Hi, fellow rotary enthusiasts....after posting a photo of the thrust bearing
mod done to my Ross PSRU, I received a couple of replies stating that a dual
thrust washer/bearing is what is needed.  I am forwarding a copy of Dave
Mix's comments and recommendations regarding my particular Ross unit. He can
explain this mod and it's reasoning  much better than I.  Thanks very much
for everyone's comments.  Keep building and flying these wonderful rotaries!
Paul Conner

----- Original Message -----
From: "David" <harborview@rivnet.net>
To: <sqpilot@earthlink.net>
Sent: Wednesday, February 12, 2003 7:46 PM
Subject: PSRU


> 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. 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
>
>