Industry wakes up to carbon fibre drive shafts for INDUSTRIAL POWER TRANSMISSIONS

Eureka magazine editor, Paul Fanning, looks at the potential of carbon fibre driveshafts for long span industrial power transmissions applications

The benefits offered by carbon fibre drive shafts have long been known in the marine sector, where large, single span shafts have eliminated the need for intermediate bearing support and offered greater speed for large boats, such as fast ferries.

This is especially true in narrow hulled catamarans, where each hull contains a staggered diesel engine. Long, lightweight shaft lines are needed, with the result that all sizable fast catamarans are now equipped with carbon fibre tubes (CFT) for transmitting power from engine and gear to water jet.

The loss of weight (carbon fibre shafts are typically 70% lighter than solid steel) also means considerable savings in terms of fuel, while the removal of high maintenance bearings has obvious benefits in terms of wear and tear and overall costs. Equally, carbon fibre’s much higher critical speeds allow much longer spans between the bearings, fewer bearing pedestals, less power loss, less maintenance, less noise and easier installation.

However, the emphasis on the use of carbon fibre shafts in marine applications has tended to obscure their suitability for other sectors. In fact, in many applications where there is a requirement for long span drive shafts, carbon fibre may offer an effective solution.
Nigel Smith, sales manager for Centa, which has been at the forefront of this technology for some years, is enthusiastic about the possibilities. “Marine is the primary application for carbon fibre driveshafts, but it is not the only one,” he says. “In Germany, for instance, they use cooling towers with motor driven fans in them. As a result, they’ve got a gearbox in the centre of the cooling tower, the fan going round inside and the motors outside. Traditionally, they’ve had a corrosion issue because the shaft is exposed to a considerable amount of water. As a result, they’ve used carbon fibre drive shafts, which don’t have a problem.”

Another instance where this technology has transferred to industry is in at least one pumping station, where the pump is situated in the pumping house, but the motor is in the roof. The traditional method of connecting the motor to the pump would be to run a universal joint (UJ) shaft all the way down the wall to another at the other end. Centa, however, has been able to span this with a single carbon fibre shaft, from the pump straight up to the motor. As well as delivering a lightweight and less cumbersome solution, this also offers considerable safety benefits. Smith explains: “If there’s a failure of the UJ shaft, there’s this thing flailing around on the end of the electric motor and it will destroy anything in its path. There are normally metal stairwells around the sides of these sites and it has been known for the shaft to flail the stairwell out. With the composite carbon fibre shaft, if it breaks – which is unlikely in the first instance anyway – all you’ve got going around is a bit of plastic.”

The use of carbon fibre driveshafts in this context is obviously far from traditional, but that doesn’t alter their potential. Says Smith: “One of the things people don’t appreciate about carbon fibre is how much torque you can transmit with it. The smallest shaft we’ve provided is 250hp and the biggest we’ve built is 23MW. The biggest we’ve quoted for is 60MW.”
This remains far from being a common industrial solution, however. “We haven’t broken into that market yet by any means,” Smith concluded. “We’ve supplied a couple of shafts, but it’s certainly not a core application as yet… people aren’t resistant to the idea as such, they just don’t know about it. I don’t think there is any resistance to the idea once it’s presented to the right people.”

DESIGN POINTERS

• Carbon fibre drive shafts can offer benefits in any application where power needs to be transmitted over a long distance.
• The larger the diameter of the tube (within reasonable values) the better its ratio of weight to stiffness and the more advantages it will confer.

Reprinted by kind permission of Eureka Magazine, February 2010 (Findlay Media Limited)