Jerry wrote:
The original and traditional Alfa prop shaft design
used in the models no had severe destructive resonance
problems cause by caused by differing rotastion massses
and the prop shaft always turning at engine speed. The
original prototypes could not manage more thas 6000K
without the likelihood of the overloaded crankshaft
being shaken to pieces by the vibration.
And George added:
Possibly, the problem with the resonances was caused
by the length of the driveline in the Alfetta series,
I don't know.
The issue is the ratio of mass to stiffness as the length of the shaft is
increased. Say you're spinning the shaft at 5,000 rpm. Start out with a
short shaft and it's fine. Then cut a new, longer shaft. At some length,
the shaft will begin to whip out at the center and be off balance. The only
way to alter that is to use a material with a higher modulus of elasticity
(smaller deformation under the same load) or increase the radius of the
shaft. I expect the Alfa engineers made the shaft as large in diameter as
possible while using a steel appropriate for the application and cost. And
by cutting the shaft length in half gave themselves a good margin on the
whipping problem. Indeed, to somewhat counter-prove the point, the shaft in
my GTV6, with its shot center bearing, whips nicely. I'd bet that, today,
we could make a single-piece shaft out of a combination of carbon fiber and
kevlar and have no problems.
Helicopters use extremely long shafts to carry power from their
transmissions back to their tail rotors. It would be an interesting
exercise to look into how their designers handle the problem.
What I do know is that Ferrari used torque
tubes as does the current C6 Corvette to transfer power
to the rear-mounted transmissions, so it is possible.
Yes, and the rear-transaxle Porsches also used torque tubes.
Rich Wagner
Montrose, CO, USA
'82 GTV6
p.s. I can't get my email spell checker's mind out of the gutter. It keeps
wanting to replace "transaxle" with "transsexual".
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