The wind turbine industry is still less developed. White etching cracks have identified to be among the tribological failures prevalently affecting the rolling elements of the wind turbine system. White etching cracks bring about unconventional rolling that results in contact fatigue mode is caused by eccentric rolling. A partial microstructure borders of white etching corresponds to cracks branching to three-dimensional networks leading to premature failure that is always unpredictable. Recent research proposes that such failure mode could be associated with numerous combinations of operating circumstances either depending on test rigs or application. Both relations with test rigs and systems converge to comparable tribological drivers relating to the evolution of hydrogen at asperity measures whose surfaces gets affected and embrittle the bearing steel. The white etching cracks keep on being delicate to continue reproducing regardless of synthetic hydrogen charging, and the underlying formation mechanisms remain unsettled.
This report aims to give an in-depth understanding of the effects of white etching cracks on the performance of wind turbine systems. It identifies the drivers including sliding kinematics, lubricant additives, electrical potential and water contamination are endlessly swapped through twin-disc machines which provide the enhanced control of the contact conditions.
White etching is defined as the appearance of the transformed steel microstructure while etching and polishing micro-sections (Benjamin et al. 2015). The affected sections are made up of ultra-fine and nano-recrystallized ferrite which is carbide free. It is seen as white on a light photosensitive micrograph because of the small etching reaction of the material (Doll, 2010). The rate of premature failures in wind turbine systems is higher and could be related to a larger number of the machines which are installed (Ramasmawi, 2016). Other industrial systems which include the paper mills, marine propulsion, constant variable drives, crusher mills and lifting gears have a lower frequency of premature failure caused by white itching cracks (Dwyer-Joyce, 2015).
The rate of failure in mechanical components in wind turbine systems has remained to be lower than in electrical components. Despite this, engineering downtimes due to driving train failures create high loses on revenue and high repair costs (Munn, 2012). The service lives of gearboxes of turbines are always not more than the deliberate 20 years. Gearboxes are used to upsurge rotor speed to that of the generator. Locations that have bearings namely the intermediate shaft, the high-speed shaft and the planet bearings (Ramaswami, 2013). Classic mechanisms of rolling initiate fatigue in the subsurface as well as on the surface.
Early cracks have occurred commonly during the first years of operational life from the first year to the third year at 5%-10% of the rating life. The first cracks appear visually as vertical axial cracks and cracks combining both the small spalls and the large spalls. Early cracks are not related to the particular bearing. The white cracks at an early life are also not linked to standard treatment of heat which is particular (Mann, 2016).
However, the white appearance of failure is allied with treatment of heat like the normal residual field stress. The progress stage of failure, operating conditions and the bearing position such as the stress field loading have high likelihood cause of failure appearance. Early white cracks in martensite rings as a particular application tends to grow into the material hence signifying the axial appearance (Erdemir, 2012). On the other hand, the concrete bainitic rings and the carburized casing hardened rings tend to have white etching cracks growing circumferentially just below the raceway indicating spalls and the flaking types of appearance. When going to the inner-ring raceways at an advanced stage of failure are at all times highly spalled regardless of heat treatment (Igba, 2015).
Gearboxes of wind turbine systems are exposed to various operating conditions. The operating conditions subjected to the operation of the systems include load, speed, lubrication either as single elements or in combination (Ramaswami, 2014). The segment of wind energy faces tough challenges to extend bearing life and reduce the occurrence of etching cracks while at the same time try to control the overall costs of energy (Wood, 2013).
Common indicators of worsening operating conditions about premature failures caused by white etching of wind turbine systems have attracted various descriptions in the available literature.
- Occasions of dynamic and cumbersome loads lead to vibrations and rapid changes in pressure.
- The rotor-axial motion of the primary shaft created additional axial and radial forces depending on the type of turbine which leads to more stresses of components, that is, dynamic loading(Wood, 2014).
- Occasional connection and disconnection of the generator from the power network result in rotational reversal and bouncing effects building impact loads.
- Failures have also been linked to rapid acceleration, rapid deceleration and quick motions of the shafts in the gearbox.
- Structural deformation and misalignments of housing and the nacelle hub.
- Compromise in lubrication between the requirements of the bearings and gears as well as between high-speed, and low-speed stages result from insufficient oil refill intervals and drain.
- At some times, the environmental conditions may be harsh coming from high changes in temperature and subsequently cause strong changes in temperature than expected between the inner ring of the bearing and the housing. Moisture, cold climate, dust and offshore winds are the other adverse external conditions.
- Surprisingly, idling leads to low conditions of the load as adhesive wear risking damage by skidding.
- Some requirements in design can be conflicting, for example, increasing the size of the rolling element will consequently increase the carrying capacity of load and thereby increase the risk of roller slip, sliding and cage damage.
A clear indication of theses available theories shows a correlation between wind speed, failure rate, and heavy fluctuating loads. Toward turbines of larger sizes which have a higher power-to-weight ratio, the trend should invariably lead greater flexibility of the supporting structures which in turn influences sharing and distribution of load over the rolling bearings and to the other drive components (Wood, 2013).
Disadvantages of the available solutions
Failure of a bearing is not always attributed to which are not connected to white etching cracks. A case of combined effects of the mechanical itching cracks and some electrical failures are difficult to measure their individual effects statistically to come up with an appropriate solution (Wood, 2013). The available evaluation in statistics apply to a partial number of wind turbines which are offshore to give a clear correlation between the rate of failure, the speed of wind and load which is heavy and fluctuating but are not applicable to all wind turbines (Vegter, 2015). Presence of cracks on bearings is has been at sometimes interpreted as an indication of uncontrolled kinematic behavior (Ruellan, 2014).
