24th International Colloquium Tribology - January 2024 183 Comparison of Different Standard Test Methods for Evaluating Greases for Rolling Bearings under Vibration Load or at Small Oscillation Angles Markus Grebe 1* , Henrik Buse 2 , Alexander Widmann 3 1 Competence Center for Tribology, UAS Mannheim 2 Tribologie Engineering Mannheim GmbH 3 Steinbeis Transferzentrum at UAS Mannheim, Department Tribology * Corresponding author: m.grebe@hs-mannheim.de 1. Introduction Rolling bearings that are often only operated at small oscillation angles or that are exposed to vibrations when stationary show typical damage after only a short period of operation. This can be classic false brinelling damage, so-called standstill marks or fretting damage. It is important to differentiate here according to the so-called amplitude ratio (x/ 2b), which indicates the ratio between the movement of the rolling element (x) and the Hertzian contact half-axis (b). Depending on this ratio, suitable laboratory test methods must be used to test the lubricating grease in a practical manner for the respective application. In the context of the lecture, the scientific fundamentals of these special operating and test conditions are explained and test results of model lubricants in these three standard rolling bearing tests as well as in a classical Fretting test under oscillating sliding friction are compared with each other. 1.1 Aim of the Test Series The aim of this in-house series of tests was to compare various known laboratory test methods for evaluating lubricating greases for rolling bearings that only perform small oscillating movements or are only subjected to vibration loads (e.g. blade bearings in wind turbines). The Fafnir wear test according to ASTM D4170 [1] and the SNR-FEB2 test [2], which is frequently required in Europe, can be mentioned here as standard procedures. In addition, an in-house test was carried out that simulates bearings at very small angles of oscillation and vibration (KTM QSST) [3], [4], [5]. The new NLGI grease specification for high performance multi-purpose (HPM) greases [6] also requires the SRV test according to ASTM D7594 [7] for greases with higher loads, so this was also included in the test series. For the series of tests, four mineral oil-based grease samples were prepared with two types of soap thickeners (lithium and calcium sulfonate), which are known to give different results in the Fafnir test. However, the aim of the series of experiments was not to find a particularly suitable grease for the application, but to show that the greases give different results depending on the tribological stress collective. In the first part of the presentation, the scientific principles of rolling bearings that are only operated under small angles of oscillation or are subjected to vibrations are explained. In these studies, it is important to consider the amplitude ratio x/ 2b, which is the ratio between the rolling element’s rolling path (x) and twice the Hertzian contact width (2b). At a ratio smaller than 1, parts of the contact are never opened, which makes the re-entry of lubricant much more difficult or even impossible. At an amplitude ratio greater than 1, reflow is possible in principle and depends strongly on the rheology of the grease [8], [9]. In the second part of the presentation, the operating and test conditions of common and partly standardized rolling bearing test methods as well as the test results are presented and discussed. 2. Results All conducted tests show that the performance of the lubricants strongly depends on the test conditions and that until today there seems to be no universal lubricant for these different operating conditions. This also confirms the results of previous tests at KTM [5]. A high oil release is advantageous for rolling bearings operating with relatively small vibration angles. However, changing the thickener can also be promising (Figure 1). On the whole, the ranking in the Fafnir test and the SNR-FEB2 test does not differ. In the Fafnir test, however, slightly different values are obtained at the industry partner and at the KTM. The results show, however, that no major differences in the rankings are to be expected for amplitude ratios significantly above 1, although the C/ P ratio in the SNR-FEB2 test, for example, is significantly smaller (Fafnir approx. 7.6; SNR-FEB2 approx. 