Aus der Praxis für die Praxis 1 Introduction The worldwide windindustry is still a key factor for the changes within the world energy markets. The price pressure coming from coal, oil, nuclear and natural gas combined with enormous worldwide production capacities for components of wind turbines makes wind energy a high competitive market. Related to those market factors costs and reliability for wind turbines and its components are the driving factors for the industry. The pressure on the costs leads the industry to search for significant savings on all component and subcomponent levels - also such as oils and greases. Two major trends are to be recognized. One is to use cheap oils with less sustainability and a borderline performance (such as mineral oils) with high frequent changing intervals such as 1.5 years or less. The other trend is to use high performance lubes with the target on long oil change intervals. The first trend decreases the initial capital costs; the second reduces the cost of energy over the lifetime of the turbine. Both trends include strong requirements on the monitoring and the treatment of the oil within lubrication systems. From this perspective the quality of oil analyses with regards to real values and the handling of the lubricant et al. become more important. Very often hot and cold climate regions have profitable wind conditions and make those sites attractive for windpark projects. Components of turbines which are located at extreme expositions are impacted by strong, gusty winds, low temperatures, ice, snow, hot or humid climates. The load on components combined with tough environmental conditions lead to strong requirements for the engineering of subsystems such as oil, grease and gearbox lubrication. 1.1 Environmental conditions Dynamic loads caused by a stochastic wind stimulation and accelerated masses as well as inertias lead to small fluid film thicknesses and mixed friction lubrication Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 35 * Dr.-Ing. Frank-D. Krull Dipl.-Ing. (FH) Stefan Schemmert, MBA Eickhoff Antriebstechnik GmbH, 44789 Bochum Lubrication and Criteria for Lubricants of Gearboxes for Windturbines F.-D. Krull, S. Schemmert* Der Beitrag liefert eine kurze Einführung in die Technologie von Windenergieanlagen, ihre Antriebsstränge und die Schmierung der Getriebe. Eine Beschreibung der Umgebungsbedingungen und des Betriebsverhaltens geben einen Überblick über die Randbedingungen für Schmiersysteme von Getrieben. Darüber hinaus setzt sich der Beitrag mit Vorteilen und Nachteilen des Gebrauchs unterschiedlicher Ölsorten im Hinblick auf die Anwendung auseinander. Offene Fragestellungen hinsichtlich der Ölreinheit, Wassergehalten sowie Schwächen, Reproduzierbarkeit und Unsicherheiten von unterschiedlichen Analysemethoden auf der Basis praktischer Erfahrung lassen Spielraum für Interpretation und Diskussion Schlüsselwörter Getriebe, Schmierung, Windenergieanlage, WEC, Öl, Oxidation, Kaltes Klima This paper gives a short introduction into the technology of windturbines, their drivetrains and the lubrication of the gearbox. A description of environmental conditions and the operating behaviour of windturbines show an overview about the boundary conditions for gearbox lubrication systems. It also discusses the advantages and disadvantages of the use of different oil types with regards to the application. Open questions out of discussions about oil cleanliness, water contents, weaknesses, reproducibility and uncertainties of different analysis methods based on experience from the practice will give room for discussion. Keywords Gearbox, Lubrication, Wind turbine, WEC, Oil, Oxidation, Cold Climate Kurzfassung Abstract T+S_4_17 07.06.17 17: 27 Seite 35 Aus der Praxis für die Praxis within the contacts of the low speed planetary stages. The wide speed range and varying speeds during low and intermediate power production conditions cause a non stationary environment for fluid films in bearings and toothings. Those conditions lead to critical lubrication conditions. 1.2 Impact on fluid films and contact lubrication The fluid film thickness within sliding and rolling contacts depend on velocity, specific load, material properties and physical oil properties. Dynamic loads and accelerating speeds have an impact on the stability of surface protecting fluid films. In case of extreme temperature environments, e. g. hot and cold climate conditions, the temperature depending fluid flow and oil supply have an additional influence on the lubrication. 2 Drive train and Gearbox Rotor blades, hub, main shaft, shrink disc, gearbox, high speed shaft coupling and generator are the main parts of a typical wind turbine drive train. Shrink disc and high speed shaft coupling connect main shaft and generator shaft with the gearbox. Rubber elements support the pins of the gearbox torque arms. They isolate gearbox vibrations and the main frame structure of the wind turbine. The floating bearing of 3 point support wind turbine drive trains. Yaw drives turn the machine head of the turbine within a geared rim around the vertical axis of the tower. Figure 2.1 shows the typical drive train of a 2 MWrange wind turbine drive train. The gearbox transforms the rotor shaft speed and torque to the generator level. Common generator concepts are based on double fed systems. A transformer adapts the variable rotor speed to constant 50 or 60 Hz grid frequency. Figure 2.2 shows a typical gearbox concept for turbines in a range between 1.5 and 2 MW. High power densities and large transmission ratios demand special gearbox concepts. Planetary-helical gearbox concepts split the input torque. This allows sharing the input loads between different gear sets. Those concepts give the opportunity to realize gearboxes with a high toque to weight ratio. Up to 2 MW typical concepts are based on 1 planetary and 2 helical stages. Above 2 MW state of the art concepts switch to 2 planetary and 1 helical stage. 