Technical Paper BEARING SELECTION TECHNIQUES AS APPLIED TO MAINSHAFT DIRECT AND HYBRID DRIVES FOR WIND TURBINES

Transcription

1 Technical Paper BEARING SELECTION TECHNIQUES AS APPLIED TO MAINSHAFT DIRECT AND HYBRID DRIVES FOR WIND TURBINES Matthew B. Turi and Christopher S. Marks

2 INTRODUCTION As wind turbine manufactures gain experience with turbine and gearbox designs, they are elevating the need to improve the reliability of drivetrains while employing an architecture that optimizes the cost structure of turbines and towers. Wind turbine generator designs have historically utilized a modular architecture (Fig. 1a, 1b). Several departures from that traditional design approach aim to improve turbine reliability and cost. Two of the most common architectures include direct drive and mid-speed hybrid drive turbines. Direct drives tend to result in more upfront cost, but can reduce complexity by eliminating the gearbox. Hybrid drives also focus on simplifying the gearbox and generally result in lower tower top mass. bearing arrangements available based on the turbine drive train architecture. Drive Type Hybrid Direct Modular Tapered Single Modular Drive Tapered Double Direct Drive Tapered Double + Cylindrical Hybrid Drive 3 & 4 Point SRB MW Table 1. Wind turbine main shaft bearing mounting arrangement general solutions. Previous technical articles have addressed concerns when using spherical roller bearings (SRB) in main shaft fixed positions as compared to preloaded double-row tapered roller bearings (TRB). Due to elevated axial loading and inability to optimize in preload, use of SRBs may result in unseating effects, abnormal load distribution between rows, roller skewing, roller retainer distress, excessive heat generation and roller smearing. Fig 1a. Modular drive train configuration. Source: NREL/TP A key consideration in turbine design is the selection of the bearing system used to support the main shaft. Options include a single bearing position system utilizing a two-row bearing or a multiple bearing position system. For each position, the bearing type and configuration must also be determined. For larger turbines, viable alternatives include combinations of spherical, cylindrical and tapered roller bearings. Table 1 lists various main shaft Fig 1b. Modular drive train configuration. Preloaded TRBs allow for improved system stiffness and are available with modified 1

3 internal geometry to operate effectively in high misalignment conditions. In addition, cylindrical roller bearings (CRB) work well with TRBs, providing additional radial capacity and stiffness that allows for a more power-dense arrangement. These and other advantages of TRB and CRB arrangements make them a better solution for multimegawatt turbines. This paper will expand on bearing selection requirements for main shaft positions in direct drive and hybrid drive turbines. DIRECT AND HYBRID DRIVES There is significant work within the industry to understand real operating loads on turbines, gears, shafts and bearings in the field. Standards have been developed to help the industry design more reliable turbines with improved performance, but there is still room for further improvement. While work continues in understanding environmental conditions and a turbine s reaction to those conditions, there are new designs focused on making a system more robust against unknown challenges and/or eliminating the sources of reliability problems. A direct drive turbine that eliminates the gearbox entirely has to meet certain considerations. To be able to generate adequate power at low speeds, generators tend to become larger, heavier and more expensive. Typical bearing solutions have been three-row CRB designs with two axially positioned rows in light preload, and one radial row mounted in clearance. Unitized two-row TRBs are also a viable and advantageous solution (see Fig. 2). Fig 2. Typical direct drive generator wind turbine design with a two-row TRB. Hybrid drives use mid-speed generators and will employ one or two planetary stages to achieve generator speeds between those typically found with direct drives (low speed) and modular designs (high speed). These designs can significantly reduce tower top mass as a ratio to power output. Also, these designs target a good balance between gearbox and generator size to achieve optimal use of space atop the tower. Fig 3. Hybrid drivetrain example with two-row TRB mainshaft. Source: DNV-GEC 2

