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Technical Paper

BEARING SELECTION TECHNIQUES AS APPLIED TO MAINSHAFT DIRECT AND HYBRID DRIVES FOR WIND TURBINESMatthew B. Turi and Christopher S. Marks

1 THE TIMKEN COMPANY

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.

Fig 1a. Modular drive train configuration. Source: NREL/TP-500-41160

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

bearing arrangements available based on the

turbine drive train architecture.

Drive Type

Tapered Single

Tapered Double

Tapered Double +

Cylindrical

3 & 4 Point SRB

Hybrid

Direct

Modular

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 1b. Modular drive train configuration.

Preloaded TRBs allow for improved system

stiffness and are available with modified

MW

1 2.5 5 10

Modular Drive

Direct Drive

Hybrid Drive


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 multi-

megawatt 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


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

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 ultra-

low 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 long-

term 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 non-

adjustable (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


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.

-400

-200

0

200

400

600

800

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Time (s)

My

Mz

Fx

Fy

Fz

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:


1. RPM (due to effects on the development

of the lubrication film thickness).

2. Pitch Moment, My

3. Yaw Moment, Mz

4. Radial Load, Fz

5. Axial Load, Fx

6. Radial Load, Fy

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.

…

Fy

Fx

…

My,1My,2My,3My,4 My,1My,2My,3My,4

Fz,3Fz,2

My,1My,2My,3My,4

Fz,1

Mz,1 Mz,2 Mz,3

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

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):


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.

Unseated Bearing Load Zone vs. Setting

0

50

100

150

200

250

300

350

400

-0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000

Setting (mm)

Low Load

Medium Load

High Load

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

increased significantly to dramatically

increase the load zone above 110 degrees.

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.

Fig. 9. Life versus bearing setting.


Fig 10. Life versus bearing setting – two-row bearing system.

A previous technical paper compared two-

row 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.

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.

SRB row (upwind) SRB row (downwind) Fig 12. Load zone in SRB. 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.


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

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 tapered-

single 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. 13. Two TS wide spread mainshaft.

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


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.

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 0.0005 radians. It is greater

than 1 for lower levels of misalignment and

will reduce life when misalignment is greater

than 0.0005 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. L10).

The required calculated L10 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 L10 will

increase. Also shown in Table 1 are the

required L10 for a 30-year design. Another

way to state this would be that the reliability

factor, a1, is multiplied by the L10 to attain the

Ln life of 175,000 or 263,000 hours for the 20-

or 30-year calculated life, respectively.


Life Reliabilit

y a1 20-year

L10 Life

30-year

L10 Life

L10 90 1 175,000 263,000

L5 95 0.64 274,000 411,000

L3 97 0.47 376,000 564,000

L2 98 0.37 478,000 717,000

L1 99 0.25 706,000 1,060,00

0

Table 2. L10 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.


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)

Included roller angle (F)

Mean roller diameter [(LED+SED)/2]

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-and-

weld cage design that avoids the closing in

process.

As mentioned previously, use of a cage will

lower the bearing rating when compared to


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.

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


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 2007

2) 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, 2003

4) Bhatia, R., and Springer, T., Using Histograms

in the Selection Process for Tapered Roller

Bearings, International Off-Highway Meeting,

Milwaukee, 1981

5) 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, 2005

6) Oyague, F. Gearbox Modeling and Load

Simulation of a Baseline 750-kW Wind Turbine

Using State-of-the-Art Simulation Codes,

NREL/TP-500-41160, Feb. 2009

Bearings • Steel • Precision Components • Lubrication •Seals • Remanufacture and Repair • Industrial Services

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Timken® is the registered trademark of The Timken Company.

© 2009 The Timken CompanyPrinted in U.S.A.

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