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Tribology Of Windmill

Home > Study Notes > Tribology Of Windmill
Published in: Mechanical
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Main objective is to identify the major tribological related issues impacting the wind energy industry leading to recommendations for future research and development strategies to improve turbine reliability and ultimately lower the cost of wind energy.

Harish C / Pune

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Qualification: M.Tech (MIT - 2016)

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  1. TRIBOLOGY OF WINDMILL I.INTRODUCTION Surface damage and failure of contacting components (i.e., bearings, and gears) are among the more frequent and costly types of failures for a wind turbine and can be the root cause of system failure for the gearbox, main rotor bearing, generator, yaw system, and blade pitch systems. Understanding the fundamental tribological factors that influence contacting element performance is important to addressing these issues. The other objective is to identify the major tribological related issues impacting the wind energy industry leading to recommendations for future research and development strategies to improve turbine reliability and ultimately lower the cost of wind energy. Harish Choudhary,B Power) Page I
  2. TRIBOLOGY OF WINDMILL 1.1 Tribological challenges in wind turbine technology Some wind turbine bearings are not achieving their desired operational lives because of life limiting wear modes. Tribological issues manifest themselves through different bearing failure modes in various systems of wind turbines. The primary mechanisms in pitch/yaw bearings, main shaft bearings, the gearbox, and the generator are false brinelling, micropitting, wear and cracking, and electrical arc damage. Micropitting and smearing are caused by large amounts of roller/raceway sliding in situations in which lambda (A), the ratio between the oil film thickness and the combined surface finishes of the parts, is low. Micropitting, smearing, and false brinelling problems can be solved with durable tungsten carbide-reinforced, amorphous, hydrocarbon thin film coatings on rollers. Coatings on rollers provide bearings with a high tolerance of debris damage. The solutions to micropitting and scuffing in gears are the same as in roller bearings. The root cause of radial cracking and wear from an irregular white Etch Area (IrWEA) is controversial, but probably mechanical in nature. Cleaner steels, higher compressive stresses on raceways, increased A, and less roller skidding can reduce IrWEA wear and radial cracking, if the IrWEA wear is of mechanical origin. In generators, less electric arc damage is shown in oils than in greases. Examples of problems without current solutions are: 1) Increasing seal life and 2) The development of a common nacelle lubricant. Harish Choudhary,B Power) Page 2
  3. TRIBOLOGY OF WINDMILL 1.2 Objective To discuss various types of bearing failures in windmill, the causes of their failure and under what conditions they fail. To find feasible solutions to prevent failure of bearings in windmill and hence to avoid windmill failure by extending its life. Harish Choudhary,B Power) Page 3
  4. TRIBOLOGY OF WINDMILL 2.Bearing Damage Modes In windmill bearings undergo following types of damages:- Macropitting (spalling) Inclusion origin Point Surface Origin (PSO) Geometric Stress Concentration (GSC) Micropitting (peeling) Wear or other damage Adhesive wear Debris denting Etching/corrosion False Brinelling/Fretting corrosion True Brinelling Heat discoloration Scuffing (smearing) Electric discharge Harish Choudhary,B Power) Page 4
  5. TRIBOLOGY OF WINDMILL 2.1 Adhesive wear Adhesive wear is due to adhesion, at the point of contact, between the two contacting surfaces having relative sliding motion. Adhesive wear take place when the two contacting surfaces having relative sliding motion are pressed against each other. When the two surfaces get pressed against each other, the contact occurs at asperities on the two contacting surfaces. Due to high contact pressure, the plastic deformation takes place at the point of contact (or real areas of contact).this lead to colds-welding or adhesion at the points of contact. These Welded contacts gets sheared off during sliding, resulting in detachments of a fragment from one surface and its attachment to other surfaces or formation of loose wear particles. This continuous formation of and shearing of weld junction which results in wear of one or both the contacting surfaces is knows as adhesive wear. 2.2 Surface fatigue wear: When the two contacting surfaces, having theoretical point or line contact, are subjected to high repetitive contacts stress, the wear which initiates as the micro- cracks and progressively lead to the formation of pits is knows as surface fatigue wear. The surface fatigue wear occurs when the contacting surfaces having theoretical point of contact are subjected to cyclic contact loads. When the hertz contact stress induced on the surface exceeds surface endurance strength of the surface, the surface failure takes place. After certain numbers of loads or stress cycles, the failure begins as micro-cracks on the surface or in the subsurface region. These cracks gradually develop in to the surface pits. The surface fatigue wear is also knows as pitting. It is important to note that, the amount of material removed by the surface fatigue wear is not as substantial as in case of adhesive or abrasive wear. However, the surface fatigue wear (or pitting) is more harmful because when the failure begins as micro- cracks, it can not be seen by naked eye and by the time cracks become visible, the failure of surface is almost over. Harish Choudhary,B Power) Page 5
