contrast, the dynamic tests consider the tribological load and are therefore well suited for detecting incompatibilities [6]. However, the disadvantages are the amount of lubricant required and the overall increased test costs. As part of the project FVA 578 III: ‘Tribologisches Ersatzsystem für dynamische Elastomer-Schmierstoff Verträglichkeitsprüfungen’, a new dynamic test methodology was investigated on a simple tribologically equivalent system for RSS, the ring cone tribometer (RFT). The aim was the development of a cost-effective dynamic compatibility test, which can already be used by interested companies at this time. In this article, the test development methodology is first presented. Furthermore, the results of the test runs on the RFT and the comparison of these with static and RSS tests are shown. Science and Research 38 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0037 1 Introduction In the lubricant development process the compatibility of a lubricant with rubber plays an important role. Incompatibility with rubber used for manufacturing rotary shaft seals (RSS), which are used to seal machines, can lead to unwanted leakage or even failure of the entire system during operation. Therefore, during the multistage lubricant development process, lubricant manufacturers carry out compatibility tests. If an incompatibility is only detected at a late stage, this means a high loss of investment and working time. Currently lubricant manufacturers have two types of compatibility tests at their disposal. These are on the one hand static tests (e.g. DIN ISO 1817), in which an elastomer sample is immerged in the test lubricant, and on the other hand dynamic tests (e.g. FLENDER and SEW tests), in which the compatibility is checked on RSS under real operating conditions [1], [2], [3]. The disadvantage of static tests is that they do not take tribological contact into account and therefore this test method does not detect incompatibilities reliably. In addition, the test conditions are only vaguely defined in some cases, meaning that good repeatability and comparability are not guaranteed [4], [5]. In A new rubber-lubricant compatibility test on a tribometer for radial shaft seals Laura Stubbe, Yvo Stiemcke, Sarah Mross, Sarah Staub, Konrad Steiner, Kerstin Münnemann, Oliver Koch, Stefan Thielen* submitted: 20.09.2024 accepted: 21.01.2025 (peer review) Presented at GfT Conference 2024 The present study deals with the development of a new dynamic test method to evaluate the elastomerlubricant compatibility on a tribologically equivalent system for radial shaft seals (RSS), the ring cone tribometer (RFT). The suitability of the RFT for compatibility testing was demonstrated based on tests on RSS and static aging tests with comparable operating conditions. There was good agreement between RSS and RFT tests, whereas static aging did not produce valid results due to the lack of tribological loading. The tests showed that the course of the friction coefficient and the optical analysis are highly informative regarding an assessment of compatibility. However, the occurrence of leakage, a change in hardness and the course of cell lowering during the test run also allow conclusions to be drawn about incompatibility. Keywords rubber, lubricant, compatibility, test method, tribometer, RSS, friction coefficient Abstract * Laura Stubbe, M.Sc. 1 (corresponding author) Dipl.-Ing. Yvo Stiemcke 1 Sarah Mross, M.Sc. 2 Prof. Dr.-Ing. Sarah Staub 3 Dr. Konrad Steiner 3 Dr. rer. nat. Kerstin Münnemann 2 Prof. Dr.-Ing. Oliver Koch 1 Jun. Prof. Dr.-Ing. Stefan Thielen 1 1 RPTU Kaiserslautern-Landau Chair of Machine Elements, Gears and Tribology (MEGT) Gottlieb-Daimler Str. 42, D-67661 Kaiserslautern Germany 2 RPTU Kaiserslautern-Landau Laboratory of Engineering Thermodynamics (LTD) Gottlieb-Daimler Str. 42, D-67661 Kaiserslautern Germany 3 Fraunhofer-Institut für Techno- und Wirtschaftsmathematik ITWM, Fraunhofer Platz 1, D-67663 Kaiserslautern Germany 2 Experimental Studies The RFT was developed as part of FVA 578 I and optimized in FVA 578 II with regard to the conformity of friction and wear with the RSS system [7], [8]. Figure 1 shows the schematic structure of a RFT test cell. The sealing contact is simulated with its air and oil-side contact angles α and β by the contact of a shaft cone with an elastomer ring sample. This has an inner diameter of 50 mm and an outer diameter of 75 mm. The rubber ring specimen is clamped in a specimen holder so that it cannot twist. This is axially movable and a line load equivalent to the radial force of the RSS can be applied to the ring specimen via weights. Above the sealing contact there is a temperature-controlled oil reservoir which can hold a lubricant quantity of approx. 