Tracy Crook wrote:

Just for a data point,

The last failure I had on my Ross drive was a failure of the prop thrust
bearing along with some spalling of the sun gear.  The thrust bearing
absorbs all of the combined thrust of the prop AND the ring gear thrust
(which is much higher).  Ring gear thrust is around 850 pounds at max engine
torque (13B). My drive has a spline between prop shaft and ring gear so
separate bearings carry the thrust.  Not sure why the bearing gave up on my
Ross since the specs indicate it should have easily handled the total
thrust.

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

I think the reason might be once the end play in the system built up it

just started

pounding back and fourth and destroyed both the front needle and the prop

thrust needle
>from impact loads.

Paul Lamar

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

 
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