Coating for bearings reduces gearbox failures

The Schaeffler Group has developed a mixed iron oxide coating for cylindrical roller bearings, which provides increased resistance to slippage during alternating, low load conditions.

Investment in renewable energy systems, whether wind, wave or tidal, requires significant financial commitment.

Protecting this investment by ensuring maximum availability of the energy system is therefore critical.

FAG rolling bearings from the Schaeffler Group are found in many wind turbine installations around the world, and more recently, in wave and tidal stream systems.

Used predominantly to support the rotor in wind turbines, these bearings are typically designed to have at least a 20-year operational service life.

Suitably designed bearings are critical in reducing wind turbine operator costs.

For example, the design of low-friction bearings for the wind turbine rotor shaft and gearbox is a key consideration, as are the associated lubrication methods for the bearings, mounting and maintenance, to ensure maintenance costs are minimised and service life is increased.

Schaeffler has worked closely with designers, manufacturers and operators of wind turbines for more than 25 years and more recently with manufacturers of wave and tidal stream systems.

In a typical wind turbine drive train, FAG rolling bearings are used to support the rotor, gearbox and generator.

In addition, plain bearings can be found in blade adjusters in pitch-controlled wind turbines and in geared motors that drive the tower slewing ring and pitch control.

The significance of these rolling bearings will increase as larger, multi-megawatt class turbines are developed.

With the implementation of increased megawatt wind turbines, higher capacity gearboxes are now required.

The operating conditions for rolling bearings therefore involve highly dynamic forces with extremes of peak and minimal loads, sudden load reversals and widely differing operating temperatures.

Schaeffler offers a wide range of rolling bearings with high static and dynamic load safety factors, including tapered roller bearings, cylindrical roller bearings, spherical roller bearings, deep-groove ball bearings and four-point contact ball bearings.

Schaeffler said that its X-life branded bearings offer increased operating life far beyond conventional limits.

X-life bearings have product characteristics such as lower noise and reduced friction, easier maintenance and higher load capacities.

FAG spherical roller bearings are also playing a part in several European tidal stream energy systems currently in development.

FAG double-row spherical roller bearings are used to support main turbine rotors with diameters up to 750mm.

Schaeffler UK is also supplying spherical roller bearings and radial ball bearings for use on drive train gearboxes for tidal energy systems.

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 Durotect B coated cylindrical roller 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 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 2um).

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 20um.

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.

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