A large diameter machined billet aluminum
prop shaft is only made possible by a major breakthrough in thin section
bearings. Namely silicon nitride balls. The torsional and bending
strength of any shaft goes up with the diameter to at least the cube if
not the fourth power so it makes sense to make the prop shaft as large
in diameter as possible. The stress on the material plummets  for a
given torque and bending load if the shaft is made larger.

The drawback to that, before high speed lite weight thin section
bearings, was the size and weight of old conventional steel ball
bearings. Tracy's front PSRU bearing alone weighs 3 pounds.

That also makes using lower strength, lighter weight aluminum shafts
possible while  keeping the weight down. The same design in steel would
weigh three times as much. One must always keep the big picture in mind.

Paul Lamar


OK ... you're talking me into it...

I guess I've seen too much chewed up aluminum in my life; housings worn
out, cases cracked, that sort of thing scares me.  I guess I am a
steel-guy at heart. So how might it work to create your new PSRU from
steel/composite?  The 4" diameter shaft/spacer might be a made from a
rather tin-walled steel tube with a composite tube inside for stiffness.
The housing might be a 6" steel tube surrounded with a composite
housing.  I think they even make/made gun barrels that way.

R. Berglund

The aluminum is treaded to take the shear and tension load of the prop flange and planet carrier. One would have to weld on a collar to accept
these bolts if steel tubing was used. Or machine a 4 inch chunk
of steel. It could be done of course. If that makes you feel safer :)
The down side is the weight of the excess meat between the bolts.
That too is no real problem if cost is no object as that could be machined away on an NC mill.

Paul Lamar

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