Wind Drivetrain Bearing Reliability


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  • Good afternoon. Thank you for attending.I am Rick Brooks, Manager of Timken’s Wind Energy Aftermarket bearing business, based at our corporate headquarters in Canton, Ohio.Joining me today are Brad Baldwin, General Manager of Timken’s global Wind Energy business, and Jonathan Glessner, Manager of our H&N Wind service business based in Pasco, Washington.Today we will be discussing the most frequently occurring bearing problems in the wind turbine drivetrain.We will dive into the root causes of the problems in each position, and explore some of the solutions that are being deployed in the industry to correct the problems.
  • Our focus today will be on the drivetrain bearings of a traditional modular wind turbine.We will start with talking about main shaft bearing arrangements.Moving on to the gearbox, we will primarily focus on Planet bearings and High Speed bearings, which is where we see the most bearing problems.Then we will finish up by touching on Generator bearings, leaving time at the end to take some questions.
  • However, before we dive into the various bearing problems, lets talk a little bit about the financial impact of the bearing failures in Wind.It is fortunate that in general there is an inverse relationship between the frequency of occurrence of bearing failures and the failures’ financial impact.For instance, we observe the highest failure rate on Gearbox high speed bearings, but because these bearings can generally be changed up tower without an external crane, the total repair cost is minimal.Likewise with generator bearings.Main shaft bearing failures are far less common, but much more costly; partly because it is a larger bearing, but mostly because an expensive crane mobilization is required.It is the same case for gearbox planetary section bearings.Unfortunately, we have lately been seeing an increase in main bearing failures in the U.S. wind turbine fleet, which is an alarming trend.
  • Over the last few years, the industry has moved aggressively in the direction of performing as much repair work up-tower as possible.The financial case for this is extremely strong.Up-tower work reduces or eliminates the crane costs, reduces the total repair time, and can either be outsourced to a number of specialized service companies, or even self-performed.However, from a bearing perspective, there are a couple of negative aspects to these field repairs that you should be aware of and try to mitigate.A repair shop is generally much cleaner environment that a wind turbine nacelle, and the shop environment lends itself to better quality procedures.Also, the ability to run a load test after the repair in a shop can assure the repair was done right.Regardless, the financial case for Up-tower repairs is so strong, that the trend toward more field work will continue.
  • So let’s begin discussing the main shaft bearing arrangements.We can split gear drive wind turbines into two categories based on main shaft bearing setup: 3-point and 4-point mount.In both cases, two of the mounting points are the two torque arms on each side of the gearbox.If there is a single main shaft bearing, it is called a 3-point mount turbine. This is the most common type of turbine in the U.S., and includes the GE 1.5MW, the Siemens 2.3MW, the Mitsubishi 1MW, the Suzlons and even one of the Vestas models. The three point mount turbines almost all have a single Spherical Roller Bearing (or SRBs) as the main shaft bearing.The four point mount turbines, which includes most of the Vestas models, Gamesa, Accionna, turbines and others, have two main shaft bearings.Most commonly this is two different types of SRBs. However, some new models of turbines use tapered roller bearings (or TRBs) and cylindrical roller bearings (or CRBs).The general trend in wind turbine design is away from SRBs toward TRBs.
  • Almost all the main shaft bearing failures we see in wind turbines are of the Spherical Roller Bearing in the 3-point mount designs.If you look at the forces acting on the bearing in this arrangement, it is easy to understand why the problems happen:The main bearing needs to support the weight of the hub and the trust from the wind, and ideally transmit only pure torque into the gearbox.An SRB is often used in this arrangement because the wind profile and pitching causes constant overturning moment forces on the main bearing.The SRB is able to handle the resulting dynamic misalignment conditions better than any other bearing type, but does a poor job accommodating the high thrust loads.As a general guideline, and SRB’s thrust load should be limited to no more the 20% of its radial load.In this application, the main bearing SRB can experience thrust ratios from 30% to 60%.The result is a variety types of bearing damaged, all based on high thrust at low speeds.
