----- Original Message -----
Sent: Sunday, April 27, 2008 4:07
PM
Subject: [FlyRotary] Re: Post Mortum on
RD-1B
Here's
my shop notes from the "this can't be good" incident.
Post
Mortum on RD-1B SN # 35B
Background
on unit:
Drive
had very little testing on installation (ground only). Builder reported some oil leaking from
rear seal and requested a new seal which was subsequently sent to and
installed by builder. Engine experienced various problems
during testing (coolant found in rotor chambers) and was rebuilt. The builder
was not able to identify the source of the stray coolant. Builder decided to examine drive while
it was off during engine rebuild and noticed that the input shaft thrust
bearing had failed (roller cage was melted and all rollers were loose in
drive). Unit was returned to RWS
for analysis and repair.
In
addition to the thrust bearing, the following problems were found during
inspection:
1. Input shaft gear drive pin had sheared
off and the gear had spun on the shaft causing the thrust bearing oil passage
to clog.
2. Sun gear pilot bearing had seized on
the prop shaft pilot and stayed there when the drive was
disassembled.
3. The rear input shaft thrust
bearing race had spin welded and seized on the shaft with evidence of extreme
heat in that area (blued metal).
I noted that the race was more or less in its normal position on the
shaft and had not seized due to moving forward onto a larger diameter portion
of the shaft as would be expected when the rollers and cage vacated their
normal location. This indicates
that the rear bearing race seized prior to the destruction of the roller
bearing cage.
4. The seized race had spun against the
adapter plate causing the aluminum to heat and extrude into the oil seal
cavity. This allowed the input
shaft to move further back against the adapter plate.
5. The oil seal was completely
destroyed by the heat of the spinning race.
6. The moly impregnated nylon snubber was
crushed on one side with evidence of heating due to friction.
7. The loose thrust bearing rollers had
bounced around inside the planet carrier housing leaving the housing and
adapter plate with the appearance of being shot peened. Surprisingly, it appears that
none of the rollers got past the planet carrier bulkhead and into the
gears. There was no evidence of
damage to any of the gear teeth.
There
are fairly obvious explanations for each of the individual failures observed
but the likelihood of all of them being present and manifesting themselves at
the same time is very small. The
analysis of the root cause reduces the problem to finding a single factor
which would fit all of the observations and failures.
The
key piece of evidence in this failure turns out to be the pinched nylon
snubber washer. It would be
possible for an error in setting the input shaft end play at time of
manufacture to cause the snubber to be crushed when assembled but this would
result in a symmetrical crush around the entire circumference of the
snubber. Operating the drive in
this condition would cause the snubber to quickly overheat and melt until the
interference was eliminated. It
is probable that the drive would operate normally at this point except for the
harsh metal to metal contact between sungear and ring gear bulkhead under
negative (abnormal) torque conditions such as engine backfire. Insufficient endplay alone would not
explain any of the other failure points.
The
only possible explanation I could come up with that would explain the
asymmetrical force sufficient to deform the snubber (it takes a force measured
in tons to do this) is for the rear race of the input shaft thrust bearing to
drop off the pilot on the shaft and get trapped between the adapter plate and
input shaft.
This can happen for a number of
reasons. Failing to push the
input shaft all the way through the damper spline and into the crankshaft
pilot is the most obvious. Too
much grease in the crankshaft pilot hole can also prevent the shaft and
bearing assembly from seating firmly against the adapter plate. It is a close tolerance fit between
input shaft pilot and the crankshaft pilot hole and the grease will not easily
extrude past the clearance.
Even if none of this has occurred and the shaft assembly is in fact
seated against the plate when the assembly is performed, the close fit of the
pilot and the grease will compress air in the cavity causing it to push the
shaft back out if the builder is not observant and does not hold the shaft in
long enough for the air pressure to bleed off. Whatever the cause, if the shaft
and bearing is not against the adapter plate, the rear race can fall off the
shaft and get stuck between the shaft and adapter plate.
In
the event that this happens and assembly continues, there is one more chance
for the builder to notice that something is wrong. When the main gear housing is slid
onto the 12 mounting bolts, it will not quite seat all the way onto the planet
carrier housing. This might
be hidden by the recess in the planet carrier housing if not checked
closely. The 12 mounting bolts
will however, have plenty of exposed thread to fasten the nuts to. When they are tightened, they
will exert a force of many tons on the pinched bearing race. This in turn causes the input shaft to
be pushed forward into the snubber seated against the ring gear bulkhead. Making matters worse, since the
pinched race is only on one side of the shaft, it is cocked sideways causing
the sungear pilot bearing to contact the prop shaft pilot at an angle. The stage is now set for a
tragic set of events when the engine is started.
The
exact sequence of events is in question but in order of most likely
occurrence, the following things happen.
The snubber is caught between two surfaces rotating in opposite
directions and is abraded in the contact patch area. This relieves some of the pressure but
it is not enough to make up for the extra 4 mm thickness of the pinched
race. The engine fires and
rpms increase rapidly. The badly
cocked sun gear pilot bearing seizes on the prop shaft momentarily causing the
drive to lock-up. The firing
chambers of the engine and inertia then shears the sun gear drive pin which
serves the same purpose as the propeller drive pin in an outboard engine, it saves the drive train and
engine from catastrophic failure in the event of a prop strike. At some point, the Loctite sleeve
retainer holding the pilot bearing in the sun gear looses its grip and the
gear starts spinning on the OD of the bearing. The input shaft is also now free
to spin inside the sun gear causing the oiling passage to be clogged with
metal and Loctite debris, shutting off oil supply to the thrust bearing.
The
engine is now free to unleash its power on the hapless thrust bearing with its
pinched race behind the input shaft. This assembly thrashes about
until friction causes the race to stick in various places which causes the
input shaft to machine itself into a crude tapered pilot and at some point
causes the race to self-center on the spinning input shaft. The compressive
forces on the assembly are finally relieved. Things appear to work normally for
awhile. However, the thrust bearing pilot on the input
shaft has been badly mangled at this point and is no longer a precision fit
inside the rear race. In addition, it has been deprived of
the forced lubrication because of the blocked oil passage in the input
shaft. It soon friction-welds
itself to the input shaft and begins spinning against the adapter plate where
it generates large quantities of heat.
The first thing to feel the effects of this heat is the oil seal which
is pressed into the adapter plate directly behind the race. It is soon destroyed but the welded
race probably keeps much oil from escaping, especially since not much oil is
being sent in its direction anyway.
The very last thing to go is the
thrust bearing roller cage. It is
made of a high temperature polysulfide material but it finally succumbs when
enough heat is transferred from the race spinning against the adapter
plate. The rollers are
flung out of the cage but at this point nothing changes other than the peening
of the surrounding housing from the flying rollers. The rear race is firmly welded to the
shaft and is not depending on the rollers to support it. The final event would have been
the eventual bouncing of one or more rollers into the gear train but it was
spared this due to the fortuitous timing of the builder's
inspection.
End
of report.
Tracy (will take about as long to repair drive as this took to
write )