3 Operating conditions of Wind turbines Availability and production gains of wind turbines depend on a couple of operating conditions. Loads and temperatures are varying depending on the turbines condition speeds. A high amount of control activities under turbulent and strong wind conditions leads to non stationary loads and speeds. 36 Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 environments, e.g. hot and cold climate conditions, the temperature depending fluid flow and oil supply have an additional influence on the lubrication. 2. Drive train and Gearbox Rotor blades, hub, main shaft, shrink disc, gearbox, high speed shaft coupling and generator are the main parts of a typical windturbine drive train. Shrink disc and high speed shaft coupling connect main shaft and generator shaft with the gearbox. Rubber elements support the pins of the gearbox torque arms. They isolate gearbox vibrations and the main frame structure of the windturbine. The floating bearing of 3 point support windturbine drive trains. Yaw drives turn the machine head of the turbine within a geared rim around the vertical axis of the tower. Figure 2 1 shows the typical drive train of a 2 MW range windturbine drive train. Figure 2 1: Drive train of a 2 MW range wind turbine The gearbox transforms the rotor shaft speed and torque to the generator level. Common generator concepts are based on double fed systems. A transformer adapts the variable rotor speed to constant 50 or 60 Hz grid frequency. Figure 2 2 shows a typical gearbox concept for turbines in a range between 1.5 and 2 MW. Figure 2 2: Gearbox concept for 1.5 2 MW wind turbines High power densities and large transmission ratios demand special gearbox concepts. Planetary helical gearbox concepts split the input torque. This allows sharing the input loads between different gear sets. Those concepts give the opportunity to realize gearboxes with a high toque to weight ratio. Up to 2 MW typical concepts are based on 1 planetary and 2 helical stages. Above 2 MW state of the art concepts switch to 2 planetary and 1 helical stage. 3. Operating conditions of Windturbines Availability and production gains of windturbines depend on a couple of operating conditions. Loads and temperatures are varying depending on the turbines condition speeds. A high amount of control activities under turbulent and strong wind conditions leads to non stationary loads and speeds. The design and suitability of the lubrication system is relevant for the reliable component lubrication. Coherences between environmental conditions and functionality: Cold climate Maintaining the lubrication and filtration system supply of the tribo contacts Frozen Gearbox Capability to start the presure fed lubrication, avoiding of oil coal at heating elements, short starting periods High speeds, low loads Avoiding of smearing Low speed, high loads Avoiding of micro pitting and wear High speed, high loads Avoiding of scuffing Blades 70-80 m Tower Torque arm Mech. brake Azimut drives Main bearing Main frame Anemometer Hub + Blades Mainshaft Gearbox Generator coupling Generator Bildquelle: REpower Systems AG Schrumpfscheibe Planetenträger Planeten Sonnenritzel Hohlwelle Rad 2. Stufe Rad 3. Stufe Ritzel 3. Stufe Ritzel 2. Stufe environments, e.g. hot and cold climate conditions, the temperature depending fluid flow and oil supply have an additional influence on the lubrication. 2. Drive train and Gearbox Rotor blades, hub, main shaft, shrink disc, gearbox, high speed shaft coupling and generator are the main parts of a typical windturbine drive train. Shrink disc and high speed shaft coupling connect main shaft and generator shaft with the gearbox. Rubber elements support the pins of the gearbox torque arms. They isolate gearbox vibrations and the main frame structure of the windturbine. The floating bearing of 3 point support windturbine drive trains. Yaw drives turn the machine head of the turbine within a geared rim around the vertical axis of the tower. Figure 2 1 shows the typical drive train of a 2 MW range windturbine drive train. Figure 2 1: Drive train of a 2 MW range wind turbine The gearbox transforms the rotor shaft speed and torque to the generator level. Common generator concepts are based on double fed systems. A transformer adapts the variable rotor speed to constant 50 or 60 Hz grid frequency. Figure 2 2 shows a typical gearbox concept for turbines in a range between 1.5 and 2 MW. Figure 2 2: Gearbox concept for 1.5 2 MW wind turbines High power densities and large transmission ratios demand special gearbox concepts. Planetary helical gearbox concepts split the input torque. This allows sharing the input loads between different gear sets. Those concepts give the opportunity to realize gearboxes with a high toque to weight ratio. Up to 2 MW typical concepts are based on 1 planetary and 2 helical stages. Above 2 MW state of the art concepts switch to 2 planetary and 1 helical stage. 3. Operating conditions of Windturbines Availability and production gains of windturbines depend on a couple of operating conditions. Loads and temperatures are varying depending on the turbines condition speeds. A high amount of control activities under turbulent and strong wind conditions leads to non stationary loads and speeds. The design and suitability of the lubrication system is relevant for the reliable component lubrication. Coherences between environmental conditions and functionality: Cold climate Maintaining the lubrication and filtration system supply of the tribo contacts Frozen Gearbox Capability to start the presure fed lubrication, avoiding of oil coal at heating elements, short starting periods High speeds, low loads Avoiding of smearing Low speed, high loads Avoiding of micro pitting and wear High speed, high loads Avoiding of scuffing Blades 70-80 m Tower Torque arm Mech. brake Azimut drives Main bearing Main frame