Grey staining – a phenomenon to date unexplained

You need to know this

Grey staining occurs as signs of wear on the surfaces of metallic components, which are permanently under high loading. Gears in gearboxes and sliding camshafts and valve lifters are mainly affected. Damage of this nature only occurs rarely in the gearboxes of motor vehicles, but mainly in industrial equipment like mill drives, rolling mills and wind power plants.

How can you detect grey staining?

Viewed with the naked eye, the damaged parts have a matt grey appearance. Only when greatly magnified, does it become apparent that many tiny pores and eruptions are responsible for the grey colour. When gears are permanently under high loading, sliding and mixed friction is created on the surfaces in contact with each other. Thus, these are plastically deformed, microscopically small cracks occur. These may develop into deeper cracks and subsequently to major eruptions. A phenomenon that is also referred to as pitting. Over time, grey staining can negatively influence the flank form of gears and change both the mesh and noise behaviour in the gearboxes.

Causes of damage by grey staining

Grey staining can have various causes. It occurs especially when components like gears are permanently heavily stressed and there is a high sliding speed combined with low film thickness. The low film thickness often results from the fact that the lubricant is exposed to high temperatures. Another reason may be an unfavourable geometry of the gears, which lead to heavy loading at certain points on the gear flank. The texture of the surface also plays an important role. If this is very rough, too thin of a lubricating film thickness leads to constant contact on the part of the metal and thus friction damage.
The choice of lubricant is also crucial, since the gear geometry, the speeds and the load are determined by the requirements of the particular gearbox design. Too low viscosity, for example, causes insufficient lubricating film thicknesses. In addition, certain additives in the oil can promote corrosion and therefore the tendency to form grey staining. Nowadays, high-performance lubricants are tested using a standard test to determine whether they are suitable for preventing grey staining.

However, there is another theory regarding the origin of grey staining. At the car makers Mercedes-Benz, wear was detected on the gear constructions of rear axles in the 1970s and 1980s. After various metallographic tests in the company’s own laboratory, the engineers came to the conclusion that the lubrication film was destroyed by vibrations with unusually high inherent frequencies of the components and consequently led to scuffing damage to the gear flanks. This explanation could also apply to roller bearings in wind turbines. This would mean that the wear is not caused by external stresses, but by a dynamic stress inside the gearbox components.

Gearboxes of wind turbines are particularly affected

Grey staining has been known for many years in gear drives. It occurs particularly often in wind turbines and thus constitutes a major problem for operators. On the one hand, it can have a significant impact on the wind power plant capacity; on the other hand, gearbox damage due to grey staining can quickly run into six figures. What is particularly unclear is who is liable for such damage.

In 2007, the German Wind Energy Association (BWE) set up a working group consisting of experts, operators, technical managers of wind turbines and lawyers to investigate the origins and effects of grey staining, as well as appropriate countermeasures. It took four years for the group to submit its final report in 2011. The conclusion: It is still unclear who may be held liable for damage caused by grey staining. The gearbox manufacturers see the problem as a normal sign of wear, which is excluded from warranties. The process of grey staining is very slow and usually only leads to gearbox failures when the warranty period has expired. To date, it is not clear from a legal point of view who is liable for damages.

This still needs to be clarified

As long as the causes are not clear, there will be no final decision on who is liable for damage to the gears of wind turbines. High-performance wind turbines with more than one megawatt capacity, which run constantly at their limit, are particularly affected. The problem of grey staining can only be partially contained by conventional lubricants and improved surface structures: the older the plant, the more difficult a satisfactory solution. Therefore, it remains unclear whether the phenomenon of grey staining will ever be seen as damage liable to recourse. The working group of the BWE therefore did not come up with any final verdict in its report. To date, there is insufficient experience on the fatigue strength of large wind turbines and the impact of dynamic forces such as changing winds, accumulation or tower shadow on the drive shafts.

The best option of all, then, is to protect new wind turbines with innovative nano-tribology from the start from grey staining and eliminate the dreaded pitting as consequential damage from the outset. However, this cannot be achieved using the highest quality lubricants, as these too, sooner or later can only reduce the frictional wear, but cannot completely rule it out. A far more effective approach is to start right on the surfaces and to coat these metal-ceramically, so that friction forces are largely avoided. The surface ceramisation is done using the oil additive DuraGear® W100. This is added to the lubricant during operation, uses it as a means of transport to friction points and stores silicates there through a chemical process on the metal surfaces, thereby refining these as metal ceramic surfaces.

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