Marcotte psru info

[Ken]

Below is a copy of my post to flysoob.

This is a maintenance heads up for anyone operating an older Marcotte
psru with a pilot bearing in the flywheel. It does not apply to the
newer units that do not have the pilot bearing.

The first few units produced had a bronze pilot bushing which was
upgraded to a 1630-2RS ball bearing race. The only rotational movement
that this bearing sees is oscillation as the rubber drive bushings flex
between power pulses. The input shaft on the old psru is solid between
the large 208 bearing in the psru and the small 1630 bearing.

I replaced my 1630 at 70 flight hours due to a very slight roughness.
After a further 110 hours, the bearing cage was found to be destroyed.
No balls had fallen out though and the shaft had not been operating
without being centered. Guy Marcotte said they have seen one of these
bearings fail in as little as 35 hours with a poorly balanced crankshaft
and flywheel assembly. He also knows one with a carefully balanced
assembly and 800 hours on it. Balancing the aluminum flywheel might be a
good idea and Guy seemed to be recommending balancing the flywheel and
crank as an assembly and of course indexing the flywheel to the
crankshaft. However the main point that I want to share is - keep an eye
on this bearing if you have one. A telltail to watch for might be a bit
of oil/grease on the inside of the bell housing which may have appeared
when the grease seal started to fail in the 1630 bearing.

My unit is from the first production run back about 10 years ago. This
unit has quite accurate machining but I can see that Guy's suppliers
have managed to improve it even more over the years. My unit has made
black metal fuzz since new, which collects on the magnet and starts to
darken the oil in my sight gauge. I had been changing the oil about
every 20 hours because of this. A magnetic plug was also installed which
I consider a must. The magnet now has some small flakes of shiny metal
which I suspect is material spalling off bearing races. Guy is not aware
of other units that also made metal like mine. However my large 208N
bearing at the input gear is very noticeably rough. I have a couple of
thoughts about this. The 208 ball bearing takes axial pounding as each
cylinder firing pulse and following negative crankshaft torque pulse
(from a 4 cylinder engine) pushes the helical angled drive gear rearward
then forward. I estimate the torque pulses at about +450 lb-ft. and -150
lb-ft. which would result in axial pulses on the input shaft (reaction
from torque pulse on the helical gear). I estimate this is well within
the capability of the 208 bearing but I haven't done statistical
lifetime calculations. I have been thinking that ANY axial movement of
the large 208 bearing will exceed the free play of the small 1630
bearing on the other end of this shaft and fail the small bearing.
However crankshaft end play of .003" is likely even more significant
here and there is over a thou of thermal expansion to contend with as
well. I don't know if a crankshaft oscillates within it's end play or
normally runs relatively fixed.

Since I do have what seems to be evidence of some slight fretting from
axial movement of the input shaft, I plan to replace my 1630 with a
cylindrical roller bearing before reassembly. That will allow axial
movement of the shaft in the flywheel pilot. This will require boring
out the flywheel as I can not locate a 1.625" OD cylindrical bearing. I
am also planning to have the flywheel balanced. I might not have further
info on how that works out anytime soon though as I have purchased a new
M300 psru and plan to run it it for awhile. I wanted to try a slightly
higher gear ratio for when I finish building my floats. Guy is very
confident of his new input bearing design which has no connection of any
kind to the crankshaft except through the six rubber drive doughnuts. I
will however be monitoring input shaft bearing play nevertheless. We
enjoyed spending most of Saturday with Guy assembling the my new psru
and discussing the design but I have no association with the company,
except as a customer. Ray and Guy are both honest, straight up, and
talented gentlemen.

I will also confirm and monitor for zero forward and rearward free play
at the prop tip when at operating temperature. I did notice a little of
this on my M200 immediately after my last flight when everything was
still hot. That is why I grounded it for inspection. My thrust bearings
are beginning to show damage if inspected carefully. The rotational free
play (gear lash) has never changed since new. In flight my unit runs
about 95C +/_ 10C. The preload on the Timken tapered roller bearings
reduces as the outer aluminum housing heats up and expands and I want to
make certain that I maintain sufficient preload. I had my M200 apart a
few times for inspection and it was running on the preload that I set.
Guy suggested about 0.005" preload on the M300 and 3 or 4 thou on the
slightly shorter M200. I imagine that the long nose drives would need more.

I will also attempt to install an oil filter. I think there might be
sufficient centrifugal pumping action available to flow some oil through
a filter. That may be excessive but a filter would have have benefited
my M200 and good psru's take a hefty amount of pocket change. Viton
seals and high quality bearings are not cheap either.

Ken




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