rolling bearing wear in wind turbineskeywords: rolling bearing, wear particles, wind turbines,...

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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PAPER REF: 5746

ROLLING BEARING WEAR IN WIND TURBINES

Beatriz Graça1(*)

, Ramiro Martins1, 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

This work focus the important information carried out by the wear particles that are present in

a lubricant sample. This information reveal the wear condition of the rolling bearings

providing effective means to increase reliability and availability of wind turbines, minimizing

maintenance costs and increasing the reliability of the machine. Wear particles from rolling

bearings in wind turbine are presented, particularly particles resulting from abrasion, fatigue

and corrosion mechanism. Root cause investigation is made, supported by microscopic

analysis.

Keywords: rolling bearing, wear particles, wind turbines, lubricant analysis.

INTRODUCTION

Wind turbine failure statistics show that most of the operating downtime is bearing related. A

recent National Renewable Energy Laboratory (NREL) study concluded that the majority of

wind turbine gearbox failures start in the bearings (Musial, 2007). High-speed bearings and

planet bearings exhibit a high failure rate and are identified as two of the most critical

components.

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.

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.

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Tribology Trends for Higher Efficiency and Reliability

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PITCH 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):

• Fretting corrosion and false brinelling - as it was pointed out by some authors

(Kotzalas and Doll, 2010), is a common issue in 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 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



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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. 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).

WEAR PARTICLE ANALYSIS

Wear particle analysis is a powerful technique for non-intrusive examination of lubricated

components in machinery. The particles contained in a lubricant carry always a great deal of

information about the operating condition of the equipment as well as provide a leading

indicator of what could be the condition if no corrective action is taken. This information may

be deduced from particle shape, composition, size distribution, and concentration. The

particle characteristics are sufficiently specific so that the operating wear modes within the

machine may be determined, allowing prediction of the imminent behavior of the machine.

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Proactive actions can securely be implemented and/or the corrective work can be well planned

and scheduled.

Some of the more common failure modes that oil analysis can detect include:

• Accelerated oil degradation;

• External contamination (water, dust particles, etc.);

• Component fatigue and wear (sliding, abrasive, corrosive, etc.);

• Incorrect lubricant use and management;

• Inadequate contamination control measures (filtration, leakage, etc.).

To understand the contents enclosed in the combined information obtained through several

lubricant analysis techniques, a certain expertise is required in either, tribological

fundamentals and in maintenance engineering. Lubricant failure causes equipment failure and

vice-versa. The lubricant analysis program should be designed to recognize both modes of

failure.

Some parts of wind power equipment often produce abnormal wear because of assembly,

severe operating conditions and poor oil quality. Thus, analyzing and identify the causes of

abnormal wear is very important for the operation and management staff.

In terms of a wind turbine gearbox, the vast majority of the particulates suspended are wear

debris which have become detached from different gearbox surface components. So, wear

failure condition diagnosis can be effectively made by the quantitative and qualitative analysis

of wear metal particles in the oil, which can guide the maintenance personal to take timely

measures to carry out condition maintenance, and to avoid further deterioration of the

accident to ensure the safe operation of the wind turbine.

Another important issue is to inspect the bearings because they often provide clues about the

causes of gear failure, such as:

• bearing wear can cause excessive radial clearance or end play that misaligns the gears;

• bearing damage may indicate corrosion, contamination, electrical discharge or lack of

lubrication;

• plastic deformation between rollers and raceways may indicate gear overloads.

FERROGRAPHY ANALYSIS

Ferrography is one of the most reliable techniques in providing valuable information about

the wear evolution in mechanical components.

The potentialities of Ferrography are not only limited to predictive maintenance strategies. Its

important contribution to tribology studies, by assisting in a better understanding of the wear

mechanisms and of the lubricant effects on the contact surfaces, turns this versatile

technology into one of the most powerful diagnostic tools to assess the machine health,

providing valuable information about the past, the current and the future condition of the

machine’s lubricated components (Graça, 2007).

Ferrographic analysis compromises two sets of instrumentation:

• Direct Reading Ferrography is the instrument that quantitatively measures the

concentration of wear particles in lubricating oil using a magnetic method. The index

readings are indicated as density small (DS) and density large (DL). DS represents all

particles measuring up to 5µm in size whereas DL indicates all particles greater than



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5µm in size. When there is a sharp increase of the DL index, it is indicative that

abnormal wear is in progress and an Analytical Ferrography is needed.

