Unacceptable wear found on gears

A potential series failure of gear boxes in wind turbine models rated at 500 kW and above is looming. Extraordinary wear has been discovered on the teeth of gears in large wind turbines operating in Denmark for less than two years. The cases of wear have come to light during routine checks of turbines prior to the expiry of their manufacturer warranties. The gear box of at least one turbine -- a 600 kW Micon unit -- has had to be replaced at a cost of DKK 800,000.

The Danish wind turbine owners association is now advising its members to take precautionary action. The pattern of wear, in the form of pitting on the surface of the teeth, has been documented by the association's technical consultant, Strange Skriver. One of his functions is to carry out inspections of members' turbines before their warranties expire.

"During 1998 I have so far been out to 83 wind turbines of 500 kW and more which have been in operation for nearly two years. On many turbines I have found wear on the gear teeth to a degree which is unacceptable after so short an operating period," says Skriver. "The problem is not restricted to a single turbine or gear box manufacturer, but is close to being a general problem," he adds.

So far the wear is restricted to areas of micro pitting on the surface of the gear teeth, but Skriver fears this can develop into far more serious macro pitting -- or holes in the surface of the gears. Skriver has inspected turbines installed with gear boxes of four different makes, by far the most popular of which is made by Flender of Germany. The remainder of the market is shared between Valmet of Finland, which supplies gear boxes to Vestas; German Jahnel-Kestermann gear boxes, used on Nordtank turbines; and Hansen gear boxes from Belgium, which are used on several makes of turbine. No pitting or other faults have been found on Hansen gears.

To date it appears that turbines sold under the Nordtank and Micon trademarks, before Nordtank Energy Group and Micon merged into NEG Micon, are the worst affected. Nordtank units are fitted with gear boxes from Flender and Jahnel-Kestermann. "I have observed micro pitting on all gears from Jahnel-Kestermann," says Skriver, but only one case on a Nordtank with Flender gears. These, however, are used on all Micon units. "I have seen a lot of these turbines and I've seen many cases of micro pitting to a greater and lesser degree," says Skriver. "It is always the second stage small gear wheel." A much lesser degree of wear is seen on Flender gears in Nordex turbines. The Valmet gears on Vestas turbines show signs of wear over their whole surface soon after starting operation, but their former polish returns after a while, a phenomenon Skriver describes as "atypical."

He cannot account for why an unusual level of wear is being seen on gears on larger turbines and has instigated the founding of a technical group to study the problem. "The manufacturers are taking this problem very seriously and together with the gear companies are seeking an explanation for the wear and methods of preventing it," says Skriver. Meantime, he advises all turbine owners to ensure their gear oil temperature is sufficiently low at all times and to have the oil content regularly tested for particles, which would indicate that pitting had started.

"The wear I have seen is not worse than that the gears can run for a number of years yet, before they need replacing, or parts of them do. But it is unacceptable that such a large degree of wear has been found within the two year guarantee period," says Skriver.