Gabriela Fedor, Frank-Olaf Mähling, Christoph Wincierz, Thilo Krapfl
Evonik Operations GmbH – Specialty Additives, Darmstadt, Germany
Evonik Industries, Horsham, USA
Evonik Korea Ltd, Seoul, S.Korea
Wind turbines are a leading technology for sustainable energy generation. For more than a decade, the number and size of turbines have grown globally, and currently, China experiences the highest growth rates with an increasing demand for high quality oil.
The lubrication of wind turbine gearboxes is one of the most challenging applications. Wind turbine gear oils have to perform over a wide range of, often challenging, operating conditions. Meteorological conditions and power-generating demands are varying. In addition, the gears suffer from system specific vibrations and are expected to perform with extraordinary reliability and durability.
Because of their robustness, synthetic ISO VG 320 lubricants are the most widely used oils in wind turbine gears. These oils are largely formulated with higher grades of polyalphaolefins (PAOs), providing wear and fatigue protection, thermal and oxidative stability, foam resistance and shear stability. Besides meeting these critical needs, synthetic lubricants have demonstrated their ability to provide long oil service lifetime and help to achieve good mechanical efficiencies.
Over the last decade, alternatives to polyalphaolefin base oils were developed and tested for wind turbine gear applications. NUFLUX™ technology, a new class of wind turbine gear oils that contain VISCOBASE® synthetic base oils, finds increasing acceptance based on the field experience. VISCOBASE® products allow to formulate oils with the solvency of Group I oils and a performance similar to Group IV based fluids. They meet, or even exceed, performance requirements as defined by national and international industry standards and OEM specifications.
Wind turbines transform wind energy into electric energy. In order to maximize the wind yield, wind turbines have increased in size, placing a great load on the gearboxes that drive these systems.
The operating conditions are challenging, and lifetime expectations are high, because of the difficulties associated with replacing gear box components. A high-quality lubricant becomes an essential element of the design of the gearbox, to ensure load handling, durability and long oil drain intervals; synthetic fluids are the preferred choice for wind turbine lubrication.
For most modern wind turbines, a gearbox attached to an electrical generator is the key mechanical component. Gears connect a low-speed shaft to a high-speed shaft and increase the rotational speeds from approximately 20 rpm to 1500 rpm, which is the typical speed required by most generators to produce electricity . In parallel, there is also a reduction of the high torque input from the rotor blades.
The gearbox usually consists of at least one planetary stage and several helical spur gears, which further reduce torque and increase rotational speed. The gears are mounted to the gearbox cage by roller and tapered roller bearings, which take up the incoming forces from the system .
It is obvious, that the properties of the lubricant used in wind turbine gearboxes are crucial in protecting the bearings and gears from mechanical damage. Recent history suggests that a number of gearbox breakdowns have occurred due to micropitting and scuffing on the gears, or because of bearing failures .
For these reasons, synthetic formulations are predominant in this application; they can deliver high protection, reliability and long lifetime. Most of the wind turbine gear oil formulations on the market with OEM approvals are based on PAO-type base stocks in combination with esters, to improve additive compatibility.
Formulations made with alternative high-viscosity base stocks are rare and are just used in niche applications. For example, fully ester-based formulations provide biodegradability in sensitive environmental situations, but are costly and have disadvantages in terms of hydrolytic stability. Polyglycols offer excellent performance in gearboxes with high levels of sliding, such as gearboxes containing worm gears. However, polyglycols are hygroscopic and have limited compatibility with conventional lubricants, often causing issues with paints and coatings.
This paper introduces new formulation options for synthetic wind turbine gear oils based on high-viscosity oligomeric methacrylate base stocks, VISCOBASE®, blended with API Group III or IV base oils. The new high viscosity base stock delivers a polarity similar to Group I base oils together with stability and performance of polyalphaolefins. The resulting formulations, NUFLUX™ technology, meet the relevant requirements of standard industrial gear oils, as well as the most relevant OEM standards for wind turbine gear box applications.