Anemometer Hub + Blades Mainshaft Gearbox Generator coupling Generator Bildquelle: REpower Systems AG Schrumpfscheibe Planetenträger Planeten Sonnenritzel Hohlwelle Rad 2. Stufe Rad 3. Stufe Ritzel 3. Stufe Ritzel 2. Stufe Figure 2.1: Drive train of a 2 MW-range wind turbine Figure 2.2: Gearbox concept for 1.5-2 MW wind turbines T+S_4_17 07.06.17 17: 27 Seite 36 Aus der Praxis für die Praxis The design and suitability of the lubrication system is relevant for the reliable component lubrication. Coherences between environmental conditions and functionality: • Cold climate → Maintaining the lubrication and filtration system supply of the tribo-contacts • Frozen Gearbox → Capability to start the pressure fed lubrication, avoiding of oil coal at heating elements, short starting periods • High speeds, low loads → Avoiding of smearing • Low speed, high loads → Avoiding of micro pitting and wear • High speed, high loads → Avoiding of scuffing • Stand Still → Avoiding of still standing Marks and “False Brinelling” • Idling → Supply of toothings and bearings above the oil sump • No Grid → Minimum supply of components during parking periods There are a couple of other operating conditions which should be addressed by testing, calculation and simulation 4 Test and qualification of the gearbox lubrication system The approval of the lubrication system is important for prototype and end of line testing. During prototype tests the expected amount of oil, temperatures, pressures and oil cleanliness have to be confirmed. In addition the suitability of emergency lubrication for idling and gridless parking conditions of the wind turbine has to be proven. During the acceptance tests of the gearboxes the confirmation of expected temperatures and pressures assure the completeness and functionality of oil pipes and leak tightness of the lubrication system. This concept excludes damages in consequence of lubrication faults and leakages. Prototype testing under CCV conditions including the verification of lubrication design assumptions is important. Tests within a cold climate chamber ensure the suitability of design assumptions and operating strategies of the wind turbine. Freezing the gearbox down to an extreme temperature of -40 °C provides the opportunity to simulate warm up and starting procedures of the gearbox. Sensors for temperatures at different oil sump and bearing positions as well as pressure sensors measure useful data which can be used to optimise the warm up procedure and components of the lubrication system. The optimisation of the warm up period between low reference temperature (-40 °C) and the start temperature of the gearbox (0 °C) increase the availability of the turbine and as a consequence the gain out of production. Figure 4.1 shows a picture of the Eickhoff 5MW test rig with climate chamber The test rig has the capability to freeze the gearbox down to a temperature of -40 °C and to start the gearbox under full load. This gives the opportunity to obtain the initial breakaway torque of the frozen gearbox at the low speed side of the gearbox. Those values are realistic compared to the situation on the wind turbine. The simulation of different warm up procedures with and without idling gearboxes is important to prove the suitability and performance of the operation strategy. Emergency lubrication can be checked with different temperatures. Figure 4.2 shows a lubricated toothing with -40 °C and -20 °C. Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 37 Stand Still Avoiding of still standing Marks and “Balse Brinelling“ Idling Supply of toothings and bearings above the oil sump No Grid Minimum supply of components during parking periods There are a couple of other operating conditions which should be addressed by testing, calculation and simulation 4. Test and qualification of the gearbox lubrication system The approval of the lubrication system is important for prototype and end of line testing. During prototype tests the expected amount of oil, temperatures, pressures and oil cleanlinesses have to be confirmed. In addition the suitability of emergency lubrication for idling and gridless parking conditions of the windturbine has to be proven. During the acceptance tests of the gearboxes the confirmation of expected temperatures and pressures assure the completeness and functionality of oil pipes and leak tightness of the lubrication system. This concept excludes damages in consequence of lubrication faults and leakages. Prototype testing under CCV conditions including the verification of lubrication design assumptions is important. Tests within a cold climate chamber ensure the suitability of design assumptions and operating strategies of the windturbine. Freezing the gearbox down to an extreme temperature of 40°C provides the opportunity to simulate warm up and starting procedures of the gearbox. Sensors for temperatures at different oil sump and bearing positions as well as pressure sensors measure useful data which can be used to optimise the warm up procedure and components of the lubrication system. The optimisation of the warm up period between low reference temperature ( 40°C) and the start temperature of the gearbox (0°C) increase the availability of the turbine and as a consequence the gain out of production. Figure 4 1 shows a picture of the Eickhoff 5MW test rig with climat chamber Figure 4 1: 5MW Testrig with climate chamber The testrig has the capability to freeze the gearbox down to a temperature of 40°C and to start the gearbox under full load. This gives the opportunity to obtain the initial breakaway torque of the frozen gearbox at the low speed side of the gearbox. Those values are realistic compared to the situation on the windturbine. The simulation of different warm up procedures with and without idling gearboxes is important to prove the suitability and performance of the operation strategy. Emergency lubrication can be checked