to ASTM D4170 [ASTM4170], which is also listed in the current NLGI high-performance multi-use specification (HPM) as a release test for lubricating greases (HPM-LL = Long Life and HPM-HL = High Load Carrying Capacity) [SHAH2020]. In Europe, the SNR- FEB2 test is frequently used [NFT1995], which is also required by many equipment manufacturers for the release of greases in blade bearings of wind turbines, among other things. In the case of standstill marks due to very small oscillation angles or vibrations, the Competence Center for Tribology Mannheim (in German: Kompetenzzentrum Tribologie Mannheim - KTM) has developed a special test (KTM-QSST) [GREB2012], which is now established in the industry. The pivoting angles vary in these three different standard tests in the range from +/ - 6° in the Fafnir test to +/ - 3° in the SNR- FEB2 test to +/ - 0.5° in the KTM-QSS test; the amplitude ratios here range from 5.5 (Fafnir) to 3.4 (SNR) to 0.55 (KTM-QSST). In addition, “classical” Fretting tests Science and Research 14 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 Introduction Rolling bearings that are frequently operated at small oscillating motions or that are exposed to vibrations at standstill can be damaged after a short time by special wear phenomena such as standstill marks or false brinelling [GREB2008, SCHA2016, GREB2017, SOSA2023, JANU2023]. Damage resulting from these phenomena can significantly reduce the fatigue life of bearings [TETO2023] and cause a wear-related increase of the friction moment [WAND2022], which endanger the function of aggregates. Since this is a special wear problem, the failures do not correlate with standard calculation approaches for life estimation such as ISO 281 [ISO281], since these are based on fatigue theories. It is therefore imperative to have application-oriented and meaningful laboratory tests available to check and evaluate the suitability of lubricants or rolling bearing modifications. In order to ensure that the results of the laboratory tests can also be used in practice, it is essential to simulate the boundary conditions prevailing in the tribological contact as well as possible [CZIC2020; GREB2021]. In the case of oscillating rolling bearings, it is therefore extremely important to consider the so-called amplitude ratio x/ 2b, which indicates the ratio between the motion of the rolling element (x) and the Hertzian contact half-axis (b) [HERT1881]. Depending on this ratio, suitable laboratory test methods must be used to test the lubricating grease in a practical manner for the particular application. For this purpose, there is the Fafnir test according Comparison of different standard test methods for the evaluation of greases for rolling bearings under small oscillating movements Markus Grebe, Henrik Buse, Alexander Widmann* Presented at the GfT Conference 2023 Rolling bearings that are often only operated at small oscillation angles or that are exposed to vibrations when stationary can show typical damage after only a short period of operation. This can be classic false brinelling damage, so-called standstill marks or fretting damage. For lubricant developers and lubricant users it is essential that laboratory test methods are available which allow a statement to be made on the suitability of a lubricant for the respective practical application. This publication explains the scientific basis for these special operating and test conditions and compares the test results of sample greases in these three standard rolling bearing tests and in a classic Fretting test under oscillating sliding friction. Keywords Rolling bearings, greases, false brinelling, standstill marks, Fafnir test, tribometry Abstract * Dr. Markus Grebe Komp.zentrum Tribologie Mannheim Dr. Henrik Buse Tribologie Engineering Mannheim Alexander Widmann Steinbeis Transferzentrum Tribologie Mannheim University of Applied Sciences Competence Center for Tribology Mannheim (KTM) Paul-Wittsack-Straße 10 68163 Mannheim - Germany according to ASTM D7594 [ASTM7594] were also carried out on the SRV test rig in point contact for this series of investigations. Scientific principles on rolling bearings at small oscillation angles or under vibrations Since the lubrication conditions in the tribologically stressed contact zone have a decisive influence on friction and wear, these must be considered very precisely within the so-called tribological system analysis [CZIC2020]. The amplitude ratio x/ 2b instead of the vibration angle allows a transfer of laboratory results also to bearings of other dimensions and loads [STAM2020]. The mere comparison of the vibration angles can lead to erroneous interpretations and thus to an unsuitable lubricant selection, since this is not solely responsible for the