Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 1 Introduction Quality requirements and supplier specifications for lubricants have become more specific in order to implement common knowledge about lubricant performance and therefore reduce the risk of downtime. One example demonstrating the importance of restricting the usage of materials is that mineral base oils can have different behaviours regarding wear protection and / or seal compatibility depending on the source they have been extracted from and the individual manufacturing process. It therefore makes sense to restrict the use in applications that are sensitive in terms of seals and wear to the use of the mineral base oils that are known to deliver good performance. Do the same rules apply for synthetic base oils, in particular for PAO? 2 Synthetic Base Oils versus Mineral Base Oils Synthetic products, e. g. gear oil formulations based on synthetic hydrocarbons are meanwhile widely appreciated in industrial applications, mainly due to their much better viscosity temperature behaviour, better cold start up behaviour and improved oil lifetime. Unlike mineral oils they are synthesised to a defined specifications. The size and structure of the molecules are very uniform. Even comparing synthetic base oil qualities from different suppliers shows very little diffe- 15 Aus Wissenschaft und Forschung * Kirsten Schwörer BP Europa SE, Geschäftsbereich Industrieschmierstoffe Mönchengladbach Dr. Martin Predel BP Europa SE, Global Lubricants Technology Mönchengladbach The effects of base oil exchange in a PAO based gear oil K. Schwörer* Eingereicht: 21. 7. 2015 Nach Begutachtung angenommen: 5. 8. 2015 Einerseits sind Qualitätsanforderungen und Lieferspezifikationen für Schmierstoffe sind spezifischer geworden, um den allgemeinen neuen Erkenntnissen über Schmierstoffleistungen gerecht zu werden und somit das Risiko von Ausfallzeiten von Produktionsanlagen zu minimieren. Andererseits sind Lieferzuverlässigeit und Produktqualität unmittelbar abhängig von der Flexibilität der Formulierungszusammensetzung. Maschinen- und Anlagenhersteller begrenzen diese Flexibilität bewusst, da man in einigen Fällen nur ein sehr eingeschränktes Wissen über die Einflüsse der alternativen Rohstoffe auf die Leistungsdaten der fertigen Formulierung hat. In diesem Artikel wird ein Fall vorgestellt, bei dem in einer bestehenden Formulierung ein Austausch eines PAO Basisöles und die Auswirkung auf die Schmierstoffleistungsfähigkeit in chemischen, physikalischen und mechanischen Tests untersucht wurden. Schlüsselwörter Schmierstoffspezifikation, PAO, Basisöl, Leistungsniveau der Formulierung, PAO Typen, synthetisches Getriebeöl, physikalische Parameter, FZG Test, FE8 Test, Grauflecken Test On the one hand, Quality requirements and supplier specifications for lubricants have become more specific in order to implement common knowledge about lubricant performance and therefore reduce the risk of downtime. On the other hand reliability of supply and product quality are directly dependent on the flexibility allowed for a lubricant formulation Equipment manufacturers limit this flexibility, due to limited knowledge about the effect on the formulation performance if components were changed. This paper describes one case based on the PAO base oil interchange and the effect on the performance in physical, chemical and mechanical tests. Keywords Lubricant specification, PAO base oil, formulation performance, PAO types, synthetic gear oil, physical parameters, FZG Test, FE8 Test, micropitting tests Kurzfassung Abstract T+S_1_16 21.12.15 10: 54 Seite 15 16 Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 rence in terms of the chemical composition. Are these specifications close enough to allow interchanging of PAO base oil types from different suppliers, to enable more flexibility and reliability in the supply chain? 2.1 PAO types investigated PAO base oils from 2 suppliers were examined in this study. As all PAO base stocks they were manufactured from olefins to an oligomer with the required viscosity. They only consist of carbon and hydrogen, without any other atoms in the molecules. A difference can be observed in the viscosity at 100 °C and the Pour point, which is caused by the different PAO base oils used to blend these two formulations. This will not lead to a much different overall performance in the application within a gearbox as we will show in the following sections, even though it might theoretically indicate slightly improved low temperature start and higher temperature film performance. 