24 O C T O B E R 2 0 0 5 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

he standard industrial gear sectorsinto which transmission and bearing

OEMs sell their equipment include the min-ing, cement, food and beverage, automotive,chemical, woodworking and material han-dling industries. In the late 1990s, there werefew transmission OEMs and no bearingOEMs with specifications in place for lubri-cants used in these industrial gear sectors.

One of the first European industrial gearlubricant specifications was published in1996 by Flender AG in Germany. This speci-fication has the notation QMA.BO and liststhe performance requirements for lubri-cants used in standard industrial transmis-sions with helical gears, bevel helical gears,planetary gear units or worm gears units or,alternatively, using gear motors. A summaryof the testing required in the current versionof this specification, QMA.BO Revision 6, isshown in Table 1. Once approved the lubri-cant is listed in the Flender recommendedlubricants list BA7300.

After the OPEC oil embargo in the 1970sand the resulting rise in the price of oil,there was a large investment in Europe inthe early 1980s into use of wind turbines asrenewable energy sources. Wind turbines ingeneral generate between 0.6 and 5megawatts (MW) of electricity depending on

By the AftonChemical Industrial Team

Practical Applications

Raising the bar for wind turbine lubrication

New lubricant specification requirements forindustrial gear applications are among the key

challenges facing transmission and bearing OEMs.

T Table 1. Testing required in the QMA.BORevision 6 Flender specification.• DIN 51517-3 issue January 2004• Compatibility with internal coating

P22-8050• Compatibility with liquid sealing

material Loctite 128068• Compatibility with elastomer seals

material NBR and FKM, static anddynamic

• Flender foam test• FVA 54/7 micropitting test• FZG scuffing test A/16.6/90

(by agreement)

Initial mechanical fail-ures for wind turbinesin the field were mainlybearing-related. For thelarge wind turbines,weight is an issue andcan lead to distortionand bearing failuresunder peak loads.

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T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y O C T O B E R 2 0 0 5 25

their size. With a realized industry growthrate of 35% per annum, the global installedcapacity to the end of 2003 was 40,301 MW,with 29,201 MW installed in Europe and theremainder shared between the U.S., India,Japan and China. If we assume that allinstalled wind turbines are 2.5 MW units,this would correspond to 16,120 wind tur-bines installed globally.

Fewer than 15% of these wind turbinesare based on direct drive technology, whichwould leave 13,702 2.5MW wind turbinesusing transmissions. At an average cost of250,000 Euros per transmission for a 2.5MWwind turbine, this corresponds to a 3425.5million Euro industry for industrial trans-mission OEMs today with a predictedinstalled capacity to the end of 2008 of over95,000 MW. This is a new lucrative market fortransmission OEMs. In 2001, the formerwind turbine division of Flender was spun offas a separate company under the name Win-ergy. Other European OEMs that supplytransmissions into standard industrial gearand wind turbine applications are BoschRexroth Getriebetechnik, Eickhoff, HansenTransmissions, Jahnel Kestermann, MetsoDrives, MAAG, Renck, Pujol and Zollern-Dorstner. An indication of the market posi-tion, and also the split in their businessbetween wind turbine and standard industri-al gear applications, is shown in Table 2.

Wind turbines have large blades from 22meters (72 feet) up to 90 meters (295 feet)long. This corresponds to a rotor diameter of44 meters (144 feet) up to 180 meters (590feet). These blades, rotating at low speeds,are connected through the gearing system toa high-speed output generator. This resultsin a torque through the system of up to 4million Nm. This extreme torque puts a largeamount of stress on the gears and bearingsin the wind turbine and on the lubricantwhich is used to lubricate both the gears andbearings.

Initial mechanical failures for wind tur-bines in the field were mainly bearing-relat-ed. For the large wind turbines, weight is anissue and can lead to distortion and bearingfailures under peak loads. In this applicationthe true load on the bearings is unknown, sothat the standard calculations for bearinglifetime do not apply. Field failures due to

CONTINUED ON PAGE 26

Table 2. OEM market position, split between sales of transmissions forwind turbine and industrial gear applications.

Split in business (% Turnover) Market

Standard position w.r.t

Wind Industrial Wind turbineturbine Gear gears sales

Bosch Rexroth Getriebetechnik 50 50 4Eickhoff 70 30 3Hansen Transmissions >50 <50 2Jahnel Kestermann 50 50 6Metso Drives Not available Not available 5Winergy 100 0 1

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(Data courtesy of Cornelia Haag, SKF)

Figure 1. Reasons for bearing failure.

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chemical attack of the gear oils on the bear-ing cage also occurred.

The first and second largest bearing man-ufacturers worldwide are SKF and FAG/INA,respectively. FAG and SKF published specifi-cations for gear oils used in wind turbinegearboxes in January 2003 and February2004, respectively.

These specifications, summarized inTables 3 and 4, contain dynamic bearingtests designed to evaluate the ability ofindustrial gear oils to prevent bearing wearand fatigue under different lubricationregimes (FAG Step I to III tests).

The FAG specification also includes adynamic bearing test to evaluate theresidue-forming tendencies of gear oilswhen low levels of water are present (FAGStep IV).