with different temperatures. Figure 4 2 shows a lubricated toothing with 40°C and 20°C. Figure 4 2: Oil under temperature conditions The consistency of the oil has with respect to its flowability different properties under temperature conditions of 40°C and 20°C. The synthetic oil which is shown in Figure 4 2 has its specified pour point above 40°C. It has still enough flowability to be distributed to the lubricated parts of the gearbox. -40°C -20°C Stand Still Avoiding of still standing Marks and “Balse Brinelling“ Idling Supply of toothings and bearings above the oil sump No Grid Minimum supply of components during parking periods There are a couple of other operating conditions which should be addressed by testing, calculation and simulation 4. Test and qualification of the gearbox lubrication system The approval of the lubrication system is important for prototype and end of line testing. During prototype tests the expected amount of oil, temperatures, pressures and oil cleanlinesses have to be confirmed. In addition the suitability of emergency lubrication for idling and gridless parking conditions of the windturbine has to be proven. During the acceptance tests of the gearboxes the confirmation of expected temperatures and pressures assure the completeness and functionality of oil pipes and leak tightness of the lubrication system. This concept excludes damages in consequence of lubrication faults and leakages. Prototype testing under CCV conditions including the verification of lubrication design assumptions is important. Tests within a cold climate chamber ensure the suitability of design assumptions and operating strategies of the windturbine. Freezing the gearbox down to an extreme temperature of 40°C provides the opportunity to simulate warm up and starting procedures of the gearbox. Sensors for temperatures at different oil sump and bearing positions as well as pressure sensors measure useful data which can be used to optimise the warm up procedure and components of the lubrication system. The optimisation of the warm up period between low reference temperature ( 40°C) and the start temperature of the gearbox (0°C) increase the availability of the turbine and as a consequence the gain out of production. Figure 4 1 shows a picture of the Eickhoff 5MW test rig with climat chamber Figure 4 1: 5MW Testrig with climate chamber The testrig has the capability to freeze the gearbox down to a temperature of 40°C and to start the gearbox under full load. This gives the opportunity to obtain the initial breakaway torque of the frozen gearbox at the low speed side of the gearbox. Those values are realistic compared to the situation on the windturbine. The simulation of different warm up procedures with and without idling gearboxes is important to prove the suitability and performance of the operation strategy. Emergency lubrication can be checked with different temperatures. Figure 4 2 shows a lubricated toothing with 40°C and 20°C. Figure 4 2: Oil under temperature conditions The consistency of the oil has with respect to its flowability different properties under temperature conditions of 40°C and 20°C. The synthetic oil which is shown in Figure 4 2 has its specified pour point above 40°C. It has still enough flowability to be distributed to the lubricated parts of the gearbox. -40°C -20°C Figure 4.1: 5MW Test rig with climate chamber Figure 4.2: Oil under temperature conditions T+S_4_17 07.06.17 17: 27 Seite 37 Aus der Praxis für die Praxis The consistency of the oil has with respect to its flow ability different properties under temperature conditions of -40 °C and -20 °C. The synthetic oil which is shown in Figure 4.2 has its specified pour point above -40 °C. It has still enough flow ability to be distributed to the lubricated parts of the gearbox. With -20 °C the oil has a consistency comparable to semifluid grease. Under this condition it is pump able and the pressure fed lubrication system is able to distribute the oil. Hence the gearbox can operate under partial load and moderate velocities. 5 Approval criteria and requested oil properties Extreme operation conditions and extreme climatic environments require high level performance characteristics for wind turbine gearbox lubricants. Logistics for the maintenance of worldwide high number of wind turbines is a challenge. Under consideration of unique maintenance and service concepts only a limited number of lubricants can be released. Approval processes for wind turbine gearbox lubricants require a high number of mechanical tests and a field test validation. Qualified test facilities and laboratories for mechanical oil tests are booked frequently and waiting times for test capacities are long. The availability of prototype wind turbines for field tests is rare. After successful field tests pre series field tests for one and two years with a higher number of turbines (19 - 20) are required. This leads to long approval processes for new oil developments. Compared to industrial gearbox oils, the supplier of oil for wind turbine gearboxes have to meet the requirements and specifications from gearbox manufacturer, bearing manufacturer, wind turbine OEM and wind energy specific national and international standards. Often test specifications are redundant with respect to the test procedures but not to the limits and test parameters. This leads to long product launches. Often time to market lasts 5 years and more. The analyses of specified performance test results such as FE8, micro pitting, scuffing and others, should be done under observation of wind turbine OEM, gearbox OEM and bearing manufacturer. According to IEC61400-4 [I-1] the approval for lubricants has to be agreed between wind turbine manufacturer, gearbox manufacturer and certification bodies based on standardized test methods. After a successful evaluation of the test results a field test with a duration of one and three years follows. The