with a die and put a locknut and stack of washers to enable
tightening the assy. 

The tapered pins are easily removed by tapping on the small end and
breaking loose the locking taper. You should coat the pins with a rust
preventative and seal the exterior to keep the connection from
corroding. It should be checked annually.

The nice thing about this connection is that if it needs to be
refurbished, you usually just need to chase it again with the reamer and
install a new pin. For this purpose it is best to do the initial
installation with more exposed pin on the big end so you have some room
for re-reaming and setting the pin in deeper.

When properly sized and executed, this connection is very reliable! 
I've seen it work on prop pitch's with big antennas, race car driveline
components under severe use, and industrial machinery. 
No failures, no slop, no worries! Easy to service too.

The mating pieces should not be too loose and need to be immobilized
while drilling and reaming. If your mast is 2" dia x .250 wall, you'll
have about .03 In radial clearance between mast and the 1.4375 dia rotor 
output shaft. This is a bit too much. You may want to shim or sleeve
this 
thing to get about half that.

Proper pinning for your particular prop pitch would be easier than 
fabricating a mast clamp connection, and will long outlast any bolts
stuffed in plain drilled holes, which appears to be the current
paradigm.


Good Luck! Can't beat a good Prop Pitch. They're worth the effort to
get'em right. Too bad so many have been going in the dumpster over the
last 
50 years! More bad practice!
We might get lucky and all die before we have to watch the last one go
down the tube!


-- 
73, Kurt, K7NV

YagiStress - The Ultimate Software for Yagi Mechanical Design
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