mechanical and kinematic contact as well as the lubrication conditions. Science and Research 15 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 Figure 1: Sketch illustrating the x/ 2b ratio at different test conditions (Based on [CATT1938, MIND1949, JOHN2003]) Figure 2: Typical wear mark with small X/ 2b ratio (here 0.55) Up to an x/ 2b ratio of 1, the contact zone between rolling element and raceway is not or not completely revealed (Figure 1). Parts of the Hertzian contact zone show micro-sliding, while the center experiences no movement up to a limit tangential force. In the case of an axial deep groove ball bearing, this leads to the typical ellipse-shaped markings (Figure 2). At x/ 2b ratios > 1, the contact is cyclically opened; typical oscillating rolling motions occur. The partial microslip movements occur only in the reversal points and are not the main cause of damage. Instead, the rolling motion leads to displacement of the lubricant (windshield wiper effect). If this cannot flow back fast enough into x/ 2b > 1. At amplitude ratios above 1, the angles are sufficiently large so that no surface element between the rolling element and raceway is permanently in contact. The contact point is thus completely opened cyclically. In general, reflow and thus relubrication of the contact point is thus possible in principle. Both from the point of view of contact mechanics [JOHN2003, GREB2012a] and from the point of view of lubrication [GREB2014, JANU2023], the tribological conditions thus differ from operating conditions in which the amplitude ratio is less than one. False brinelling occurs when a loaded oscillating rolling contact with slippage is insufficiently lubricated (starved lubrication). Adhesion and tribocorrosion result in a large quantity of wear particles, which further restrict the lubricant flow and thus replenishment. As a result, the wear particles act as an abrasive medium and, over time, deep depressions are formed, which are frequently observed in practice. According to the authors, however, the term false brinelling is thus firmly associated with the phenomenon of trough formation as it occurs only under these operating conditions. A detailed description of the causes as well as numerous results of lubrication tests is given in [GREB2020]. The extremely comprehensive publication by DE LA PRESILLA et al. summarizes the current state of science with regard to oscillating rolling bearings [PRES2023]. At amplitude ratios less than 1, visible raceway damage is clearly different from trough-shaped false brinelling damage. A typical mark that has not been altered by overrolling has an undamaged central stick zone and a damaged outer partial slip zone (Figure 2). According to GREBE [GREB2008], this damage is called the standstill mark. The damage starts at the ends of the main axis of the ellipse, i.e. in the areas with the largest microslip. Thereafter, damage development continues along the ellipse contour towards the ends of the minor ellipse axis. In the further course, the damaged and affected area enlarges due to secondary wear effects. Although the elliptical standstill marks caused by small oscillation angles, vibration, or mere elastic deformation may appear relatively harmless at first glance, there is significant localized surface damage caused by various wear mechanisms. The outer end of the elliptical wear marks exhibits tribochemical reactions and surface disruption. No changes are visible in the central sticking zone of the contact, where neither plastic damage nor sliding movements occur. At the boundary between the sticking zone and the micro-slip zone, micro-cracks occur due to large tangential forces introduced locally into the surface (Figure 3). This can significantly reduce the service life of the bearing, as tests on an FE8 testing machine with pre-damaged bearings have shown [TETO2023]. Therefore, the authors propose to clearly distinguish the terminologies and to use the term fretting for oscillating Science and Research 16 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 the raceway between the two reversal points, deficient lubrication phenomena occur [GREB2017, JANU2023]. Oil release, base oil viscosity and shear thinning effects (structural viscosity) play a crucial role [TETO23]. The sketch in Figure 1 illustrates the relationships using the example of the axial deep groove ball bearing 51206 and typical load collectives. In operational practice, the damage mechanisms are typically identified on the basis of the signs of wear on the bearing raceways. In the best case, this allows conclusions to be drawn about the operating conditions (e.g. whether the bearing is exposed to unknown vibrations). In the literature, the designation of wear damage is