Russell Kent wrote:

Paul,
I know you're trying to shutdown the list, so this can wait, but I'm trying to wrap my brain around why you think a small number of thick ribs will safely transfer the torque (from PSRU to engine) better than merely increasing the overall thickness of the bell housing
by the amount of the ribs?  I mean that instead of making six 0.5 inch ribs, make the housing 0.0625 thicker overall.  Can you explain this to me?

Or maybe you're concerned about the gyroscopic bending moments that the PSRU is trying to transfer to the engine.  If that's the case, then wouldn't skinnier, taller ribs on the outside work better than the squat fat ones on the inside?

Russell Kent

Its a weight issue. For the same reason they use a lot of ribs
with thin skins in aircraft wings. The stiffness of a beam
is proportional to the depth cubed. 

Yes skinnier ribs would work better on the out side.
The depth of the bell housings is determined 
by the diameter and position of the ring gear, the length
of Tracy's input shaft and where the starter needs to be mounted.
Also since Tracy uses a sun gear thrust bearing that abuts the inner
1/2 thick seven inch diameter wall located just aft of Tracy's 
PSRU that too must be stiff. That inner plate also has the
function of locating the input shaft oil seal. To take
full advantage of deep & narrow outside ribs one would need a large 
counter bore the diameter of Tracy's PSRU. As it is now we 
can just face off the PSRU mounting surface.

I am trying to keep the pattern as simple as possible
and still accommodate all these needs.

Never the less your suggestion is a good one and I will try
to implement it in Rhino. I suspect it would be a wash 
weight wise given the boundary conditions. 

BTW 1/2 inch flat plate is overkill. Jerry Hey proved that with 
an actual static deflection test. A 16 inch diameter flat aluminum 
plate would weigh about 10 pounds alone. Similar to what Tracy 
is using now. Not counting the weight of the spacers and 4 inch 
long 10 mm bolts.

We won't be saving a lot of weight here compared to
what Tracy is using but we will be eliminating other motor mount 
bracket schemes such as the Schertz beam, the oil pan sandwich
plate and the wider PSRU plate trick. The later two schemes
require very expensive NC machine time to get the weight down.

If anybody feels this is not worth while please let me know.
It takes a lot of time on my part and a lot of expense getting
the castings made and machined.

It would be better and lighter to start from scratch and integrate 
the psru and the bell housing. That is not practical as there
are a bunch of PSRU's already out there.

Paul Lamar
 
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