suitability of a lubricant for their application. They ask the bearing and / or gearbox manufacturers to release a lubricant. To release a lubricant, an approval is required and mostly it is related to minimum requirements for the lubricant candidate. The choice for a suitable lubricant includes the requirement for the properties of minimizing friction, remove heat and provide sufficient viscosity, to separate contact surfaces, without harming the gears and bearings. Based on these requirements, test criteria and requirements are defined. For rolling bearings and gears, an example for those requirements can be found in [18]. What is missing are requirements for the damage mode WEC. The proposed FE 8 test rig A lot of research results are available for the axial cylindrical roller bearing 81212. In this paper 139 data files are available for the 81212 and 2 for the 81206, reported in 7 publications. See table A-1. With WEC, 101 bearings failed and without WEC, 40 bearings are reported. For the assessment of the possibility of occurrence of WEC, the calculation model [3, 4, 5] is used. With this model it is possible, to calculate a mechanical basic safety for the tested bearings, based on design, load, bearing capacity and speed, which can be used, to compare test results among themselves, a so-called compa- Science and Research 32 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0015 Introduction Since nearly 2011, WEC bearing damages have been observed. WEC damages are reported for instance by Johan Luyckx [1] in the industrial application wind turbine gear unit. In his presentation, failure rates of up to 70 % during the first 2 years are mentioned for non-coated bearings. These damages are identified as “White etching Cracks“ so called WEC. First research investigations with respect to WEC failures are published in 2014 by the Forschungsvereinigung Antriebstechnik FVA [2]. In the Bearing World Journal Vol. 5 (2020) [3], a knowledge based analytic and AI suitable calculation method is published by Leimann, to evaluate the risk of WEC occurrence in industrial applications. The method can adjust the quality of the results in a self-learning process by each added test result. WEC damages on rolling bearings, their damage patterns, hypotheses of origin and influences on the origin are deeply discussed in the GfT presentation, reported in 2023 [17]. In this presentation, also design proposals are done to reduce the risk of the occurrence of WEC, for instance, the choice of bearing material and coatings as black oxide. With respect to the influence of lubricants, a lot of research tests are carried out. Unfortunately, a direct assignment for these lubricants between technical or chemical component data as the type and quantity of chemical elements is, with view to the state of the art of research, hardly possible. There is a lot of research done to find a direct assignment of chemical elements and their volume share to the occurrence of WEC, but until now, there is no clear result. Also, field observations of damaged bearings cannot help to differentiate lubricants with respect to their behavior of creating or not creating WEC, because the operating and ambient conditions differ a lot during the runtime and influences as water or current or voltage go through are difficult to capture in the field. End customers and OEMs expect from bearing, gearbox, and lubricant manufacturers recommendations about the Thoughts on a standardized FE-8 Test for the assessment of the WEC-carrying capacity of lubricants in rolling bearings Dirk-Olaf Leimann * Presented at the GfT Conference 2024 For an amount of 141 WEC-Tests on axial cylindrical roller bearings on a so-called FE 8 test equipment, a mechanical WEC safety is calculated from the time to failure value compared with the L 10h life. From these values, a factor is derived to get an idea about the influence of the tested lubricants on WEC forming. Furthermore, a study on relevant influence parameters as load and speed is done. Out of these data, a proposal is done for a standardized WEC-test for lubricants. Keywords WEC-Tests, axial cylindrical roller bearings, wind turbine gearboxes, WEC damage, FE 8 test rig Abstract * Dipl.-Ing. Dirk-Olaf Leimann, Düsseldorfer Straße 4, 47441 Moers rative reliability or safety S WEC . Out of this comparison it is possible, to define test conditions, which are suitable for lab testing at research institutes and test labs from lubricant suppliers on available FE 8 test equipment. The paper presents results of the mechanical WEC safety S WEC for the 141-bearing data in the table A-1. It is visible that, without taking the lubricant into account, all bearings would not fail. With the time to failure of the WEC damaged bearings, a second factor is calculated for the bearings with WEC damage. Out of these results a factor is formulated, to capture the influence of a lubricant. Furthermore, a parameter study is done with variations on load, speed, and number of rollers to determine suitable test parameter. To complete the investigations on influence