bearing for wind turbine

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NTN TECHNICAL REVIEW No.71（2004）

[ New Product ]

Bearings for Wind Turbine

1. Introduction

Worldwide electricity production reached about31,000MW by the end of 2002. This is a 27％ increaseover the previous year. In the past few years windpower generation, which emits no carbon dioxide, hasgained widespread acceptance as the cleanest andmost environmentally friendly form of energy.Technical issues remaining in the development ofwind power generation include increasing reliability ofthe system while reducing the cost of operation.Bearings are one of the most important components ofwind turbines and require designs that optimizereliability and economic efficiency while taking intoaccount the characteristics of the applications. Thisreport introduces a method to optimize wind turbinebearing design and the features of bearingsdeveloped for wind turbines.

2. Structure of Wind Turbines andBearings

Fig. 1 shows the nacelle of 1 to 2MW wind turbines.Bearings are used in various places of the nacelle:rotor shaft, gearbox (step-up gear), generator, yawgearbox (reduction), yaw slewing table, blade pitchrevolving seat and hydraulic pump.

3. Bearing Operating Conditions

The rotor shaft bearing supports the blades androtor and transmits torque to the gearbox. The bearingloads and rotating speeds vary considerably due toconstantly changing winds.

At wind speeds below the cut-in wind speed (i.e. theminimum wind speed required for power generation),the rotor shaft will idle resulting in low-speed, low-loadoperation. At wind speeds above the cut-in speed, the

*Industrial Sales Headquarters

In 2002, worldwide electricity production was about 31,000MW. This is a 27％ increase over the previous year.

In the past few years, the wind turbine generating system, which emits no carbon dioxide, has gained widespread

acceptance as the cleanest and most environmentally friendly energy. The technical trend for wind turbines is to

increase reliability and efficiency while reducing the cost of operation. The bearings, which are one of the most

important components for wind turbines, require designs that optimize reliability and economic efficiency while

considering the characteristics of this applications.

This report introduces special characteristics for wind turbine bearings and a method to optimize wind turbine

bearing design.

Souichi YAGI*


rotating speed increases above the rated speed,resulting in average loads. In the case of wind gusts,the blades and rotor will exert large loads on the rotorshaft bearing. Fig. 2 shows the load and momentexerted on the rotor shaft bearing.

Such changes in the load, moment and rotatingspeed also affect the gearbox bearing. One of thefeatures of wind turbine bearings is that they operatein a wide range of loads from light to heavy load(when exposed to gusts). Fig. 3 shows examples ofmeasured rotor load and moment over one minutewhen a 700-kW wind turbine is operated at windspeeds of 23 to 24m/s.

Rotor shaft bearings repeat start, acceleration,deceleration and stop operations irregularly as theyare exposed to fluctuation of load. Therefore, theoptimal specifications for various parameters,including bearing type, clearance, number of bearingrollers, crowning and cage must be examined for eachcondition (minimum load, average load, maximumload).

Fig. 1 Nacelle

Fig. 2 Rotor load schematic

Blade

Generator

Rotor shaft Gearbox (step-up gear)

Yaw gearbox (reduction gear)

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Fig. 3 Measurements of rotor load and moment

F：LoadM：Moment

MZN

FZN

FYN

MYN

MXN

FXNZN

YNXN

FXN

MXN

MYN

MZN

FYN

FZN

140

250

280

220

160

-250

130

100

0 2010 30 50 60（秒） 40

0 2010 30 50 60（秒） 40

0

300

200

-200

-300

0

0

100

60

（kN）

（kNm）


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When designing the bearing, first calculate thebearing life for the maximum required strength of thehousing and the average deformation of the housing.Then design a slim housing and choose a shaftbearing that meets the required calculated life.

With deformation of the housing and outer ringraceway taken into account, calculate the load oneach rolling element to obtain the life of the rotatingand stationary rings.

Table 1 Wind turbine rotor shaft bearing assembly(with gearbox)

4. Rotor Shaft Bearings

Table 1 shows the structures of the shafts that usea gearbox to increase blade speed to the rated speedof the induction generator. Bearings suitable for eachrotor shaft type are also shown. Table 2 shows thestructures of the shaft of synchronous generators notequipped with a gearbox.

Structure FeaturesBlade-sidebearing

Generator-sidebearing

SRBSRBSRB

SRBCRBDTRB

SRB CRB

SRB

TRRB

DTRB

CRB

―

¡Two bearings are used.

¡The gearbox is supported on the rotor shaft.

¡The generator-side bearing is also used as the gearbox's input bearing.

¡The generator-side bearing is also used as the gearbox's input bearing.¡The load on the blade-side bearing is supported by the nacelle.

¡No rotor bearing is used and the rotor load is borne by the gearbox bearing.

