ROLLING BEARING FAILURES IN WIND TURBINES

Transcription

1 Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal July 2016 Editors J.F. Silva Gomes and S.A. Meguid Publ. INEGI/FEUP (2016) PAPER REF: 6394 ROLLING BEARING FAILURES IN WIND TURBINES David Gonçalves 1(*), Beatriz Graça 1, Jorge Seabra 2 1 Institute of Science and Innovation in Mechanical and Industrial Engineering (INEGI), University of Porto, Porto, Portugal 2 Faculdade de Engenharia (FEUP), University of Porto, Portugal (*) bmg@fe.up.pt ABSTRACT In order to improve reliability in wind turbines several approaches are being applied and continually developed. One of those, rely on the lubrication field: the lubricating oil must assure a good lubrication of the wind turbine components independently of the operating conditions. For such purpose, it is essential to keep the lubricant free of contamination and with capability to protect surfaces from abnormal wear, extending the life of the wind turbine. This paper will focus on the important information that an effective failure analysis can provide to contribute for an increase in reliability and availability of wind turbines, minimizing maintenance costs associated with oil change outs, labour, repairs and downtime. A case study of a rolling bearing failure in a wind turbine will be presented, showing evidences of White Etching Cracks (WEC), as a fatigue mechanism related with microstructural alteration. Root causes investigation are made and supported by optical and electron microscopic surface analysis. Keywords: Wind turbines, rolling bearing, failure analysis, lubricant analysis. INTRODUCTION The fast-growing wind industry is developing larger, increasingly efficient and reliable wind turbines that require equally capable and durable lubricants (Magats, 2007). The most expensive components of a wind turbine, besides tower and blades, are gearboxes and bearings, requiring about 13% of the total costs. So, wind turbine manufacturers and operators are consistently faced with tribological issues that drastically reduce the lifetimes of gearbox systems. Most gearbox failures do not begin as gear failures or gear-tooth design deficiencies. The observed failures appear to initiate at several specific bearing locations under certain applications, which may later advance into the gear teeth as bearing debris and excess clearances cause surface wear and misalignments. So, the bearings are a vital part of wind turbines. They have to operate continuously under variable load and frequently intermittent lubrication. All of the forces generated by the wind directly affect the bearings. Highly dynamic forces with extreme peak and minimum loads, sudden load changes and strongly varying operating temperatures place high demands on the bearing lubricant. The long-term exposure to high vibrational stresses has an especially negative effect on rolling bearing cages presenting great challenges for bearing tribology in wind turbines. The bearings are also exposed to high speeds and temperatures as well as the risk of current passing through them