final approval will be available in case that all data obtained from the field tests are evaluated. The international standard IEC61400-4 [I-1] contains a list with international and national standardized test methods for oils. In addition the standard provides a list of non standardized but available state of the art in-house and “community” tests (FVA, GFT, ASTM, …) 6 Additional future wind turbine related oil property requirements Within the last 8 years new failure mechanisms at bearings have been observed within wind turbine gearboxes. One of the most discussed mechanisms is white etching cracks called WEC. Up to now a couple of root causes have been allocated. Those influences and interactions are known as 1. Dynamics and vibration 2. Hydrogen diffusion 3. Electricity 4. Slippage 5. Lubricant additive systems 6. Adiabatic shear bands Most of the known influence parameters are part of the environment of wind turbines. Within the expert community detailed coherences and the damage propagation within the bearing steels are still in discussion. The discussed failure propagation theories can be divided into Hydrogen diffusion phenomena, crack propagation from the surface and crack propagation starting within the subsurface of the material. Meanwhile WEC can be produced under specific test conditions reproducible on test rigs. One of the most commonly used test rigs for such tests is the FE8 test rig. Figure 6 1 shows White Etching Cracks within the subsurface material structure of a bearing inner ring produced on an FE8 test rig [S-2]. Based on parameter studies on these test rigs new requirements for lubricants are introduced to the wind industry. Bearing manufacturer are publishing coherences between special types of corrosion inhibitors from conservation 38 Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 T+S_4_17 07.06.17 17: 27 Seite 38 Aus der Praxis für die Praxis fluids used during the manufacturing process and anti wear additives which might be responsible for having an influence on the occurrence of WEC. One of the most common technologies to decrease the influence on WEC is the use of bearings with a black oxide treated surface. This specific treatment passivates the metallic surface and reduces the friction within the sliding contacts. Those coherences lead to better lubrication properties and reduce the transmission of heat and energy into the material. The dangerous reaction of some additive chemistry might also be positively influenced. 7 Lubrication conditions within Toothings and bearings of wind turbine gearboxes The efficiency of rotor blades is strongly influenced by the ratio between wind speed and rotor speed. To receive the optimum power for different wind speeds the rotor speed must be controlled. Electrical transformers adapt the variable speed of the generator to the constant grid frequency. The maximum allowable rotor speed is restricted by the noise emission of rotor blades. Exceeding the limiting blade tip velocity of 320 km/ h the aerodynamic noise emission will increase above average. This leads to lower rotor speeds with an increasing inclination of the gearbox ratio for larger wind turbines. Due to low input speeds Lambda values of large wind turbine gearboxes are typically below 1 (λ < 1), that means boundary lubrication [S-1]. In case of standard oils and surface technologies the contact of roughness peaks combined with high pressure values and sliding surfaces may lead to surface initiated fatigue and wear. 8 Micro pitting at toothings and bearings Boundary lubrication related effects are micro pitting and wear. Micro pitting or grey staining is usually observed as grey shadowed area of the polished tooth surfaces. Under special light conditions those areas are visible for experienced eyes. The avoidance of micro pitting in wind turbine gearboxes is very important. Compared to the calculation of pitting or scuffing application limits and calculation parameter are not well defined. Complex coherences of influence parameters such as toothing technologies, surface finish, operational environment and lubricant parameters are challenging the handling of working tribological systems. Micro pitting leads to a removal of material in the critical area of the tooth flank. Shear stresses at the roughness peaks cause surface initiated fatigue. In consequence of the shear stresses micro cracks occur. Periodic loads on the crack lips will rip off the edges. As a result small pittings (micro pittings) at the bottoms of the cracks will develop. The light refraction at the edges of the pores give areas of micro pittings their characteristic dim grey look. Figure 8.1 shows micro pittings on a metallographic specimen 200-times magnified by a REM. Micro pitting on bearings is a new phenomenon within the gearbox industry. Tribologie + Schmierungstechnik 64. 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Sh face initiate of the shea n the crack tings (micro lop. tion at the e eir characte ows microp imes magnifi ro pitting 20 bearings is ustry. may lead to things and effects are g is usually ished tooth hose areas g in windtur tion of pit lation param fluence par urface fini parameters logical syste oval of mate ar stresses fatigue. r stresses mi lips will rip o ittings) at th ges of the p istic dim gre ittings on ed by a REM. times mag new pheno surface initi bearings icro pitting bserved as surfaces. U are visible ine gearbox ing or scu ter are not meters such h, operati are challen s. rial in the cri t the rough ro cracks oc f the edges. bottoms of res give area look. metallogra ified enon within ted and rey der for s is fing ell as nal ing ical ess cur. s a the s of hic Figure 6.1: WEC within the subsurface material [S-2] Figure 8.1: Micro pitting 200-times magnified T+S_4_17 07.06.17 17: 27 Seite 39 Aus der Praxis für die Praxis It was introduced to the FVA research group. The research project “micro pitting on