often ambiguous, as various terms such as false brinelling, fretting or fretting corrosion are used quasi arbitrarily for the observed wear characteristics. Some of these terms describe the resulting wear based on visual perception, while others refer more to the underlying damage mechanisms. However, a clear assignment of damage terminology is necessary, since the wear processes are based on different fundamental mechanisms, for which entirely different mechanical or lubrication approaches are required. The term fretting is usually used in German to describe a damage mechanism that occurs predominantly in reciprocal sliding contacts [GfT72009]. In combination with tribochemical oxidation, this is also referred to as fretting corrosion. The definition of fretting goes back to TOMLINSON, who in 1927 investigated the influence of vibrations between a spherical and a flat steel surface [TOML1927]. First mathematical descriptions of the processes in the contact zone were made by CATTANEO [CATT1937] and MINDLIN [1949]. Important experimental investigations on the subject of fretting with different vibration amplitudes were carried out by VINGSBO and SOEDERBERG in a ball-on-disc contact [VING1988]. They determined so-called “wear-maps”, which show the influence of partial slip and macroscopic slip (gross slip) on wear. All these investigations have in common that they were carried out under oscillating sliding friction. However, the rotation of the ball around an axis perpendicular to the normal of the contact, as is the case in a rolling bearing even at small angles, leads to different conditions from the point of view of contact mechanics [JOHN2007] and lubrication [GREB2012]. In oscillating rolling contacts, locally occurring micro-sliding motions (fretting) are usually accompanied by other mechanisms, and their individual contribution to the overall wear process can vary significantly depending on operating conditions [GREB2008, GREB2021]. The term false brinelling was first used by ALMEN in 1937 and originally referred to the formation of depressions in the bearing raceway, which can easily be mistaken for plastic indentations (“true brinelling”) as produced by Brinell hardness testing [ALME1937]. This trough-shaped damage occurs at amplitude ratios contacts with short stroke under pure sliding conditions, to revert to the term false brinelling damage for x/ 2b ratios > 1 and standstill marks for x/ 2b ratios < 1 for oscillating rolling contacts as they occur in rolling bearings. These terms describe the perception of the damage mark and indicate the underlying damage mechanisms. Grease samples used Four greases provided by an industrial partner were used in this extensive comparative study. The greases differ in terms of their NLGI class (3x NLGI 2; 1x NLGI 3), their soap thickener (lithium, calcium sulfonate) their oil release and their individual additivation. Unfortunately, only individual characteristic values may be published here. For the selection it was important that they are known to show significant differences in the Fafnir test. Fafnir test according to ASTM D4170 The American standard ASTM-D4170 [ASTM4170] was the first standardized test method for oscillating rolling bearings. It is based on the extensive investigations of HUDSON et al [HUDS1946] at the rolling bearing company FAFNIR and was introduced in 1982 by the American Society of Testing and Materials, now ASTM International, and is also known as the “Fafnir wear test”. In this test, special test bearings (thrust ball bearings similar to type 06x65, di = 16 mm; Da = 35.69 mm; 9 balls; D = 7.142 mm) are subjected to a normal force of 2450 N applied by springs (max. Hertzian contact pressure approx. 1.87 GPa, i.e. a Cdyn/ P ratio of approx. 7.9) or 4450 N (approx. 2.28 GPa, C dyn / P = 4.3) applied at a frequency of 30 Hz with a vibration angle of ± 6° for 22 hours. After testing, the bearings are visually inspected and the mass loss on the raceways is determined. According to [KLUE2010], mass losses below 5 mg are considered acceptable in the Fafnir test. The ASTM test is referred to as both the “false brinelling test” and the “fretting test.” However, due to the oscillating angles of ± 6°, which corresponds to an amplitude ratio of about 5.5, there is no reason from a scientific point of view to speak of a fretting test here, even if tribooxidation is triggered by deficient lubrication. Overall, the test has a relatively high scatter, which is why an alternative SRV test (high-frequency, linear-oscillating test machine; SRV = in German: Schwing-Reib-Verschleiß) was introduced in 2016 as a possible alternative (ASTM D7594), which will also be described later. Especially on the American market, data on Fafnir values can be found for