3.2 Wear behaviour In this section data relating to the following performance and typical properties regarding wear-specific behaviour is compared between formulations based on two alternative base oils: • FZG scuffing wear • Micropitting protection • Bearing wear performance 3.2.1 FZG scuffing wear Both formulations have passed the FZG scuffing wear test DIN ISO 14635-1 with a result of >14 and therefore Aus Wissenschaft und Forschung Figure 3.2.1.1: Test gear tooth profile A Table 2.1.1: Typical data of alternative raw material PAOs used to blend the synthetic gear oil formulations Parameter Units Method PAO A PAO B Kin. Viscosity mm 2 / s ISO 3104 404 406 at 40 °C Kin. Viscosity mm 2 / s ISO 3104 40.5 48.5 at 100 °C Viscosity - ISO 2909 151 181 Index specific Denkg/ m 3 DIN 51757 840-850 840-850 sity at 15 °C Flashpoint °C ISO 2592 > 250 > 250 COC Table 3.1.1: Typical data of synthetic gear oils A and B Synthetic Synthetic Gear Oil Gear Oil Parameter Units Method A B Kin.Viscosity mm 2 / s ISO 3104 325 326.5 at 40 °C Kin.Viscosity mm 2 / s ISO 3104 34.1 40.8 at 100 °C Viscosity - ISO 2909 152 179 Index specific Denkg/ m 3 DIN 51757 853 849 sity at 15 °C Flashpoint °C ISO 2592 >250 >250 COC Pour point °C ISO 3016 -36 -45 3 Test schedule & results Two ISO VG 320 gear oils were blended with the 2 alternate PAO base stocks. All other components and the manufacturing process remained unchanged. We looked for any effects on the final performance of the PAO base oil change in terms of the following criteria: • Basic physical properties • Wear behaviour • Further investigations 3.1 Basic physical properties In this section data relating to the following performance and typical properties is compared between formulations based on two alternative base oils: • Kinematic viscosity • Density • Flash point • Pour point T+S_1_16 21.12.15 10: 54 Seite 16 Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 Many of today’s modern gear oil formulations pass this test with the load stage equal to 10. Excellent formulations pass with the result higher than 10. The tested gear oil formulations pass the Micropitting test at 90 °C with a result of higher than 10 with a very similar trend which is fully within the repeatability of this test method. Below is an example of the trend regarding the size of the micropitting area on the test gears, which is one of the criteria, used to rate the micropitting protection capability. The results are within the usual test repeatability and remain well below failure limits, indicated by the shaded areas in the plots. 17 Aus Wissenschaft und Forschung Figure 3.2.2.1: Test gear set from FVA-Nr. 54 after load stage 10 pass Graph 3.2.2.1: Development of micropitting area with Synthetic Gear Oil A Graph 3.2.2.2: Development of micropitting area with Synthetic Gear Oil B exceed the requirements specified for standard industrial gear oils in DIN 51517 part 3 [D-2] or other international standards defining the physical and chemical characteristics of EP gear oils. No difference was observed in the performance behaviour of both product formulations. 3.2.2 Micropitting protection Micropitting is a damage that is likely to appear very early in gearboxes, leading to small micropittings and micro cracks on the surface that tend to grow. The results will be growing pittings and / or a change of the tooth profile, leading to further wear. This failure mode was discovered to be very typical for wind turbine gear box applications, and the main causes of this failure were identified to be a too low viscosity of the lubricating oil, a too high roughness of the surfaces and lubricants with additives non capable of preventing this failure. To address these phenomena a micropitting test based on the FZG-test rig with a minimum test result of load stage 10 at a test temperature of 90 °C is required from the majority of gear suppliers and some of major machine manufacturers. Failures due to micropitting are still severe failure modes in some applications in the field. They lead to long and costly downtimes of wind turbines. The lubricant plays a major role in the prevention of micropitting. Usually the oils are rated using the results of the FVA-micropitting test [F-1] at 90 °C. This test is a load stage test, where after each stage the profile form deviation, the micro pitted area and the wear rate are measured and compared to the state of the art limits. After the load stage test, the lubricant is then tested in an endurance test phase with duration up to 400 hours at load stage 10. This determines the long term capability of the gear oil to prevent micropitting under severe operating conditions. T+S_1_16 21.12.15 10: 54 Seite 17 18 Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 3.2.3 Bearing wear performance Another important characteristic is the ability of the lubricant and its additive system to cope with the severe and broad operating conditions in bearings used in wind turbine gear boxes. In the past experiences in practice have shown that some gear oils in the market were showing quite excellent protection of gears but bearing damages caused by a certain aggressiveness of the oil against bearing steels and brass cages could be observed. the prevention of wear at boundary lubrication; FE8- 80 h; 7.5/ 100-80. In the test mode step 1 the bearing is loaded with 100 kN and rotating at a speed of 7.5 m/ s. The test temperature of 80° is achieved by heating the oil and the test runs over a period of 80 hours. The bearing manufacturer basically rates the wear rate Mw50 on the rollers as well as if rippling