4 DIRECT/HYBRID DRIVE CONSIDERATIONS Fig. 4 shows loads in a coordinate axis imposed by the rotor blades on a typical wind turbine. Fig 4. Loads and working system of axis. Some challenges faced by direct drive turbine manufacturers and bearing suppliers in managing the loads and stresses in a compact space. Stress internal to the bearing is a function of the weight of the hub/blade and rotor assembly, along with external loading during operation. Therefore, for any type of wind turbine architecture, it is critical that wind turbine manufacturers provide an accurate assessment of field loading to the bearing manufacturer. Inadequate inputs into bearing life models may result in improper bearing life analysis and potentially lead to premature bearing damage. Lubrication of critical race/roller surfaces is another issue requiring special design consideration. Most bearings in the direct drive mainshaft market are grease lubricated. Care needs to be taken to select the proper grease that will not migrate away from roller/race surfaces and lead to seal leakage. This may need to be balanced with the ability of lubrication control systems to work with the specified grease. These systems should be designed to ensure proper lubrication of 3 each row and prevent bearing lubrication starvation due to flow blockages. Whether to supply a bearing with a full complement of rollers or to include a cage or separator is another critical design decision. Full complement designs will use more rollers in the same design space, thus will have increased load carrying capacity. Rollers will contact each other at the roller body, so appropriate surface treatment may be necessary to avoid surface damage during use. For full complement designs, surface treatments can be incorporated to provide surface hardness improvements and ultralow surface finishes allowing improved lubricant film thickness generation at relatively low speeds. The type and method of lubrication will also influence the decisions on applying a full complement bearing. Incorporating a cage on ultra-large bearings may provide benefit in roller guidance, lubricant distribution and elimination of roller body contact. Direct drive main shaft bearings also need to have properly designed features that allow for efficient handling and installation. The size of the bearings can create logistical challenges and bearings need to be installed properly to avoid issues that can cause longterm performance problems. Some bearings are designed to have bolt-on features for attachment to the nacelle structure, hub and rotor assemblies. Without an external shaft or a press fit into the housing, bolt designs are critical to maintain bearing clamp, and in some cases, alignment of bearing races. Bearing setting is another critical aspect for proper performance. In a tapered nonadjustable (TNA) design, bearing suppliers can carefully control the designed setting. In fact, the only factor outside the bearing supplier s control that can impact the operating setting is external clamp load. For a turbine mainshaft application with two separate rows, setting is the responsibility of the turbine assembler. Several methods for achieving a desired final setting may be

5 employed, but bearing size needs to be considered for several reasons, including proper measurement of initial parameters, accurate assessment of adjustments needed to achieve final setting and determining the final assembly effect on setting. We will cover the importance of bearing lateral setting later in this paper. BEARING SELECTION FOR DIRECT DRIVES AND HYBRID MAINSHAFTS BEARING FATIGUE DUTY CYCLE The bearing fatigue duty cycle received from the customer can have a significant influence on the size and geometry of the mainshaft bearing designs. A concern is that adding conservatism by oversimplification of the duty cycle will result in a negative cost structure. Some manufacturers use hundreds of conditions in the duty cycle. Others may use tens or only a single condition in the duty cycle. Duty cycles usually are generated using design programs to model the wind turbine system, typically with an output at 20-Hz. The high frequency of data provides a vast number of snap shots of the system, even for short time intervals. All this data must be sorted and binned in useful categories, using the arithmetic average bin value, for fatigue analysis. A five second excerpt of data from the graph has been added in to show the variation of the data. Variation in this short time is graphically shown in Fig. 5. The complete data is then sorted into bins and the time durations in each bin is summed to determine the percent of time each condition contributes to the duty cycle My Fx Fz Mz Fy Time (s) Fig. 5. Five second snapshot of data from design program. In order to develop a duty cycle from time series data for these load conditions, two methods can be utilized to generate duty cycles an independent or dependent reduction. In an independent reduction, each load is binned separately for a specific RPM bin. A load histogram can then be generated for each load using the previously discussed technique. An equivalent load for each resulting load histogram can then be calculated. Finally, a duty cycle can be constructed with the corresponding combinations of independent equivalent loads. While an independent duty cycle is simpler to create it may not always maintain the proper relationship between specific load combinations. This type of load case may result in an over-predicted bearing life due to lost load/moment relationships. This is where a dependent duty cycle reduction can be beneficial. In a dependent reduction, loads are binned dependently based on importance of effect to bearing life, where low importance loads can generally be equated to as few or as little as one equivalent load. Bin size should be determined methodically for the speed and loads by understanding the effect on the bearing system. The following recommended order of importance of the data for proper bearing analysis can be utilized in either reduction case: 4