bearings” [F-1] ended in 2013. The main conclusions are that the causes of micro pittings in bearings need very specific operation conditions and a specific additive chemistry. The research group concludes on differences between lubrication conditions on bearings and toothings. The main parameters with regard to specific fluid film thicknesses are comparable but the limit values differ. Compared to toothings the lubricating conditions must be worthier to cause micro pitting failures on bearings. In addition to critical fluid film thicknesses a high amount of slippage is needed. A major difference seems to be the influence of additive chemistry. In case of toothings additives decrease the risk of micro pitting. In case of bearings a specific additive chemistry increases the risk of micro pitting. Future requirements for lubricants might include that fact. 9 Requirements on oil cleanliness and oil monitoring With the introduction of the IEC61400-4 standard (design requirements for wind turbine gearboxes) the requirement for continuous oil monitoring became established with the end of 2012. The continuous monitoring shall include the analyses of 1. Oil cleanliness 2. Oil viscosity 3. Water content 4. Wear metals 5. Measurement of oil oxidation 6. Key additive elements from the additive package The standard states that according to ISO/ TS16281 the oil cleanliness expressed by the factor ec has a large impact on the reliability of bearings. Therefore specific requirements for allowable cleanliness levels are defined. The oil cleanliness levels under operating conditions shall at least meet the cleanliness level of -/ 17/ 14 according ISO4406 which was developed for hydraulic fluids. Figure 9 1 contains the recommended cleanliness levels according to IEC61400-4 [I-1] The difficulty to meet such accurate recommended oil cleanliness levels is the accuracy of the testing methods which are not standardized. The recommendation differentiates for different gear oil conditions more or one class in-between the different cases. The practical analytical methods with automatic particle counting are only suitable for low viscosity fluids. Fluids and oils of higher viscosity grades such as 320cSt must be diluted with solvents such as Toluene for chromatography and particle counting. In practice different laboratories use different dilution ratios und different solvents. In consequence the results of the particle counts can differ up to 4 numbers within each class. Verification by manual counts is therefore recommended. The detection of water content as monitoring value needs a precise specification to the laboratories. The coherence which should be addressed by this value is the moisture of the oil. The moisture is important with respect to oil ageing and the lifetime of bearings and toothings. If it occurs for a longer time a saturation level of 80 % indicates critical conditions for the gearbox and the lubricant. Each gearbox oil has its own temperature depending saturation behaviour. That means a difference of up to e. g. 500 ppm between two standard gearbox oils which has 40 Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 Figure 9.1: Recommended oil cleanliness level T+S_4_17 07.06.17 17: 27 Seite 40 Aus der Praxis für die Praxis to be kept in mind. In reality a critical water content has to be specified with 300 ppm for an oil A and 800 ppm for an oil B. Controversial positions between different parties on the condition of the oil, based on the measured water content, are often based on a fundamental misinterpretation of those results. Figure 9 2 shows two artificial saturation curves to explain this specific topic. The measurement of water content requires the use of a specific and suitable method. Karl Fischer is the method which is typically used for wind turbine oils. Using Karl Fischer method it is to differentiate between two methods 1. Karl Fischer direct 2. Karl Fischer indirect Based on the working principle of this method it is necessary to use Karl Fischer indirect method to receive comparable and reliable results. With some oils and additive packages the result of the direct method is mainly influenced by the additives and the value for the water content is wrong. The difficulty of measuring the oil oxidation is similar. The method which is typically used is based on the measurement of the Total Acid Number (TAN). This value gives an indication about the acid ratio within the oil. The TAN differs not only for the modified base oil molecules it also indicates specific values for additive chemistry (means different additive packages have different base values - with production tolerances). Different base oils show a different behaviour during the critical oxidation process. That means in consequence: To use this method as monitoring method for oil oxidation/ ageing it is necessary 1. To have base values for the reference condition of each oil filling. 2. It is necessary to monitor the progression of the TAN Values. 3. To have the knowledge about the specific ageing behaviour during the oxidation process. Figure 9 3 shows the schematic oxidation curves of examples for different oil types Based on examples for three different oil types, time spots for an oil analyses from 1 to 3 figure 9 3 very clearly state the potential mistake that could be made in the interpretation of oil analyses without a continuous history. In this case “Pure PAO” means a very mild additivated oil. Spot 1: All oils are in a good condition -> a high performance PAO with a high level of additive chemistry looks critical; Reality: It is just the initial condition of this specific oil Spot 2: Mineral oil is out of scope, both other oil types look like that they are in a very good condition Reality: The Synthetic oils could be close to the critical slope Spot 3 (Not a long duration after spot 2): The oil analyses for both synthetic oils are too late - these oils have exceeded their critical