2. Technical requirements
In order to give users of gearboxes a basis for an appropriate lubricant selection, the industry has set standards in various national and international bodies, to define minimum requirements for various applications. Wind turbine gear oils must meet general industry specifications, like the DIN 51517-3 in Europe, or AGMA 9005-F16 in North America and GB/T 33540 in China. There is no formal approval process in place for these standards.
Learning from field experience, including failures , OEMs and gearbox manufacturers have set up their own criteria and approval lists. These specifications are more stringent, and more accurately reflect true operating conditions, including low temperature requirements  and material compatbility with gearbox components. These specifications open the door for differentiation: lubricant manufacturers and marketers can differentiate from competition by performance considering the challenges of technical developments, and end users demand for higher performance, reliability and durability.
OEM and gearbox manufacturer specifications are based on four pillars as visually depicted in Figure 1:
1- Major Industry standards such as DIN 51517-3 CLP, AGMA 9005-F16, ISO 12925-1 or IEC 61400-4.
2- Gearbox manufacturer specific tests and additional standard tests: gearbox manufacturers develop tests for their components and materials, such as bearings, seals and coatings. Flender being an industry baseline with their transparent complex testing program considering individual components of the gearbox.
3- Bearing manufacturers, such as FAG Schaeffler that have their own test program for wind turbine application focused on multi-faceted bearings protection.
4- Gearbox manufacturers might require full gearbox testing on their in-house test rigs as part of their approval for a field test.
5- Wind turbine manufacturers and those wind turbine gearbox manufacturers, who have own lubricant approvals, require field testing over extended periods of time. Often wind turbine OEMs ask to test several combinations of types and sizes of wind turbines with gear boxes from different manufacturers for giving full approvals. Some OEMs and gearbox manufacturers for non-wind applications ask for field testing as part of their approval as well.
Lubricant approvals from OEMs and gearbox manufacturers are typically valid for the period of time that formulation remains unchanged. Time validity of individual tests might however vary, but the time span is usually between 5 and 10 years. OEMs and gearbox manufacturers publish either a separate list of approved lubricants or put the list of approved lubricants in their manuals, which are regularly updated.
For wind applications, the Flender specification is widely accepted as a performance baseline, although additional tests and approvals of the bearing suppliers such as Schaeffler, Svenska Kullager Fabriken (SKF) and Nippon Seiko K.K. (NSK) are needed to receive an approval from a wind turbine gear box manufacturer for the field test. A field test is the final step of a wind turbine gearbox manufacturer approval. Approvals of the gearbox manufacturers supplying the OEM are needed before running field trials on wind turbine / gearbox combinations over several years. Field trial phase is usually between one and three years and is based on a regular oil monitoring, primary focused on parameters such as viscosity (to confirm stay in grade requirement), analysis of wear particle concentration and monitoring of potential additive depletion.
Figure 1: Technical requirements of industrial gear oils on different approval levels
3. Fluid alternatives
The majority of commercial wind turbine gear oils are formulated to a viscosity grade of ISO 320. In most cases, these fluids are based on higher PAO or mPAO grades, such as PAO 40 and 100. The lubricants contain a number of additives, including those for antiwear, anti-corrosion, EP and antifoam. In order to incorporate additive packages into the fluid, the solvency of the base oil needs to be adjusted to provide a stable environment for surface active components. PAO-based fluids are typically adjusted by addition of ester oil or alkylated naphthalin to shift the polarity of the base stock. The Polyalphaolefins usually equal up to 80-90 w.-% of the final formulation.
Ideal solvency for additive packages is inherently given for Group I base oils. A closer look at the Anilin points of Group I, Group IV and NUFLUX™ technology reveals the low polarity and poor solvency power of PAOs, Figure 2. This is interpreted as a disadvantage for various lubricant properties.
Non-polar synthetic oils can have a difficult time suspending varnish-forming degradation by-product. Material compatibility issues can occur with certain seals, metals, paints or coatings. On the other hand, many ester-type synthetics do not perform well in the presence of water and have the tendency to hydrolyze.