almost all high-performance greases. In Europe, the test is not so widely used. Despite the known problems with comparability, the test has been included in the current NLGI grease specification for high performance greases (High-Performance Multiuse Greases with High Load Carrying Capacity - HPM-HL) [SHAH2020]. In addition to the standard test according to the norm, two further modifications of the test were carried out in this series of investigations in order to be able to evaluate certain influencing factors. In one test variant, the total grease quantity was reduced. According to the standard, the raceway grooves of the bearings are completely filled with grease and then drawn off. This results in a grease quantity of approx. 0.35 g. In addition, 0.65 g should be evenly distributed in the cage, resulting in a total grease quantity of 1.0 g. In another test variation (“test 2”), the Science and Research 17 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 Figure 3: FIB section at the transition between the adhesion and microslip zone shows a crack running into the depth [GREB2006] Figure 4: Overview of greases used with relevant data with the significantly lower oil release rate and the higher NLGI class takes second place in this test. In the case of calcium sulfonate samples 7 and 15, the grease is extremely thickened as a result of the particle input. Very deep troughs can be seen. Increasing the amount of grease from 0.35 g to 1 g, as specified in the standard, leads to a significant reduction in wear for all specimens, except for sample 1, which already gives a good result with the small amount of grease. The grease differences become much smaller with the larger grease amount. The mass losses are compared in detail in the overview diagram in Figure 13 and in the final discussion of the results. Science and Research 18 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 Fafnir rolling bearing was replaced by the SNR-FEB2 standard bearing 51206. The bearing adaptation in the Fafnir test rig were modified accordingly. The number of balls was reduced from 12 to 8 and the normal force was adjusted to give similar contact pressure ratios to those used in the ASTM D4170 standard test. In this test, too, the raceway grooves were drawn off flat (without further grease in the cage). The tests with 0.35 g grease show clear differences between the four greases (Figure 6). The lowest mass loss is found in sample 1, the lithium soap grease with the highest oil separation rate. Nevertheless, clear troughs can already be seen here. The other lithium soap grease Figure 5: KTM’s Fafnir tester with standard parameters and damaged test bearing Figure 6: Parameters and photo documentation of the test bearings after the test (top uncleaned, bottom cleaned) SNR-FEB2 test The SNR-FEB2 test and the associated test rig were developed by the French rolling bearing company Société Nouvelle de Roulements (SNR, now NTN-SNR Group). The test is described in the now withdrawn French standard NFT60-199 [NF1995]. The test is widely used in Europe and in the wind industry and is repeatedly requested as a clearance test. It reproduces small oscillation movements of the bearings with x/ 2b ratios significantly above 1 well. In the test, thrust ball bearings of type 51206 are tested in two units (left and right) at an angle of +/ -3° at 25 Hz and a normal force of 8000 N (max. contact pressure approx. 2.3 GPa, C dyn / P = 3.1) for 50 hours. In this test rig, too, the movement is initiated by eccentrics; the normal force is applied by springs. The test temperatures are Science and Research 19 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 Figure 8: Typical signs of wear and mass losses in SNR-FEB2-test Figure 7: Modern SNR-FEB2 test rig at the Mannheim Tribology Competence Center [Source: KTM] (9000 LW), 80 min (120000 LW)) to investigate the change over time. For each of these three individual tests, a new set of 4 balls is used, which are positioned at an unstressed location on the races. At the end of the overall test, each bearing ring thus shows 12 marks. Always 3 marks show the development over the number of cycles. The following microscopy pictures show the results after one minute (Figure 10) and 80 minutes (Figure 11). SRV fretting test according to ASTM D7594 Due to problems with the repeatability and comparability of the Fafnir test, at the urging of the U.S. aerospace industry, a new friction test on the SRV test rig (SRV = Oscillating Friction Wear Tribometer; high-frequency, linear-oscillating testing machine) was developed [FACI2007] and subsequently standardized (ASTM D7594, pure sliding, point contact, 100 N, amplitude 0.3 mm, 50 Hz, 80 °C [ASTM7594]). The authors would refer to this test as a “true” Fretting