190 ml. During the test run, the shaft cone is set in rotation and the frictional torque generated in the sealing contact can be recorded using a torque measuring hub. The wear of the ring sample can already be estimated during the test run (on-line) by means of eddy current sensors via the wear-related lowering of the sample holder [9]. As a result, the course of the change in the sensor-cell distance can be determined. A total of six rubber-lubricant combinations were tested, whereby each combination was tested twice. As rubbers, 75 FKM 585 and 72 NBR 902 were selected. For the lubricants, two practical lubricants (“High-Ref”) and two low-additivated base oils (“Low-Ref”) based on PAO or mineral oil were used, all with an ISO VG 220 viscosity. The shaft cones were made from 16MnCr5. Table 1 provides an overview of the resulting combinations and the corresponding test conditions. The oil sump temperature was set to 80 °C for combinations with NBR and 110 °C for combinations with FKM. The material-specific temperature was selected in such a way that chemical damage to the elastomer is avoided, but thermal acceleration according to A RRHENIUS takes place. Thereby, according to V AN’T H OFF , a temperature increase of 10 °C increases the reaction rate by a factor of two to three [11]. The test duration was 1008 h and the sliding speed was set to 6 m/ s in each case. Both parameters were determined experimentally. For this purpose, preliminary investigations were carried out in which the test duration (240, 1008 and 2016 h) and the speed collective were varied. It turned out that clear intolerance reactions can be identified after 1008 h and that the tests can be carried out at a stationary speed, which reduces the complexity of the tests compared to carrying them out with a speed cycle. The curves of the friction coefficient and the sensor-cell distance, the planimetric sealing edge wear and the change in hardness were evaluated. The determination of the values is described in the following. To determine the line load required for the ring specimens on the RFT, the radial force F r on the RSS heated Science and Research 39 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0037 Figure 1: Scheme of the ring cone tribometer test rig (RFT) and the contact angles based on [10] # Rubber Lubricant v slide / m/ s T Oil sump / °C Duration / h 1 75 FKM 585 High-Ref PAO 6 110 1008 2 75 FKM 585 Low-Ref Min 6 110 1008 3 75 FKM 585 Low-Ref PAO 6 110 1008 4 72 NBR 902 High-Ref Min 6 80 1008 5 72 NBR 902 Low-Ref Min 6 80 1008 6 72 NBR 902 Low-Ref PAO 6 80 1008 Table 1: Summary of test parameters IRHD hardness measurement was carried out based on the standard DIN ISO 48-2 [13] on the “digi test II” device from Bareiss ® , whereby method M (micro hardness) was selected. The hardness of the sealing edge of the ring samples and the RSS is measured before and after the tests at 8 points distributed around the circumference and the mean value is calculated from this. In addition, the analysis methods NMR, DSC, DMTA and optical analysis were evaluated, and a possible leakage was considered. These criteria were evaluated according to the evaluation scheme shown in Table 2. Values colored green indicate good compatibility and values colored red indicate incompatibility. Orange colored means that the result points to slight incompatibility. In addition to the tests on the RFT, static aging tests and RSS tests with comparable test conditions were carried out in order to compare the different methods with regard to the reliability of detecting an incompatibility. The RSS tests were carried out on the multi-shaft test bench [14]. RSS according to DIN 3760 A-80-100-10 were used. The static aging tests were carried out based on the standard DIN ISO 1817 [1]. 3 Results The test results of two elastomer-lubricant combinations are shown below. The results of a well-compatible combination are presented on the one hand and of an incompatible combination on the other. This should make it clear which criteria can be used to assess incompatibility in the RFTtests. No leakage occurred with the combination FKM - High-Ref PAO (#1), for which good compatibility was expected. The optical analysis shows no defects at the sealing edges. The friction coefficient curves show a steady course with few fluctuations within the first 400 h (Figure 3 (l)). Figure 4 shows the course of the sensor-cell distance. This shows a slight lowering, which Science and Research 40 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0037 to test temperature is first determined. The line load p l can be calculated from this and the diameter of the ring specimen D using the following formula. (1) The friction coefficient µ is calculated using the following formula from the friction torque M r measured during the test run, the sealing edge radius of the sample r and the line load p l . This eliminates the influence of the