Figure 2: Anilin points of base oils and their mixtures
High-quality mineral base oils perform well in most gear oils formulated for general industrial applications. However, many of these applications do not have stringent requirements for wide temperature operating window, unlike wind turbine application. NUFLUX™ technology is able to address the same temperature operating window as PAO based fluids, Figure 3.
Figure 3: Operating temperature span for different classes of IGO
Mineral formulations have higher pressure-viscosity coefficients than common synthetic oils, giving greater film thickness at operating viscosities. However, many synthetic base oils show inherently better oxidative and thermal stability, making them preferable for applications with high-operating temperatures and where long service intervals are required.
Synthetic gear oils with high viscosity index and low pour points had seen increasing use, and are a common choice for wind turbine gear boxes. Their excellent low temperature viscosity allows for a wide temperature operating window. Examples for high VI synthetic oils are PAO, PAG and VISCOBASE® high viscosity base stocks.
Stable viscosities, excellent seal compatibility, coating compatibility, and oxidation resistance contribute to long drain intervals in harsh operating conditions.
A combination of base oil with higher and lower viscosity is used to formulate the desired wind turbine gear oil. The comparison in Table 1 demonstrates that an ISO VG 320 oil can be formulated with a 6 or 8 cSt Group III base oil and a high viscosity base oil, i.e. low molecular weight polyalkylmethacrylate (VISCOBASE®), achieving viscosimetric properties similar to those of traditionally used PAO/ester - formulations. In fact, in terms of viscosity-temperature behaviour, the resulting NUFLUX™ technology has advantages over mineral oil and PAO/ester formulations.
Table 1: Comparison of ISO VG 320 wind turbine gear oils, formulated with the same commercially-available additive package at identical treat rate as used in several industry-approved wind turbine gear oils
The versatility of NUFLUX™ technology is shown in Table 2 by using different low viscosity base oil components. The desired KV 40 can be adjusted by the ratio of high and low viscosity base oil. Higher Viscosity Indexes are achievable with higher fractions of VISCOBASE® or by using a low viscosity base oil component with high Viscosity Index, like PAO8.
NUFLUX™ formulations do not require the addition of polar compatibilizers to dissolve the additives, unless used in combination with PAO based base stocks, but still with treat rates significantly lower than full PAO formulations. PAO-based wind turbine-formulations typically contain about 10 to 25 w.-% of esters or alkylated naphthaline to increase additive solubility and to avoid haze.
Table 2: Comparison of NUFLUXTM ISO VG 320 formulation options. “Additives” stand for the sum of performance package, PPD and demulsifier.
4. Endurance tests with NUFLUXTM technology
The performance of NUFLUX™ has been tested in the field in the United States, Europe, Africa and Asia. Performance demonstrations were conducted in areas such as in the mining industry, water treatment industry as well as the wind industry.
In-house testing by Moventas
NUFLUX™ technology was successfully tested in a gearbox by Moventas. Test setup consisted of two PLH-1400 gearboxes that were run back-to-back. One gearbox was filled with NUFLUX™ ISO VG 320 and the other with a commercial PAO gear oil. Temperature was measured at 8 different points and oil pressure was measured at 3 points. Gears were painted to examine the contact patterns, Figures 4 a,b. Moventas has stated that, contact patterns did not show any abnormal behavior. All contact patterns were good and no hard end contacts or marks of particles were detected. Therefore, NUFLUX™ ISO VG 320 passed the factory test and is permitted in field tests.
Figure 4 a: NUFLUX™ ISO VG 320 tested at Moventas. Painted gears before the test run, on the left is generator side and on the right is rotor side.
Figure 4 b: NUFLUX™ ISO VG 320 tested at Moventas. Gears after the test run, on the left is generator side and on the right is rotor side. Contact patterns were fully intact.
NUFLUX™ ISO VG 320 has demonstrated its performance on more than 40 wind turbine gears. Most customers have conducted the field trials by themselves with the support of Evonik.
