RE: carbon fibre driveshafts

["Robert K. Davis" <bob@btv.ibm.com>]

Actually, I was a little hasty in my assumption that the carbon fiber driveshaft would fail with 10 degrees of twist.  The driveshaft I used in the example had 3.2mm wall thickness, 102mm I.D. by 3050mm long. For a 102mm I.D. by 3050mm long shaft of the same material with 0.2mm wall thickness, the  critical buckling torque is about 30,000 Nm.  Now, the critical buckling torque is dependent on the wall thickness raised to the 2.25 power.  So, for a 3.2mm wall thickness, the critical buckling torque would be on the order of 15,000,000Nm.  Given that the torsional stiffness is 45,000Nm/rad, this relates to 341rad, which does seem excessive.  If the claim is 2*pi rad of windup, then the critical buckling torque need only be 283,000Nm.  Clearly it seems that such a driveshaft is within the realm of possibility.

--
Bob Davis