The SKF specification contains a dynamicbearing corrosion test which is run on thegear oil mixed with either standard water orwith saltwater. The testing with saltwater isto mimic the problems seen with saltwatercorrosion of bearings in off-shore windfarms. Most of the transmission OEMs have

CONTINUED FROM PAGE 25

Table 3. Requirements for gear oils in wind turbine gearboxes.• Oil operating time up to 5 years• Filterability• No sludge formation• Wear resistance• Cleanliness during operation• Corrosion resistance• Oxidation stability• Decrease/avoid air bubbles• Start-up temperature -20C/-40C• Temperature resistance up to 120C

• Biodegradability• High VI • Avoid slippage• Wide operating temperature• Scuffing load and micro pitting

carrying capacity• Wide speed range• WGK 1• High circulation with low

volumes

Table 4. Summary of the bearing tests in the FAG specification.

Test Description Rig Duration

Step I Wear behavior at boundary lubrication FAG FE-8 80 hrs

Step II Fatigue behavior under mixed friction conditions FAG FE-8 800 hrs

Step III Fatigue behavior under EHL conditions FAG L-11 >= 700 hrs

Step IV Fatigue behavior and residue forming with water added FAG FE-8 >= 600 hrs

(Data courtesy of Cornelia Haag, SKF)

Figure 2. Corrosive attack on bearings in FAG Step III Test.

(Photos courtesy of Richard Karbacher, Schaeffler Group)

Test Description Rig Duration

SKF roller test Chemical and thermal stability at 100 C SKF in-house 8 weeks Copper corrosion DIN EN ISO 2160 at 120 C Laboratory test 3 and 24 hrsFAG FE-8 DIN 51819 FAG FE-8 80 hrsSKF Emcor Test acc. ISO 11007 SKF EmcorSKF Filterability Test SKF in-house < 15 minutes

Additional / Optional tests

For gear boxes without coolingSKF roller test Chemical and thermal stability at 120 C SKF in-house 8 weeks

For Off-shore applicationsSKF Emcor Test 0.5% NaCl acc. ISO 11007 SKF Emcor

Table 5. Summary of the bearing tests in the SKF specification.

26 O C T O B E R 2 0 0 5 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

23-29 tlt prac. apps 10-05 9/26/05 4:55 PM Page 26

now included these specifications from thebearing OEMs in their specifications.

There also have been wind turbine fieldproblems due to excessive micropitting on thegear teeth. All the latest OEM specificationsrequire that the FZG micropitting testing,according to FVA 54/7, is run not just at thestandard test temperature of 90 C but also at60 C, as this is approximately the temperatureof the gearbox reservoir in wind turbines withcooling systems.

Another reported problem in the field isblocking of the gearbox filters. There are manylubricant-related sources of deposits that canblock filters. For example, sludge is createddue to thermal or oxidative decomposition ofthe lubricant, precipitates formed due to thepoor solubility of the fresh additive systemover time or poor solubility of the additive sys-tem after use at elevated temperatures in thepresence of water and/or contaminants. OEMslike Winergy are now looking to the suppliersof the filters such as Hydac and to the oil andadditive companies to work together to identi-fy a filtration test that can predict poor per-formance in the field.

Regardless of the reason for the precipitateformation, there is a common result (apartfrom filter blocking and additive depletion),namely the reduction in the ability of the oil toprevent foaming and separate away fromwater. The antifoam additives used to give pro-tection against foaming and the demulsifiersused to allow the oil to separate out water arevery surface active. If any precipitate falls outof the oil, these antifoam and demulsifiersalso will fall out with the precipitate. As a firststep, Winergy has included in their specifica-tion a variation on the dynamic Flender foamtest developed by Hydrac.

The oil is evaluated for foaming at varioustemperatures between 0 C and 60 C, but theoil is also filtered through a 5-micron filter andthe testing is rerun. This ensures that theantifoam system is not depleted on use andafter filtering.

The common tests in the specificationspublished in the last year from the transmis-sion OEMs, who supply into the wind turbinesector listed in Table 2, are shown in Table 6(see page 28).

These new specifications also are beingused by the OEMs to approve lubricants for

Figure 3. End of test parts from FAG Step IV Test.

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y O C T O B E R 2 0 0 5 27

(Photos courtesy of Richard Karbacher, Schaeffler Group)

(Data courtesy of Dr. Armin Schmidt, Hydac).

CONTINUED ON PAGE 28

Figure 4. Filterability problems caused by oil aging products

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28 O C T O B E R 2 0 0 5 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

use in standard industrial gear applications. This has significantly raised thebar on lubricant requirements for standard industrial gear applications andalso raised the cost of developing new technology meeting OEM require-ments. The key challenge in the new specifications is combining good EPproperties (to eliminate surface fatigue on the gear teeth) with excellent cor-rosion and bearings protection. <<

For more information or if you have questions about this article, contact Ian Macpherson,marketing manager-industrial additives for Afton Chemical Corp. in Richmond, Va., at(804) 788-5800.

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Figure 5. Foam formation on the oilsump of the gearbox.

Figure 6. Hydac In-House Tests Filter-ability Multi-Pass Filterability TestsAssessment with Flender Foam Tester.

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Table 6. Common requirements in the transmission OEMs new specifications

Test DescriptionDIN 51517 Part IIIFVA 54/7 Micropitting test at 90 C and 60 C => 10 ISO 220VGFZG Scuffing test A/8 3/90 and double speed A/16.6/90Freudenberg seals static and dynamicNBR (Nitrile) and FKM (Flouroelastomer)Foam test acc. ASTM 892 or flender foam testCompatibility with all coating and liquid sealing materialsMeets FAC Specification for Wind turbine Applications*Meets SKF Specification for Wind turbine Applications*

*Required by 60% of the OEMs

CONTINUED FROM PAGE 27

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