A consistent theory is not available regarding premature failures caused by white etching cracks on wind turbines even though universities, manufacturers, independent institutions and suppliers investigating the issue. The studies have kept around the local change in the microstructure of the bearing material. Industrial experiences are at some points significant causes of failure of turbines. There is the unavailability of heavily loaded systems which have a significantly innovative lifecycle design. In other terms, sufficient experiences are absent about machines' endurance (Solano-Alvarez, 2015).
Include heat treatment, microstructure residual stresses, cleanliness, and hydrogen content.
It comprises of overloads, impact loads, vibration, peak loads, torque reversals, structural stresses, slip and electrical currents.
Lubricant, corrosion, additives, tribochemical effects, temperature gradients, contamination and hydrogen generation.
The other aspects are mounting which may lead to scratches, quality issues, and transport.
A lot of the influencing factors are found to be correlated. The factors that drive the failures will either operate as single elements or a combination of the factors. The bearings will fail by spalling as a consequence of nucleation sites of white etching cracks. The hypotheses are divided up to hydrogen enhanced white etching cracks development, load and stress-related white etching crack development and the overlapping ones. The damages identified above influences generator bearings and alternator applications by damaging current and corrective actions using special greases bearings and special steels (Š?epanskis, 2015).
Most early failures in bearings are related to issues of the surfaces mostly lubrication. Most white etching cracking failures in the wind turbine gearbox is positioned to originate at the surface or close to the surface and then propagate into the steel or case material corrosion fatigue. Wind turbine gearboxes bearings are always large. The initiation and spread mechanism differ as likened to small bearings. Deeper radial cracks have been reported for larger bearings with restrained loads because of residual stress and that of a higher loop.
For the case of premature bearing failure suggests a fast propagation of the crack. The high-speed branching and spread will be explained by the existence of chemical elements such as oxygen and aging lubricant products at the face or tips of the cracks.
A system with subsurface cracks experiences vacuum conditions and hence has a slower rate of crack growth from mechanical fatigue wholly. At earlier stages, the cracks and crack systems connected to the surface to permit entry of lubricant and oxygen.
The hydrogen-enabled fatigue has same effects and also the classic rolling fatigue from contact. Conversely, it requires aggressive and corrosive environment or continuous passage of electric currents of high frequency. The presence of free water facilitates the creation of a highly erosive situation. Turbine manufacturers have claimed to have control over contents of elevated water in the lubricants. The investigation does not identify moisture corrosion in wind turbine gearboxes. If all factors are considered and humidity ignored, the growing back passivating tribolayers provide a barricade layer to preventing corrosion and the absorption of hydrogen into steel remains to be intact and continuous. Absorption of hydrogen into steel is detrimental. Severely mixed contacts of friction cause local generation of hydrogen. The maintenance of a constant production of hydrogen, the interacting metal surfaces must be fresh and could lead to a weakening effect on the surface thus facilitating generation of surface cracks.
Failed raceways hardly reveal severe wear that could allow permeation of hydrogen. Therefore, diffusion of hydrogen through the raceway is not likely to occur without the addition of any other factor. The potential additional factor is the relatively corrosive oils combined with contaminants eventually. Operation of grease initiated failure is therefore distinguished from failure mechanisms which are initiated by the surface like distress. Further investigations are required for relevance quantification. Currently, the role played by hydrogen generation can be seen to be a local effect which is generated by the system of cracks because of entry of lubricant leading to the mechanism of corrosion fatigue etching cracking.
Furthermore, conditions of moderate bearing load in wind turbine gearboxes, there is a build-up of residual stress together with a decrease in the line of X-rays diffraction in failed bearings which broadens close to the raceways. The mixed friction of vibrations and shear stress as shown by the material is also an indication supporting initiation of failure on the surface and near the surface. Inadequate conditions of lubrication and some vibrations affects with higher frequency, reduce the thickness of the film and therefore raise the risk for the local mixed friction.
The discussed conditions roots to the following to counter the challenge of white etching cracks.
- Special passivation.
- Stabilization of structures near surfaces.
- Increasing resistance of bearings to hydrogen and chemical attacks.
- Improvement of running-in.
- Lowering the levels of micro-friction at in peak loading.
- Use of special clean steel.
- Extensive strengthening processes for the surfaces.
- Allowing the conditioning of the components.
- Raising resistance against the surface and subsurface crack initiation and propagation.
- Reduction of some additions that act as surface and material stress raisers.
The high growth in wind industry goes hand in hand with increasing the sizes of the turbine, and the turbulent conditions give unique challenges on rolling bearings. The industry is young and is faced with the problem of premature failures. In addition to bearing and heat treatment, wind conditions interfere with bearing kinematics, lubrication, and loading. The phenomenon of white etching cracks has been observed. Due to the high wind of the appliance, different locations of the turbine are affected, and the conditions that lead to differentiated kinematics should be avoided to reduce tensile stresses and micro-wear.
In general, bearing manufacturers are supposed to shift their focus to modifying the applications in a way that will aim to lower the risk of premature failures and increase robustness under the specific conditions of wind turbine gearboxes.
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