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Prop Pitch

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Subject: Prop Pitch
From: (David & Barbara Leeson)
Date: Mon, 14 Oct 96 10:16 EST
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

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