Bill Freeman wrote:

Please do not try to run these gearboxes in the air! Once the smooth surface is
gone, you are very close to catastrophic failure.  2 hours or 20 hours? hard
to tell,
but SOON.

 The failure in Chuck's picture is spalling (as he correctly stated) and is
caused by
contact stresses higher than the material can stand over the long term
(local subsurface
fatigue).  There are a number of ways to accomplish this, breaking down into two
general categories, which are opposite views of the same thing.

           One:  Material is OK (i.e. as intended by the tranny maker), load
is higher than
                    the material can stand.

           Two:  Load is OK (i.e. not higher than the tranny maker intended)
and the
                     material is weaker than it is supposed to be.

Things that can cause condition One:   inadequate gear size for the load,
torsional
resonance condition, improper gear alignment.

Things that can cause condition Two:  gears "de heat treated" by welding,
poor lubrication
leading to local overheat, faulty original manufacture.

Another possibility is that the gears were used and had a lot of cycles
(miles) on them,
so they fatigued sooner than expected, since they were possibly already near
their limits
on fatigue.

ALL PSRUS NEED A MAGNETIC DRAIN PLUG IN THE LUBE OIL CIRCUIT !!
Gears and bearings will almost always dump off chunks before a hard failure.
Regular
checks of the magnet should be a normal drill.  A chip detector would be a
really good idea, too.

Sorry to hear about two PSRU failures, but VERY glad they didn't fail in
flight.

Bill Freeman

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