  • Now that I have explained the most common main shaft bearing failure modes, let take a look at the most common enhancements that are being used in the industry to try to address these problems.A variety of changes to the physical internal geometry of the bearings are being used to try to partially offset some of the damage modes.These are the changes that are the least visible, but can have an impact on performance of the bearing.But controlling the exact geometry of the rollers and races, it is possible to improve the component contact conformity, reduce stress, and delay some of the problems we have seen, particularly edge loading.But by itself, these changes will no eliminate main bearing failures.
  • Another enhancement is the improve the surface finish of the rolling elements or races through various types of super finishing.This lowers torque, and more importantly, reduces asperity contact and stress that leads to race wear and micropitting.
  • Another enhancement, that is relatively straight forward, is to use a two piece cage design.This allows the two rows in the bearing to operate more independently from each other, lowering operating forces and stress.
  • The most important enhancement, in my opinion, is the use of a wear resistant “DLC” or Diamond Like Coating on the rollers.If done properly, this technology can mitigate the surface initiated wear and micropitting which is the primary root cause of most of the main shaft SRB bearing failures we see.It does this by preventing the adhesion of the asperities between the rollers and races.As a side benefit, such coatings also increase fatigue life, and greatly increase the components ability to operate in a heavy debris environment.
  • The Timken Wear Resistant bearing incorporates all of the enhancements I just mention.The goal is to try to make a replacement bearing that has all the features to address the specific failure modes we see in the field.However, all these features mitigate damage, but they cannot eliminate the application conditions that exist in the 3 point mount main shaft spherical roller bearing.
  • We have had excellent field results, and after 4 years of operation, there have been no failures of signs of the damage that lead to failures.The grease is in excellent condition, the races show only normal wear, and the DLC coating is intact.
  • There are a few other factors to consider regarding the health of your main shaft bearings that I want to mention:Given the high crane costs of a main shaft bearing failure, Condition Monitoring will let you know it is coming, and allow you to schedule it in a way in minimize crane costs. This should include at a minim, Statistical analysis and trending of bearing temperatures, or ideally, Vibration or Shock Pulse analysis.Lubrication practices can significantly delay the onset of main bearing damage.It is worth re-examining yourgreasing practices, including amount & frequency.Auto-Lubrication systems can also help, if used properly.The selection of the grease itself can extend the life of the bearing.Investigate characteristics of current grease and alternatives other alternatives.If condition monitoring or inspections indicate that bearing damage is progressing, a grease flush is worth considering.This will get rid of the accumulated debris, and extend the life of bearings in early or medium stages of wear.
  • Let’s move on now to gearbox bearings.High Speed and intermediate shaft bearings failures are by far the most common.The most common failure mode for these positions is axial cracking on the inner rings.We will cover this in some detail.Low speed and planet carrier bearings have the lowest rate of occurrence.When there is damage to one of these positions, it is usually thrust of debris damage.Since we are focusing on the common bearing damage modes today, we will not be going into these.The planet bearings position itself has a medium occurrence rate, and one that we have recently begun to receive more questions about as the U.S. wind fleet ages.Planet positions are prone to debris damage and load related damage.
  • So starting with the planet bearing position, here is a quick summary of the type of planet bearing arrangements.Many Killowatt class turbines used spherical rollers bearings in the planet position.As the turbines go bigger, this caused many problems.Currently, not major megawatt wind turbine has this arrangement.So development progressed to Cylindrical roller bearings, or CRBs, and to Tapered roller bearings, or TRBs.In an effort to increase power density, both of these designs are often what we call integrated designs.That means that the outer race of the bearing is actually machined into the inside of the gear.
  • This chart illustrates the types of damage we see with each planetary bearing design.We do tend to see certain failures common to particular turbine / gearbox combinations.It is important to understand that a TRB design is preloaded, which eliminates the possibility of the rollers skidding and smearing when not in the load zone.The preload of the TRB also forces the load to be distributed evenly accorss the rows.Uneven load distribution is one of the most common problems with planet CRBs.The TRB, with its angled roller contact is also able to accommodate thrust much better that the CRB.The integrated designs for both CBRs and TRBs, in addition to be able to transmit more power, eliminate the possibility of the outer race turning or creeping within the gear.Debris damage is a possibility regardless of resign.
