P040 NTN TR71 En

User Manual: P040

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NTN TECHNICAL REVIEW No.71（2004）

[ New Product ]

Bearings for Wind Turbine

1. Introduction

Worldwide electricity production reached about

31,000MW by the end of 2002. This is a 27％increase

over the previous year. In the past few years wind

power generation, which emits no carbon dioxide, has

gained widespread acceptance as the cleanest and

most environmentally friendly form of energy.

Technical issues remaining in the development of

wind power generation include increasing reliability of

the system while reducing the cost of operation.

Bearings are one of the most important components of

wind turbines and require designs that optimize

reliability and economic efficiency while taking into

account the characteristics of the applications. This

report introduces a method to optimize wind turbine

bearing design and the features of bearings

developed for wind turbines.

2. Structure of Wind Turbines and

Bearings

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, yaw

gearbox (reduction), yaw slewing table, blade pitch

revolving seat and hydraulic pump.

3. Bearing Operating Conditions

The rotor shaft bearing supports the blades and

rotor and transmits torque to the gearbox. The bearing

loads and rotating speeds vary considerably due to

constantly changing winds.

At wind speeds below the cut-in wind speed (i.e. the

minimum wind speed required for power generation),

the rotor shaft will idle resulting in low-speed, low-load

operation. 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*

Bearings for Wind Turbine

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 rotor

shaft bearing. Fig. 2 shows the load and moment

exerted on the rotor shaft bearing.

Such changes in the load, moment and rotating

speed also affect the gearbox bearing. One of the

features of wind turbine bearings is that they operate

in a wide range of loads from light to heavy load

(when exposed to gusts). Fig. 3 shows examples of

measured rotor load and moment over one minute

when a 700-kW wind turbine is operated at wind

speeds of 23 to 24m/s.

Rotor shaft bearings repeat start, acceleration,

deceleration and stop operations irregularly as they

are exposed to fluctuation of load. Therefore, the

optimal specifications for various parameters,

including bearing type, clearance, number of bearing

rollers, crowning and cage must be examined for each

condition (minimum load, average load, maximum

load).

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：Load

M：Moment

MZN

FZN

FYN

MYN

MXN

FXN

YN XN

FXN

MXN

MYN

MZN

FYN

FZN

140

250

280

220

160

-250

130

100

02010 30 50 60（秒） 40

300

200

-200

-300

100

（kN）（kNm）

NTN TECHNICAL REVIEW No.71（2004）

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When designing the bearing, first calculate the

bearing life for the maximum required strength of the

housing and the average deformation of the housing.

Then design a slim housing and choose a shaft

bearing that meets the required calculated life.

With deformation of the housing and outer ring

raceway taken into account, calculate the load on

each rolling element to obtain the life of the rotating

and 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 use

a gearbox to increase blade speed to the rated speed

of the induction generator. Bearings suitable for each

rotor shaft type are also shown. Table 2 shows the

structures of the shaft of synchronous generators not

equipped with a gearbox.

Structure Features

Blade-side

bearing

Generator-side

bearing

SRB

CRB

DTRB

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 bearing

DTRB :

Double-row tapered roller bearing

TRRB :

Tr iple-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 Features

Blade-side

bearing

Generator-side

bearing

TRRB

DTRB CRB

SRB

DTRB

CRB

¡Direct drive

¡Outer ring rotation

¡The load on the

blade-side bearing

is supported by the

nacelle.

¡Inner ring rotation

QR＝（QRJ）

J＝1

1Σ wi 1/ wi

QS＝（QSJ）

J＝1

1Σ we 1/ we

QR, Qs：Average load on rotating and

stationary rings

Z：Number of rolling elements

wi, we ：Constant

LR＝（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 point

p：For ball bearing 3

For roller bearing 10/3

e：For ball bearing 10/9

For roller bearing 9/8

Spherical roller and tapered roller bearings are

mainly used. However, spherical roller bearings, that

feature low misalignment rates, are widely used.

Normally, misalignment of ±0.5˚ needs to be taken

into account.