limit and could have created damages Conclusion of this case study: A forecast of the usability of mineral oils could be done based on spot results in case that the reference curve for fresh oil and the critical limits are known. Synthetic oils have a longer durability but they have to be monitored with more care. The characteristic ageing curve must be known, reference values from fresh oil Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 41 Figure 9.2: Artificial temperature depending saturation curves for two oils on a level of 80 % saturation Figure 9.3: Schematic Oxidation/ Time curves of examples for different oil types T+S_4_17 07.06.17 17: 27 Seite 41 Aus der Praxis für die Praxis must be present and an additional criterion is necessary to detect the critical oxidation ratio. This additional criterion could be observed within IR spectroscopes of used oils. Spectra of aged oils point out a wavelength peak in within the range of 1740-1730 1/ cm which is caused by the oxidized alcohols (Carbonyl groups) [F-2]. Figure 9 4 shows typical IR spectra of a fresh and an aged oil as well as the difference of both spectra. 10 Standard requirements on lubricants Low film thicknesses within the low speed planetary stage, extreme temperature changes, humidity and other climatic influences as well as shock loads and high specific loads result in high requirements on the performance of lubricants used in wind turbine gear boxes. Those high requirements regarding the performance ratings need to be balanced with secondary requirements. The load carrying capacity of a gear set is ensured by using lubricants with high scoring protection properties as well as high micro pitting carrying capabilities. Test temperatures at 60 °C need to be considered in addition to the usually higher oil test temperatures. The cooling and heating systems control the oils sump operating temperature in wind turbine gearboxes to a maximum of 65 °C. Also different available Additive technologies in the market might show a low reactivity of one component at lower operating temperatures, this means that important performance criteria might be met at higher test temperatures, but at lower temperatures the lubricant fails. To ensure the reliable operation of gear oil in wind turbine gearboxes and the specific operating conditions it is recommended to test those with methods simulating the practical conditions and applications as close as possible. Micro pitting test at 60 °C and 90 °C nowadays are seen as basic requirements of gearbox manufacturers regarding the qualification process of gear oils. For the evaluation of the results it is necessary not only to compare the quantitative results but also to consider the change on the surface of the test gears. Very often the achieved load stage does not show a real differentiation between the performance and behaviour of gear oils, especially with regards to the reserves on the load carrying capability. Figure 10.1 shows examples comparing two different oil types performing the micro pitting test with only one load stage difference. The potential to protect the gears from micro pitting shows a very wide difference compared to the test result. Bearing manufactures qualify oils via test, that also include the influence of water and saltwater on the tribological system. The long term influence of water and salt differentiates lubricants based on industry standardized test methods, e. g. the FE-8 test machine with regards to the suitability for offshore Applications. Often the technologies of used additive systems to achieve highest performance values compete with compatibility requirements. The development of thoroughly balanced oils needs a know how of the lubricant developer with regards to segment specific solutions, limits, calculation methods and field experience. The continuous dialogue between lubricant developers and gearbox manufacturers are necessary to ensure development of appropriate lubrication solutions. Typical relevant secondary criteria are 42 Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 Figure 9.4: IR spectra of fresh and aged oil with a carbonyl wavelength peak in the range of 1740-1730 1/ cm [F-2] Spectra of aged oils point out a wavelength peak in within the range of 1740 1730 1/ cm which is caused by the oxidized alcohols (Carbonyl groups) [F 2]. Figure 9 4 show typical IR spectra of a fresh and an aged oil as well as the difference of both spectra. Figure 9 4: IR spectra of fresh and aged oil with a carbonyl wavelength peak in the range of 1740 1730 1/ cm [F 2] 10. Standard requirements on lubricants Low film thicknesses within the low speed planetary stage, extreme temperature changes, humidity and other climatic influences as well as shock loads and high specific loads result in high requirements on the performance of lubricants used in wind turbine gear boxes. Those high requirements regarding the performance ratings need to be balanced with secondary requirements. The load carrying capacity of a gear set is ensured by using lubricants with high scoring protection properties as well as high micropitting carrying capabilities. Test temperatures at 60 °C need to be considered in addition to the usually higher oil test temperatures. The cooling and heating systems control the oils sump operating temperature in wind turbine gearboxes to a maximum of 65 °C. Also different available Additive technologies in the market might show a low reactivity of one component at lower operating temperatures, this means that important performance criteria might be met at higher test temperatures, but at lower temperatures the lubricant fails. To ensure the reliable operation of gear oil in windturbine gearboxes and the specific operating conditions it is recommended to test those with methods simulating the practical conditions and applications as close as possible. Micropitting test at 60 °C and 90 °C nowadays are seen as basic