Carbon-Fiber Drives Shaft Systems

Carbon-fiber drive shafts have graced racing cars for years, reducing weight and eliminating vibrations. But concerns about crash performance and costs have, with a few exceptions, kept them out of production vehicles. Toray Composites America (http://torayusa.com/tca/) of Tacoma, WA, has found ways to address both issues with an improved epoxy-and-carbon fiber drive shaft design that features a new winding pattern and simplified joining technology.

According to Tetsuyuki Kyono, director of Toray's composites development center, these patented crashworthy epoxy-and-carbon fiber drive shafts offer an energy-absorbing progressive fracture mode not found on previous carbon-fiber shafts.

To bump up the shaft's ability to absorb axial crash forces, Toray engineers added an extra layer of helical windings to the shaft ends, sandwiching it between an inner hoop layer and an outer lapping layer. During a crash, a flange on the drive shaft's steel yoke wedges itself between these inner and outer layers in what Kyono describes as a "wedge effect." At first, the wedging causes only a delamination of the inner and helical layer. Later on, as a sufficiently serious crash progresses, the tube cracks around the yoke.

Kyono contrasts this fracture with the sudden buckling and fracture of conventional carbon-fiber shafts. And he cites a 55-kph crash test result in which the new shaft design reduced peak axial compression loads to 10 kN from 330 kN with a conventional carbon-fiber shaft.

Another key to the crashworthy drive shaft system is its joint with the yoke. Kyono notes that previous carbon-fiber shafts tended to be joined to their yokes with adhesives, which not only increased assembly cost but also axially constrained the yokes »during crashes. Toray engineers, however, eliminated the need for adhesives by coming up with a robust press-fit joint. It relies on serrations on the outer diameter of the yoke to grip the inner diameter of the shaft. The interference between serrations and shaft was carefully optimized with the help of finite element modeling. Kyono says the resulting joint easily meets 2,000-Nm torque in static and fatigue tests.

Toray's carbon-fiber shafts, which Kyono says weigh 40-50% less than comparable two-piece steel designs, have started to appear on some production vehicles, including the Mitsubishi Montero, Nissan 350Z, and Mazda RX-8.