When it comes to the maintenance of wind turbines, the cost of replacing a bearing on the tower is more favourable than having to replace a complete gearbox due to a bearing failure. Over the next few years, it is predicted that MRO companies will increasingly have to carry out routine inspection and overhaul of gearboxes on wind turbines. This will inevitably result in the replacement of bearings that are deemed as being damaged or likely to fail in a short time period.
For this reason, Schaeffler has developed Durotect B, a new black oxide coating for its FAG range of cylindrical roller bearings. Field tests have demonstrated that significantly lower failure rates in wind turbine gearboxes are experienced if these units are fitted with suitably treated bearings.
Durotect B is a matt black, mixed iron oxide layer that is produced in a dip solution. The surface structure, magnetic and electrical characteristics of the base material are maintained. The coating is highly resistant to bending and stress.
As well as excellent anti-corrosion protection, Durotect® B also offers increased resistance of the bearing during slippage and sliding, as well as during alternating, low load conditions that are common in wind turbine and tidal stream energy applications. The coating also provides improved run-in behaviour of the bearings. Geometrical interchangeability is also provided due to reduced thickness of the coating (between 0.4 and 2.0 µm).
Schaeffler also recommends Durotect B for use on other types of roller bearings, including bearings for high speed shafts and couplings, as well as for full complement bearings.
In a wind turbine, virtually all forces and moments induced by the wind act directly on the rotor shaft. The rolling bearings are therefore subjected to highly dynamic loads and sometimes harsh operating conditions. This means that the design of the rotor shaft, as well as the bearings that support it, are critical.
A rolling bearing solution will typically comprise a locating/floating bearing design with spherical, cylindrical and tapered roller bearings. Spherical roller bearings, for example, can now be protected with another coating developed by Schaeffler.
Durotect P is a dark grey coloured PTFE coating that reduces friction and provides improved anti-wear protection of main rotor bearings in wind turbines. The thickness of the coating is between 10 and 20 micrometers. The coating provides improved sliding capacity of the bearing outer ring, with a corresponding reduction in the friction factor to less than 0.1 under axial displacement of the bearing rings.