or micropitting could be observed on the surface of the washers of the test bearings. In addition to this the test result also gives information about the friction torque at the bearings during the test Aus Wissenschaft und Forschung Figure 3.2.3.1: FE8-Test rig [D-3], setup for FE8wear tests with gear oils Figure 3.2.3.1.1: Test bearing for FE8-Test [D-3], setup for FE8-wear test under mixed friction conditions In consequence bearing manufacturers set up a couple of requirements to observe and compare the behaviour of a gear oils regarding the corrosion protection (also under the influence of salt water), the formation of residues under the influence of water and temperature, several wear tests on FE 8-test-rig [D-3; D-4] and endurance tests on test benches for antifriction bearings 3.2.3.1 FE8 step 1 The most common bearing test that is nowadays also used for the validation of used oils is the FE8 step 1 test. This tests proves the behaviour of a gear oil regarding Table 3.2.3.1.1: FE8-step 1 requirements for new oils and test results Requirement to be rated Synthetic Synthetic with a grade Gear Oil Gear Oil 1 A B bearing roller < 10 mg < 1 mg 2 mg wear Mw50 rippling small none none micropitting none none none The rating would be a grade 1 according to Schaeffer’s requirements for the evaluation and rating of those test results. 3.2.3.2 FE8 step 2 A second very important test is the FE8 stage 2 test. In this test the lubricant is running under mixed friction lubrication conditions in two roller thrust bearings at a defined temperature of 70 °C. After 800 hours without failure of the bearings due to pitting formation the roller wear is measured. The nominal lifetime Lh10 of the bearings is calculated to be 300 hours, so a test time of 800 hours is a strong test requirement and a good criterion for the differentiation of the lubricants. Both formulations pass this test with good results of 16 mg and < 1 mg roller wear. 3.2.3.3 FE8 step 4 In this test the lubricant is tested towards its long term suitability for bearing lubrication and the behaviour in the presence of water, specifically the residue formation under the presence of water and elevated temperature. The test results from this test are shown in Table 3.2.3.3.1. Results are very comparable, if not the same. Looking in further detail at the evaluation of the condition of the T+S_1_16 21.12.15 10: 54 Seite 18 Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 preheating system the condition for both oils after the test are shown in the following two figures. The appearance of residues is within the repeatability of this test and therefore both products are showing the same performance. 3.3 Further investigations In this section data relating to the following performance and typical properties regarding further performance is compared between formulations based on two alternative base oils: • Copper corrosion • Rust behaviour • Foam Properties • Heat capacity • Shear stability 3.3.1 Copper Corrosion & Rust behaviour PAO base oil do not affect the influence on rust and corrosion as these types of base oils do not have any active chemical polar functions. The properties of PAO gear oil on corrosion and rust is dominated by the additive system. As these remained unchanged in both formulations no differences were observed. 3.3.2 Foam behaviour The foam behaviour of lubricants is usually determined by two test methods: • Foam test according to ISO 6247, where the foaming properties at different test temperatures is determined in a model test, by blowing air through a porous ball (Figure 3.3.2.1) [I-2] • Flender Foam Test Method, which also gives an indication about the air separation of the immersed air bubbles into the oil test reservoir. (Figure 3.3.2.2) No foam was observed in either of the two gear oil formulations in the ISO 6247 test, in which air is blown into the oil at 23.5 and 95 °C and again at 23.5 °C. 19 Aus Wissenschaft und Forschung Figure 3.2.3.3.1: Residue in the preheating system for Synthetic Gear Oil A Figure 3.2.3.3.2: Residue in the preheating system for Synthetic Gear Oil B Figure 3.3.2.1: An ISO foam test according to ISO 6247 in process Table 3.2.3.3.1: FE8-Step 4 requirements for new oils and test results Requirement to be rated Synthetic Synthetic with a grade Gear Oil Gear Oil 1 A B Running time > = 600 > 600 > 600 Filter blocking 0 0 0 Wear of < 10 0 3 rollers [mg] Wear of < 20 8 10.9 cage [mg] Fatigue no no no damages Residues at slight slight slight the bearings Residues at the slight slight Slight/ preheating system moderate T+S_1_16 21.12.15 10: 54 Seite 19 20 Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 In the Flender foam test the oil sample is mixed with air by a pair of gear wheels turning at high speed (half-immersed, 1400 min -1 ) at a defined temperature (usually 25 °C). After 5 minutes running time the drive is switched off. After switching off the increase in oil volume due to air inclusion and the surface foam are measured on a scale. The volume and the surface foam are observed for a period of 90 minutes