2 Topic_F: Tribology and Surface Engineering Most bearings fail within 10% of their lifetimes predicted by current standards (Evans, 2012). Many factors influence bearing life but load and cycles are required for failure. After a sufficient number of rotations, the bearing will fail from fatigue. And the higher the load, the sooner it fails. Other factors that accelerate the process include poor row-to-row load sharing, poor oil condition (such as high water content, debris, additive depletion) and skidding. If damaged bearings are not replaced promptly, significant harm to other mechanical components may result. High-speed bearings, planet bearings and intermediate-shaft bearings exhibit a high rate of premature failure and are considered to be some of the most critical components in wind turbines. Considering the limited accessibility of wind turbines and the long lead times for supplying gear-box components, oil and failure analysis offers an attractive, proactive way of maintaining and servicing wind turbine units, leading to improved operating efficiency. ROLLING BEARING WEAR It is well known that at least 60% of premature bearing failures are due to incorrect lubrication (Tudose, 2013). So, the lubricant plays a vital role in the performance and life of rolling element bearings. A lubricant that is designed for specific operating conditions will provide a load bearing wear protective film by separating the friction surfaces. In addition, bearing lubricant has to ensure dissipation of heat, elimination of contaminants, flushing away wear debris, lubricate the seal lips and fill the labyrinth seal gaps. When they fail, it is usually a critical event, resulting in costly repair and downtime in a wind turbine. There are numerous causes for lubricant failure, including: Insufficient lubricant quantity or viscosity; Deterioration due to prolonged service without replenishment; Excessive temperatures; Contamination with foreign matter; Use of grease when conditions dictate the use of static or circulating oil; Incorrect grease base for a particular application; Over lubricating. Excessive wear on rolling elements, rings and cages follows, resulting in overheating and subsequent catastrophic failure. In addition, if a bearing has insufficient lubrication, or if the lubricant has lost its lubricating properties, an oil film with sufficient load carrying capacity cannot be generated. The result is metal-to-metal contact between rolling elements and raceways, leading to surface damage. There are five dominant surface damage modes in wind turbine rolling bearings (Errichello et al, 2011): False brinelling and fretting corrosion - as it was pointed out by some authors (Kotzalas and Doll, 2010), is a common issue in yaw and pitch systems when the bearings and gears are not rotating and are subjected to structure-borne vibrations caused by wind loads and/or small motions from the control system, termed dither. Under these conditions, lubricant is squeezed from between the contacts and the relative motion of the surfaces is too small for the lubricant to be replenished. Natural oxide films that normally protect steel surfaces are removed, permitting metal-to-metal contact and causing adhesion of surface asperities. Fretting begins with an incubation period during which the wear mechanism is mild adhesion and the wear debris is -408-

3 Proceedings of the 5th International Conference on Integrity-Reliability-Failure magnetite (Fe 3 O 4 ). Damage during this incubation period is referred to as false brinelling. If wear debris accumulates in amounts sufficient to inhibit lubricant from reaching the contact, then the wear mechanism becomes severe adhesion that breaks through the natural oxide layer and forms strong welds with the steel. In this situation, the wear rate increases dramatically and damage escalates to fretting corrosion. Micropitting - in bearings, it is typically caused by sliding or skidding during unsteady operation. Micropitting is commonly a precursor to larger surface failures. In general, the major factors influencing micropitting include inadequate EHL film thickness, surface roughness, unsteady operating conditions and anti-wear lubricant additives; Scuffing and smearing - this is surface damage caused by sliding contact friction caused by inadequate lubrication. In lightly loaded roller bearings, pure sliding between rolling elements and inner ring can occur when there is a large mismatch between the inner ring and roller set rotational speed. For demanding applications such as wind gearbox high-speed shafts, idling conditions and changing of load zones can sometimes lead to high sliding risk. In radially loaded roller bearings, the most critical zone where sliding can occur is the entrance of the rollers into the load zone. While rotating, the rollers slowdown in the unload zone of the bearing because of friction and subsequently have to be suddenly accelerated as they re-enter the load zone. Electric discharge - this occurs when factors such as faulty insulation or improper grounding allow electric current to pass through the bearing and damage the surface. Wind turbine bearings might be damaged by lightning strikes. When an electrical arc occurs, it produces temperatures high enough to melt bearing surfaces. Microscopically, the damage appears as small, hemispherical craters. Edges of the craters are smooth and they might be surrounded by burned or fused metal in the form of rounded particles that were once molten. Overall, damage to bearings is proportional to the number and size of the arcing points. Depending on its extent, electric discharge damage might be destructive to bearings. Associated microcracking might lead to subsequent Hertzian fatigue or bending fatigue. If arc burns are found on bearings, all associated gears should be examined for similar damage; Microstructural alteration - this includes white etching area (WEA) cracks and can lead to axial cracking and macropitting early in relatively new bearings. This is one of the more critical and least understood wind turbine failure modes. While not unique to the wind industry, it is found to be much more prevalent than in other applications. There are several theories about the cause of WEA cracks, including hydrogen induced embrittlement from lubricant decomposition (Uyama, 2014), mechanically induced, from high stress and slip conditions (Evans, 2012), mechanical impact loading (Luyckx, 2012), or multiple influencing factors, without one root cause (Holweger et al, 2015). Another concern related with bearing surface damage is the fatigue failure caused by lubricant debris (Dwyer-Joyce, 2005). The wear particles suspended in the lubricant passing through the contact will cause some damage to the bearing surfaces. Debris particles from steel bearing components are highly cold-worked, and can be produced as spalls or delaminate flakes from cold-worked surface layers. Since the hardness of debris particles is equal to or greater than the surfaces that they come into contact with, they can cause abrasion, denting, and sometimes embedment especially in softer metals like the bronze roller separators. The presence of debris particles, either loose, or embedded, leads to a localized disruption in the function of the inter-element lubricating film. Depending on when the debris indenting occurred, etches of shallow pits can be sharp or rounded from subsequent plastic deformation