  • Enhancements for planetary bearing arrangements are all done during a rebuild.Some enhancements require an extensive re-design, such as going from a non-integrated to an integrated design, or changing from a CRB or SRB to a TRB.Other changes are less extreme, such as use Case Carburized replacement bearings, DLC coated CRB rollers, or modified coontrolled clearance in the CRBs to help better share the load across the rows.
  • Let’s move onto high speed bearings.There are three different bearing arrangements in the helical parallel section of the gearboxes.Generally, there is a Cylindrical Roller Bearing on the hub side of each shaft, which is a floating or non-locating position.The generator side of each shaft will then have one of three arrangements: a CRB plus a Ball Bearing, a double row TRB, or a specially designed single row TRB, as the fixed or locating position.We tend to see lower rates of axial cracks in the designs that use TRBs in the locating position, and the industry is primarily using this arrangement for new models.
  • The damage modes we commonly see for intermediate and high speed bearings are:Skidding and smearingInclusion related axial cracking or spallingAnd White Etch Area axial cracking or brittle flaking.In the next few slides, we will talk about each.
  • This slide tracks a roller on its path during 1 orbit around a cylindrical roller bearing operating with a typical radial internal clearance (RIC) producing approximately a 90 degree load zone. This means that only the rollers B in the load zone are supporting the load while the other rollers D outside the load zone do not. An unloaded roller D will continue to lose its rolling velocity and operate with increasing sliding accumulated during the time interval from C to A. At A, the roller experiences its highest sliding. Entering the load zone at A, it quickly regains traction in the acceleration zone. Testing confirms that a new bearing surface is most vulnerable to smearing damage when it passes through this acceleration zone. Once the roller has passed through the acceleration zone and regained full traction, its velocity normally climbs to that required for pure rolling. This phenomenon can result in race surface damage, or be a initiating factor for cracking or spalling.
  • Sometimes, the cracks that form are due to subsurface or near-surface inclusions in the steel.Put simply, an inclusion is something other than steel, either metalic or non-metalic, that remains in the steel through the manufacturing process.The Inclusion acts as an initiation point for a material transformation caused by the forces at play in the wind turbine gearbox application.This sometimes appears as a so called White Etch Butterfly when the material is sectioned.A crack forms at the inclusion, growing through the transformed area.When it reaches the surface, it can either crack through or spall out, depending on the material.
  • The cause and formation of white etch area cracks is a subject that is hotly debated within the industry.I am going to walk you through what our research has told us about this common phenomenon.The problem begins with the hoop stresses from the original press fit on the shaft.
  • The roller skidding and skewing we discussed a few moments ago, in addition to damaging the race surfaces, also create tensile stresses.
  • In various load conditions, such as e-stops, generator engagements and disengagements, and constant speed changes cause the load zone in the bearing to shift frequently. The combination of the residual hoop stresses and the applications’ inherent tensile stresses, combines with these highly dynamic conditions in a wind turbine gearbox. The result is the subsurface transformation of the of metallic structure.
  • These repeated dynamic events cause plastic deformation subsurface.
  • The transformed area is far more hard and brittle than the original material.Eventually, a crack forms in this brittle area and propagates to the surface.This sequence of events can happen all around the bearing inner race, resulting in numerous cracks.
  • So based on the common gearbox high speed shaft damage modes we just reviewed, this chart summarizes the effectiveness of various bearing enhancements that can be used to address these problems.As you can see, it really takes a combination of solutions to address all three problems.Black Oxide treatment has become minimum standard for high speed bearings over the last few years, but black oxide alone does not solve the issue.The best combination is very Clean Steel to address the Inclusions, the right DLC coating to address the smearing, and Case Carburized heat treatment to address the white etch area formation.