With B type spherical roller bearings, the rollers are

guided by the inner ring's center rib. This enables

operation with stable torque, low skew and low heat

generation in a wide load range from the minimum to

the maximum load. Fig. 4 shows the structure of

NTN's spherical roller bearings (both B and C types).

B type C type

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Since rotor shaft bearings are exposed to vibration

of blades and gearbox, fretting corrosion may occur.

Therefore, selection of appropriate bearings and

grease, as well as optimization of clearance and fitting

are important factors.

Photo 1 shows an external view of a bearing at the

end 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：24126CL1

Vibration acceleration：9G

Vibration cycles：10 million cycles

Maximum surface pressure：1080MPa

Fig. 5 shows an example of the deformation of a

rotor shaft bearing outer ring and housing. The

bearing and housing are optimized by calculating the

bearing life, including housing deformation and

bearing clearance and also confirming the housing

strength based on a stress analysis. In this example, a

bearing for 1.5-MW wind turbines was used, and the

maximum deformation in the axial direction was

0.07mm. The difference in the bearing life calculated

with this deformation and bearing clearance taken into

account and the one calculated with the housing and

outer ring considered to be non-deformed was within

5%, which is not problematic in actual use. If this

difference is excessively large and the life is short, the

design of the housing needs to be changed to improve

rigidity.

5. Planet Bearings for Gearbox

A gearbox consists of an input shaft, planet gear,

low-speed shaft, intermediate shaft and high-speed

shaft. Fig. 6 shows the structure of a gearbox, and

Photo 2 shows an external view of a full complement

cylindrical roller bearing, that is used for input and low-

speed shafts.

Fig. 7 shows an example of planet gear

mechanism.

Bearings for Wind Turbine

Spherical roller bearings or full complement

cylindrical roller bearings are used as planet bearings.

An analysis model of the planet bearing is used to

calculate the life of bearing with deformation of the

outer ring taken into account. When calculating, make

sure that the load on the bearing is considered to be

the one exerted on rolling elements and the gear

engagement point between the ring gear and sun gear

is fixed.

Fig. 8 shows an example of deflection analysis

results of a planet bearing used for 1.5-MW gearbox.

The planet bearing used for analysis consists of two

double-row cylindrical roller bearings with four rows of

rolling elements, and the maximum deflection was

0.21mm.

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NTN TECHNICAL REVIEW No.71（2004）

Planet gear

Carrier

Ring gear

Sun gear

(Low-speed shaft)

Fig. 7 Planetary gear model

High-speed

shaft

Intermediate

shaft

Low-speed

shaft

Planet gear

Planet bearing

Carrier

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 with

elastic deformation of the rings taken into account,

varies by a maximum of 58% among rows. The

carrier-side rows have higher load ratio than the

others, resulting in shorter calculated life. Fig. 9

illustrates the arrangement of bearings, and Table 3

shows the calculated results.

12 34

Generator side

Carrier side

Row No. Life ratio

143

133

100 Inner diameter φ220mm

Double-row cylindrical roller bearing

The maximum contact surface pressure (Pmax) for

gearbox bearings is calculated based on point contact

for spherical roller bearings, and line contact for

cylindrical roller bearings. In most cases misalignment

is taken into account when specifying the maximum

contact surface pressure.

Pmax＝KrcKmPline (Cylindrical roller bearing)

rc：Crowning correction factor

Km：Misalignment factor

Pline ：Maximum line contact surface pressure

In the case of spherical roller bearings, if the

maximum contact surface pressure exceeds the limit,

the bearing size needs to increase. This will increase

the calculated life and reduce the maximum contact

surface pressure. However, in the case of light loads

the rolling elements may not roll properly on the

raceway and will begin to slide. This sliding may

cause damage to the raceway. Because of this, NTN

recommends the minimum load be at least 4% of the

basic 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 planet

bearing is exposed to insufficient lubrication when it

begins to move.

The problems concerning the planet bearings can

be summarized into the following three points.