4 Topic_F: Tribology and Surface Engineering Raised lips around pits can penetrate into the oil film and lead to localized solid contact or disruption in smooth flow between surfaces. Ductile particles causes smooth rounded, relatively shallow indents, whilst brittle particles cause deep steep sided dents (Blau et al, 2010). The operating life of a bearing is, in a large extent, influenced by lubrication. A correct lubricant, its performance capability, the effect of additives, cleanliness in relation to contaminants, and adherence to the specified lubrication intervals, contribute towards determining the overall reliability of the bearing. However, finding the balance between one or several base oils in a lubricant and the right additive package is very complex considering all the different tribological conditions that can arise in a contact and the potential tribochemical interactions between additives themselves and with the steel substrate of all bearing elements. Although, the main functions of a lubricant in a bearing are the following: separating the contacting surfaces in order to avoid friction and wear due to metal to metal contact; accommodating the surface sliding velocities; transmitting the normal damping vibrations and transient pressure spikes; dissipating and evacuating frictional heat out of the contact; evacuating contamination particles and wear debris out of the contact. Failure analysis of rolling bearings is complicated due to the fact that one failure mode may initiate another. The different bearing failure modes will have different time-dependent limit state functions. As a first step towards minimizing bearing failure, the process should include avoiding the wrong installation (e.g., misalignment), contamination, inadequate lubrication (the wrong type of lubricant or an insufficient amount of lubricant), as well as misuse of the bearing. CASE STUDY Rolling Bearing Failure Diagnostic The main purpose in this case study was to investigate the nature of element bearing surfaces damage and to determine possible root cause(s). The bearing elements analyzed were: the inner ring of a tapered roller bearing (HR 30326J) and the copper cage of the cylindrical roller bearing (NU 2324) from the high speed shaft of a wind turbine (GE Wind Energy 1.5s). An additivated synthetic gear lubricant was been used to lubricate this gearbox, containing sulphur (S), phosphor (P), calcium (Ca), molybdenum (Mo) and boron (B). The following figures show the damaged areas of the bearing elements submitted to analysis. Detailed studies including visual inspection, Optical Microscope, Scanning Electron Microscope (SEM) and Energy Dispersive Spectrum (EDS) analysis were performed on the damaged bearing surfaces. In the surface of inner ring of the tapered roller bearing (see Figure 1) are observed flaking relatively deep at the edge of the raceway. This contact fatigue mechanism resulted from geometric stress concentration (GSC) (Bruce, 2012), and it is often associated with overload in a misaligned tapered bearing