  • This is a good point to briefly discuss the various types of engineered surfaces.The chart above shows the results of laboratory test comparing the effects of super finishing, black oxide, and Wear Resistant, or DLC coated bearings in a test with high smearing.As you can see, both the standard finish and the honed sample had about the same performance.The black oxide was slightly better, until the treatment wore off. Then it preformed just like the other two samples.Only the Wear Resitant version showed no indication of smearing, even after running for 3 times as long as the other 3 samples.
  • So when you compare Black Oxide and DLC coated surfaces, it is important to understand that they are very different.Not all bearings that look black are the same.Black Oxide is a conversion treatment that involves putting the components, usually both rollers and races, into a hot bath of oxidizing salts.It builds up a protective layer of Fe3O4 on the surface.Black Oxide has a number of benefits in many applications including wind, but unfortunately it wear off.A DLC coating on the other hand, is a Tungsten carbide amorphous hydrocarbon composite coating that is applied in a vacuum chamber.It is normally applied only to the rollers, not the races, because you want to the advantage of the interaction of two dissimilar materials.DLC has a lot of advantages over no coating or Black Oxide, and if done properly, does not wear off.Another big difference between the two however is cost. Black Oxide is far less expensive to apply, which is the reason it is the first thing that is tried.
  • So, here are some field results of a Timken upgraded bearing.This bearing was put into a high speed position of a wind turbine gearbox that had previously had axial cracks.After 18 months of operation, we cut it up, and there was no sign of the subsurface material transformation that leads to axial cracks.Also, the DLC coating on the rollers was 100% intact.
  • So here a a few other factors that can impact the health of your gearbox bearings that we didn’t have time to dive into today:Condition Monitoring of various types is becoming far more common, and can small problems before they cause a total gearbox failure. The proper selection of gearbox oil can have a huge impact on the health of the bearings. In partiuclar, it is important to consider Anti-Foaming properties, as well as other Additives and Properties.A variety of Oil System Accessories, such as Moisture removal systems and upgraded Oil filtration systems can help.Also, when it comes time to change the oil in the gearbox, they when and how you go abou changing the oil can have a significant impact.
  • Finally, let’s briefly touch of generator bearings.The primary failures we see in the field are electrical fluting caused by current going through the bearings, and damage from inadequate lubrication.
  • To address the lubrication concerns, take a good look at your maintenance practices.For the electrical fluting, there are a number of enhancements that can be used, including:Add on shaft grounding ring Upgrade the ground brushes from carbon to metal fiber, ORUpgrade bearings from metal/ceramic coated to ceramic balls
  • To summarize
  • Wind Drivetrain Bearing Reliability

    1. 1. Wind Drivetrain Bearing Reliability
    2. 2. Before We Start  This webinar will be available at & email  Q&A at the end of the presentation  Hashtag for this webinar: #WindWebinar
    3. 3. Moderator Presenter Paul Dvorak Windpower Engineering & Development Rick Brooks The Timken Company
    4. 4. Wind Drivetrain Bearing Reliability Presented by The Timken Co. Richard Brooks, Mgr. Wind North America
    5. 5. WIND TURBINE DRIVETRAIN - COMMON BEARING DAMAGE Generator Bearings Gearbox High Speed Bearings Gearbox Planet Bearings Main Shaft Bearings
