Grey spotting - a hitherto unexplained phenomenon

What you should know

Grey spotting occurs as a sign of wear on the surfaces of metallic components that are permanently under high stress. The main areas affected are gear wheels in gearboxes and the sliding camshafts and tappets in valves. Damage of this type rarely occurs in vehicle gearboxes, but mainly in industrial systems such as mill drives, rolling mills and wind turbines.

How can grey spotting be recognised?

Viewed with the naked eye, the damaged parts have a matt grey appearance. Only under high magnification does it become apparent that the grey colour is caused by many tiny pores and breakouts. In the case of gears, sliding and mixed friction occurs on the contacting surfaces under permanently high loads. This causes plastic deformation and microscopic cracks appear. These can develop into deeper cracks and then into larger breakouts. This phenomenon is also known as pitting. Over time, grey spotting can have a negative impact on the flank shape of gears and change both the gear dynamics and the noise behaviour in the gears.

Causes of damage due to grey staining

Grey spotting can have various causes. It mainly occurs when components such as gear wheels are subjected to permanent heavy loads and there is a high sliding speed combined with a low lubricant film thickness. The low lubricant film thickness is often the result of the lubricant being exposed to excessively high temperatures. Another reason can be an unfavourable geometry of the gears, which results in heavy loads at individual points on the tooth flank. The condition of the surface also plays a role. If it is very rough, a lubricant film that is too thin will lead to constant contact of the metal and thus to friction damage.

The choice of lubricant is also crucial, as the gear geometry, the speeds and the load are determined by the requirements of the respective gear design. Too low a viscosity, for example, results in insufficient lubricant film thickness. In addition, certain additives in the oil can promote corrosion and thus the tendency to form grey spots. Today, high-performance lubricating oils are tested using a standard test to determine whether they are suitable for preventing grey spotting.

However, there is another theory for the development of grey spotting. In the 1970s and 1980s, the car manufacturer Mercedes-Benz noticed signs of wear on the gearwheel constructions of the rear axles. Following various metallographic tests in the company's own laboratory, the engineers came to the conclusion that the lubricating film was being destroyed by vibrations with unusually high natural frequencies of the components, resulting in seizure damage to the gear flanks. This explanation could also apply to the roller bearings in wind turbines. This would mean that the wear is not caused by external loads, but by dynamic stress inside the gear components.

Wind turbine gearboxes are particularly affected

Grey spotting has been known for many years in gearboxes. It occurs particularly frequently on wind turbines and is therefore a major problem for operators. On the one hand, it can have a lasting effect on wind turbine performance; on the other hand, gearbox damage due to grey leakage can quickly cost a six-figure sum. Above all, it is unclear who is liable for such damage.

In 2007, the German Wind Energy Association (BWE) founded a working group consisting of experts, operators, technical managers of wind turbines and lawyers to investigate the development and effects of grey spotting, as well as appropriate countermeasures. It took four years for the commission to present its final report in 2011. The conclusion: It remains unclear who can be held liable for damage caused by grey spotting. Gearbox manufacturers see the problem as a normal sign of wear and tear that is excluded from warranties. The process of grey spotting occurs very slowly and usually only leads to gearbox failures once the warranty period has expired. To date, the courts have not clarified who is liable for damages.

There is still a need for clarification

Until the causes are clarified, there will be no final decision on who has to pay for damage to the gearboxes of wind turbines. High-performance wind turbines with an output of more than one megawatt, which are constantly running at their limit, are particularly affected. The problem of grey staining can only be partially contained with conventional lubricants and improved surface structures: The older the system, the more difficult it is to find a satisfactory solution. It therefore remains to be seen whether the phenomenon of grey spotting will even be recognised as damage requiring recourse. The BWE working group has therefore not reached a final judgement in its report. So far, there is a lack of sufficient experience regarding the fatigue strength of large wind turbines and the effects of dynamic forces such as changing winds, pre-stowage or tower shadows on the drive shafts.

The very best thing to do is to protect new wind turbines from grey staining right from the start using innovative nanotribology and to rule out the dreaded pitting as consequential damage from the outset. However, this cannot be achieved even with the highest quality lubricants, as even these can only reduce friction wear sooner or later, but cannot completely eliminate it. A far more effective approach is to apply a metal-ceramic coating directly to the surfaces so that frictional forces are largely avoided. Surface ceramisation is achieved by 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 deposits silicates on the metal surfaces through a chemical process, thus refining them into metal-ceramic surfaces.