factors, the 141 data are evaluated with respect to lubricant technical and chemical properties. Taking all results into account, a standardized FE-8 test for the assessment of the WEC-carrying capacity of lubricants in rolling bearings is proposed to discuss. The proposed test parameter reflects to the axial load, speed, number of rollers, oil flow, material, oil viscosity at 100 °C and test temperature. Additional test parameter from research results should be discussed to add. The so-called FE 8 test design (Image 1) is a well-known and accepted test equipment with a high availability at research institutes and industry to test lubricants regarding their suitability for the use with rolling bearings. On FE 8 test rigs, test bearings can be axial cylindrical roller bearings 81212, taper roller bearings 31312 and angular contact ball bearings 7312. Information about the FE 8 test rig with more details can be found in the GfT data sheet “GfT FE8 N 060” [6]. The mentioned bearing types represent a large spectrum of industrial applications. The standard material of these bearings is 100 Cr 6. In the research investigation 707 I of FVA [2] a lubricant “Low Ref SAE30” is used with a charge 1 and 2 and an axial cylinder roller bearing 81212 on a FE 8 test rig. By using this lubricant, WEC damages could be reproducible produced with the axial cylindrical roller bearing. Tests with a radial cylindrical roller bearing with this lubricant and similar load, but higher speed conditions, could not produce WEC damages. Especially with research on wear behavior, WEC damages could not be observed on FE 8 test rigs, even with a big range of different lubricants all kind of. For instance, in the cited FVA investigation Nr. 327 „Entwicklung experimenteller Grundlagen für eine schmierstoffabhängige Verschleißlebensdauerberechnung für Wälzlager (Ölkennwerte für Wälzlager)“ [7], 10 very different lubricants were examined with bearing types 81212, 31312 Science and Research 33 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0015 Image 1: FE 8 test rig with angular contact ball and axial cylindrical bearing [2,6] Table 2 shows, that non coated bearings have failure rates up to 70 % in between 2 years run time. Problem description and existing recommendations for rolling bearing tests The GfT presentation “White Etching Cracks” (WEC) [17] lists possible influences to generate WEC damages. Mainly, these could be current or voltage go through, hydrogen embrittlement and chemical elements or compounds. In tests, a correlation between damage occurrence and quantity of hydrogen, amperage or current voltage could be found. Unfortunately, direct correlations for lubricants between chemical elements or compounds and quantities could rarely be seen, because of too much possible components and elements and their chemical possibilities. So, the only judgment could be, WEC damage occurred or not. From the IEC 61400-4 [8] it is known, that a lubricant is suitable for rolling bearings in the wind turbine application, if the lubricant fulfills the recommendations, shown in the tables E5 and E6. Rolling bearings should fulfill those requirements with respect to wear and fatigue behavior, determined on FE 8 tests. Table 3 is a summary of test methods, load parameters and requirements. The load parameters are like those, reported in table A-1 in the annex. Similar recommendations from the industry and specifications are shown in table 4. Based on the approach of the IEC 61400-4, the availability of test specimen and test equipment, a sufficient data base on WEC tests for the axial cylindrical roller bearing 81212 with and without WEC damages and the possibility, to cover the mechanical basic safety, it is obvious, to define test parameters for a WEC lubricant test on FE 8 test rigs. Science and Research 34 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0015 and 7312 without reported subsurface damages. Contact stress and test duration are comparable to the data from [2]. The oil viscosity at 100° C was between 7 and 16 mm 2 / s. Speed values were lower with a factor of 4 to 40. There were also test times with a duration of more than 500 hrs. In literature it can also be found, that WEC damages can also be generated on test rigs with radial bearings. But these test rigs differ a lot from each other in design, bearing type and size and are not standardized as the FE 8 test rig [21] with the bearing 81212. For wind turbine gear boxes, a list of recommendations for the assessment of gear lubricants are mentioned in tables E5 and E6 [8] with respect to FE 8 tests and the use of the lubricant in the wind turbine application. The calculation method to evaluate the risk of WEC occurrence In the cited papers [3, 4, 5], there are detailed information’s about the applied calculation method. The method is a knowledge based analytical model, based on real field data and results from research test data. The method compares permissible contact stresses with occurring contact stresses and from those, WEC safety factor S WEC is formed for the assessment of the