different specimen diameters on the RFT and the RSS system and the results can therefore be better compared with each other. (2) On the RFT, the wear of the ring sample can already be estimated during the test run (on-line) by means of eddy current sensors via the wear-related lowering of the sample holder [9]. This results in the course of the cell lowering during the test run. After the test run, the planimetric wear is determined as described in [12]. The micro Criteria Evaluation Leakage / ml 0 > 0; ≤ 5 > 5 Friciton coefficient curve Slight changes / Stationary course Unsteady course without fluctuations Strong fluctuations Change of sensor-cell distance* Slight swelling, low wear Minor fluctuations in the course Significant fluctuations in the course Increased wear Planimetric wear / mm 2 < 0,005 0,005 < x < 0,01 > 0,01 Hardness change / IRHD-M <+2; >-6 <+5 ; >-10 > +5 / < -10 Optical analysis Inconspicuously Discoloration Cracks, blistering, chemical erosion Table 2: Evaluation scheme for assessing the analytical methods regarding compatibility. Figure 2: Optical analysis of the sealing edge of the samples for the combination NBR - Low-Ref Min.. L.: Blistering and discoloration (RFT), m.: Cracks and discoloration (RFT), r.: Blistering and discoloration (RSS). * The investigated combinations did not show a critical case of swelling, therefore no red category was defined. indicates low wear, and also a partial raising of the test cells, which indicates swelling. The results of the planimetric wear measurement confirm the moderate wear expected from the course of the sensor-cell distance (Figure 5). The change in hardness (Figure 6 (l)) shows a clear decrease in hardness for all ring samples, which indicates swelling. This is confirmed by the NMR, as shown in Figure 7. The NMR measurements of the samples (for details see Ref. [10]) show a clear lubricant signal at the sealing edge. The resolution of the NMR measurement of ~0.16 +-0.03 mm/ pt allows a rough determination of the penetration depth based on the length of the signal peak. This results in a penetration depth of approx. 0.7 mm. Together with the reduced hardness and the decrease in distance in the measurement of the sensor-cell distance, the test results confirm a slight swelling in the FKM - High-Ref PAO combination. However, this does not appear to affect the sealing function. As with all analytical methods, it is therefore necessary to determine limit values from which a change is classified as negative. For the combination NBR - Low-Ref Mineral oil (#5), for which an incompatibility was expected, significant leakage was detected. All ring samples show clear discoloration, cracks and, in some cases, blistering of the sealing edge (Figure 2 (l),(m)). The friction coefficient curves of the first 400 h are shown in Figure 3 (m). The tests show a strong change in the friction coefficient, especially at the beginning of the test run. This is an indicator of incompatibility [15], [16]. The progression of the sensor- Science and Research 41 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0037 Figure 3: Friction coefficient curves of the first 400 h. L.: FKM - High-Ref PAO on RFT (on 3 samples), m.: NBR - Low-Ref Min. on RFT (on 3 samples), r.: NBR - Low-Ref Min on RSS (on 2 samples). Figure 4: Course of sensor-cell distance for the combinations FKM - High-Ref PAO and NBR - Low-Ref Min. on RFT. During some test runs, the data acquisition system failed, which is why no curve can be shown for this time period. Figure 5: Planimetric wear comparison of the combinations FKM - High-Ref PAO and NBR - Low-Ref Min. (average of three values). ples of the combination NBR - Low-Ref Min. (#5) showed a slight shift of T g by 1.3 °C or 2 °C in a positive direction. Due to the low informative value of the DSC with regard to the evaluation of compatibility in the RFT tests, this analysis method is omitted from further consideration, just like the DMTA. In total, the evaluation of the RFT test runs showed clear incompatibility reactions for selected combinations and criteria, as shown in Table 3. According to the results, for combination 3-6 premature failure would be expected during usage. The criteria “Optical analysis” and “Friction coefficient curve” in particular can be used to draw reliable conclusions about incompatibility in the RFT tests. Thereby significant fluctuations in the friction coefficient curve indicate incompatibility (compare Figure 3), as also shown in [15]. When observing the change in the sealing edge, clear cracks, blistering and discoloration were found in the incompatible combinations 4-6 (compare Figure 2), and chemical erosion of the sealing edge in combination 3. The comparison of the different test methods regarding the evaluation of the compatibility of the combinations shows a good agreement between the