6 1. RPM (due to effects on the development of the lubrication film thickness). 2. Pitch Moment, M y 3. Yaw Moment, M z 4. Radial Load, F z 5. Axial Load, F x 6. Radial Load, F y Once the low priority load bins have been defined, higher importance load data can then be binned in subset histograms of appropriate size for each lower importance load bin. A duty cycle can be constructed from the dependent relationships and analyzed with an advanced bearing fatigue calculation program with Miner s Rule to determine the bearing L10a fatigue life. Fig. 6 below illustrates a generic relationship structure based on the author s recommended importance of reduction. Typically, bearing manufacturers are provided the binned duty cycle from wind turbine OEMs and/or gearbox manufacturers. Equally important as the correct time series data is the method of the reduction. While each manufacturer can have its own method for the reduction of time series data, it is also important that they understand the significance of the reduction methods on the load/moment relationship on predicted bearing life. My,1My,2My,3My,4 Fz,1 Mz,1 Mz,2 Mz,3 My,1My,2My,3My,4 My,1My,2My,3My,4 Fy Fx Fz,2 Fig. 6. Sample dependent duty cycle relationship structure. BEARING LIFE CALCULATIONS Bearing life calculations have evolved from basic catalog calculations (load and speed effects) to very sophisticated calculations that include many different environmental conditions that impact life. The catalog calculations were sufficient in very basic Fz,3 5 bearing sizing but would not model actual operating conditions and many assumptions made for catalog calculations do not hold true in real world operation. Bearing companies have developed in-house analytical programs to better evaluate the environmental effects influencing bearing life. It is suggested that wind turbine manufacturers contact their approved bearing suppliers for advanced bearing life analysis. There are several life adjustment factors included in advanced bearing analysis in Syber, a proprietary finite element based computer simulation software of the author's company. In addition to load and speed, other major life influencers are: 1. Load zone (bearing fits and setting) 2. Thermal effects (operating temperatures, thermal gradients, lube sump temperatures) 3. Lubrication effects 4. Misalignment/race stress (functions of housing and shaft stiffnesses radial, axial, and tilting) 5. Fatigue propagation rate 6. Bearing geometry factors BEARING LOAD ZONE Load zone is an angular measurement of the load distribution in a bearing and is a direct indication of how many rollers per row share the applied load. There are a vast list of factors that determine what the operating load zone is, including initial lateral setting, applied load, operating temperature, structural properties of the shaft/housing and bearing fitting practice. The following diagram (Fig. 7) shows a graphical representation of load zone, with the blue arrow indicating an approximate 250 degree load zone):

