Under the retrofit program and in co-operation with Flender, the German supplier of the gear boxes, the planet bearing and the high speed bearing "of all relevant gear boxes" will be replaced by heavier duty bearings "utilising the good operating experience from gear boxes used before 1996," states NEG Micon. Flender will carry out the work at a new facility in Germany and there will be a renewed product warranty of two years on the bearings.
"There's no doubt that the chosen solution is the right one, but only if NEG Micon solves the pitting problem on the gear teeth at the same time," says Strange Skriver, technical consultant to the Danish association of wind turbine owners. Skriver has routinely checked hundreds of turbines under warranty for association members, gaining first hand knowledge of gear and bearing problems -- usually even before NEG Micon knew about them. "On gears where pitting damage is already present, it's crucial the damage is eliminated together with the bearing retrofit," he says, adding that a new shaft, new tooth gearing and a second polishing are the only effective solutions.
Gunnerskov assures that all gear boxes will be completely dismantled and that all parts will be checked during installation of the heavier duty bearings and before the gears boxes are reassembled. Only by doing this will technicians be able to see the extent of any damage. So far, about 120 bearings have shown early signs of wear.
"The use of heavier duty bearings in all gears is only one part of the retrofit," Gunnerskov says. "All parts of the gear box will be checked, damaged components will be replaced and gear teeth will be ground in the critical areas where it is needed. Besides this, all gear boxes will be equipped with an improved lubrication function. In fact, we are continuing with the lubrication program we first launched to solve the problem, which was the right approach, but was just not adequate."
Until April this year, NEG Micon denied that it had any serious gear box problems, attributing unusual wear to incorrect gear box oil (Windpower Monthly, May 1999). In 240 NEG Micon 750 kW turbines, the company then changed the oil, mounted oil filters, lowered the oil temperature and on some machines established high pressure lubrication, work that cost DKK 10 million but did not get down to the real problem.
Not our fault
Danish national laboratory Risø along with Det Norske Veritas (DNV) have type certified the affected NEG Micon turbines and their gears. Risø's Poul Højholdt denies that the two organisations have relaxed any of their technical criteria to issue certificates.
"Both Risø and DNV have the same safety factor demands, which we always have had -- that is from 1.2 to 1.5, varying from component to component. It's not the certification that has caused a weak foundation. And you can't say that the gear boxes are under dimensioned for the large turbines, or poorly built, or that the fault lies with the manufacturer of the turbines or gear boxes," Højholdt says.
"There are many factors that are working together here," he adds. "Other companies have had gear box problems as well. Flender gear boxes are just those that until now have been hardest and most soundly hit." Højholdt notes that the wind industry in general must be more meticulous with lubrication and with sizing gear box bearings.
Risø, in co-operation with Skriver, has now begun to create a database of information from routine inspections he makes of the turbines of association members.