test, since a pure oscillating sliding motion is used here (no rolling! ) and the stroke is less than the Hertzian contact width (2b). The NLGI specification for high performance multipurpose greases (HPM-LL = Long Life and HPM-HL = High Load Carrying Capacity) lists this test as an alternative to the Fafnir test according to ASTM D4170. The SRV test rig is now so widespread that a more detailed presentation is omitted here. Science and Research 20 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 usually either room temperature or -20 °C. After the run, a visual inspection and weighing of the rolling bearing parts are also performed here. In SNR testing, values below 2 mg are considered very good; values up to 5 mg are still considered good [KLUE2010] (see Figure 8). When damage starts, it progresses and escalates very quickly, which can then lead to a relatively large scattering of results. Since it was known from preliminary tests that all four grease samples did not perform particularly well under the SNR-FEB2 conditions, the test duration was shortened to one hour (instead of 50 hours). In this way, differences that are later lost to secondary wear effects can still be readily seen. Grease sample 1 also performs best in this test. It can be seen that the grease is still reasonably fluid. In contrast to all three samples, no dry wear particles are visible here yet. Nevertheless, depressions can already be seen here as well (Figure 9). Quasi standstill test of the KTM (KTM-QSST) The KTM-QSST test was developed to map the lubricant influence in a macroscopically stationary bearing with alternating tangential forces, which are initiated due to very small oscillation motion or due to vibrations (QSST-quasi-stationary test) [GREB2008]. In the test, 8 of the 12 balls are removed. For this, three test times are run (for greases 1 min = 1500 load cycles (LW), 6 min Figure 9: Parameters and photo documentation of the test bearings in the SNR-FEB2 test with a test time of one hour Evaluation and discussion of the test results The following chart (Figure 13) compares all tests with a larger amplitude ratio (SNR-FEB2 and ASTM D4170 standards and variations). “ASTM D4170 (Industr.)” denotes the industry partner’s Fafnir results listed for comparison. The third row of bars (Test 1: D4170 0.35g) shows how a reduced amount of grease affects the wear result (0.35 g instead of 1.0 g as per standard). The fourth row of bars shows the results with the SNR standard bearing ARKL 51206 in the Fafnir tester with test conditions adapted to ASTM D4170 and also reduced grease quantity (only running grooves filled) (“Test 2”). It can be seen that sample 1 (lithium soap grease with high oil release) achieves a very good result in all five tests. The differences are surprisingly small, even though different test bearings, test methods and grease quantities are used here. The grease thus appears to be very well suited for all conditions. Sample 2 (lithium soap grease with lower oil release and NLGI 3) shows a significantly greater scatter. Only in the ASTM standard test at the KTM did it perform similarly well to grease 1. However, a higher Fafnir value was measured at the industry partner. This coincides with the poorer values in tests with a reduced amount of grease. In the classic SNR-FEB2 standard test, the sam- Science and Research 21 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 Figure 11: Exemplary wear marks in the KTM quasi-still test (amplitude ratio approx. 0.55) - test duration 80 min = 120,000 load cycles. Figure 10: Exemplary wear marks in the KTM quasi-still test (amplitude ratio approx. 0.55) - test duration 1 min = 1500 load cycles. in the Fafnir test with 0.35 g grease. The clear influence of the thickener is evident here, as the grease has an even lower oil release than sample 2. For grease samples 1 and 15, two greases with different thickener types and different oil release perform best on average. This shows that oil release seems to be important for the lithium soap greases. However, with a different thickener, other variables might be more important. Thus, it should be noted that the overall rheological properties of a lubricant affect performance under these particular operating conditions and the results cannot simply be reduced to a single measured value. For example, the two calcium sulfonate greases clearly show that it is not just oil release that is important. For the lithium soap greases, on the other hand, oil release is a critical factor, as other researchers have already demonstrated [SCHA2016, TETO2023, JANU2023]. The result is completely different for the tests with a small amplitude ratio. The clear but unfortunately subjective evaluation based on school grades (1 very good, 