RFT tests and the RSS tests, as shown in Tables 3 and 5. With the incompatible combination NBR - Low-Ref Min., the RSS tests also show clear fluctuations in the friction coefficient curve (Figure 3 (r)) as well as discoloration and blistering on the sealing edge (Figure 2 (r)). However, it was shown that incompatibility reactions are probably provoked more quickly on the Science and Research 42 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0037 cell distance shows a clear increase in the axial position of the test cells, which indicates increased wear (Figure 4). This is confirmed by the measurement of the planimetric wear (Figure 5). It should be noted that the measurement was falsified by the damage to the ring samples (cracks, blistering) so that the absolute values are not valid for this combination. The hardness measurement shows a slight decrease in hardness for the samples on average (Figure 6 (r)). In the signal of the NMR analysis no change can be seen for this combination, which indicates that no lubricant has penetrated the sample. The DMTA measurement shows a clear decrease in the storage modulus of the FKM-ring sample and a shift in the peak of the loss factor (shift in T g ). The NBR-ring sample shows a decrease in the storage modulus, a shift in T g is not recognizable here. Although the DMTA showed changes in the elastomers, no causality between the changes and an incompatibility could be determined here due to the small amount of data. The results of the DSC are unremarkable for most samples, only the sam- Figure 6: Comparison of the change of IRHD hardness throughout the investigation for the combinations FKM - High-Ref PAO (l) and NBR - Low-Ref Min. (r) (average of three values). Figure 7: Result of the NMR for a sample of the combination FKM - High-Ref PAO. RFT than with RSS tests. In contrast, there is less agreement with the results of the static aging test (Table 5). This does not reliably identify incompatibility, as the tribological load is missing here. The results were used to develop a test recommendation for carrying out compatibility tests on the RFT. The criteria were evaluated regarding their suitability for assessing incompatibility. It was found that the friction coefficient curve and the optical analysis in particular provide reliable results. 4 Conclusion and Outlook As part of the project FVA 578 III [17], a new type of compatibility test was developed on the RFT, which reliably reveals incompatibilities. For this purpose, a test cycle and the test duration required to reliably detect incompatibility reactions were first determined using tests on the RFT. This is 1008 h and the tests can be carried out at a stationary speed. Afterwards, all test-relevant combinations were tested with the developed test parameters. The evaluation showed some clear signs of incompatibility, such as cracks and blistering formation at the sealing edge. Furthermore, clear reductions in hardness due to the tribological load could be determined, which, in combination with the course of the sensor-cell distance, indicated swelling of the corresponding samples. This assumption was finally confirmed by NMR analysis. The course of the friction coefficient was identified as another reliable indicator for recognizing incompatibility. Significant fluctuations in this curve indicate incompatibility. Overall, incompatibility between lubricant and elastomer in dynamic tribological contact on the RFT can already be simulated in early phases of lubricant development with laboratory-scale samples. By testing with tribological load, incompatibilities are detected much more reliably than with static tests. This has been shown by studies with comparable test parameters. The good comparability of the results with tests on RSS was also demonstrated based on the tests carried out. Another advantage of testing on the RFT is that it is cost-effective. As part of the FVA projects 578 I/ II [7], [8], the design of the RFT was optimized for simple production, mainly from standard parts. Interested companies were provided with all the necessary design drawings as well as a manual for setting up, carrying out tests, sample production and measurement. Due to the compact design of the RFT, it allows a very space-saving, cost-effective use of several test cells with only one drive. The lubricant requirement is significantly lower than for comparable RSS test rigs and the preparation of elastomer ring specimens from readily available test plates is very cost-efficient. Incompatibilities between elastomer and lubricant can be easily and reliably identified after the test by observing the friction coefficient curve and optical analysis of the test specimens. The changes in hardness and the course of the sensor-cell distance can also be used to draw conclusions about changes in the elastomer, in