A worn generator shaft occurs when the inner ring of a generator bearing damages the shaft surface through friction. At the location where the bearing of the generator comes into contact with the shaft, the metal rubs against the shaft surface for an extended period of time. The shaft is then no longer quite so smooth and becomes smaller in diameter. Even small deviations in the diameter can lead to increased imbalance, heating, and damage progression: The bearings no longer fit tightly, the connection becomes imprecise, and wear continues to increase. Due to the load imbalance, the turbine runs more unevenly, it can vibrate or become louder. In the end, there is a risk that the generator might fail and need to be stopped. For this reason, it is a good idea to detect worn shafts as early as possible and fix any damage that has already begun to occur.   

Precise analysis of fault messages provides clarity  

"The first error message we usually get from the technical department is an overheated bearing. Our first assumption is that the bearing is defective," said Lorenz-Theo Feddersen, Engineer Generator Transformer at Deutsche Windtechnik. "However, if our service teams on site cannot detect any noises from the bearing, this is an indication that the bearing itself is not defective. Shafts that are starting to show signs of wear, on the other hand, are initially still quiet. Only later do severe vibrations occur in conjunction with a banging noise." Further troubleshooting requires a more sophisticated approach: "Only when the bearing cover is removed and a visual inspection is carried out do we know for sure that the shaft is affected and that there is damage due to wear."  

Repair directly on the turbine instead of using a crane and replacing components  

If the shaft has become worn, Deutsche Windtechnik offers a special laser welding process that is also known as laser deposition welding in technical jargon in cooperation with a partner.  Since no electrical current is applied to the shaft with this method, it can be repaired directly on the turbine inside of the nacelle. The generator is not affected. Lorenz-Theo Feddersen said: "The procedure, in which the shaft is welded on and both bearings are always replaced, usually only takes a few days to complete. The fact that the generator does not have to be replaced also means that the crane and the corresponding logistical personnel are no longer required."  

How does the welding process work in detail?  

Laser deposition welding is a process in which the base material, in this case the generator shaft, is melted using a focused beam of light, and new material is applied using a filler material (welding wire).  

Before the welding process, impurities and cavities must be removed from the generator shaft by grinding, milling or turning. During the welding process, between 0.6 and 1 mm of new material is then applied per welding layer. The welding layers are thin so that the layers thermally bond together and a structure is formed. The heat created by the highly concentrated laser beam only affects a tiny area: The generator shaft only heats up to a maximum of 45 - 50 °C, so the shaft can still be touched without suffering burns. This rapid application of minimal heat to the rotating generator shaft produces even heating, thereby preventing distortion. With a shaft diameter of 170 mm and a width of 85 mm, the total welding time is only between 80 and 100 minutes.  

The result has the same properties as the original generator shaft  

Lorenz-Theo Feddersen said: "Laser deposition welding is an alternative repair method for areas that are subjected to high levels of strain, such as the bearing seats of large gearbox or generator shafts, because a homogeneous structure is created during welding. This structure has the same properties as the generator shaft. Hardness and tensile strength can also be added to the welding rod through additional materials, for example to create a hard surface."