6 very poor) shows that grease sample 1 also has a clear advantage in this test (Figure 13). Nevertheless, the result is not particularly good. Calcium sulfonate grease 15, which performed very well, especially at larger angles, is rather unsuitable under these conditions. Samples 7 and 15 (both calcium sulfonate greases) differ only slightly. Sample 2 is completely unsuitable. The low oil release in combination with the higher NLGI class seems to be extremely unfavorable for the lithium soap grease. Whether calcium sulfonate greases in general perform relatively poorly under these conditions cannot yet be Science and Research 22 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 ple performed worst. Thus, the grease already has problems providing proper lubrication under these conditions. Smaller amounts of lubricant (only raceway grooves filled) seem to be particularly problematic. Sample 7 (a commercial calcium sulfonate grease) performs the worst in almost all tests. This grease also appears to have problems with small amounts of lubricant, as evidenced by the high mass losses in both tests on the Fafnir test rig at a gerase amount of 0.35 g. The sample also performs relatively poorly in the standardized ASTM D4170 or SNR-FEB2 tests. Sample 15 (modified calcium sulfonate grease) often shows the second best result. Only in the Fafnir test with reduced grease quantity has extreme wear been measured. The results clearly show that for tests with a relatively large amplitude ratio of >>1, the oil release, the thickener type and the grease quantity are decisive. Sample 1 performs by far best with an oil release of 4.64 % according to DIN 51817 [DIN51817] and is also the most reproducible. For the other samples, the statement is not quite so clear. Sample 2, with the same thickener but a higher NLGI-class and much lower oil release of 1.18 %, performs consistently much worse. Results at reduced grease levels are particularly poor. Sample 7, with the lowest oil release but based on calcium complex, also shows significant weakness at low grease levels. The results with larger amounts of lubricant are better, but also do not come close to the results of grease sample 1. Sample 15 shows good results except for the possible outlier Figure 12: Test results - comparison of different tests with an amplitude ratio of 3.3 (SNR-FEB2) or 5.5 (ASTM D 4170) generalized based on the two samples. Unfortunately, no studies by other researchers under similar conditions are known for this modern thickener type. The comparison with the “classic” Fretting test under oscillating sliding friction in the SRV shows the problem of the very high contact pressure at the beginning of this test due to the point contact. Two of the four grease samples could not be tested under these standard test conditions, as adhesive failure (seizure) already occurred in the run-in phase of the test (reduced normal force 50 N) for the two lithium soap greases investigated (Figure 14). The two calcium sulfonate greases, on the other hand, gave almost identical results (mean wear scar diameter 415 µm for sample 15 versus 397 µm for sample 7). The high contact pressure in point contact requires a special lubricant or additive chemistry for these initial conditions, which are 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. Greases may be suitable for friction conditions in plain contact (e.g. for shaft-hub connections [BUSE2021, BUSE2022]) or for rolling bearings, even if they do not pass the SRV test according to ASTM D7594. Summary The aim of this in-house test series was to compare different 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 and the SNR-FEB2 test frequently required in Europe can be mentioned here as standard procedures. In addition, an in-house test was carried out to simulate bearings at very low oscillation angles and vibrations (KTM QSST). The new NLGI grease specification for high perfor- Science and Research 23 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 Figure 13: Subjective Evaluation of the standstill markings by means of school grades (1 very good, 6 very poor) Figure 14: Friction coefficients in the SRV fretting test; blue and gray line: failure to run-in for samples 1 and 2 (both lithium soap greases); green sample 7, red sample 15 This series of tests was intended to show that 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 grease behavior in a rolling bearing. If necessary, results from several tests must also be taken into account if different conditions may prevail in practice. For example, the conditions in the blade bearings of wind turbines are very different. During the adjusting motion, large x/ 2b ratios occur, while during standstill under vibration, small x/ 2b ratios have to be