7 increased significantly to dramatically increase the load zone above 110 degrees. Fig 7. Load zone. Load zone influence on catalog life is determined through the use of a life multiplication factor. The factor is 1 at 180 degree load zone. The factor increases in slight preload. Since TRBs are usually mounted in pairs, their individual load zones are interdependent. Thus, system life depends on the operating setting in each row under a given condition. In multiple condition duty cycles, the load zone can change dramatically and will affect bearing performance. This factor takes into account the change in roller loading on bearing life. Fig. 9 shows a typical bearing life versus lateral setting curve. Peak life tends to be in slight preload where optimum roller sharing occurs. When analyzing bearing life for a two-row arrangement, it is more appropriate to focus on system life, which is a measure of the life associated with both bearings and accounts for the likelihood of either bearing reaching a failure point. This can be seen in the system life curve for a given condition in Fig. 10. In a two-row TRB system, a net thrust force will exist that will cause one row to be seated while the other is unseated. This directionally-dependant net thrust force is the sum of the external thrust applied to the system plus the tow-induced thrusts generated by radial loads on the TRBs. By design, a radial load applied to a TRB will create thrust forces with magnitudes relative to the outer raceway angle. Fig. 10 includes individual row life for seated and setup (unseated) bearings. Unseated Bearing Load Zone vs. Setting Low Load Medium Load High Load Setting (mm) Fig. 8. Varied loads and setting effect on load zone. Figure 8 shows that a reduction in bearing preload on the unseated bearing will lead to a reduction in load zone for a range of conditions. One might conclude to increase the dimensional preload beyond 0.30 mm to ensure both rows are well-seated under the heaviest loads, the preload would need Fig. 9. Life versus bearing setting. 6

8 SRB row (upwind) SRB row (downwind) Fig 12. Load zone in SRB. Fig 10. Life versus bearing setting tworow bearing system. A previous technical paper compared tworow TRBs versus two-row SRBs in the fixed position of a wind turbine mainshaft. One focus of the paper was load zone and the impact on bearing life. A two-row TRB solution can be installed with initial preload in the system. Controlled preload is advantageous from the standpoint of optimizing bearing life through load sharing between rollers for a given duty cycle. Fig. 11 includes examples of TRBs in a tapered double inner (TDI) arrangement for a given load condition. Optimization of bearing load zones in wind turbine applications has several benefits. Loads can be balanced among available rollers to reduce loads on the maximum loaded roller in certain conditions. When a system is not optimized or uses bearing types which don t allow for the load zone control similar to TRBs, fewer rollers may be carrying the bulk of the load. Keeping rollers engaged with race surfaces also prevents premature damage from skidding/smearing. This happens when rollers move through the unloaded zone and are being pushed by the cage, rather than being driven by traction from the rotating raceway. Roller surface and race surface will then see contact when the roller moves back through the loaded zone. This contact will cause adhesive wear, and also increased tensile shear forces beneath the surface of the race/rollers. The tensile shear forces can lead to formation of axial cracks. The basic design of a TRB, plus the ability to optimize setting in preload, will work to avoid skidding/smearing damage and also help balance load between the rollers of both rows. TRB row (upwind) TRB row (downwind) Fig. 11. Load zone in typical TRB. A comparable spherical two-row bearing (Fig. 12) will tend to have one row-carrying load while the other may be unloaded. This is mainly due to the inability to set the bearing in initial preload. Lack of roller load sharing could cause reduced fatigue life in service. 7

9 THERMAL EFFECTS Temperature can impact bearing life in multiple ways, all of which must be taken into account when trying to perform advanced life calculations. Areas in which thermal gradients can impact are listed below: Lubricant viscosity Operating setting Bearing arrangement Dissimilar material thermal expansion Fig. 13. Two TS wide spread mainshaft. Because lubricant viscosity is a function of temperature it is important to properly assess operating temperatures in order to predict proper film thickness. Thermal gradients between shaft and housings impact axial shaft expansion/contraction which can result in a change of setting between two bearings. In addition to axial shaft expansion, radial expansion of the bearing raceways can occur. Because TRB raceways are designed on an angle, a radial expansion of the raceway can be equated to an axial movement of the raceway. Both of these thermal effects will ultimately impact the operating setting of the bearing. In a case where two bearings are wide spread, the change in relative shaft and housing length due to thermal expansion, L, is large compared to a close couple TDO or TDI style bearing assembly. In order to illustrate the effects of thermal gradients, an example with a two taperedsingle roller bearing (2 TS) arrangement for a wind turbine main shaft (Fig. 13) was analyzed with and without thermal gradients between the shaft, housing and bearing raceways. From the subsequent life plot (Fig. 14) it is evident at maximum setting there is a significant difference in predicted life, which may not meet the acceptable life requirements for the application. Fig. 14. Life versus setting with and without thermal gradients. Finally, differences in material properties can mean larger relative displacements for even small thermal gradients when compared to similar materials, making thermal effects even more important to consider for proper advanced life prediction. LUBRICATION For direct drive mainshaft bearings, grease is a very viable solution due to low operating speeds. Although grease may result in a thinner film thickness, it is the preferred option for direct drive applications. It will have a lower chance of leakage, will not migrate as easily, and will exclude contaminants more effectively than oil. Common considerations for the grease selection process include: Higher viscosity (ISOVG 460 or 320) is better for maintaining good film strength 8

