EVOLUTION A SELECTION OF ARTICLES PREVIOUSLY PUBLISHED IN EVOLUTION, A BUSINESS AND TECHNOLOGY MAGAZINE FROM SKF WIND POWER TECHNOLOGY KNOWLEDGE SHARING, PROVIDED BY SKF 28 30 ContentsA selection of articles previously published in Evolution, a business and technology magazine issued by SKF. 03 Robust measures Premature bearing failures in wind gearboxes 32 14 and white etching cracks. 14 Dark forces How black oxide-coated bearings can make an impact on cutting O&M costs for wind turbines. 20 A flexibile choice SKF’s high-capacity cylindrical roller bearings are specifically designed for wind turbine gearbox applications. 23 Marble arts Developments in ceramic bearing balls for high- performance applications. 28 Wind powering the US The United States is increasingly looking to the skies to find new energy. 30 Spin doctors Engineering company Sincro Mecánica helps to make sure ageing turbines stay productive at Spain’s leading wind farms. 32 Improved efficiency A successfull cooperation of companies within the SKF Group has led to the development of an improved pump unit for centralized lubrication 20 systems. 2 evolution.skf.com 3 PHOTO: ISTOCKPHOTO Premature bearing failures in wind gearboxes and white etching cracks (WEC) Wind turbine gearboxes are subjected to a wide variety of operating conditions, some of which may push the bearings beyond their limits. Damage may be done to the bearings, resulting in a specific premature failure mode known as white etching cracks (WEC), sometimes called brittle, short-life, early, abnormal or white structured flaking (WSF). Measures to make the bearings more robust in these operating conditions are discussed in this article. evolution.skf.com | Published in EVO 2.2013 Fig. 1: Standard Multi Megawatt (MMW) wind gearbox (for 3-point suspension) having a low- speed planet stage and two spur wheel sections (high-speed intermediate shaft and high-speed shaft) with highlighted bearing locations that can be affected by premature bearing failures. 4 A MBITIOUS WORLDWIDE renew- ings, intermediate shaft and high- fine nano-recrystallized carbide- able energy targets are pushing speed shaft bearings (fig. 1). free ferrite, appear white in a light wind energy to become a main- Much premature wind gear- optical micrograph due to the low stream power source. For example, box bearing damage results in a etching response of the material. the Global Wind Energy Council, failure mode that is not caused by Known to occur only occasionally GWEC1, expects that the currently the classic rolling contact fatigue in some industrial applications such installed wind energy capacity of (RCF) mechanisms (fig. 2). While as paper mills, continuous variable 200 GW will double within three these classic mechanisms are sub- drives, marine propulsion systems, to four years, keeping open the surface initiated fatigue as well as crusher mill gearboxes or lifting aspir ational goal of 1,000 GW of surface initiated fatigue and can be gear drives, in wind applications installed capacity by 2020. predicted by standard bearing-life the frequency of premature failures Despite high wind turbine avail- calculation methods (refer to ISO seems to be higher (but might be ability (> 96 %, depending on tur- 281 and ISO/TR 1281-2), premature also related to a larger population bine), and a relatively low failure rate crack failures are not covered by of installed machines). Commonly, of mechanical components com- these methods. However, attempts early cracks have occurred within pared with electrical components, to calculate bearing life have been the first one to three years of oper- failures on mechanical drive trains made when detailed information of ational time or at 5 to 10 % of the still create high repair costs and rev- the case is available (e.g., local effect calculated rating life (fig. 3). enue loss due to long downtimes2. of hoop stresses)37. Mostly occurring on the inner In most wind turbine concepts, ISO 15243 describes the visual ring, as shown in fig. 4, the visual a gearbox is commonly used to step appearance of the classic rolling appearance of early cracks var- up the rotor speed to the generator contact fatigue mechanisms. ies from straight cracks (“axial speed. Today, the actual service life White etching refers to the cracks”) to cracks in combination of wind turbine gearboxes is often appearance of the altered steel with small spalls and large/heavy less than the designed 20 years. Fail- microstructure when polishing spalling. Based on SKF’s knowledge ures can be found at several bearing and etching a microsection. The from increased field experience, it locations, namely the planet bear- affected areas, consisting of ultra is concluded that early failures by Published in EVO 2.2013 | evolution.skf.com Red= Blue= Premature Industrial failure application 100 10 Classical fatigue <-> cracks & WEC Failed [%] 1 1 10 100 1,000 Sub-surface initiated fatigue Surface initiated fatigue Cracks & WEC Calculated L [%] * ** 10 Fig. 3: Typical in premature fail- ing of industrial machinery is that bearings from identical machines in the same specific surroundings are failing within a consistent and short time period. The slope differ- ence is a predictor of “other than ISO 281; ISO/TR 1281-2; ISO 15243 classic fatigue”. Those having had a short service life are likely to have a short service life again if no further Fig. 2: Classic fatigue failure modes versus cracks and WEC actions on the bearing-shaft-hous- *micrograph according to reference 5, **micrograph according to reference 6 ing system are taken. 