  6. TRIBOLOGY OF WINDMILL 2.3 Brinelling Brinelling is a material surface failure caused by Hertz contact stress that exceeds the material limit. This failure is caused by just one application of a load great enough to exceed the material limit. The result is a permanent dent or "brinell" mark. It is a common cause of roller bearing failures, and loss of preload in bolted joints when a hardened washer is not used. False brinelling is damage caused by fretting, with or without corrosion, that causes imprints that look similar to brinelling, but are caused by a different mechanism. Brinell damage is characterized by permanent material deformation (without loss of material) and occurs during one load event, whereas false brinelling is characterized by material wear or removal and occurs over an extended time from vibration and light loads. The basic cause of false brinelling is that the design of the bearing does not have a method for redistribution of lubricant without large rotational movement of all bearing surfaces in the raceway. Lubricant is pushed out of a loaded region during small oscillatory movements and vibration where the bearings surfaces repeatedly do not move very far. Without lubricant, wear is increased when the small oscillatory movements occur again. It is possible for the resulting wear debris to oxidize and form an abrasive compound which further accelerates wear. Harish Choudhary,B Power) Page 6
  7. TRIBOLOGY OF WINDMILL 3. Review of the literature on the bearings in windmill 3. 1 Increase in windmill capacity over the years aomo 3S.n zsmo 20.n 1996 1997 1998 1999 2000 200t 2002 2003 2004 2005 2006 2007 2008 2009 1280 1.530 2.520 3.440 3.760 6300 770 8.133 11331 tsaas t9.86S 26.029 38.343 Fig 3.1.1 Global annual installed wind capacity between 1996 and 2009 Over the years windmill are designed for huge capacity for high energy generation and therefore their size have also increased in recent years. This increase in size of windmill also means increase their in bearing sizes. Due to increase in bearing sizes they have become highly susceptible for failure, hence at most precaution should be taken while designing and selecting bearings 3.2 Maintenance and repair of windmill Wind Energy Siting and Maintenance Costs. Wind energy projects may be on land or offshore, and can vary in scale from small projects of one to a few turbines to large, multi-turbine projects (denoted as utilityscale or wind farms). Utility-scale projects can consist of up to hundreds of wind turbines. These turbines are normally operated by independent power producers who sell the generated power to the local utility provider. Wind turbine operation and maintenance (O and M) costs, which are known to be the predominant costs that contribute to the cost of wind energy, are generally attributed to a limited number of components, including insurance, land usage, maintenance, repair, spare parts, and administration. For most wind turbines, maintenance and repair account for the largest share of O and M costs. These costs include the following: Harish Choudhary,B Power) Page 7
  8. TRIBOLOGY OF WINDMILL • Downtime: The revenue lost from turbine downtime is factored into the overall cost of repair. Downtime includes the logistics time for organizing a repair crew and supplies, as well as travel time and the actual time needed to repair the affected component. • Labor Costs: The cost of a service crew is factored into O and M. • Crane: If major repairs or component replacements are necessary, a crane may be needed. The cost of transporting a crane (normally to a remote location) and operating it contributes significantly to O and M. • Materials and Consumables: The magnitude of this cost may vary significantly depending on the component which has failed and the extent of the damage. It must be noted that O and M costs in offshore wind turbines tend to be significantly higher than that of comparable land-based turbines because they are more difficult to access. Less frequent access for maintenance and repair can lead to large reductions in downtime costs. 3.3 Bearing and Gear Failure Modes Seen in Wind Turbines • Bending fatigue, originating from non-metallic inclusions • Micropitting, due to rough surfaces or lubricants with inadequate micropitting resistance • Subcase fatigue, due to grind temper or inadequate case depth • Adhesion or abrasion, due to contaminated lubricants • Fretting corrosion during parking Harish Choudhary,B Power) Page 8
