The Gearbox Problem

They were a mule hair from payment suspension in 2000 when a failure in the rollers inside the gearbox was showed its ugly face around the world.

For one time, the whole leadership and all the business people were hiding under their desk, while few clever engineers in development department fought to come up with a solution who could save the company. The cold fact was that no one could see other solutions than dismantling the whole nacelle, put it on the ground for replacement with a new gearbox and reinstall it.

Even the girls in the canteen could figure out that it would be their death.

And no no no! Andersen was not a part of the solution. They just ask me, if I would follow them with my calculation on the sideline to secure a safe connection in their design.

They chose the material alumec from Swedish Uddeholm. An aluminium material there was new for me.

An overturning force that was extremely difficult to build into the mathematical model, that in the end, should tell us that the equipment was safe. That uncertain force must be transmitted through the threads to the nacelle-frame as indicated on my picture second up to the left.

As you have tensile stress from that forces, it must add to calculated preload force on rods, and if your model is right, it will tell you that an overload fracture must occur in the bolt shaft and not in the threads and here it began to be difficult. Alumec was not steel material but an aluminium material with a tendency to oxidize on its surface. Oxidation that gives it a very high ultimate stress point in a very fragile film on its surface and a more softcore below. That was clear after my full-scale test of the material.

I was concern about the combination of plastic deformation in the cutting threads during the preload process without knowing it and a slipping possibility when the outside load was added. An external load, who was unpredictable, because the wind was unpredictable.

The devil is in the detail, as they say. This time we allowed him to live down there between the threads and instead restrict the whole operation to be executed within a given frame of wind speed.

In that sense, was a part of the safety factor moved to the site. Fewer hours the windmill was dismantled, the less possibility was it, that it was precisely on their site, the wind will jump, and overstress the interims equipment. An intensive training program brought the whole operation down to two working days on its site for a site manager, two fitters and a small mobile crane there could handle the gearbox.

N.E.G Micon was safe from suspension, and the leadership and all the business people began slowly to raise their heads to the level with their desk. From there they scanned the space around them until they convince them self, that their livelihood was safe by forces the did not fully understand. Came on their feet and slowly began to operate again.