¡Influences by plastic deformation of gears and

bearing

¡Influences by misalignment of planet bearing

caused by twisting of carrier

¡Influences by dry start up (insufficient lubricating

oil)

The specifications for the optimal bearing must be

take into account these properties. NTN has been

working on the design of ribs that have high load

capacity and that provide sufficient resistance against

sliding and scuffing under light loads, as well optimal

axial clearance. In addition, NTN has also been

working to prolong the life of bearings by employing

special heat treatment.

Furthermore, NTN has introduced the special test

machines shown in Figs. 10 and 11, to promote the

development of next generation planet bearings.

2150 1450

1990

Test bearing

Bearings for Wind Turbine

6. Insulated Bearings for Generator

To improve the reliability of bearings used in

generators, it is necessary to prevent sparks (galvanic

corrosion) caused by electric current passing through

the bearings. NTN has produced a new single-layer

bearing having sufficient insulation capability and

reliability by adopting special ceramics and improving

the spray-coating method.

This bearing provides insulation resistance of 100MΩ

or higher and dielectric breakdown voltage of 2kV or

higher, meeting the insulation performance required

for wind turbines.

Photo. 3 shows an external view of this insulated

bearing.

For details, refer to "Insulated Bearing "MEGAOHM"

Series" in this book.

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NTN TECHNICAL REVIEW No.71（2004）

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 capable

of conveying large torque, the bearings to be used for

them must be compact and have high load capacity.

Because of this, angular contact ball bearings with thin

inner/outer rings and tapered roller bearings are often

used.

With angular contact ball bearings, which are

exposed to large axial load, the resistance to axial

load has been improved by increasing the groove

depth on the inner and outer rings. Fig. 12 shows the

cross-sectional view of the standard bearing and that

of special design bearings for yaw gearboxes. The

special design bearing has a high axial load

resistance, approximately 9 times higher than the

standard bearing.

Standard bearing Special design bearings

for yaw gearbox

Fig. 11 NTN inner ring rotating test machine

1400 1300

1850

Test bearing

8. Conclusion

Compared with Europe and North America, where

the use of wind turbines is widespread, Japan often

suffers from considerable atmospheric turbulence,

severe tropical storms and lightening (winter). Thus,

wind turbines that are reliable and suit Japan's climatic

conditions are desired. NTN has been working on

improving the reliability of bearings designed for wind

turbines. By choosing optimal bearing specifications

that satisfy wind turbine manufacturers and users and

supplying high-quality products, NTN will contribute to

the development of an eco-friendly wind power

generation.

References

1) Germanischer Lloyd

Regulations for the Certification of Wind Energy

Conversion Systems

2) B.Schlecht

Moderne Simulationstechniken zur dynamischen

Auslegung von Triebstraengen in Multi- Megawatt-

Windenergieanlagen

3) B.Schlecht et al

"MULTIBODY-SYSTEM-SIMULATION OF DRIVE

TRAINS OF WIND TURBINES"

4) B.Niederstucke et al

LOAD DATA ANALYSIS FOR WIND TURBINE

GEARBOXES

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Souichi YAGI

Industrial Sales Headquarters

Photo of the author

Fig. 13 Comparison of life of ETA tapered roller bearing

and standard bearing (with contamination)

Table 4 Test conditions (30206, ETA30206)

For tapered roller bearings, carbonitrided ETA

bearings are used to provide longer life.

ETA bearings are long-life bearings with high

thermal stability gained by optimizing distribution of

retained austenite and carbide present on the surface

through special heat treatment. They feature high

resistance to contaminants contained in lubricating oil

and high peeling resistance. Table 4 shows the life

test conditions and Fig. 13 shows the results.

Such implementation has enabled NTN to provide

compact, highly reliable bearings.

Normal

lubricating oil

Contaminated

lubricating oil (Reference)

Radial load (kN)

Rotating speed (min-1)

Lubricating oil

17.64

2000

Turbine oil 56 Turbine oil 56

NTN

's standard contaminants

100101102103

Cumulative percent failed (%)

標準軸受

ETA軸受

Life (h)

Standard bearing

ETA bearing

Bearings for Wind Turbine

P040 NTN TR71 En

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