• Analytical Ferrography is often referred to as the oil analysis equivalent of criminal

forensic science (Huysman, 2011). Analytical Ferrography utilizes microscopic

analysis to identify the composition of the material present. This technology will

differentiate the type of material contained within the sample and determine the

wearing component from which it was generated. Debris wear can indicate the degree

and type of damage that is being experienced by different machine components. The

size and shape of particles are dependent on the type of wear they have experienced

whilst the number can signify the degree of damage that has occurred (Maslach,

1996). In a windmill gearbox several wear mechanism may occur simultaneously (see

Table 1). A trained ferrographic analyst is able to use the size, shape, concentration

and composition to identify the wear mode and status of the equipment. This allows a

skilled diagnostician to determine the root cause of a specific tribological problem.

Not only the overall equipment condition can be determined, but the lubricant

condition can also be analysed by looking for particles such as those listed in Table 2

(Maslach, 1996).

Table 1 - Wear types and particles which identify abnormal wear

Wear type Particle characteristics Size

(µµµµm) Possible cause

Rubbing Wear Platelets < 15 Sliding of components

Cutting Wear Fine chunks/spirals like machining

swarf

> 5 Misaligned components, abrasive

contamination, cracks in the wear

surfaces

Severe Sliding Wear Deep grooves within irregular-shaped

particles, chunks

> 15 Poor lubrication, severe operating

conditions (speed/load)

Bearing Wear Fatigue spalls

Laminar particles

> 15 Abnormal rolling wear in rolling

bearings or between gear teeth

Gear Wear Fatigue chips from gear teeth

Scuffing/Scoring particles

> 15 Scuffing and scoring of the gear

teeth around the pich line

Spherical Wear Smooth and rough spheres < 5 Early indication of an abnormal

rolling contact

Table 2 - Particles which identify lubricant condition

Particle type Characteristics Possible causes

Black Oxides

(Fe3O4)

Black and irregular in shape High operating temperatures due to

inadequate lubrication

Red Oxides

(Fe2O3)

Red and irregular in shape Rust (water presence) or dirt contamination

Corrosive Wear

(FeO)

< 1 micron in size Acidic lubricant due to additive depletion

Lubricant

Degradation

Irregular shape amorphous matrix

containing ferrous particles

Overstress causing breakdown of the

lubricant structure

Dirt Sand, dirt, fibers, etc. Filter or breather leak

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The advantage of Analytical Ferrography is that the source, cause and scope of equipment

wear can be determined with an overall view of the problem. The analysis determines both the

type and metallurgy of the wear particle, allowing to ‘see’ inside the operating equipment.

However, the lubricant sample has to be taken correctly and in a location strategically

selected. Additional information concerning the lubricant and equipment/component is always

a valuable means to achieve a successful diagnostic of the Ferrography results.

CASE STUDIES

Rolling Bearing Wear Particle Analysis

A grease sample obtained from the pitch thrust bearing of a blade wind turbine has been

analysed through Ferrography. Since this technique is designed to analyse lubricating oils, the

grease sample was submitted to a dissolution procedure using an appropriated solvent

mixture.

The wear indexes obtained by Direct Reading Ferrography were extremely high (DL=100,1

and DS=35,7) showing the presence of an abnormal wear condition. To identify the cause of

the high wear indexes obtained, Analytical Ferrography was performed. The wear particles

observed under microscopic analysis are presented in Figure 1 (Photos 1 to 6). The wear

particles were submitted to heat treatment (325°C during 90sec.) to identify their metallurgy.

The blue and straw color transitions show that they are from AISI 52100 steel and medium

alloy steel, respectively (Photo 4 and Photo 6).

Fig. 1 - Wear Particle analysis from a wind blade thrust bearing grease lubricated

Photo 1

Photo 3 Photo 4

Photo 5

Photo 2

Photo 6

(a)

(b)



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All the microphotography’s shows a very high concentration of ferrous particles, with a

predominance of small size oxidized wear particles.

