Black oxide coated bearings can deliver significant performance improvements at an acceptable cost. They can resist a range of damaging operating conditions.

During operation, wind turbines are subjected to a wide range of varying temperatures, wind speeds and loads. These operating conditions combine with potential detrimental influences from solid contamination and cause tribochemical attack. The attack is due to oil chemistry and water contamination, which can result in bearing damage which would considerably shorten bearing service life. Hence, wind gearbox bearings ideally must endure widely varying temperatures, speeds and loads, plus exposure to contaminants, moisture and chemicals. These conditions can limit bearing life cycle and create a non-classical failure mode resulting in white etching (WEC) and/oraxial cracks.

Featuring an enhanced black oxidation surface treatment applied to the rings and rollers, the black oxidised bearings help prevent premature failure due to WEC/axial cracks.

Black oxidation can contribute to protect the bearing components from the aforementioned failure mechanism. This is where Black oxide coated bearings from SKF can help. SKF offer manufacturers, operators and maintenance providers an affordable solution, which will improve gearbox uptime. These bearings are suitable for new installations or as a replacement for untreated conventional bearings

A better black oxidation process

Black oxide is a surface treatment that is formed by a chemical reaction at the surface layer of the bearing steel and is produced when parts are immersed in an alkaline aqueous salt solution operating at a temperature between approximately 130 and 150°C. The reaction between the iron of the ferrous alloy and the reagents produces an oxide layer on the outer layer of the bearing components, consisting of a well-defined blend of FeO, Fe2O3 and resulting Fe3O4. The result is a dark-black surface layer of approximately 1–2 μm in thickness. The total process consists of about 15 different immersion steps, in many of which it is poss-ible to vary chemical contents, concentrations, temperatures, immersion times and fluid behaviour within the tanks

Protection from the causes of premature bearing failure

Black oxide coated bearings can deliver significant performance improvements at an acceptable cost. They can resist a range of damaging operating conditions. They offer better performance in poor lubrication situations because their treatment delivers improved lubricant adhesion and enhanced smearing resistance. The risks of fretting, micropitting and cracks can be limited. Resistant to corrosion and chemicals, black oxide bearings reduce the effects of moisture and aggressive oil ingredients. They also help improve friction behaviour and reduce wear, particularly under mixed friction conditions, which are present in wind gearboxes.

A proven solution

SKF’s enhanced black oxidation surface treatment is just one of a number of cost-saving, high performance solutions that SKF has developed during our extensive involvement with the wind energy industry. For wind gearboxes in particular, black oxide has proven to be an extremely effective solution, resulting in reduced turbine O&M costs by enhancing bearing reliability and performance.

Proven to be extremely effective against WEC/axial cracks

More than 80,000 SKF black oxide coated bearings are operating in the field with no reported failures due to WEC/axial cracks. In addition, a wind gearbox manufacturer has reported, in a sample of 1,000 standard cylindrical roller bearings in a gearbox application, a failure rate ranging from 40 - 70% (after two years). Subsequently, in a sample of 1,150 black oxidized cylindrical roller bearings for a similar application, the failure rate was 0.1% over the same period.

Applications

These below mentioned bearing types from SKF are used in key wind gearbox systems:

*Tapered roller bearings

*Cylindrical roller bearings

*Ball bearings.

Over the years, SKF has accumulated a wealth of evidence that this cost-effective process can deliver appreciable performance improvements for the wind energy industry that can result in fewer early failures and hence contribute to lower overall O&M costs.