3.1). Figure 1: Results of the bearing tests with x/ 2b > 1 The result is completely different for the tests with a small amplitude ratio (Figure 2) (here 0.55). The evaluation on the basis of the school grades shows that grease sample No. 1 with the highest oil release also shows certain advantages in 184 24th International Colloquium Tribology - January 2024 Comparison of Different Standard Test Methods for Evaluating Greases for Rolling Bearings under Vibration Load or at Small Oscillation Angles this test. Calcium sulfonate grease No. 15, which performed very well under wider angles, is hardly better than the other two samples under these conditions. The lithium soap grease with the lower oil release (sample 2) shows the greatest damage. It is noticeable here that the damage is already pronounced after just one minute. Figure 2: Results of the standstill tests; Evaluation by means of (subjective) school grades A comparison with a „classic“ Fretting test under oscillating sliding friction in the SRV test rig shows the problem of the very high contact pressure at the beginning of the test. Two of the four grease samples could not be tested under these standard test conditions. For both lithium soap greases, adhesive failure already occurred during the run-in phase with reduced normal force. Both calcium sulfano greases, on the other hand, ran through without any major differences in friction and wear behavior. The high contact pressure in point contact requires a special lubricant or additive chemistry to prevent seizure, which is ultimately of secondary importance in rolling bearings. The test should therefore only be used if high local contact pressures and pure sliding friction are to be expected in practice. 3. Conclusion It can be shown that the performance of the lubricants strongly depends on the test conditions and that so far there does not seem to be one universal lubricant for these different operating conditions. This series of tests was intended to show that, for meaningful screening, a laboratory test must be selected that reflects practical conditions as closely as possible. This was clearly demonstrated using the specially formulated model greases. In particular, small x/ 2b ratios of less than 1 represent a major challenge for the lubricants used. Greases that still perform very well at x/ 2b ratios greater than 1 can be completely unsuitable here. The SRV fretting test (ASTM D7594) is not suitable for reliably predicting the lubrication behavior of a grease in a rolling bearing. If necessary, results from several tests must also be taken into account for a final decision, if different conditions may prevail in practice. For example, the conditions in the blade bearings of wind turbines are very different. Large x/ 2b ratios occur during the adjusting motion of the blades, while small x/ 2b ratios must be considered when the blades are stationary under vibration [10]. Therefore, the lubricant selected must always represent a compromise. References [1] American Society for Testing and Materials. ASTM D4170: 2016 - Standard Test Method for Fretting Wear Protection by Lubricating Greases, 2016. [2] Normalisation Francaise. Nft 60-199 - aptitude à résister au faux effet brinell, 1995. [3] M. Grebe, P. Feinle. Brinelling, False-Brinelling, „false“ False-Brinelling? - Ursachen von Stillstandsmarkierungen und geeignete Laborprüfungen; Jahrestagung der Gesellschaft für Tribologie, (GfT) Tagungsband, 2006. [4] M. Grebe. False Brinelling - Standstill marks at roller bearings. PhD thesis, Slovak University of Technology, Bratislava, 2012. [5] M. Grebe. False-Brinelling und Stillstandsmarkierungen bei Wälzlagern - Schäden bei Vibrationsbelastung oder kleinen Schwenkwinkeln. Expert-Verlag, Renningen-Malmsheim, ISBN 978-3-8169-3351-9; 2017. [6] R. Shah, J. Jinag, and J. Kapernik. Next-generation NLGI grease specifications. NLGI Spokesman, 83(4): 63-73, 2020. [7] American Society for Testing and Materials. ASTM D7594: 2019 - Standard Test Method for Determining Fretting Wear Resistance of Lubricating Greases Under High Hertzian Contact Pressures Using a High-Frequency, Linear-Oscillation (SRV) Test Machine, 2019. [8] C. Schadow and L. Deters. Abschlussbericht Forschungsvorhaben Nr. 540 I: False Brinelling - Stillstehende fettgeschmierte Wälzlager unter dynamischer Belastung. FVA-Forschungsheft 951, 2010, Arbeitskreis Schmierstoffe und Tribologie, 951: 127, 2010. [9] S. Tetora, C. Schadow, and D. Bartel. Abschlussbericht Forschungsvorhaben Nr. 540 III: Stillstehende fettgeschmierte Wälzlager unter dynamischer Belastung. FVA-Informationsblatt, 2022, Arbeitskreis Schmierstoffe und Tribologie, 1500: 4, 2022. [10] M. Stammler. Endurance Test Strategies for Pitch Bearings of Wind Turbines. Ph.D. Thesis, Fraunhofer Verlag, Stuttgart, Germany, 2020.