SRB : Spherical roller bearing CRB : Cylindrical roller bearingDTRB : Double-row tapered roller bearing TRRB : Triple-row cylindrical roller bearing

Table 2 Wind turbine rotor shaft bearing assembly (without gearbox)

Fig. 4 Spherical roller bearing, B type and C type

Structure FeaturesBlade-sidebearing

Generator-sidebearing

TRRBDTRB

CRB

SRBDTRB

CRBCRB

¡Direct drive

¡Outer ring rotation

¡The load on the blade-side bearing is supported by the nacelle.

¡Inner ring rotation

QR＝（QRJ） J＝1

Z

Z

1Σ wi

1/ wi

QS＝（QSJ） J＝1

Z

Z

1Σ we

1/ we

QR, Qs ：Average load on rotating andstationary rings

Z ：Number of rolling elementswi, we ：ConstantLR＝（Cn／QR）p：Life of rotating ring LS＝（Cn／Qs）p ：Life of stationary ring L＝（LR

-e＋LS-e）-1/e：Life of bearing

Cn ：Dynamic rated load on contact pointp ：For ball bearing 3

For roller bearing 10/3e ：For ball bearing 10/9

For roller bearing 9/8

Spherical roller and tapered roller bearings aremainly used. However, spherical roller bearings, thatfeature low misalignment rates, are widely used.Normally, misalignment of ±0.5˚ needs to be takeninto account.

With B type spherical roller bearings, the rollers areguided by the inner ring's center rib. This enablesoperation with stable torque, low skew and low heatgeneration in a wide load range from the minimum tothe maximum load. Fig. 4 shows the structure ofNTN's spherical roller bearings (both B and C types).

B type C type

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Since rotor shaft bearings are exposed to vibrationof blades and gearbox, fretting corrosion may occur.Therefore, selection of appropriate bearings andgrease, as well as optimization of clearance and fittingare important factors.

Photo 1 shows an external view of a bearing at theend of vibration test.

Photo 1 Vibration test results

Fig. 5 Bearing outer ring and Pillow Block deformation

(1) Fretting on inner ring bore

(2) Fretting on inner ring raceway

《Vibration test》Bearing：24126CL1Vibration acceleration：9GVibration cycles：10 million cyclesMaximum surface pressure：1080MPa

Fig. 5 shows an example of the deformation of arotor shaft bearing outer ring and housing. Thebearing and housing are optimized by calculating thebearing life, including housing deformation andbearing clearance and also confirming the housingstrength based on a stress analysis. In this example, abearing for 1.5-MW wind turbines was used, and themaximum deformation in the axial direction was0.07mm. The difference in the bearing life calculatedwith this deformation and bearing clearance taken intoaccount and the one calculated with the housing andouter ring considered to be non-deformed was within5%, which is not problematic in actual use. If thisdifference is excessively large and the life is short, thedesign of the housing needs to be changed to improverigidity.

5. Planet Bearings for Gearbox

A gearbox consists of an input shaft, planet gear,low-speed shaft, intermediate shaft and high-speedshaft. Fig. 6 shows the structure of a gearbox, andPhoto 2 shows an external view of a full complementcylindrical roller bearing, that is used for input and low-speed shafts.

Fig. 7 shows an example of planet gearmechanism.


Spherical roller bearings or full complementcylindrical roller bearings are used as planet bearings.An analysis model of the planet bearing is used tocalculate the life of bearing with deformation of theouter ring taken into account. When calculating, makesure that the load on the bearing is considered to bethe one exerted on rolling elements and the gearengagement point between the ring gear and sun gearis fixed.

Fig. 8 shows an example of deflection analysisresults of a planet bearing used for 1.5-MW gearbox.The planet bearing used for analysis consists of twodouble-row cylindrical roller bearings with four rows ofrolling elements, and the maximum deflection was0.21mm.

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Planet gear

Carrier

Ring gear

Sun gear(Low-speed shaft)

Fig. 7 Planetary gear model

High-speedshaft

Intermediateshaft

Low-speedshaft

Planet gear

Planet bearingCarrier

Input shaft

Support bearing

Fig. 6 Gearbox for wind turbine

Photo 2 NTN Full complement cylindrical roller bearing

Fig. 8 Deflection of planet bearing

Fig. 9 Arrangement of bearings

Table 3 Life ratio of each row for planet bearing

The life, calculated from the analysis results withelastic deformation of the rings taken into account,varies by a maximum of 58% among rows. Thecarrier-side rows have higher load ratio than theothers, resulting in shorter calculated life. Fig. 9illustrates the arrangement of bearings, and Table 3shows the calculated results.

1 2 3 4

Generator sideCarrier side

Row No. Life ratio

1

2

3

4

85

143

133

100Inner diameter φ220mmDouble-row cylindrical roller bearing

The maximum contact surface pressure (Pmax) forgearbox bearings is calculated based on point contactfor spherical roller bearings, and line contact forcylindrical roller bearings. In most cases misalignmentis taken into account when specifying the maximumcontact surface pressure.

