Upload
others
View
15
Download
0
Embed Size (px)
Citation preview
Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal 24-28 July 2016
Editors J.F. Silva Gomes and S.A. Meguid
Publ. INEGI/FEUP (2016)
-407-
PAPER REF: 6394
ROLLING BEARING FAILURES IN WIND TURBINES
David Gonçalves1(*)
, Beatriz Graça1, Jorge Seabra
2
1Institute of Science and Innovation in Mechanical and Industrial Engineering (INEGI), University of Porto,
Porto, Portugal 2Faculdade de Engenharia (FEUP), University of Porto, Portugal (*)Email: [email protected]
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.
Topic_F: Tribology and Surface Engineering
-408-
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
Proceedings of the 5th International Conference on Integrity-Reliability-Failure
-409-
magnetite (Fe3O4). 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.
Topic_F: Tribology and Surface Engineering
-410-
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.
Proceedings of the 5th International Conference on Integrity-Reliability-Failure
-411-
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.
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.
(a)
Topic_F: Tribology and Surface Engineering
-412-
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.
Proceedings of the 5th International Conference on Integrity-Reliability-Failure
-413-
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-01-0145-FEDER-000022 - 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/50022/2013.
Topic_F: Tribology and Surface Engineering
-414-
REFERENCES
[1]-Magats, Shalini, Wind Turbine Oil Trends and Best Practices, Monograph, Eric Bevenino,
Industrial Lubricants & Solutions, North American Lubricants. Chevron Texaco, USA, 2007.
[2]-Evans, M. H., "White structure flaking (WSF) in wind turbine gearbox bearings: effects of
'butterflies' and white etching cracks (WEC)," Materials Science and Technology, pp. 3-22,
2012.
[3]-Tudose, L. and Tudose C., “Proper Lubricant Selection for Rolling Bearing Applications”,
RKB Bearing Industries – Advanced Software Engineering Unit, 2013.
[4]-Errichello R., Sheng, S., Keller J., Greco, A., “Wind Turbine Tribology Seminar Report”,
sponsored by NREL, ANL and U.S. Department of Energy, Colorado, USA, 2011.
[5]-Kotzalas M. N. and Doll G. L., “Tribological advancements for reliable wind turbine
performance”, Phil. Trans. R. Soc. A 368, 4829–4850, 2010
[6]-Dwyer-Joyce, R. S., “The life cycle of a debris particle”. Tribology and Interface
Engineering Series, 48, 681-690, 2005.
[7]-P. J. Blau, L.R. Walker, H. Xu, R. Parten, J. Qu, and T. Geer, “Wear Analysis of Wind
Turbine Gearbox Bearings” Final report by Materials Science and Technology Division, Oak
Ridge National Laboratory, 2010.
[8]-Kenred Stadler and Arno Stubenrauch, Premature Bearing Failures in Industrial
Gearboxes, SKF GmbH, Germany, March 2013
[9]-Gegner, J., Tribological Aspects of Rolling Bearing Failures, Tribology - Lubricants and
Lubrication, Dr. Chang-Hung Kuo (Ed.), ISBN: 978-953-307-371-2, 2011.
[10]-Uyama H., and Yamada, H., White Structure Flaking in Rolling Bearings for Wind
Turbine Gearboxes, American Gear Manufacturers Association, ISBN: 978-1-61481-072-8,
USA, September 2013.
[11]-Kenred Stadler and Arno Stubenrauch, Premature Bearing Failures in Industrial
Gearboxes, SKF GmbH, Germany, March 2013.
[12]-Arnaud Ruellan Du Crehu. Tribological analysis of White Etching Crack (WEC) failures
in rolling element bearings. Mechanics of materials [physics.class-ph]. INSA de Lyon, 2014.