5 cracks are neither linked to a par- ticular type of bearing (fig. 5) nor to a particular standard heat treat- ment (fig. 6) 6, 7, 8, 9, 10. The failure appearance, however, 4a is associated with the heat treat- ment (e.g., residual stress field), the stage of failure progress and very 4b 4c likely also to the operating condi- tions or bearing position (e.g., stress Fig. 4: Failure appearance: a) straight cracks, b) straight cracks and small spalls, and c) spalls. field from loading). As can be seen in fig. 6, for early cracking in this specific application, cracks in mar- tensite rings tend to grow straight into the material (suggesting the straight “axial” crack appearance, e.g., fig. 6a), whereas in bainitic (fig. 6b) as well as in carburized case hardened rings, the cracks tend to grow circumferentially below the raceway (explaining the spalling/ 5a flaking type of appearance, e.g., fig. 6c). Nevertheless, in a very 5c advanced failure stage, the inner ring raceways are often heavily 5b spalled, independent of the heat Fig. 5: Examples of typical bearing types that can be affected: a) tapered roller bear- treatment. ing, b) cylindrical roller bearing, and c) spherical roller bearing. evolution.skf.com | Published in EVO 2.2013 6b 6a 6c Fig. 6: Crack growth patterns in standard heat treatments: a) martensite, b) bainitic and c) case hardened (case carburized)6. Challenges due to operat- • depending on turbine type, up, dust, cold climate, offshore, ing conditions in wind tur- additional radial and axial forces moisture23 6 bine gearboxes by the rotor, axial motion of the • idling conditions – leading to low Wind turbine gearboxes are sub- main shaft – leading to dynamical load conditions and risk of skid- jected to a wide variety of operating loading, higher stresses of gearbox ding damage (adhesive wear)23 conditions that may push the bear- components especially at the first • some design requirements can be ings beyond their limits (e.g., with stage19, 20 conflicting, e.g., increasing rolling respect to load, speed, lubrication • occasional connecting and discon- element size will increase the load and combinations of these). The necting of the generator from the carrying capacity but simultane- wind energy segment faces some of power grid – leading to torque ously increase the risk of cage and the toughest challenges for extend- reversals and bouncing effects (e.g., roller slip and sliding damage6, 7, 17, 23. ing bearing life and reducing the can lead up to 2.5 - 4 times higher occurrence of premature failures nominal torque, impact loads)12, 15, 21 As stated, bearings may fail for while at the same time reducing the • rapid accelerations/deceler­ other reasons not attributed to overall cost of energy. ations and motions of the gearbox falling below best practice stand- There are many opinions in the shafts13, 15 ards24, 25 and from other industrial public domain summarizing com- • misalignment, structural defor- experiences. Statistical evaluations mon indications of severe operat- mations (nacelle hub, housings)11 of a limited number of offshore ing conditions in conjunction with • lubricant compromise between wind turbines2 indicate clearly a premature failures in wind turbine needs of gears and bearings as correlation between failure rate, applications. These include: well as between low- and high- wind speed and heavy and fluctuat- speed stages, insufficient oil ing loads. The trend towards larger • periods of heavy and dynamic drains and refill intervals22 turbine sizes with higher power-to- loads/torques – leading to vibra- • harsh environmental conditions weight ratios will invariably lead to tions and rapid load changes (e.g., – eventual large temperature more flexible supporting struc- transient raceway stress exceed- changes and consequently larger tures11 that, in turn, will influence ing 3.1 GPa, heavy loads of 15,000 temperature differences between the load sharing and load distribu- per year, impact loads)6, 7, 11, 12, 13, 14, the bearing inner ring and hous- tion within the rolling bearings as 15, 17, 18 ing than expected when starting well as on other drive components. Published in EVO 2.2013 | evolution.skf.com According to reference 26, in fig. 7. Many papers (for example, Thus, driven by a single factor “young”, heavily loaded applications reference 10) discuss a local change or by a combination of several fac- having a highly innovative product in the bearing material microstruc- tors, WEAs develop locally in the design life cycle, sufficient experi- ture into WEC by certain influen- bearing steel matrix.