  9. TRIBOLOGY OF WINDMILL • Case/core separation, due to excessive case depth at tips of teeth • Axial cracks in bearing inner rings. Fig 3.3.1 Axial failure of windmill blade due to cracks in inner ring 3.4 Microstructural Alterations in Rolling Contact • The alterations are nano-grained ferrite (e.g., cell ferrite) • Crack faces in rolling contact are subject to fretting displacements and high pressure • Surface cracks are typically associated with local plastic deformation, due to reduced plastic constraint Hydrogen generated by decomposition of the lubricant. Formation mechanisms of white structures related to hydrogen embrittlement, and accelerating factors of hydrogen generation, were reported. Our research shows the following: Harish Choudhary,B Power) Page 9
  10. TRIBOLOGY OF WINDMILL • Bearing failures in wind turbine gearboxes may be classified as white structure flaking • White structure flaking is induced by hydrogen that diffuses into the bearing's steel • White structures are caused by localized, microstructural changes. The presence of white structures indicates that hydrogen-induced, localized plastic deformation is present in rolling contact fatigue • The type of lubricant, slip, static electricity, and material influence white structure flaking. Harish Choudhary,B Power) Page 10
  11. TRIBOLOGY OF WINDMILL 3.5 The Bearing Axial Crack Root Cause Hypothesis of Frictional Surface Crack Initiation and Corrosion Fatigue-Driven Crack Growth Looking from Results of failure analysis and research,some medium and large size bearings, such as those used in wind turbine gearboxes, suffer from premature failures due to axial raceway cracks. Root cause hypotheses from the literature were reviewed. Surface initiation, and the subsequent chemically-assisted propagation of the cracks, occurs as brittle spontaneous fracture and corrosion fatigue, respectively. Local microstructural changes result from hydrogen impacts due to aging reactions of the lubricant at the tip and the rubbing faces of the advancing crack. Material response in the form of cleavage-like surface cracking suggests there are causative tangential tensile stresses. Weaker areas with inhomogeneities and edge-zone embrittlement were considered. The tensile stresses are caused by sliding friction in the rolling contact, induced by vibrations, for example. A tribological model was presented. The tangential tensile stresses are estimated to be high enough for cracking. A combination of cold working, because it generates compressive residual stresses, together with black oxidizing and final low-temperature reheating, is proposed as an effective countermeasure. Conclusions are: • Axial cracks initiate from the surface by brittle, spontaneous, cleavage- like fractures • The root cause of the axial cracks is tangential tensile stress, due to high local sliding friction • Surface cracks propagate by corrosion fatigue cracking (CFC) • CFC is caused by lubricant decomposition at the crack tips and on the rubbing crack faces. Harish Choudhary,B Power) Page 11
  12. TRIBOLOGY OF WINDMILL 4. R&D Activities 4.1 Surface Treatment and Nano Lubricant Bearings The objective of the current work has been to develop, test, and implement innovative surface engineering and nano-lubrication technologies that can increase the reliability of wind turbine drivetrain components; primarily through an ultra-fast boriding process, lubricant- derived, diamond-like carbon (DLC) coatings, and nano-boron lubrication technology. The ultra-fast boriding process was developed to enhance the hardness of gear and bearing surfaces, by more than a factor of two, compared to standard carburizing (1800 HK to 600 HK). This was achieved through an advanced electrochemical process that increases efficiency by 80% compared to current boriding, making it economically practical for wind applications. The sliding wear performance of the borided surface is shown to be improved by an order of magnitude compared to that of standard carburized gear steels. The lubricant-derived DLC technology introduces a paradigm shift in lubricant/material technologies. The concept is to design coatings that catalyze a reaction with base lubricants to form a protective boundary film. The advantage of this technique is that it can generate boundary films from oils that do not contain organometallic additives. This simplifies the lubricant formulation and removes additives, which in some cases can cause early failure in some wind applications. The results presented showed scuffing performance of these designer coatings that exceeded the limits of the test rig, Surface analysis showed evidence of diamond-like, sp3 bonding in the boundary film. Finally, the nano-particle-based lubricant additives have been under development for several years to enhance performance compared to traditional additive packages. Nano-particle additives, mostly based on boron or carbon, are engineered to interact with the contacting surface to produce a low friction/protective boundary film. Results demonstrate the friction reduction qualities and surface analysis shows evidence of the tribofilm. Micro-pitting performance also was significantly enhanced with nano-boron additives compared to commercially-formulated oils. In the current test, micro-pitting was nearly eliminated. Harish Choudhary,B Power) Page 12