Photo 2, which is a 1000x magnification of the Photo 1, makes clear that most of the particles

are black ferrous oxides. These particles are typical from a fretting wear process that occurs as

the result of a low amplitude oscillatory relative motion of the contacting bearing surfaces

under load. Small wear particles are formed through the mechanism of adhesive wear.

Because of the small amplitude of motion and the bearing is grease lubricated, the wear

particles are not carried out of the contact area and removed from the system. So, the particles

produced contribute also to the wear through abrasive action and corrosion.

Besides fretting wear debris, it can be observed in Photo 3 and Photo 5, other ferrous particles

typically from:

• fretting fatigue (a) - which resulted from the friction and a high local temperature in

the fretted area and, as a result, decreases fatigue strength of the material operating

under cycling stress. Cracks are initiated and propagated to the surface very rapidly

with continued fretting, leading to a fatigue spall;

• fatigue cracks (b) – which are the source of small spherical particles (from 1 to 10

microns), generated from tongues of metal removed by a cavitation erosion process

due to the application and release of extreme pressure in the lubricant entrapped in the

propagating crack by rolling contact (Scott, 1973). A marked increase in spherical

particles indicates possible surface distress.

This case study identifies an advanced fretting corrosion wear process promoting a

premature fatigue failure in the bearing elements.

Gearbox Bearings Wear Particle Analysis

The lubricant of a wind turbine gearbox has been evaluated through various analyse

techniques, including Ferrography. The wear indexes obtained by Direct Reading Ferrography

were very high (DL=80,3 and DS=31,6) showing the presence of an abnormal wear condition.

To identify the cause of the high wear indexes obtained, Analytical Ferrography was

performed. The wear particles observed under microscopic analysis are presented in Figure 2.

The microphotography’s presented shows that the wear debris deposited on this ferrogram

were generated by a very severe wear.

Under low magnification (Photo 1 at 200x), it can be observed that the size, the shape and the

concentration of particles (ferrous and non-ferrous) are typical from:

• (c) severe fatigue wear;

• (d) gear scuffing;

• (e) severe sliding wear (chunk).

These particles were submitted to heat treatment (325°C during 90sec.) to identify their

metallurgy. As can be observed in Photo 2, the fatigue and scuffing wear particles are from a

low alloy steel material (gear teeth) and the large sliding particle is a copper alloy, resulted

from a severe surface damage in the rolling bearing, through abrasion action.

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Fig. 2 - Wear Particle analysis from a wind turbine gearbox

Additional information about the wear mechanisms present can be reached using higher

magnification (1000x) and analyzing different areas of the ferrogram (entry, middle and end

region):

• Photo 3: high magnification view (1000x) of the ferrogram entry, shows the oxidized

surface of a severe fatigue particle (c), evidencing high temperatures in the contact;

• Photo 4: high magnification view (1000x) of the mid-section of the ferrogram, shows

a large sliding chunk from copper alloy and small ferrous wear particles;

• Photo 5: low magnification view (200x) of the ferrogram end, shows the presence of

very small ferrous debris. These particles are typical from a corrosive wear;

• Photo 6: high magnification of Photo 5 (1000x) shows a long copper alloy debris over

the corrosive wear particles.

This case study identifies an abnormal wear condition in the rolling bearing elements of

the gearbox.

As already evidenced by the National Renewable Energy Laboratory (NREL) in USA, the

majority of the wind turbine gearbox failures appear to initiate in the bearings. A corrosion

process is taking place in the bearing elements. This will cause an increased bearing clearance

which could be sufficient to result in an unacceptable misalignment in the bearing behavior.

The ferrous wear particles analyzed shown an advanced fatigue wear process in the gear teeth,

inducing that the gearbox operation entered in an abnormal wear condition which could be

related with a main shaft misalignment. A strong presence of wear debris in the lubricant,

promotes an increase of the gearbox wear severity and fatigue. If this problem is not been

Photo 1 Photo 2

Photo 3 Photo 4

Photo 6 Photo 5

(c)

(e) (d)



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detected and located early on, further damage can occur. A strategic and corrective action

should be implemented to avoid a potential gear failure and/or additional damages in the wind

turbine mechanical components.

Rolling Bearings 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 analysed 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). Additivated synthetic gear lubricants were 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, 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 3) 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.