10 Synthetic base oil with high viscosity index (VI) will provide better lubrication over a larger temperature range Excellent water, rust, oxidation, and corrosion resistance is important for extended grease life Low-temperature operation with adequate pumping may be required in some applications Lubrication control systems are a way to ensure effective re-lubrication over time and to make sure each bearing row is receiving grease. Newer systems have features that will inject grease with two separate ports, directing lubrication at each bearing row. Also, bearings can be designed with features that take a more active role in removing used grease from the bearing rather than relying on back pressure to force it out. This can also keep internal pressures lower and may help increase expected life of contacting lip seals. MISALIGNMENT/RACEWAY STRESSES Fig 15. Misalignment. Bearing life can be negatively affected by excessive shaft and housing misalignment. High loads and overturning moments can cause this to happen. Misalignment will increase edge stresses in roller bearings and could cause early damage in the bearing in the form of geometric stress concentration (GSC) spalling. TRBs and CRBs can be designed with special profiles to alleviate edge stresses under given conditions. This is another reason for the importance of an accurate assessment of wind turbine loading. 9 Basic stress profiles are shown below in Fig. 16. Stresses are higher near the center due to race and roller crowning. Relatively high loading can cause load truncation at the ends of the contact area and misalignment can cause stress imbalance along the raceway. The final graph shows typical stress plots for edge stress conditions. Fig. 16. Raceway stresses. For catalog calculations, the impact on bearing life is handled through the use of a life factor and this factor is generally 1 for a misalignment of radians. It is greater than 1 for lower levels of misalignment and will reduce life when misalignment is greater than radians. RELIABILITY REQUIREMENTS There have been many bearing life expectations from various customers. Some have used 150,000 hours, while others have used 175,000 or even 200,000 hours life calculation for which 90 percent of the population will reliably survive (e.g. L 10 ). The required calculated L 10 for a 20-year design life would improve with increasing reliability requirements. As seen in Table 2, taken from ISO281:2007, in order to obtain the required reliability of 150,000 hours at a higher reliability level, the calculated L 10 will increase. Also shown in Table 1 are the required L 10 for a 30-year design. Another way to state this would be that the reliability factor, a 1, is multiplied by the L 10 to attain the L n life of 175,000 or 263,000 hours for the 20- or 30-year calculated life, respectively.