considered [STAM2020, SCHW2020]. The selected lubricant must therefore always represent a compromise. References ALME1937 J O Almen. Lubricants and False Brinelling of Ball and Roller Bearing; Journal of Mechanical Engineering, 59(6): 415-422, 1937 BUSE2022 H. Buse. Fretting wear tests on tribometers - basics, industrial relevance and test realisation. Promotion an der TU Bratislava; Material Sciences and Technology; Reg. No.: MTF-114753- 82998; 2022 BUSE2021 Buse, H.; Schueler, F.; Hodúlová, E. Planar Contact Fretting Test Method Applied to Solid Lubricants. Lubricants 2021, 9, 58. https: / / doi.org/ 10.3390/ lubricants9060058 CATT1938 C Cattaneo. Sul contato di due corpo elastici. Mat. Nat. Rend., 27, page 342 - 348, 1938. CZIC2020 Czichos, Habig. Tribologie-Handbuch - Tribometrie, Tribomaterialien, Tribotechnik, Springer Vieweg Wiesbaden, ISBN 978-3-658-29483-0; Edition 5, 2020 FACI2007 Faci, H. Fretting Test by SRV. In Proceedings of the ASTM Meeting Miami Beach, Naples, FL, USA, 20 June 2007. GFT72009 GfT-Arbeitsblatt Nr. 7: Tribologie - Verschleiß, Reibung - Definitionen, Begriffe, Prüfung; Gesellschaft für Tribologie: Jülich, Germany, 2009. GREB2008 M Grebe, P Feinle, and W Hunsicker. Einfluss verschiedener Faktoren auf die Entstehung von Stillstandsmarkierungen (False Brinelling Effekt). Tribologie und Schmierungstechnik, 55. Jahrgang, Heft 1-08, Expert-Verlag, Renningen, 2008, 55: 12, 2008. GREB2012 M Grebe. False Brinelling - Standstill marks at roller bearings. PhD thesis, Slovak University of Technology, Bratislava, 2012 Science and Research 24 Tribologie + Schmierungstechnik · volume 71 · issue 3/ 2024 DOI 10.24053/ TuS-2024-0013 mance multi-purpose greases (HPM) also requires the SRV test according to ASTM D7594 for greases with higher loads, so this was also included in the test series. Modifications to the tests were added to the test series to investigate the influence of grease amount and bearing type. For the test series, four mineral oil-based grease samples were prepared with two types of soap thickeners (lithium soap and calcium sulfonate), which gave different results in the Fafnir test. The aim of the test series 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 publication, the scientific principles of rolling bearings operating only at small angles or subjected to vibration were explained. In these investigations, it is important to consider the amplitude ratio x/ 2b, i.e. the ratio between the displacement of the rolling element (x) and twice the Hertzian contact width (2b). For 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, replenishment is possible in principle and depends strongly on the rheology of the lubricating grease. In the second part of the publication, the operating and test conditions of common and partly standardized rolling bearing test methods were presented. Four grease samples with known formulations were compared in these three standard rolling bearing tests, two modified tests and a classic Fretting test under oscillating sliding friction. It can be seen that the performance of the lubricants is highly dependent on the test conditions and that, to date, there appears to be no universal lubricant for these different operating conditions. A high oil release is advantageous for rolling bearings operating with relatively small vibration angles. But a change of the thickener can also be promising. It is important to note that greases with a high oil release rate may not achieve the desired grease operating life, which makes early relubrication necessary. A comparison with a “classical” Fretting test under oscillating sliding friction in the SRV test rig shows the problem of the very high contact pressure at the beginning of this test. Two of the four grease samples could not be tested under these standard test conditions. 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. GREB2012a M Grebe, P Feinle, W Hunsicker. Kontaktmechanik Kontaktmechanische Beschreibung von False-Brinelling (Stillstandsmarkierungen bei Wälzlagern); Jahrestagung der Gesellschaft für Tribologie, (GfT) Tagungsband, 2010 GREB2014 M Grebe; P Blaškovitš, P Feinle. Failure of roller bearings without macroscopic motion - Influence of the pivoting angle on the contact mechanics and the wear mechanisms in the contact between roller and raceway 19th International Colloquium Tribology: Industrial and Automotive Lubrication, Esslingen, 2014; Proceedings on DVD GREB2017 M Grebe. False-Brinelling und Stillstandsmarkierungen bei Wälzlagern - Schäden bei Vibrationsbelastung oder kleinen Schwenkwinkeln. Narr Francke Attempto Verlag, ISBN: 9783825251604; DOI: 10.36198/ 9783838551609. 2017 GREB2018 M Grebe, J Molter, F Schwack, and G Poll. 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