Tracy Crook wrote:

Bill Freeman wrote:

  Tracy's spalling of the sun gear is a VERY important observation.
Spalling is caused
by subsurface fatigue failure, due to Hertzian contact stresses above

what

the material
can take.  This is probably due to higher torque than the gears can

stand,

altho misalignment
might account for some of this.  The really incredibly neat things about

a

plantary gearset
like this is that you can realtively easily increase the number of

planet

gears and linearly
reduce the geat contact stresses.  Going to a 4 planet set from a 3

planet

set will
cut the contact force by 25%, so if the peak gear stress is just

reaching

into the
short life fatigue zone, you should move the operating point out to MUCH

higher

fatigue (spalling) life or possibly even hit the infinite fatigue life

leve.

Certainly, better
oiling will help spread the gear loads slightly over the tooth

(beneficial)

and cool the
gears better, too.  Of course, having the planet float slightly to

equalize

the load is
critical.

Bill

Bill, maybe you can help me better diagnose what was happening on my Ross
drive.  I'm not sure if I'm using the technically correct term (spalling)
for what I saw.  It looked like what happens when two pieces of metal rub
together without sufficient lubrication.  Ross said it was due to an
improperly machined oil passage in the drive which blocked oil from getting
to the sun gear.   Even more confusing to me was the fact that the spalling
was only on half (of circumference) of sun gear.  It was most sevear at one
point and faded away on either side of this point for about 90 degrees and
was completely gone on the other half.  I was unable to imagine any senario
which would explain this other than some odd harmonic oscillation along with
some misalignment.  This might have been possible because the prop shaft
thrust bearing race was like a wash board.

Tracy Crook
rws@altavista.net
www.rotaryaviation.com

Bill is incommunicado for awhile Tracy. Up in the Colorado mountains. 
Spalling as I recall is a pitting action where the failure is a 
surface compressive failure of the material. Like Bill said. To quote the Sky Ranch
Engineering Manual "spallig is a metal fatigue failure and not a lubrication failure."
It is commonly seen on camshafts or valve lifters due to high contact stresses.

If it was due to resonance you probably would see the failure on both sides of the
sun gear as the rotary fires twice per rev.

Did this happen before or after you epoxied the planet carrier into the housing?
Sounds like a misalignment of the sun gear and planets.

Paul Lamar
 
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