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

1. IntroductionThis article about the general andfundamental requirements anddemands made on gear oils for windturbines starts with an overview ofthe industrial gear oils market andcovers the classification of industrialgear oils, their specifications and thecurrent standardisation activities.Also covered are the demands onwind turbine gear oils, especially withregard to mechanical-dynamic andchemical tests. Moreover, the lowtemperature aspects of industrialgear oils for wind turbines are alsodealt with.

As a rule, specially approved industrialgear oils are used for the main gears(ISO VG 320) – mostly multistage

planetary spur gears, and for theazimuth gears (ISO VG 220). Viscosityselection is based on requirements ofthe gear tooth meshing (slow on therotor side and fast on the generatorside) and the requirements of thecorresponding bearings. ISO VG 320gear oils thus represent a compromisebetween the specific ‘needs’ of theplanetary gears, the spur gears and theroller bearings. In addition, such gearoils have to satisfy special demandsregarding compatibility with seals,paints, filters, corrosion preventives,running-in oils etc. (Figure 1)

2. Industrial Gear Oils – The MarketIn 2011, industrial gear oil sales inGermany totalled 35,525 tonnes.Compared to the previous year (28,248

tonnes), this was an increase of about25%. Industrial gear oils are thus animportant part of the entire lubricantsmarket. In terms of volumes, gear oilsales have returned to 2007 / 2008levels. Of these volumes, mineral oil-based gear oils account for about 75-80% followed by fully-synthetic,PAO-based gear oils with a share ofabout 10-15%. The rest is made up byfully-synthetic gear oils based onpolyglycols (about 5%) and syntheticester oils and other special gear oilswhich make up less than 5%. There isa clear trend towards the use of fully-synthetic lubricants in gearboxes anddrive-train components. Only theselubricants can fulfill the extremetechnical demands of the machineryand thus the customers.

Fig. 1: Wind turbine gearboxes,Demands on the gear oils

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3. Classification of Industrial Gear OilsThe most important European standard forindustrial gear oils is the German DINstandard 51517 issued in 2011. There arealso other international standards such asISO 6743, Part 6 together with ISO 12925-1.The new AGMA Standard 9005/E02 isalso largely based on the DIN and ISOnorms. It mainly describes physicalthresholds as well as demands regardingwear protection of the tooth flanks,scuffing / scoring protection (FZG Test A/8,3/90 according to DIN-ISO 14635-1). Therequirements for roller bearings are basedon the FE8 roller bearing wear test(according to DIN 51 819-3). Finally, anew specification for wind turbine gearoils is ISO 61400/4 (Figure 2).

4. Working Temperature Range andLife of Industrial Gear OilsIn very many cases, operators want toknow details of the lubricant’s maximumworking temperature and expectedservice life. Such questions can only beanswered with generally-applicablestatements because numerous fringe andambient parameters influence industrialgear oils and especially their service life.In general, it is best to observe therecommendations and experiences of theleading gearbox manufacturers. Figure 3shows the Flender guidelines on workingtemperature range and service life at anmedian sump temperature of 80°C.Naturally, negative influences such aswater (both free and dissolved),

contaminants, assembly chemicals suchas e.g. Loctite products, dirt, abrasion,wear and acidic gases need to beconsidered when evaluating thecondition of an industrial gear oil orwhen calculating service life.

5. Base Oils for Industrial Gear OilsThe most important base oils for windturbine gear oils are fully-synthetic polyal-phaolefins (PAO). These fully-syntheticbase oils can be manufactured withdefined chemical structures by synthesis.To optimise elastomer compatibility andadditive solubility, PAO formulationsusually contain polar, e.g. ester oilcomponents (about 15-20% ester oil).These polar components keep additives,additive reaction products and ageing by-products in solution. Polyalphaolefinsdisplay very good low-temperaturebehaviour as well as excellent viscosity-temperature behaviour. Their highViscosity Index is generally shear stablewith insignificant shearing losses of about1-2% in the DIN 51 350-6 taper bearingshear test. Polyalphaolefins also have avery low coefficient of friction and areusually miscible and compatible withmineral oils whereby residual mineral oilwhen changing over should not exceed10% because otherwise, in certaincircumstances, the mineral oil could havean excessive impact on the performanceof the PAOs.

A niche group among wind turbine gearoils is those based on saturated, syntheticesters. Compared to mineral oils andpolyalphaolefins, these offer rapidbiodegradability of over 60% after 28days in the OECD 301C test and are thusan alternative to conventional hydrocarbonoils in ecologically sensitive applications.

Ester oils have a very high Viscosity Indexand their viscosity-temperature behaviouris generally good. The products are alsoshear stable with shearing losses of 1-2% in the taper bearing shearing testbeing insignificant. Ester oils havepowerful self-cleaning properties due totheir high polarity. This polarity keepsageing by-products and contaminants insuspension until they are trapped incorresponding filters.