    6. 6. FINANCIAL IMPACT - WIND DRIVETRAIN BEARING DAMAGE $0 $500,000 Low High T O T A L R E P A I R C O S T Frequency of Occurrence Generator Bearings Gearbox High Speed Bearings Gearbox Planet Bearings Main Shaft Bearings $$$ $
    7. 7. UP TOWER VS. DOWN TOWER REPAIRS Up Tower Down Tower Crane Costs +++ --- Self Perform ++ - Clean Environment - ++ Load Test --- ++ Per Event Cost ~$10-$20k ~$250k-$500k
    8. 8. Two TS TDI and CRB Ultrawind Two SRBs Single SRB WIND TURBINE MAIN SHAFT MOUNTS
    9. 9. Hub Weight Wind Thrust Torque Overturning Moments Spherical Roller (SRB) Main Bearing MAIN SHAFT APPLICATION REVIEW-3 POINT MOUNT SRB
    14. 14. MAIN SHAFT SRB ENHANCEMENTS Enhanced Geometry • Reduce edge loading • Improves roller/race contact • Decreases roller stresses
    15. 15. MAIN SHAFT SRB ENHANCEMENTS Surface Finish • Reduced asperity contact and stress • Higher surface film, lower torque • Reduce friction and wear
    16. 16. MAIN SHAFT SRB ENHANCEMENTS Two Piece Cage: • Lower operating forces/stress
    17. 17. MAIN SHAFT SRB ENHANCEMENTS Wear Resistant “DLC” Coatings • Mitigate surface wear damaged • Increase fatigue life • Increase debris resistance
    18. 18. TECHNOLOGY DESCRIPTION BENEFITS Roller Finishing Low Roughness, Isotropic Finish Reduced Asperity Contact & Stress Roller Coating WC/aC:H Coating 1 µm thick Increased Wear Resistance, Increased Fatigue Life, Increased Debris Resistance. Internal Geometry Roller/IR Conformity Decreases Roller Stress, Reduces Potential Roller Skew Creates Favorable Traction Split Cage Two-Piece Machined Brass Cage Lowers Possible Operating Forces TIMKEN WEAR-RESISTANT SRB Wear Resistant SRB
    20. 20. MAIN SHAFT BEARING HEALTH- OTHER FACTORS • Condition Monitoring • Statistical analysis and trending of bearing temperatures • Vibration or Shock Pulse analysis • Lubrication Practices • Variation in manual greasing practices (amount & frequency) • Value of Auto-Lubrication systems • Grease Selection • Investigate characteristics of current grease and alternatives • Grease Flush • Designed to extend the life of bearings in early stages of wear 20
    21. 21. High Speed & Intermediate shafts: • High occurrence Rate •Axial cracks on Inners Planetary positions: • Medium occurrence Rate • Debris & load damage WIND GEARBOX BEARINGS Planet Carrier & Low Speed Positions: •Low occurrence Rate •Thrust damage
    23. 23. PLANET BEARING - OBSERVED DAMAGE MODES 23 Non-Integrated Integrated TRB • Debris damage • Creeping Outer • Debris damage CRB • Debris damage • Thrust load • Heavy / Uneven loading • Smearing • Creeping outer • Debris damage • Thrust Loading • Smearing
    24. 24. PLANET BEARING ENHANCEMENTS • SRB: • Replace with new design • CRB: • Case carburized • DLC coating • Modified controlled clearances • Redesign as TRB or integrated • TRB: • Case carburized • Redesign as integrated
    25. 25. CRB CRB + ball bearing HIGH SPEED AND INTERMEDIATE COMMON GEARBOX BEARING ARRANGEMENTS Non-locating Position Locating Position Options Double TRB Single TRB
    26. 26. HIGH SPEED & INTERMEDIATE BEARINGS – OBSERVED DAMAGE MODES I R r R r F 1. Smearing 3. White Etch Area Axial Cracking / Flaking 2. Inclusion Related: Axial Cracking / Spalling
    27. 27. ROLLER SLIDING / SMEARING IN CRB IR r R I R R r Outer Race Inner Race Cage Roller D - roller sliding increases outside the load zone C – load zone exit – most of the traction forces are lost beyond this point F B - traction forces restore roller velocity to pure rolling motion in load zone A– smearing as roller accelerates entering the load zone Load distribution in load zone
