| chrisl |
12-10-2013 01:29 PM |
Quote:
Originally Posted by Captain Snooze
(Post 1380064)
Why is this the correct weave and by inference the others incorrect weave?
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Carbon fiber is a lot like wood - it's not uniformly strong in all directions. It's much stronger along the grain/weave than it is perpendicular to it. For a shaft loaded in torsion (in other words, pretty much every driveshaft ever made), the maximum loads will be at a 45 degree angle to the shaft. So, for a carbon fiber part (or fiberglass), you want the fibers to be wound at a 45 degree angle for the best strength with the minimum amount of material. Since it can be loaded in either direction (acceleration or engine braking), you want fibers wound around it in both directions, at 45 degrees. You can see this in the shaft above.
For some reason (that I do not understand), many carbon driveshafts instead have the fibers wound very nearly circumferentially, at least at the surface of the driveshaft. This gives very little strength against a twisting load (but it would be great for a pressure vessel, like a composite scuba tank). They must have fibers in some other orientations internally as well, to give them the necessary strength, but even so, the fibers wound circumferentially at the surface are doing almost nothing aside from adding weight - for a driveshaft, you really want all of the fibers at or near 45 degrees (you could maybe make a case for varying the winding angle between perhaps 30 and 60 degrees in the various layers, but even that isn't really necessary).
This also probably explains why they can make it lighter than other CF driveshafts. They also could be using a higher grade carbon - all CF isn't created equal. A standard carbon fiber might have a modulus (stiffness) of around 230GPa and a strength of around 3.5GPa, while a high modulus carbon will have a similar strength and a modulus of around 550-600GPa (so it's 2 or 3 times stiffer, but not a lot stronger). A high strength carbon fiber on the other hand might have a modulus around 300GPa (only a bit stiffer than the basic stuff), but an ultimate strength of around 6GPa or even a bit more (so nearly twice as strong as the basic carbon fiber). Of course, the high stiffness and high strength carbon fibers are a lot more expensive than the ordinary carbon, but using them can significantly improve the strength and stiffness (or reduce the weight for the same strength and stiffness) compared to a component made with basic CF. |