Pmax＝Krc Km Pline (Cylindrical roller bearing)Krc ：Crowning correction factorKm ：Misalignment factorPline：Maximum line contact surface pressure

In the case of spherical roller bearings, if themaximum contact surface pressure exceeds the limit,the bearing size needs to increase. This will increasethe calculated life and reduce the maximum contactsurface pressure. However, in the case of light loadsthe rolling elements may not roll properly on theraceway and will begin to slide. This sliding maycause damage to the raceway. Because of this, NTNrecommends the minimum load be at least 4% of thebasic static load rating.

Concerning lubrication, some measures need to be

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Fig. 10 NTN outer ring rotating test machine

implemented to prevent a situation in which the planetbearing is exposed to insufficient lubrication when itbegins to move.

The problems concerning the planet bearings canbe summarized into the following three points.

¡Influences by plastic deformation of gears andbearing

¡Influences by misalignment of planet bearingcaused by twisting of carrier

¡Influences by dry start up (insufficient lubricatingoil)

The specifications for the optimal bearing must betake into account these properties. NTN has beenworking on the design of ribs that have high loadcapacity and that provide sufficient resistance againstsliding and scuffing under light loads, as well optimalaxial clearance. In addition, NTN has also beenworking to prolong the life of bearings by employingspecial heat treatment.

Furthermore, NTN has introduced the special testmachines shown in Figs. 10 and 11, to promote thedevelopment of next generation planet bearings.

2150 1450

1990

Test bearing


6. Insulated Bearings for Generator

To improve the reliability of bearings used ingenerators, it is necessary to prevent sparks (galvaniccorrosion) caused by electric current passing throughthe bearings. NTN has produced a new single-layerbearing having sufficient insulation capability andreliability by adopting special ceramics and improvingthe spray-coating method.

This bearing provides insulation resistance of 100MΩor higher and dielectric breakdown voltage of 2kV orhigher, meeting the insulation performance requiredfor wind turbines.

Photo. 3 shows an external view of this insulatedbearing.

For details, refer to "Insulated Bearing "MEGAOHM"Series" in this book.

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Photo 3 Insulated Bearing Fig. 12 Special design angular contact ball bearings

7. Bearings for Yaw Gearbox

Since yaw gearboxes need to be small and capableof conveying large torque, the bearings to be used forthem must be compact and have high load capacity.Because of this, angular contact ball bearings with thininner/outer rings and tapered roller bearings are oftenused.

With angular contact ball bearings, which areexposed to large axial load, the resistance to axialload has been improved by increasing the groovedepth on the inner and outer rings. Fig. 12 shows thecross-sectional view of the standard bearing and thatof special design bearings for yaw gearboxes. Thespecial design bearing has a high axial loadresistance, approximately 9 times higher than thestandard bearing.

Standard bearing Special design bearingsfor yaw gearbox

Fig. 11 NTN inner ring rotating test machine

1400 1300

1850

Test bearing

8. Conclusion

Compared with Europe and North America, wherethe use of wind turbines is widespread, Japan oftensuffers from considerable atmospheric turbulence,severe tropical storms and lightening (winter). Thus,wind turbines that are reliable and suit Japan's climaticconditions are desired. NTN has been working onimproving the reliability of bearings designed for windturbines. By choosing optimal bearing specificationsthat satisfy wind turbine manufacturers and users andsupplying high-quality products, NTN will contribute tothe development of an eco-friendly wind powergeneration.

References1) Germanischer Lloyd

Regulations for the Certification of Wind EnergyConversion Systems

2) B.SchlechtModerne Simulationstechniken zur dynamischenAuslegung von Triebstraengen in Multi- Megawatt-Windenergieanlagen

3) B.Schlecht et al"MULTIBODY-SYSTEM-SIMULATION OF DRIVETRAINS OF WIND TURBINES"

4) B.Niederstucke et alLOAD DATA ANALYSIS FOR WIND TURBINEGEARBOXES

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Souichi YAGI

Industrial Sales Headquarters

Photo of the author

Fig. 13 Comparison of life of ETA tapered roller bearingand standard bearing (with contamination)

Table 4 Test conditions (30206, ETA30206)

For tapered roller bearings, carbonitrided ETAbearings are used to provide longer life.

ETA bearings are long-life bearings with highthermal stability gained by optimizing distribution ofretained austenite and carbide present on the surfacethrough special heat treatment. They feature highresistance to contaminants contained in lubricating oiland high peeling resistance. Table 4 shows the lifetest conditions and Fig. 13 shows the results.

Such implementation has enabled NTN to providecompact, highly reliable bearings.

Normallubricating oil

Contaminatedlubricating oil (Reference)

Radial load (kN)

Rotating speed (min-1)

Lubricating oil

17.64

2000

Turbine oil 56Turbine oil 56

+NTN's standard contaminants

99

80

50

20

10

100 101 102 103

5

1

Cum

ulat

ive

perc

ent f

aile

d (%

) 標準軸受

ETA軸受

Life (h)

Standard bearingETA bearing


bearing for wind turbine

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