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 -1961- PAPER REF: 5746 ROLLING BEARING WEAR IN WIND TURBINES Beatriz Graça 1(*) , Ramiro Martins 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 (*) 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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ROLLING BEARING WEAR IN WIND TURBINESKeywords: rolling bearing, wear particles, wind turbines, lubricant analysis. INTRODUCTION Wind turbine failure statistics show that most of the
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Proceedings of the 6th International Conference on Mechanics and Materials in Design,
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
-1962-
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
Proceedings of the 6th International Conference on Mechanics and Materials in Design,
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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Tribology Trends for Higher Efficiency and Reliability
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
-1970-
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
wind turbines, and is not associated with the classical mechanism for rolling contact fatigue
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
(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
be explained by the presence and influence of certain chemicals in the lubricant, such as
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.
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
wind turbines, and is not associated with the classical mechanism for rolling contact fatigue
ral analysis (EDS) in the interior of a micro crack (Z1 in Figure 6),
. Should be noted that copper
(Cu) is an element compound of the cage material which was also diluted into the lubricant.
, 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
ropagation inside the material. This rapid crack
2011, Uyama, 2013), can
be explained by the presence and influence of certain chemicals in the lubricant, such as
and its degradation resulting compounds (hydrogen sulfide - H2S,
among others). Hydraulic effects will additionally drive the crack propagation quickly in
(a) of inner ring.
Proceedings of the 6th International Conference on Mechanics and Materials in Design,