The problem in the gear boxes, made by Jahnel Kestermann of Germany, is concentrated on the spherical roller bearings -- a type of bearing that has otherwise been abandoned by most manufacturers today and which experts have warned against (Windpower Monthly, July 2000).
Skriver, who has inspected more than 1000 gear boxes in the last decade, was one of the first to warn the industry against future gear box failures. In September 1998, he failed to convince the Danish wind sector to launch a campaign to deal with gear box problems before they got out of control. "But it proved to be impossible to find the financial backing for such an initiative," says Skriver. "And neither wind turbine manufacturers nor gear box manufacturers were particularly interested." Turbines were instead erected at record speed and with the increasing numbers of machines, the problem grew. In 1999, both NEG Micon and Vestas announced major problems with gear boxes in specific turbine models, resulting in expensive retrofit programs (Windpower Monthly, July 2000).
Skriver says that a third of the 377 gear boxes he inspected in Denmark between January 1998 and January 2000 were expected to run into serious problems. He bases his judgement on the already visible pitting damage on them. "But pitting is overshadowed by bearing problems, and bearings on Jahnel Kestermann gears in Nordtank 600 kW turbines are some of the worst." He adds that nobody as yet can explain why these bearings fail.
The Nordtank 600 kW units are operating in Denmark, Spain, the Netherlands and Germany, says NEG Micon's Ole Gunneskov. He assures that the gear boxes of these machines will be retrofitted as failures occur. Since repairs can be done without dismantling the turbine, they are expected to take about a day, meaning that production loss will be minimal, says Gunneskov.
In Denmark owners of turbines are concerned that insurance may not cover component failures on a machine after it has been retrofitted and which is out of its guarantee period. Denmark's around 6000 turbines are mainly insured by three companies: Codan, if/Skandia and Topdanmark. Representatives from all three companies, speaking at a seminar last month for wind turbine owners held at Vestas, agree they cannot guarantee that insurance will always cover bearing or gear box failures, whatever the details of the policy agreed with an owner. For insurance to cover a failure it must be "unpredictable," they add.