Good and bad news on bearings

Following the severe criticism from Danish wind turbine owners in 2000 of NEG Micon's proposed retrofit of gearboxes for 1250 of its wind turbines the world over, the company changed course. Instead of using spherical bearings in the Flender gearboxes to be retrofitted, it decided to use cylindrical bearings. The decision proved to be a good one, averting an epidemic of further failures, says Strange Skriver, expert consultant to the Danish association of wind turbine owners. Flender subsidiary Winergy has used cylindrical bearings in its gearboxes ever since.

"With so many Flender gearboxes in operation the world over, failures will of course still happen, but not in the numbers once feared and certainly nothing that could be called a series failure," says Skriver. He does not imagine that retrofitted gearboxes on 600 kW and 700 kW turbines supplied by NEG Micon will fail in the near future. NEG Micon has since merged with Vestas.

Skriver is not as optimistic about gearboxes from other suppliers installed in thousands of wind turbines of the same size, where spherical bearings are still in use. "They are used by all other gearbox companies. That means these gearboxes are in the danger zone for accelerated wear and failure," says Skriver. Gearbox failure has typically hit wind turbines in the 500 kW to 900 kW size range that were most sold in the late 1990s and the first years of the century. These owners should be prepared for gearbox retrofits, says Skriver, which typically cost EUR 67,000-EUR 80,000.

While the gearbox problem is either solved or understood, these days the most pressing technical problem on new turbines is accelerated wear of the main bearing, says Skriver. It occurs on main bearings with wear marks inflicted on the bearing when the wind turbine has been stationary. To alleviate the problem, automatic greasing has been introduced on some of the wind turbines where the main bearing has not been retrofitted. Most recently an experiment is being conducted with oil lubrication on the main bearing in a wind farm in Denmark. It may prove to be an effective solution, says Skriver.

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