11 Life Reliabilit y a 1 20-year L 10 Life 30-year L 10 Life L , ,000 L , ,000 L , ,000 L , ,000 L ,000 1,060,00 0 Table 2. L 10 life requirement for various reliabilities. It is important to understand that the reliability requirements are defined for failure by subsurface fatigue spalling. There are other types of bearing failures that may occur in the application that are not considered using traditional fatigue durability analysis. These include, but are not limited to: Scoring: Scoring may occur on a roller bearing if the end of the roller contacts an improperly lubricated flange or if a high rib contact stress or improper contact geometry exists. Scuffing: Scuffing traditionally occurs when there are insufficient traction forces between the roller and the raceways resulting in gross sliding at the contact. As the heat generation increases, the surfaces adhere and cause transfer of the material. The sliding is caused by low bearing preload or a low load zone, high speeds and/or light loads. Micropitting: Micropitting is similar to macropitting, except occurring on the micrometer scale. The small pits on the surface are due to the increased stresses that occur on the microscale when lubricant films are thin compared to the surface texture resulting from the finishing process. This issue is grossly accelerated when sliding occurs on the surface simultaneously with the thin lubricant films. Structural issues: Structural issues may be related to sections of the inner or outer raceways that may be used as structural members to transmit the load instead of using a housing or shaft to transfer the load. Brinelling and false brinelling: Brinelling results from permanent deformation or yielding in the part. False brinelling is commonly seen when the rollers are not rotating and oscillate back and forth along the direction of the rotational axis of the roller. DESIGN OF THE TRB TRBs achieve true rolling motion by being designed on apex as in Fig. 17. Lines drawn extending the inner and outer raceways towards the centerline will intersect on the centerline. The roller s size (body length, small- and large-end diameters, and body included angle) along with its relative position to the centerline, will define the bearing series. A single roller could be used in many different series by adjusting its angular position relative to the centerline. This allows for optimization of the radial and axial load carrying capability. The forces acting on and generated by the TRB are shown in Fig. 18. Resultant forces act perpendicular to the raceway. Since race surfaces are not parallel, there will be an effective seating force that pushes the roller into the rib. The seating force aids in roller alignment during operation. Excessive seating forces can cause sizeable rib forces resulting in increased heat generation and early bearing damage. Fig. 17. On-apex design of a TRB. 10

12 Included roller angle (F) Mean roller diameter [(LED+SED)/2] Fig. 18. Forces acting in a TRB. A typical double-row TRB single main bearing for mainshaft applications is composed of a double outer race [A] (or cup), two inner races [B] (or cones), two rows of rollers [C] and a retainer [D] (cage) for each roller row as shown in Fig. 19. The intersection of the bearing centerline and the angled dashed lines in Fig. 19 define the bearing spread for counteracting the overturning moments. Fig. 19. Typical TDO bearing components and features. There are many design considerations required for two-row TRB for mainshaft applications. Designs should be balanced in order to obtain a bearing that is optimized for performance, price and manufacturing. The primary features (Fig. 19) of the bearing that must be considered in the design phase are: Mean pitch diameter (average of the bore and outside diameter of the bearing) Included cup angle (E) 11 Optimization of the overall design takes skill and experience because these factors are closely interrelated. Bearing envelope size will usually be dictated by turbine designers, but upfront work with bearing suppliers will make the most effective use of available space. Designers and application engineers will balance features affecting load carrying capability relative to radial, axial and overturning moments, combining predicted bearing life, system stiffness, powerloss and heat generation, load zone maintenance, setting, lubrication, and handling and maintenance issues into an optimized solution. RETAINERS AND UNITIZATION There are several options in bearing designs for mainshaft bearings in regards to roller unitization. Bearing cages can have some performance benefits. Full complement designs (no cage or separators) have power density benefits, but need to be engineered with care due to roller body contact during operation and also can complicate assembly and setting procedures. Manufacturing of "L" style cages in sizes typical for mainshaft bearings in direct and hybrid drives may be accomplished through precision cut processes such as: Full machining Forming technology CNC controlled precision cutting A traditional closing in process may not be feasible in this size range. This can be overcome with a means of axial retention to hold the rollers in place after assembly. The inner race assembly can then be handled separately from the outer race without a need of unitization. Another option is a cut-andweld cage design that avoids the closing in process. As mentioned previously, use of a cage will lower the bearing rating when compared to