    28. 28. INCLUSION RELATED AXIAL CRACKING • Caused by non-ferrous contaminants in the steel • Commonly manifests as White Etch Butterflies • Material transforms in the area of the inclusion • Crack forms and prorogates to the surface or spalls
    29. 29. WHITE ETCH AREAS CRACKS 1. Hoop Stress from original Press Fit on shaft 2. Tensile stresses from roller sliding & skewing 3. Dynamic conditions begin the subsurface transformation 4. Crack forms in the hard & brittle transformed area 5. Crack propagates to the surface
    30. 30. WHITE ETCH AREAS CRACKS 2. Tensile stresses from roller sliding & skewing IR r R r F
    31. 31. WHITE ETCH AREAS CRACKS 3. 3. Dynamic conditions begin the subsurface transformation
    32. 32. WHITE ETCH AREAS CRACKS Dynamic Event Dynamic Event Normal Load Normal Load Plastic Deformation Transformation Plastic Deformation Transformation 4. 4. Crack forms in the hard & brittle transformed area
    33. 33. WHITE ETCH AREAS CRACKS 5. 5. Crack propagates to the surface
    34. 34. BEARING ENHANCEMENTS - HIGH SPEED & INTERMEDIATE Enhancement White Etch Inclusions Smearing Steel Cleanliness + +++ Black Oxide + (early stages) ++ (early stages) ‘DLC’ Coatings + +++ Case Carburized +++ ++
    35. 35. 0 1 2 3 4 CompletedSmearingTestCycles Smeared during first cycle ENGINEERED SURFACE: PERFORMANCE COMPARISON Ground Honed/ES20 Honed/ES20/Black Oxide Wear-Resistant
    36. 36. ENGINEERED SURFACES (COATINGS & TREATMENTS) Black Oxide Surfaces • Black oxide is a conversion treatment: • Metallic parts are put into a hot bath containing oxidizing salts • Steel reacts with the oxidizing salts to form a thin layer of magnetite on the surface (Fe3O4). • Black oxide is a sacrificial surface • Benefits: • Aesthetics • Corrosion prevention • Run-in and/or performance improvement • Prevents mild adhesive wear damage • Improve rolling contact fatigue life DLC Coated Surfaces • DLC is a hard coating: • Tungsten carbide incorporating an amorphous hydrocarbon composite. • Applied with a Physical Vapor Deposition process in a vacuum chamber. • Benefits: • Improve rolling contact fatigue life. • Mitigate life limiting wear damaged caused by metal adhesion. • Increased fatigue Life • Increased debris resistance • Prevent scuffing or smearing damage (Timken Wear Resistant coating)
    37. 37. Inner Race Subsurface Condition Roller coating intact RESULTS OF TIMKEN CRB (CASE CARBURIZED AND WEAR RESISTANT) AFTER 18 MONTHS
    38. 38. GEARBOX BEARING HEALTH – OTHER FACTORS • Condition Monitoring • Oil analysis programs and Online oil monitoring systems • Vibration or Shock Pulse analysis • Oil Selection • Anti-Foaming • Additives / Properties • Oil System Accessories • Moisture removal systems • Oil filtration systems • Quality Oil Changes • Frequency based on oil monitoring condition • Vendor oil change features 38
    39. 39. GENERATOR BEARING OBSERVED DAMAGE MODES •Electrical Fluting •Inadequate Lubrication
    40. 40. GENERATOR BEARING ENHANCEMENTS • Add on shaft grounding ring • Upgrade the ground brushes from carbon to metal fiber • Upgrade bearings from metal/ceramic coated to ceramic balls
    41. 41. Generator Bearings Gearbox High Speed Bearings Gearbox Planet Bearings Main Shaft Bearings WIND TURBINE DRIVETRAIN - SUMMARY BEARING DAMAGE AND UPGRADES •Wear Resistant coating •Case Carburized •Controlled clearance •Grounding improvements •Ceramic balls •Case Carburized •Wear Resistant coating •Wear Resistant coating •Two piece cage •Geometry enhancements
    42. 42. Questions? Paul Dvorak Windpower Engineering & Development Twitter: @Windpower_Eng Rick Brooks The Timken Company Phone: 330-471-7812
    43. 43. Thank You  This webinar will be available at & email  Tweet with hashtag #WindWebinar  Connect with Windpower Engineering & Development  Discuss this on the