5 Proceedings of the 5th International Conference on Integrity-Reliability-Failure The surface of the cylindrical roller bearing cage (see Figure 2), presents an intensive chemical corrosion. Through optical microscopic can be observed numerous rounded cavities (craters) covered by black oxides. (a) Fig. 1 - Inner ring of the tapered roller bearing - HR 30326J: dashed line signifies the axial plane from which the cross-sectional (a) analysis was conducted Fig. 2 - Cage of the cylindrical roller bearing (NU 2324) showing intensive chemical corrosion in its surface (magnified 200x) Polished cross-sections (a) of the inner ring observed under Scanning Electron Microscope (SEM) revealed the fine network of subsurface micro-cracks propagation (see Figure 3). After Nital etching, the optical microscope photomicrography, shows some evidences of "White Etching Cracks" (WEC) resulted from microstructural alteration in the multi branching cracks networks. This type of microstructural changes of steel bearings often occurs in gearboxes of wind turbines, and is not associated with the classical mechanism for rolling contact fatigue (RCF). Energy Dispersive Spectral analysis (EDS) in the interior of a micro crack (Z1 in Figure 4), shows the presence of sulphur (S), phosphor (P) and copper (Cu). Should be noted that copper (Cu) is an element compound of the cage material which was also diluted into the lubricant. According to recent studies published by SKF (Stadler et al, 2013), WEC can be related with hydrogen induced microstructure transformation by means of hydrogen release from the composition products of the penetrating oil. Premature failure of bearings in gearboxes for wind turbines is associated with rapid crack propagation inside the material. This rapid crack propagation and branching, according to several authors (Gegner, 2011, Uyama, 2013), can be explained by the presence and influence of certain chemicals in the lubricant, such as oxygen (O2), hydrogen (H2) and its degradation resulting compounds (hydrogen sulfide - H2S, among others). Hydraulic effects will additionally drive the crack propagation quickly in different directions, which depends on the surface crack orientation

6 Topic_F: Tribology and Surface Engineering Fig. 3 - SEM view (left) and optical view (right) of the cross-sectional surface (a) of inner ring. Fig. 4 - EDS analysis inside the micro crack (Z1) of the inner ring. Fig. 5 - SEM/EDS analysis in the cage surface of the cylindrical roller bearing

7 Proceedings of the 5th International Conference on Integrity-Reliability-Failure The cage surface shows a large extension of wear caused by chemical action, with the generation of a large number of craters. Those craters are covered by a residue mainly composed by sulfur (S), phosphorous (P) and calcium (Ca) (Z1 and Z2 in Figure 5). These elements are compound additives of the gear oil. CONCLUSION The following important deductions were establish from the outcomes of this case study: the results shown that a corrosion process was present in the rolling bearing elements, causing an increase of the bearing clearance which could be sufficient to result in an unacceptable misalignment in the bearing behaviour; lubricant formulation and decomposition should be considered either regarding to corrosion wear processes and hydrogen generation and penetration into the bearing materials; micrographic surface analysis show a typical wear mechanism of incorrect alignment of gears causing load distribution unevenly across the face width, promoting overload in a small area (GSC), that ultimately cause macropitting and bearing failure. This failure corresponds to an unconventional fatigue failure mode called White Etching Cracks (WEC) that is often qualified as the least understood failure mode experienced in wind turbines and as the most critical since it remains unpredictable using bearing models and some condition monitoring techniques (Arnaud, 2014). While not unique to the wind industry, it is found to be much more prevalent than in other applications, namely in terms of frequency and impact on O&M costs. Considering the limited accessibility of wind turbines and the costly interventions that rolling bearing failures can cause, tribological analysis of the failure modes is a powerful tool to identify problems in wind turbines and to understand the lubricant chemistry alterations that could be related with the problem origin. Condition monitoring of wind turbine rolling bearings operating under harsh conditions is becoming increasingly necessary to detect bearing defects at an early stage preventing catastrophic failure, high replacement costs and lower farm efficiency. This means that maintenance can be planned and costly consequential damage avoided. ACKNOWLEDGMENTS The authors gratefully acknowledge the funding from: National Funds through Fundação para a Ciência e Tecnologia (FCT), under the project EXCL-II/SEM-PRO/0103/2012; NORTE FEDER SciTech - Science and Technology for Competitive and Sustainable Industries, cofinanced by Programa Operacional Regional do Norte (NORTE2020), through Fundo Europeu de Desenvolvimento Regional (FEDER); LAETA under the project UID/EMS/50022/

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