Fig. 3: Working Temperature Range and Life of Industrial Gear Oils (according to Flender)

Fig. 2: Classification of Industrial Gear Oils

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Table. 1: Siemens Flender Specificationsfor Industrial Gear Oils (Revision 13) –Excerpt

6. Demands on Wind Turbine Gear OilsOf all the demands on gear oils, theSiemens Flender specifications areviewed as fundamental requirementsand are among the most importanttogether with DIN 51 517,Part 3 (Table 1). Prominent among thephysical/chemical demands arecompatibility with internal gearboxpaints, static elastomer tests (NBR – 100°C, 1008 h, FKM – 110°C,1008 h) and the Freudenbergdynamic elastomer test. Experiencehas shown that compatibility withgearbox paints under specific testconditions is not an unimportantfactor. Paint compatibility is affectedby the base oil as well as by theadditives and their components. Asregards seals, FKM materials arerelatively uncritical. NBR compatibilityis a problem insofar as thecorresponding tests are carried-outat very high temperatures for 1008hours (a drop in test temperature to95°C is planned). The dynamic testsare performed in line withFreudenberg in-house procedures.At present, a Working Group isexamining the demands, testtemperatures and test durations anda change to the current demandsand test parameters is currentlyunder discussion. Apart fromFlender and Freudenberg, a numberof leading lubricant and additivemanufacturers are represented inthis Working Group to discuss andperhaps practically modify thepresent specifications.

In addition, important mechanical-dynamic tests – demands on windturbine gear oils with regard tomicro-pitting and scuffingavoidance (standard test and testwith higher speeds), roller bearingwear test FE8 as well as diverseFlender foam tests, also withcorrosion preventive contaminationshould also be mentioned.

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Of particular significance is good micro-pitting protection as well as scuffingprevention / protection. Scuffingprevention influences gearbox design, i.e.gears are designed considering theHertzian stress in rolling contact (pitchpoint) according to FZG scuffing tests(DIN-ISO 14635-1). The same applies toroller bearing selection. Thecorresponding FE8 test parameters aredefined in the area of extreme mixedfriction at very high axial loads and highoil temperatures.

An important selection criterion forindustrial gear oils is the protection ofthe teeth against micro-pitting. Any gearoils used must offer a high degree ofprotection against micro-pitting. Thisproperty is tested according to FVA 54 I-IV with the therein specified C-meshing,pitch line velocity of 8.3 m/s and oilinjection temperatures of 90°C and

60°C. Micro-pitting is tested in both aload stage test as well as an endurancetest. In the load stage test, the mediantooth profile deviation must be less than7.5 µm and the maximum threshold forthe endurance test is 20 µm. Also, thetooth profile deviation must not increaseunder load. Figures 4 and 5 show anoverview of the demands regardingmicro-pitting for industrial gear oils forwind turbines.

Scuffing is generally tested with the FZGtest according to DIN-ISO 14635-1. Thestandard scuffing test, performed at apitch line velocity of 8.3 m/s and aninitial oil sump temperature of 90°C, canbe run at up to load stage 14 which isequivalent to a Hertzian stress of 2138N/mm2. This test highlights the highlypositive influence of fully-synthetic gearoils based on polyalphaolefins. Lowcoefficients of friction, a high Viscosity

Index and optimum mechanical efficiencycan lead to a possible lowering of oilsump temperatures by about 20°C atcorrespondingly high loads whencompared to mineral oils. This meansthat fully-synthetic, polyalphaolefin-basedgear oils offer lower power losses andbetter efficiency under load incomparison to mineral oils. This leads toan increase in the load-bearing viscosity.Lower oil sump temperatures are alsorecorded in practice. A drop in oil sumptemperature in the gearbox housing ofabout 5-7°C was achieved in a windfarm when a CLP 320 mineral oil wasreplaced with a fully-synthetic, polyal-phaolefin-based lubricant: RENOLINUNISYN CLP 320. Other advantagesincluded high viscosity at equivalentoperating conditions, greater lube filmstability, less oxidation and longer servicelife of the fully-synthetic gear oil (Figures6 and 7).

Fig. 4: Micro-Pitting Test According to FVA 54 I-IV – Test Parameters

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Fig. 5: Micro-pitting according to FVA 54 I-IV – Example of the test results with Fuchs Renolin Unisyn CLP 320

Fig. 6: FZG Scuffing test – Oil sump temperatures in the standard FZG A/8,3/90 test

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Fig. 7: Field test results – Wind turbine gear oils

References• Bundesamt für Wirtschaft und

Ausfuhrkontrolle: Mineralöldaten fürdie Bundesrepublik Deutschland2011

• DIN 51 517, Teil 1-3: Schmierstoffe,Schmieröle C, CL, CLP,Mindestanforderungen, August2011

• ISO 6743-6, Lubricants, IndustrialOils and Related Products (Class L) –Part 6: Family C (Gears), 1990 (E)

• ISO 12925: Lubricants, industrial oilsand related products (Class L) –Family C (gears) – Specifications forlubricants for enclosed gear systems,1996

• GfT-Arbeitsblatt 5,Zahnradschmierung, Gesellschaft fürTribologie, D-47443 Moers

• Siemens (Flender) requirements –Eignungsnachweie für Öle, die inFLENDER-Stirnrad-, Kegelrad- undPlanetengetrieben sowieGetriebemotoren eingesetzt werden– Rev. 13 (28.02.2011)

• FVA, Informationsblatt 54-II/IV:Testverfahren zur Untersuchung desSchmierstoffs auf die Entstehungvon Grauflecken bei Zahnrädern,1993

• DIN ISO 14635-1: Gears – FZG testprocedure A/8.3/90 for relativescuffing load carrying capacity ofoils, May 2006

To be continued in LubeTech No.86

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