and documented in a graph. The criteria used for the evaluation of foaming behaviour is the percentage of volume increase found in the initial phase of the observation period 1 minute after switching off the foam tester. The requirements of new oils for the Flender foam test are usually specified according to the below table, as well as the test results determined for the test candidates: ing systems in gear boxes. A suitable test to determine the specific heat capacity is a test according to ASTM E1269 using Differential Scanning Calorimetry. Test results are shown in table Aus Wissenschaft und Forschung Table 3.3.2.1: Results of the Flender Foam Test Test Foam development temperature Require Synthetic Synthetic ment Gear Oil Gear Oil A B 25 °C 15 % 7 % 6 % Table 3.3.2.3: Specific heat capacity Specific heat Synthetic Synthetic capacity Gear Oil A Gear Oil B Specific heat at 2.105 2.105 40 °C in J/ (K*g) Specific heat at 2.354 2.344 100 °C in J/ (K*g) Shear stability Synthetic Synthetic DIN 51350-06-KRL/ A Gear Oil A Gear Oil B Change of kinematic < 0.5 % < 0.5 % viscosity at 100 °C Figure 3.3.2.2: Flender foam test Product formulation candidates show same performance with respect to the foaming properties. 3.3.3 Heat Capacity Gear box manufacturers need to know about the calorimetric properties of gear oils to design cooling and heat- 3.3.4 Shear Stability The lifetime of gear oils is often limited by a low viscosity limit due to shearing. Gear oils using polymers and viscosity boosters are more sensitive to this failure mode. As both gear oil formulations were blended only from pure PAO base stocks they gave both low values in the four ball-KRL test with tapered roller bearings. 4 Conclusion Reliability of supply of lubricants is directly dependent on the flexibility allowed for a lubricant formulation. Equipment manufacturers tend to limit this flexibility, as in some cases there is no knowledge available about how the formulation performance would be affected if components were to be changed. This presentation demonstrates one case based on the interchange of the PAO base oil. The performance is not affected, when an alternative PAO base stock is used to blend this synthetic gear oil. 5 List of References [D-1] DIN 51502; Lubricants and related materials; Designation of Lubricants and marking the containers for lubricants, lubrication equipment and lubrication points; Published: 1990-08; Beuth Verlag [D-2] DIN 51517 Lubricants - Lubricating Oils - Part 3; Lubricating Oils CLP; Specifications; Published: 2009- 06; Beuth Verlag T+S_1_16 21.12.15 10: 54 Seite 20 Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 [D-3] DIN 51757 Lubricants - Lubricating Oils - Part 3; Lubricating Oils CLP; Specifications; Published: 2009- 06; Beuth Verlag [D-4] DIN 51819-1; Testing of lubricants - Mechanicaldynamic testing in the roller bearing test apparatus FE8 - Part 1: General working principles; Published: 1999-12; Beuth Verlag [D-5] DIN 51819 part 3; Testing of lubricants - Mechanicaldynamic testing in the roller bearing test apparatus FE8 - Part 3: Test method for lubricating oils, axial cylindrical roller bearing; Published: 2005-03; Beuth Verlag [D-6] DIN ISO 14635-1; Gears - FZG test procedures - Part 1: FZG test method A/ 8,3/ 90 for relative scuffing loadcarrying capacity of oils (ISO 14635-1: 2000); Published: 2006-05; Beuth Verlag [D-7] DIN ISO 3448: 2010-02; Industrial liquid lubricants - ISO viscosity classification (ISO 3448: 1992) Published: 2010-02; Beuth Verlag [F-1] FVA Informationsblatt 54/ I-IV, Testverfahren zur Untersuchung des Schmierstoffes auf die Entstehung von Grauflecken bei Zahnrädern, Published: 1993 [I-1] ISO 2592: Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity (ISO 3104: 1994 + Cor. 1: 1997); Published: German version EN ISO 3104: 1996 + AC: 1999; Beuth Verlag [I-1] ISO 2909: Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity (ISO 3104: 1994 + Cor. 1: 1997); Published: German version EN ISO 3104: 1996 + AC: 1999; Beuth Verlag [I-1] ISO 3016: Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity (ISO 3104: 1994 + Cor. 1: 1997); Published: German version EN ISO 3104: 1996 + AC: 1999; Beuth Verlag [I-2] ISO 3104 Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity (ISO 3104: 1994 + Cor. 1: 1997); Published: German version EN ISO 3104: 1996 + AC: 1999; Beuth Verlag [I-3] ISO 6247; Petroleum products - Determination of foaming characteristics of lubricating oils; Published: 1998- 06, Beuth Verlag 21 Aus Wissenschaft und Forschung Bestellcoupon Tribologie und Schmierungstechnik „Richtungsweisende Informationen aus Forschung und Entwicklung“ Getriebeschmierung - Motorenschmierung - Schmierfette und Schmierstoffe - Kühlschmierstoffe - Schmierung in der Umformtechnik - Tribologisches Verhalten von Werkstoffen - Minimalmengenschmierung - Gebrauchtölanalyse - Mikro- und Nanotribologie - Ökologische Aspekte der Schmierstoffe - Tribologische Prüfverfahren Bestellcoupon Ich möchte Tribologie und Schmierungstechnik näher kennen lernen. 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