requirements of gearbox manufacturers regarding the qualification process of gear oils. For the evaluation of the results it is necessary not only to compare the quantitative results but also to consider the change on the surface of the test gears. Very often the achieved load stage does not show a real differentiation between the performance and behaviour of gear oils, especially with regards to the reserves on the load carrying capability. Figure 10 1 shows examples comparing two different oil types performing the micropitting test with only one load stage difference. The potential to protect the gears from micropitting shows a very wide difference compared to the test result. Figure 10 1: Micropitting load carrying reserves of two different test gear sets SKS>10 left; SKS=9 right. Bearing manufactures qualify oils via test, that also include the influence of water and saltwater on the tribological system. The long term influence of water and salt differentiates lubricants based on industry standardized test methods, e.g. the FE 8 test machine with regards to the suitability for offshore Applications. Often the technologies of used additive systems to achieve highest performance values compete with compatibility requirements. The development of thoroughly balanced oils needs a know how of the lubricant developer with regards to segment specific solutions, limits, calculation methods and field experience. The continuous dialogue between lubricant developers and gearbox manufacturers are necessary to ensure development of appropriate lubrication solutions. Typical relevant secondary criteria are different materials used for the production and mounting of gearboxes, e.g. sealing materials and yellow metals in thrust washers as well as bearing cages, paints, liquid seals, etc. These materials need to provide long term compatibility with Graufleckenfläche Figure 10.1: Micro pitting load carrying reserves of two different test gear sets SKS >10 left; SKS = 9 right T+S_4_17 07.06.17 17: 27 Seite 42 Aus der Praxis für die Praxis different materials used for the production and mounting of gearboxes, e. g. sealing materials and yellow metals in thrust washers as well as bearing cages, paints, liquid seals, etc. These materials need to provide long term compatibility with the lubricant. This fact requires compatibility investigations, which also include the behaviour if the gear oil is mixed with contaminations, water or salt water. The foaming properties of a lubricant also plays a major role, as usually labyrinth seals are used at the rotor shaft and the medium speed stage of the gearbox needs to turn in the oil sump. The foaming behaviour stands in contradiction to a required good air release property, which is required due to the fact that high dispersed air in gear oils will influence the lubricating gap. Standstill and transportation require a good corrosion protection of the gear oils, making sure other performance criteria, e. g. EPand wear properties, are not influenced in a negative sense. In addition the lubricant needs to provide a high level of filterability also with finest filter technologies down to filter mesh sizes of 2 µm and a good biodegradability and/ or low toxicity. High Hertzian pressures in bearings and gears require an excellent shear stability to provide a stable viscosity at operating temperature. For the formation of the strongest lubricating film at higher temperatures it is recommended that the viscosity-temperature curve is as flat as possible. Synthetic Hydrocarbons, synthetic esters or polyglycols have much better viscosity temperature behaviours compared to mineral oils. Figure 10.2 shows the viscosity behaviour of different base oil types based on temperature changes Due to the requirement to cover the wide range of operating temperatures in those areas usually synthetic gear oils are considered. 11 Conclusion Highest requirements on lubricants and production techniques remain the basics for a reliable operation of tribological systems in wind turbine applications. The development and strict application of performance tests for lubricants applied during the development of new lubricants as well as for the benchmarking and suitability of market-available lubricants helps to increase reliable operation, also for units aiming to achieve higher power outputs. Combining both will help to set up drive train systems based on gear technologies and lubricants, which are able to cope with the latest and future requirements on wind turbine operation, reliability and availability. References [H-1] Hamrock, Bernhard J: Fundamentals of Fluid Film Lubrication; International Editions; ISBN 0-07-113356- 9; McGraw Hill, Inc; New York, St. Lois, San Francisco, Aucland, Bogotá, Caracas, Lisbon, London, Madrid, Mexico City, Milan, Montreal, New Delhi, San Juan, Singapore, Sydney, Tokyo, Toronto; Singapore 1994 [I-1] IEC 61400-4: Wind turbines - Part 4: Design requirements for wind turbine gearboxes [F-1] Bongardt, C.; Beilicke, R.: Einfluss von instationären Betriebszuständen zur Graufleckenbildung in Wälzlagern und Klärung von Mechanismen. Abschlussbericht, Forschungsheft zum Forschungsvorhaben 627 I der Forschungsvereinigung Antriebstechnik e. V., Frankfurt, 2013. [F-2] Prof.-Dr. Hans Meier: Getriebeöldiagnose, Ölprobenuntersuchungen als Beurteilungskriterium für Betriebssicherheit und Verschleißzustand von Getrieben, Abschlussbericht, Forschungsheft zum Forschungsvorhaben 88/ I, Heft 176, Forschungsvereinigung Antriebstechnik e.V., Frankfurt, 1984 [S-1] Schönbeck, G.: Einfluß der Schmierstoffe auf die Zahnflankenermüdung (Graufleckigkeit und Grübchenbildung) hauptsächlich im Umfangsgeschwindigkeitsbereich 1..9 m/ s, Diss. TU-München 1984 [S-2] Schaeffler: MOEWE Zwischenbericht Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 43 Figure 10.2: Differences of viscosity-temperature-behaviour of base oil types T+S_4_17 07.06.17 17: 27 Seite 43