probability of WEC failures. This method is used to find design and mechanical test parameter as load, speed, contact stress, nominal life time and number of rollers, which enables to find conditions, that could make a serious influence of a lubricant visible. The safety factor judges the risk of the occurrence of a WEC damage. WEC damages on Wind Turbine Gear Boxes Johan Luyckx [1] reports in 2011 serious bearing damages due to WEC on wind turbine gearboxes. He compares black oxide coated bearings with non-coated bearings. ! " # $# # $# % & Table 1: Definition of the safety factor S WEC with respect to the probability of occurrence Table 2: Failure rates versus gearbox population Image 2: WEC damages  The data pool Out of various publications, available data for at least 141 WEC tests were collected. These files include design data, load data, data about the lubricant and other informative data. In total 49 specific data for each tested bearing were available. The main data regarding design, load, speed, and lubricant are summarized in the table annex A-1. With the data from the 141 bearing tests, the following tables and diagrams are extracted. Compared are the results of all 141 data files with 101 data files from bearings with WEC and the 40 files without WEC. The life time factor S time to failure in table 5, 6 and diagram 2 is calculated from the time to failure according to the equation (1). (1) with: L t time to failure in hrs L 10 h calculated life in hrs The diagrams show the results of the cases with WEC. Table 5 shows the minimum, maximum and average values for the main parameters and an estimation of the influence of the lubricant. The 141 data files include 139 data sets for the axial cylindrical roller bearing 81212 and 2 sets for the bearing 81206. 137 tests were done with an oil flow between 0,1 and 0,25 l / min and 4 tests were done with an oil flow of 1,2 l / min. With the higher oil flow of 1,2 l / min, WEC damages were not observed. 1 3346 73879 4: ; 7 7 7 7 Science and Research 35 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0015 Table 3: Test recommendations in IEC 61400-4 “Design requirements for wind turbine gear boxes” Table 4: Examples for the recommendations from industry for tests with FE 8 [19,20,21] 346 73879 4: ; 7 Table 5: Evaluation of the 141 data files mentioned in [8] and failure rates mentioned in [1]. With these estimated data, the average damage factor is 1,90 (see diagram 2). That means, that a FE 8 test could representative and be used, to give a judgment on the suitability of a lubricant with respect to WEC damages. Science and Research 36 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0015 The results of the 101 data sets contain 22 different lubricants, where WEC damages could be created. As visible in the diagram 2, the average of the estimated damage factor is about 2,1. In table 6, an estimation of the damage factor is done for a three-stage planetary gear unit for the application wind under the conditions Table 6: Possible WEC damage factor with the assumptions from [8] and [1] Diagram 2: Results of the possible influence of lubricants to create WEC Diagram 1: Data and results for the 101 bearing tests with WEC damage Design and load influence The evaluation of table 5 shows, that WEC damages can occur at contact stresses between 1750 and 2219 MPa and loads between 25 and 100 kN. In table 7 in IEC 61400-4 [8] it is recommended, that the maximum contact stress should not exceed values between 1300 and 1650 MPa. Looking to the speeds in table 5, it can be recognized, that they are in the range of table 6 and that the average and minimum values of non WEC test sets are higher, than with WEC. In the research project FVA 707 I [2] also tests with the radial cylindrical bearing NU 206E in combination with the lubricant “Low Ref SAE30-2” were carried out. While using the same contact stress and same lubricant, no WEC damage could be created. The axial cylindrical roller bearing 81206 had 12 rollers and the bearing 81212 was tested with 15 rollers. In other cited documents, also 19 rollers are reported for the 81212. Influence of a lubricant The cited publications on WEC tests on FE 8 test rigs show, that the lubricant could have a serious influence for WEC creation. How these lubricants behave in the field is difficult to estimate because of variable ambient and load conditions. Data for field experience are rarely published. One very good description on field experience is done by O.L. Jensen [15], but more detailed information’s are not available. For 76 from 141 WEC tests, data about the lubricant and the possible influence of mechanical data and data about chemical elements and compounds are available. 