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Lube-Tech No.86 page 1

Part 2

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7. Special Tests for Wind TurbineGear OilsFAG Wind Turbine Four-Stage Test(Schaeffler-FAG)The FAG FE8 four-stage test was speciallydeveloped for wind turbine gear oils. Inthe past (about 10 – 15 years ago), gearoils contained highly-active phosphorous-sulphur compounds. In the FE8 test rig,these generated roller wear rates of 200-300 mg. These days, industrial gear oilsare formulated with mild phosphorous-sulphur compounds to meet the rollerwear specifications of less than 30 mg(Figure 8).

The FAG FE8 four-stage test attempts toreplicate different load and mixed frictionconditions at different speeds, temper-atures and test parameters.

Stage 1 can be described as a short-term test and is performed on the FE8test rig according to DIN 51 819, Parts 1to 3 at 80 KN axial load, 80°C for aduration of 80 hours.

Stage 2 describes a fatigue test withmoderate mixed friction and is performedon the FE8 test rig at 75 rpm, 100 KN axialload, 70°C for a duration of 800 hours.

Stage 3 is a so-called fatigue test underEHL conditions (10 bearings). The test isperformed in the FAG Test Rig L11 at9000 rpm, an axial load of 8.5 KN, about80°C and for a duration of 700 hours.

Stage 4 involves a deposit test at highertemperatures in the presence of water.This modified PM paper-making machineoil test from FAG is performed on aspecial FAG test rig at 750 rpm, an axialload of 60 KN, at up to 140°C for aduration of 600 hours.

Fig. 8: FE8 Roller Bearing Wear Test

Lubricants for Wind TurbinesGear Oils-Demands and CharacteristicsFuchs Europe Schmierstoffe GmbH, Wolfgang Bock, Henrik Heinemann

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A wind turbine gear oil must pass allthese different test procedures with goodresults. Figure 9 shows a summary of theFAG wind turbine four-stage test (withan example of results of a tested windturbine gear oil).

SKF Specifications for Wind TurbineGear OilsIn its WTGU specification for wind turbinegear oils, the company SKF placed anemphasis on the high chemical andthermal stability of the lubricant. To

summarise, it can be noted that SKFfocuses on chemical stability (SKF RollerTest, SKF Oil Film Ageing Test) along withFE8 performance (wear protection),filtration and corrosion protection (Figure 10).

Lube-Tech No.86 page 2PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE

Fig. 9: FAG Wind Turbine Four-Stage Test

Fig. 10: SKF Specifications for Wind Turbine Gear Oils – SKF Test Report on Fuchs Renolin Unisyn CLP 320

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Lube-Tech No.86 page 3PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE

8. Low-Speed Wear Behaviour ofIndustrial Gear OilsGearboxes in wind turbines usually haveslow and faster-running stages. The low-speed wear behaviour of industrial gearoils must be considered. This feature canbe examined using the DGMK 377method and/or the FZG method. At aperipheral speed of 0.05 to 0.57 m/s anda high Hertzian stress (load stage 12),this involves determining total wear pertest stage in mg. Figure 11 shows acomparison of industrial gear oils basedon mineral oils, polyalphaolefins, estersand polyglycols. The high Viscosity Indexof synthetic oils and the correspondingly

thicker lubricating film during the testcan significantly lower the low-speedwear when compared to mineral oils.

9. Low-Temperature Viscosity ofIndustrial Gear OilsIndustrial gear oils in wind turbines haveto perform under a number of differentconditions and temperatures. Thequestion of which maximum viscosity ispermissible is still vigorously discussed invarious specialist journals. Following theautomotive sector, a low temperaturespecification threshold of 150,000 mPa*shas been accepted in some circles.However, the behaviour of gear oil

formulations with regard to the pourpointshould also be considered (Figure 12).

As far as the low-temperature viscosity ofmineral oils is concerned, it should beremembered that very large viscositydeviations (measured values compared tocalculated values) can occur near to thepourpoint. In principle, the given viscosityvalues for temperatures under 0°C shouldbe those actually measured.In the case offully-synthetic, polyalphaolefin-based gearoils, a relatively good correlation existsbetween the calculated and themeasured viscosity values even fortemperatures of -10°C, -20°C and -30°C.

Fig. 12: Low TemperatureViscosity – Threshold ValuesAccording to AGMA 9005-E02

Fig. 11: Slow-Running Wear Behaviour of Industrial Gear Oils

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Figure 13 shows the low temperatureviscosity (dynamic viscosity in mPa*s) offully-synthetic, polyalphaolefin-based ISOVG 220 and ISO VG 320 gear oils takinginto account the permissible (andpresently controversial) threshold viscosityof 150,000 mPa*s.

10. ConclusionThis article discusses the specificationsof wind turbine gear oils from alubricant manufacturer’s andcomponent manufacturer’s point ofview.

The highest demands are made on thetechnical performance of such gear oilswith regard to mechanical-dynamicwear protection, chemical stability andlong-term stability. Comprehensivemanufacturer, DIN and ISO tests mustbe passed, and compliance with rollerbearing and gearbox manufacturers’specifications must be given before awind turbine gear oil is approved. Suchtesting activity generates high testingand laboratory costs.

Apart from laboratory trials inmechanical-dynamic test rigs, qualifyingoils must pass coç field trials in variousambient conditions. Wind turbine gearoils are mostly based on fully-syntheticbase fluids with polyalphaolefinspredominating. Wind turbine gear oils

are specialties with the highest qualitydemands. Specific testing and specifi-cation criteria are currently beingcritically discussed, especially withregard to elastomer and chemicalcompatibility.

References• DIN 51 819: Mechanisch-dynamische

Prüfung auf demWälzlagerschmierstoff-Prüfgerät FE8- Teil 3: Verfahren für Schmieröl

• DGMK 377, FZG Slow Speed WearTest FZG C/0,05/90:120/12, 1997Hamburg, Germany

• Bartels / Bock: Reduzierung derReibung und Absenkung derÖltemperatur durch gezielteAuswahl von Getriebeölen, ATKAachen, 2001

• Bock: Leistung durch Synthese –Hochleistungsgetriebeöle: dasZusammenspiel von Mechanik undChemie, ATK Aachen, 2007

• Bock / Heinemann / Braun:Synthetische Industriegetriebeöle inder Antriebstechnik –Energiesparpotentiale, verbesserterVerschleißschutz - neuesteErgebnisse aus der Praxis und demPrüfstand, ATK Aachen, 2009

• Theo Mang, Wilfried Dresel (Hrsg.):Lubricants and Lubrications, Wiley-VCH Verlag, Weinheim, Germany,2001

LINKwww.fuchs-oil.de

Fig. 13: Low Temperature Viscosities of Industrial Gear Oils based on Polyalphaolefins

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