13 an identically sized full-complement design, but there could be other advantages related to better grease distribution including elimination of contact between roller bodies (rollers will contact cage which is made of softer material and generally will not wear roller surface) and roller guidance through unloaded zones. For full-complement designs, there are several considerations that must be taken into account during the design process, including: Maximum allowable speed is limited to prevent metal transfer from roller to roller/race. Engineered coatings on rollers will allow for increases in speed and will enhance bearing performance by altering the surface finish and improving the lambda ratios. The bearing life should be improved, particularly in low lambda conditions, by reducing adhesive metal transfer. Unitization will simplify bearing setting, installation and removal, and may help eliminate incidental damage to rollers during turbine assembly. The use of CRB/SRB designs in mainshaft configurations, especially hybrids which may have a very large outside diameter (OD) size, is related to roller size. Large rollers operating in a system with excessive clearance may be more prone to skidding/smearing damage compared to a preloaded TRB. SEALS Sealing is more critical in direct drive generator wind turbines than hybrid and other drivetrain designs. The seals need to control grease/oil leakage and also exclude contaminants from entering the bearing. Direct drive generators can be damaged if lubricants leak from the bearing seals into the generator. Seals are also critical in off-shore applications where exposure to salt water spray causes a harsh operating environment. 12 Contacting lip polymer seals are likely to control leakage better than non-contacting labyrinth seals, but care must be taken in designing the seal for ability to meet life expectations for wind turbines in the field. Non-contacting labyrinth seals, when designed and applied properly, should give more confidence in meeting long-life targets. Concerns that must be addressed for labyrinth seals are control of lubricant leakage and robustness to system deflections to avoid labyrinth element contact. A two-row TRB bearing supplied with a preset lateral setting, seals and lubrication takes complexity out of the turbine manufacturer s assembly process and allows the bearing manufacturer to maintain tight control of the characteristics that factor into final bearing assembly. CONCLUSION There is a strong drive in the industry to improve wind turbine reliability. Proper bearing design and application are key factors in helping to increase turbine uptime and reducing maintenance costs. Accurately defining system loading and environmental conditions and translating them for use into advanced analytical programs is a key first step to achieving improvements. For mainshaft designs in mid-speed hybrids or direct drive turbines, TRBs provide features that address concerns relating to bearing life/capacity, stress and roller load management, reduction of skidding and smearing, improving system stiffness and simplifying the turbine assembly process. The authors company has significant experience in advanced analysis to help achieve the desired improvements. Involving bearing suppliers in the design process can lead to better use of available package space for the bearings and allow for a more optimized turbine design. ACKNOWLEDGMENTS The authors would like to extend sincere appreciation to several individuals who

14 helped formulate the ideas discussed in this paper, including Timken associates Jim Charmley, Gerald Fox, Michael Kotzalas, Doug Lucas and David Novak. REFERENCES 1) Butterfield, S., McNiff, B., and Musial, W., Improving Wind Turbine Gearbox Reliability, European Wind Energy Conference, May ) Dinner, H., Trends in Wind Turbine Drive Trains, KISSsoft GmbH, Switzerland 3) Lucas, D., and Pontius, T., Designing Large Diameter Close-Coupled Two-Row Tapered Roller Bearings for Supporting Wind Turbine Rotor Loading, Hannover Fair, ) Bhatia, R., and Springer, T., Using Histograms in the Selection Process for Tapered Roller Bearings, International Off-Highway Meeting, Milwaukee, ) Ionescu, L., and Pontius, T., Mainshaft Support for Wind Turbine with Fixed and Floating Bearing Configuration: Tapered Double Inner Row Bearing vs. Spherical Roller Bearing on Fixed Position, ) Oyague, F. Gearbox Modeling and Load Simulation of a Baseline 750-kW Wind Turbine Using State-of-the-Art Simulation Codes, NREL/TP , Feb

15 Bearings Steel Precision Components Lubrication Seals Remanufacture and Repair Industrial Services Timken is the registered trademark of The Timken Company The Timken Company Printed in U.S.A.