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United States

Researchers tackle gearbox failure

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America's National Renewable Energy Laboratory (NREL) is trying to solve the mystery of why some wind turbine gearboxes will fail at a given wind farm while others nearby run without a hitch, a problem that occurs across designs and suppliers. The solution lies in finding a basic common cause for micro pitting and other damage that occurs on gearwheel teeth and bearings. "Very similar issues are occurring at wind farms all across the country," says Jim Johnson, a senior engineer at NREL. Researchers have some leads. "It looks like the bearings are the issue," says Johnson. But why the damage occurs has yet to be established. "There are machines that are particularly hard on the gearboxes and for no particular reason," he says.

NREL's project, known as the gearbox reliability collaborative, brings together a spectrum of industry participants -- including gearbox makers, wind farm operators and lubrication system designers -- that have signed non-disclosure agreements to encourage free exchange of information without fear of legal repercussions. NREL researchers modified a pair of older gearboxes to simulate typical drivetrain operation on today's multi-megawatt turbines. To avoid implicating individual manufacturers, they drew from across a range of common designs. Instruments measure stress and loads on the gearboxes and bearings on a purpose-built testbed (main story).

"We've spent a lot of time harmonising and calibrating the different analysts and their tools and now we're stepping forward to the more complex load cases and that's producing some very interesting results -- but nothing I would go public with yet," says NREL chief wind turbine engineer Sandy Butterfield. The team is working out how effective each method of analysis is, he adds. "These gearboxes are instrumented to a level that's very rare and when those start producing data that we can compare with the analysis, then I think light bulbs will start to go on. And that should happen within weeks or months," predicts Butterfield.

The gearboxes will later be installed on a pair of generically revamped 750 kW turbines erected near each other at NREL's campus outside Boulder, Colorado. One gearbox will eventually be moved to the Ponnequin wind farm at another site in Colorado for a real-world comparison.

Even with their imperfections, Johnson says gearboxes, which in wind turbines are needed to increase the speed of the rotor to that of the generator shaft, remain more economic than direct drive turbines, in which rotors are coupled directly to slow speed generators. "Those designs, if they're looked at from a cost-of-energy standpoint, still cannot match the conventional drive trains," Johnson says. "It's not necessarily true for each individual machine, or worse for one than another. But as a general statement, direct drive is still very expensive."

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