In Europe, 20 wind turbines operated with NUFLUX™ ISO VG 320 for more than 6 years without an oil change on gearboxes of Winergy, Moventas and ZF Wind. Compared to incumbent fluids, NUFLUX™ has shown reduced operating temperatures and has delivered reliable lubrication and an extended oil drain interval. In Texas, Iowa and California (USA), more trials were conducted on gearboxes of GE, Winergy and Rexroth (all 1,5 MW), with many of them already switching to normal operation mode. The observations showed superior oxidation stability, inhibited deposit formation and excellent equipment protection. More recently, performance demonstrations have been started in India, China and more in the USA. The endurance tests have proven the advantages of NUFLUX™ technology in the field.
End users observed bearing temperatures up to 5 °C lower compared to incumbent WTGO formulations, shown in Figure 5. Due to more efficient gear operations, lower equipment operating temperatures contribute to extended oil service life, less oil degradation and chemical attack with lower wear metal levels and cleaner bearings.
Figure 5: Wind turbine bearing temperatures, measured with NUFLUX™ ISO VG 320 versus a PAO-based VG320 reference oil.
→Lower bearing temperatures
→Less oil degradation and chemical attack
Some of the field trial gearboxes contained high amounts of residues before starting the trial with the NUFLUX™ fluid. The cleaning effect of NUFLUX™ technology is visualized in Figure 6. Less varnish, deposits and residues were observed in several field trials.
Figure 6: NUFLUX™ ISO VG 320 cleans and prevents gears from deposits and varnish. The pictures show gears before (2014) and after the oil change (2015) to NUFLUX™ ISO VG 320.
Good wear performance of NUFLUX™ technology is confirmed by a low level of ICP wear metals over the full length of wind turbine performance demonstrations. A detailed investigation was made over 18 months during an official wind turbine gear test. Test result is shown in Figure 7.
Figure 7: Measured concentrations of Fe wear over an 18 months field trial with NUFLUX™ ISO VG 320
The concentrations of iron, copper, and other metals have been found on the same or lower levels compared to high reference PAO-based ISO VG 320 fluids. There were no signs of abnormal wear. Concentrations of water and the antiwear package elements, sulphur and phosphorous, were stable over the full length of an 18 month field trial.
This paper presented formulations and performance of a new class of wind turbine gear oils. NUFLUX™ technology is based on a blend of a synthetic high viscosity polymeric base oil, VISCOBASE® 5-220, blended with low viscous hydrocarbons, either group III or IV base oils. The resulting formulations perform on the level of PAO and Ester-based formulations using the same additive package. They meet or exceed the technical requirements for industrial gear oils, as well as, the specific requirements of component OEMs and wind turbine manufacturers.
Table 3: Performance ranking of different industrial gear oil technologies in important characteristics
Table 3 compares the performance levels of mineral based, PAO based and NUFLUX™ technology based industrial gear oils. Compared to PAO based oils NUFLUX™ technology performs well without the addition of solvency improvers, like esters, that increase cost of such formulations. The NUFLUX™ ISO VG 320 formulation also fulfils all material compatibility demands in wind turbine applications.
Results from long term studies in real life wind turbines demonstrate that NUFLUX™ might contribute positively to the overall gearbox efficiency and durability of the overall system.
NUFLUX™ technology is a proven alternative to fully PAO-based wind turbine gear oils.
 Wind Turbine Oil Trends and Best Practices by Shalini Magats, North American Lubricants, ChevronTexaco.
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 Improving Wind Turbine Gearbox Reliability; W. Musial, S. Butterfield, B. McNiff; 2007 European Wind Energy Conference; Milan, Italy
 http://www.renewableenergyworld.com/rea/news/ article2010/06/wind-turbine-gearbox-reliability
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Challenge; Machinery Lubrication, 9/2002.
End of Part 1.
Performance, material compatibility test results and introduction of WTGO approval process will be followed in Part 2.