  13. TRIBOLOGY OF WINDMILL 4.2 Update on the Development of a Full Life Wind Turbine Gearbox Lubricating Fluid The goal of this project is to prove and implement an alternative chemistry for the lubricating fluid used for wind turbine gearboxes. Due to the chemical robustness of the altered lubricating fluid, the expectation is that the need for condition monitoring and/or maintenance of the gearbox is reduced significantly and could potentially be eliminated. The fluid technology to be expanded into gearbox lubrication has been successfully used in "lube for life" specialty applications. In this study, bench testing like FZG gear and FE 8 bearing have been carried out to document the suitability of the fluid compared to existing gearbox oil specifications. The benefits, such as superior viscosity temperature profile with viscosity indices above 300, and material compatibility have been demonstrated. The target viscosity's superior viscosity temperature profile is lower than that currently used gearbox oils and results in friction reduction and reduced wear of the internal gears and components. Full- scale wind turbine gearbox trial results indicate improved power output efficiency. In addition to the increase of efficiency, the other potential impact is increased operating reliability through reduction in the downtime by eliminating planned and unplanned gear oil changes. Harish Choudhary,B Power) Page 13
  14. Rotor Wind direction Blades TRIBOLOGY OF WINDMILL Pitch Low. shan Gear box Generator Anemomctcr Controller Brake Yava drive o Wind va Naccllc Yaw motor Tower shan Fig 4.2.1 Elements of windmill with moving parts and surfaces in contact. 1 2 3 4 5 6 7 8 9 10 11 12 13 electric motor belt transmission shaft power unit flexible coupling engine head with camshaft, followers and valves lubricating oil pipe oil filter oil pump with overflow valve and electric motor lubricating oil outlet pipe control and stability system of lubricating oil temperature measurement, control and stabilitysystem of shaft rotational speed power measurement system system of measurement and analysis of amplitude, speed and acceleration Harish Choudhary,B Power) Page 14
  15. TRIBOLOGY OF WINDMILL 5. Conclusions The reliability of a wind turbine is highly dependent on tribological issues associated with blade pitch systems, main shaft bearings, yaw systems, gearboxes, and generators. Many of the failure modes that occur in these systems such as Hertzian fatigue, adhesion, abrasion, corrosion, fretting corrosion, polishing, electric discharge, and scuffing are influenced by tribology. Lubricant base oil, additives, and cleanliness must be correctly specified for each of these systems to achieve their design life. Currently, bearings in blade pitch systems, main shafts, yaw systems, gearboxes, and generators suffer early failures despite well maintained systems, proper lubricant selection, and clean oil. Furthermore, micropitting continues to attack gear teeth and bearing components. False brinelling and fretting corrosion are the primary failure modes for blade pitch and yaw bearings, and electric discharge is often the root cause of failures of generator bearings. Lubricant contamination by solid particles is a principle mechanism that causes debris dents on bearing components. These eventually lead to micropitting and macropitting. A recent critical bearing problem manifesting as axial cracks is occurring primarily in the inner rings of bearings, though also sometimes in outer rings. Currently, there are several controversial hypotheses for the root cause of this failure mode, but none of the hypotheses have been widely accepted across the entire tribology community. Harish Choudhary,B Power) Page 15
  16. TRIBOLOGY OF WINDMILL 6. REFERENCES 1. B.C. Babu, K.B. Mohanty, Doubly-fed induction generator for variable speed wind energy conversion systems modeling and simulation. Int. J. Comput. Electr. Eng. 2(1), 141 147 (2010) 2. H. Chandler (ed.), Wind Energy The Facts, in European Wind Energy Association (2003) 3. Y. Amirat, M.E.H. Benbouzid, B. Bensaker, R. Wamkeue, Condition Monitoring and ault Diagnosis in Wind Energy Conversion Systems: A Review in Electric Machines and Drives Conference. IEMDC '07, IEEE International, 2007 4. Global Wind 2009 Report, Global Wind Energy Council, 2010 5. Wind Energy by 2030, U.S. Department of Energy Technical Report, 2008 6. Strategic research agenda: market deployment strategy from 2008 to 2030, European Wind Energy Technology Platform, 2008 7. Mid and long range plan for renewable energy development, Chinese Committee for National Development and Reform, 2007 8. B. Lu, Y. Li, X. W u, Z. Yang, in A review of recent advances in wind turbine condition monitoring and fault diagnosis (Power Electronics and Machines in Wind Applications, PEMWA, IEEE, 2009), pp. 1-7 9. 2009 wind technologies market report, U.S. Department of Energy, 2010 10. J. Ribrant, L. Bertling, Survey of failures in wind power systems with focus on Swedish wind power plants during 1997—2005. IEEE Power Engineering Society General Meeting, 2007 Harish Choudhary,B Power) Page 16
  17. TRIBOLOGY OF WINDMILL Harish Choudhary,B Power) Page 17
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