Fig. 3 - 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. 4 - Cage of the cylindrical roller bearing (NU 2324) showing intensive chemical corrosion in its surface

(magnified 200x)

(a)

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Polished cross-sections (a) of the inner ring observed under Scanning Electron Microscope

(SEM) revealed the fine network of subsurface micro

optical microscope photomicrography,

(WEC). This type of microstructural changes of st

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

shows the presence of sulphur (S), phosphor (P)

(Cu) is an element compound of the cage material which was also diluted into the lubricant.

According to recent studies published by SKF

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 p

propagation and branching, according to several authors

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

among others). Hydraulic effects will additionally drive the crack propagation quickly in

different directions, which depends on the surface crack orientation.

Fig. 5 - SEM view (left) and optical view (right)

Fig. 6 - EDS analysis inside the micro crack (Z1)

Higher Efficiency and Reliability

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of the inner ring observed under Scanning Electron Microscope

(SEM) revealed the fine network of subsurface micro-cracks propagation (see Figure 5

ptical microscope photomicrography, shows some evidences of "White Etching Cracks"

f microstructural changes of steel bearings often occurs in


ral analysis (EDS) in the interior of a micro crack (Z1 in

shows the presence of sulphur (S), phosphor (P) and copper (Cu). Should be noted that copper


According to recent studies published by SKF (Stadler et al, 2013), WEC can be related with



wind turbines is associated with rapid crack propagation inside the material. This rapid crack

propagation and branching, according to several authors (Gegner, 2011, Uyama


and its degradation resulting compounds (hydrogen sulfide


different directions, which depends on the surface crack orientation.

and optical view (right) of the cross-sectional surface (a)

EDS analysis inside the micro crack (Z1) of the inner ring.

of the inner ring observed under Scanning Electron Microscope

cracks propagation (see Figure 5). The

some evidences of "White Etching Cracks"

eel bearings often occurs in gearboxes of


ral analysis (EDS) in the interior of a micro crack (Z1 in Figure 6),

. Should be noted that copper


, WEC can be related with



ropagation inside the material. This rapid crack

2011, Uyama, 2013), can


and its degradation resulting compounds (hydrogen sulfide - H2S,


(a) of inner ring.



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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 7).

Fig. 7 - SEM/EDS analysis in the cage surface of the cylindrical roller bearing.

CONCLUSION

The following important deductions were obtained from the results of these case studies:

• the results from both wear particle analysis cases shown a corrosion process taking

place 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 wear mechanism that is typical of incorrect

alignment of gears causing load distribution unevenly across the face width promoting

overload in a small area (GSC) and lead to premature failure.

Considering the limited accessibility of wind turbines and the costly interventions that rolling

bearing failures can cause, wear particle analysis offers an attractive, proactive way of

maintaining and servicing wind turbine units, leading to improve their reliability. The

combination of wear particles analysis with failure diagnosis is a powerful tool to identify

problems in wind turbine and to understand the lubricant chemistry alterations that could be

related with the problem origin.

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ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Fundação para a Ciência, Tecnologia,

FCT, Portugal, within the project EXCL/EMS-PRO/0103/2012. This work was co-funded by:

COMPETE, QREN, EU.

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Proceedings of the European Wind Energy Conference, 2007.

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‘butterflies’ and white etching cracks (WECs)”, Materials Science and Technology, Volume

28, Issue 1, pp. 3-22, January 2012.

[3]-Tudose, L. and Tudose C., “Proper Lubricant Selection for Rolling Bearing Applications”,

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[9]-Luyckx, J., “White Etching Crack Failure Mode in Roller Bearings: From Observation via

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[10]-Holweger W., Wolf M., Merk D., Blass T., Goss M., Loos J., Barteldes S., Jakovics A.,

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[11]-Dwyer-Joyce, R. S., “The life cycle of a debris particle”. Tribology and Interface

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[12]-Scott D., Mills G.H., “Spherical Particles formed in Rolling Contact Fatigue”, Nature

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[13]-Bruce, R.W., Handbook of Lubrication and Tribology: Theory and Design, Volume II,

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rolling bearing wear in wind turbineskeywords: rolling bearing, wear particles, wind turbines,...

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