Hi, Paul.....regarding the use of two thrust bearings on my Ross PSRU,  I
would rather forward Dave Mix's comments rather than possibly misstating his
reasoning for doing the mod this way.  Here is a copy of the message he sent
to me regarding the options.  Thanks for your interest and comments. Hope to
see you at Sun-N-Fun.  Paul Conner

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Hi Paul,
    I've been inside your psru and have done the necessary measurements.

Here's a rundown of what I discovered, and a proposal for the bearing
installation.
    The good news is that your shaft seal is a better design than the
early seals that
Ross used, but the downside is that it uses the 1.750 diameter of the
shaft as its
sealing surface, therefore this dia must stay at 1.750. There is a
thrust bearing available that will fit that dia but things are never
that easy. The shaft seal fully
seated in the bellhousing cavity protrudes .006 and needs to be
protected
from any compression, so in order to provide a path for the thrust load
to
pass into the housing I will make a "bridge" that will support the
thrust bearing
on one side and straddle the seal body on the other side. ( All 360
degree
circular of course ).
    Mr.Ross did a fine job of space utilization and left me with .127
space to
put in .180 +/-.004 worth of stuff. I will take a minimal cut into the
planet
carrier (approx. .053 deep) to accomodate the new parts.
     Now the dilemma, all this puts one bearing set in place ( bearing
plus
hardened & ground washer on each side) To put an additional bearing in
place will require the planet carrier cut to be .162 deep (vs .053). The
carrier in that area is .255 thick HOWEVER the cut would be VERY close
to the planet
gear pinions (.035 wall more or less, this would be at 4 places).

    This is totally your call, and I don't want to influence you, but,
DON'T DO IT.
The thrust bearing is rated at 5600 pound load at 4500 rpm. Within
reason
ratings run inverse to each other (the lighter the loading the higher
the allowable
speed). The max pure thrust load you could ever possibly put on the
bearing
in this application is equal to the max aerodynamic drag of the
aircraft. 

Nothing to do with the prop thrust.

Operating
at 6000 rpm at such a minimal load is reasonable. Also remember that the
aft
thrust bearing is a single race also and has a similar rating and is
potentially seeing
a greater load due to the gear tooth spiral effect.
    That was easy enough. The bearings are ordered as are the races and
material to make the bridge. Also, please measure the bellhousing/psru
gasket thickness for me.
    Please let me know if you want the second bearing, its do-able, but
if it were my
gearbox I would opt to maintain the integrity of the planet carrier and
forgo the
second bearing.
Thanks,                            Dave

This bearing has nothing to do with the prop thrust.
This bearing takes the thrust load from the helical sun gear
and operates at up to 6500 RPM on take off and 5000 RPM continuously.

Here are some drawings of devices that will do the job and that
is not dependent on the distance from the sun gear to the Ross
bell house casting.

Paul Lamar
 
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