65 bearing tests failed with WEC and 11 tests had no WEC damages. Table 7 shows a summary of some of those more specific lubricant data. Suitable test parameter Out of an analysis of all data it turns out, that a test bearing 81212 with 15 rollers is suitable for a standardized test on a FE 8 test rig with load parameters, like research test and field experience. The proposal is supported by the calculated WEC safety S WEC . The variation of parameters and their results are shown in table 8. From table 8 it can be seen, that a test speed of 350 rpm in combination with a bearing load of 60 kN, resulting in a contact stress of 1750 MPa, are suitable test parameters. • Bearing 81212 • Number of rollers 15 • Oil inlet volume 0,2 l/ min • Material 100 Cr 6 Science and Research 37 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0015 Table 7: results of the evaluation of specific lubricant data Table 8: Suitable test parameters for a standardized WEC test [7] N. van de Sandt et al, „Entwicklung experimenteller Grundlagen für eine schmierstoffabhängige Verschleißlebensdauerberechnung für Wälzlager“, Vorhaben FVA 327, Heft 665, 2011 [8] NN, Wind Turbines - Part 4, Design requirements for wind turbine gear boxes, IEC 61400-4, Genève, 2012 [9] H.K. Danielsen et al, “Accelerated White Etch Cracking (WEC) FE8 type tests of different bearing steels using ceramic rollers”, Wear 494-495 (2022) 204230 [10] H.K. Danielsen et al, “FE8 type laboratory testing of white etching crack (WEC) bearing failure mode in 100Cr6”, Wear (2019) 202962 [11] J.Loos, „Einfluss der Reibbeanspruchung auf die WEC- Bildung in Wälzlagern“, Tagungsband GfT Fachtagung, Göttingen, 2014 [12] A.D. Richardson et al, “The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 steel”, Springer Verlag, Tribology Letters (2018) [13] H. Surborg, „Einfluss von Grundölen und Additiven auf die Bildung von WEC in Wälzlagern“, Dissertation Universität Magdeburg, Shaker Verlag 2014 [14] F.G. Guzmán, White Etching Cracks (WEC) in ölgeschmierten Wälzkontakten, Verlag Mainz, 2020, ISBN 978-3-95886-340-8 [15] A. Ruellan, K. Stadler, “Can oil chemistry accelerate WEC-associated damage in thrust & radial bearings? ”, Bearing World 3 rd international FVA-conference, 2020 [16] O.L. Jensen et all, “Prevention of “White Etching Cracks” in rolling bearings in Vestas wind turbines” Tagungsband CWD ATK, Aachen, 2021 [17] F.G. Guzmán et all, “GfT-Positionspapier „White Etching Cracks - WEC“ Tagungsvortrag 63. Tribologie-Fachtagung, GfT, Göttingen 2022 [18] D. Leimann, “Hansen selection criteria for lubrication oils for gearboxes in wind turbines” Tagungsband 17 th International Colloquium Tribology, TAE, Esslingen, 2009 [19] NN.: Druckschrift 7300de, “Getriebeschmierung und Getriebekonservierung”, Flender Bocholt, 2022 [20] NN.: Druckschrift PI 4-1104, “Produktinformation Fuchs Renlolin Unisyn CLP“ Mannheim [21] NN, DIN 51819-3 „Prüfung von Schmierstoffen - Mechanisch-dynamische Prüfung auf dem Wälzlagerschmierstoff-Prüfgerät FE8 - Teil 3: Verfahren für Schmieröl - einzusetzende Prüflager: Axialzylinderrollenlager”, DIN-MEDIA 2016 Science and Research 38 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0015 Proposal To evaluate a lubricant regarding the suitability in an industrial application and to give a judgment about the WEC behavior, the test parameters from table 8 should be further discussed. The proposal is based on the best practice approach of the IEC 61400-4 [8] for recommendations on lubricants for the use in wind turbine gearboxes Citation from [8]: “Wind turbine gearboxes can be quite varied in their design and configuration. As such, lubrication requirements may vary among gearboxes and also by their operating environment, i.e., ambient conditions, duty cycle, etc. The information contained in this annex is designed to provide users (end users, owners, turbine manufacturers, and component suppliers) with a guideline for a minimum level of lubricant performance for this application. Additionally, guidelines for condemning limits of selected lubricant parameters are offered based on experience in the industry.” So, a judgment could be: If a test according the parameters from table 8 is carried out on a FE 8 test rig and the lubricant did not lead to the formation of WEC damages, the minimum conditions of the test are fulfilled and the tested lubricant is suitable for the application. Literature [1] J. Luyckx, “WEC failuremode on rollerbearings”, VDI- Wissensforum, Tagung Gleit- und Wälzlager Schweinfurt, Düsseldorf 2011, ISBN 978-3-18-092147-1 [2] C. Bongardt, M-O Özel et al The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 steel, Risse auf Lageringen, Gefügeveränderungen in Wälzlagerringen mit Rissen als Folgeschaden, Vorhaben FVA 707 I, Heft 1121, 2014 [3] D. Leimann, “Calculation Method to Evaluate the Risk of WEC Occurrence in Industrial Applications”, Bearing World Journal Vol. 5 (2020), VDMA, Frankfurt 2020 [4] UK Patent Applikation GB2585272A “Comparable stress for rolling bearings”, Intellectual Property Office, Newport, United Kingdom, 2021 [5] Offenlegungsschrift DE102021128876A1 “Vergleichspannung für Wälzlager 2”, Deutsches Patent und Markenamt, München, 25. Mai 2022 [6] N 060 mod 1, GfT Arbeitsgruppe „Datenbank Tribologische Prüfstände“ - Datenblatt FE 8 Prüfgerät, Science and Research 39 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0015 Annex Table A-1: bearing test data for 81212 and 81206