particular Science and Research 43 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0037 Table 3: Results of the tests on the RFT, evaluated in terms of compatibility. Green colored: Combination is well compatible, orange colored: Slight incompatibility reaction, red colored: Combination is incompatible Table 4: Results of the tests on RSS, evaluated in terms of compatibility. Green colored: Combination is well compatible, orange colored: Slight incompatibility reaction, red colored: Combination is incompatible Table 5: Results of the static tests, evaluated in terms of compatibility. Green colored: Combination is well compatible, orange colored: Slight incompatibility reaction Combination-# Leakage Friction coefficient curve Change in hardness Optical analysis NMR Compatible? 1 Yes 2 Yes 3 No 4 No 5 No 6 No Combination-# Leakage Friction coefficient curve Change in hardness Optical analysis NMR Compatible? 1 Yes 2 Yes 3 No 4 No 5 No 6 No Combination-# Leakage Friction coefficient curve Change in hardness Optical analysis NMR Compatible? 1 Yes 2 Yes 3 Yes 4 Yes 5 Yes 6 Yes Elastomeren für Dichtungsanwendungen in der Antriebstechnik,“ Forschungsvereinigung Antriebstechnik e. V. (FVA), Forschungsvorhaben 578 I, Abschlussbericht, FVA-Heft Nr. 979, 2011. [8] D. Weyrich, „Praxistaugliche Prüfmethodik für Reibungs- und Verschleißuntersuchungen am tribologischen Ersatzsystem von RWDR,“ Forschungsvereinigung Antriebstechnik e. V. (FVA), Forschungsvorhaben 578 II, Abschlussbericht, FVA-Heft Nr. 1374, 2020. [9] C. Burkhart, T. Schollmayer, B. van der Vorst, M. Sansalone, S. Thielen und B. Sauer, „Development of an online-wear-measurement for elastomer materials in a tribologically equivalent system for radial shaft seals,“ Wear, p. 203671, 2021. [10] D. Bellaire, S. Thielen, C. Burkhart, K. Münnemann, H. Hasse und B. Sauer, „Investigation of Radial Shaft Seal Swelling Using a Special Tribometer and Magnetic Resonance Imaging,“ ACS Omega 7/ 14, pp. 11671-11677, 2022. [11] S. Pongratz, „Alterung von Kunststoffen während der Verarbeitung und im Gebrauch,“ Universität Erlangen-Nürnberg, 2000. [12] T. Schollmayer, C. Burkhart, W. Kassem, S. Thielen und B. Sauer, „Verschleißanalyse an Radialwellendichtringen und weiteren Maschinenelementen mittels Laserprofilometrie,“ in Gesellschaft für Tribologie e. V. (Hg.): 62. Tribologie-Fachtagung, Online-Konferenz, 2021. [13] DIN Deutsches Institut für Normung e. V., DIN ISO 48- 2: Elastomere oder thermoplastische Elastomere - Bestimmung der Härte, Berlin: Beuth Verlag, 2021. [14] S. Thielen, P. Breuninger, H. Hotz, C. Burkhart, T. Schollmayer, B. Sauer, S. Antonyuk, B. Kirsch und J. C. Aurich, Improving the tribological properties of radial shaft seal countersurfaces using experimental micro peening and classical shot peening process, Tribology International, 2021. [15] C. Wilbs, D. Frölich, M. Adler und A. Heinl, „Radial lip seal friction torque - a suitable lubricant-elastomer compatibility indicator? ,“ in Nextlub, Düsseldorf, 2023. [16] A. Hüttinger, M. Woeppermann und J. Hermes, „Dynamische RWDR Tests neu definiert! ,“ in 60. Tribologie-Fachtagung, Göttingen, 2019. [17] L. Stubbe, „Tribologisches Ersatzsystem für dynamische Elastomer-Schmierstoff-Verträglichkeitsprüfungen,“ Forschungsvereinigung Antriebstechnik e. V. (FVA), Forschungsvorhaben 578 III, Abschlussbericht, 2024. Science and Research 44 Tribologie + Schmierungstechnik · volume 71 · issue 5-6/ 2024 DOI 10.24053/ TuS-2024-0037 swelling can be detected. This can be confirmed using NMR. The knowledge gained was used to develop a test recommendation for carrying out compatibility tests on the RFT. 5 Acknowledgement This work was funded by the German Federal Ministry of Economics and Climate Protection within the framework of Industrial Collective Research (IGF 21650N). The authors would like to thank the Forschungsvereinigung Antriebstechnik (FVA) for their support within the framework of the research project FVA 578 III “Dynamische Elastomer-Schmierstoffverträglichkeit”. References [1] DIN Deutsches Institut für Normung e. V., DIN ISO 1817: 2015: Elastomere oder thermoplastische Elastomere - Bestimmung des Verhaltens gegenüber Flüssigkeiten, Berlin: Beuth Verlag, 2016. [2] Freudenberg Sealing Technologies, Dynamic oil compatibility tests for Freudenberg radial shaft seals to release the usage in FLENDER-gear units applications (Table T 7300), 2018. [3] SEW Eurodrive, Prüfvorschrift 97 118 03 15: Statische und dynamische Prüfungen von Radialwellendichtringen (RWDR), 2016. [4] J. Braun, „Elastomerverträglichkeitsunterscuhungen von Schmierstoffen - Reicht die bestehende Normung aus,“ Tribologie + Schmierungstechnik, Bd. 56, Nr. 6, pp. 24- 29, 2009. [5] J. Braun, „Aktuelle Elastomerverträglichkeitstest-Standards - eine kritische Betrachtung,“ in Proceedings of the 16th International Sealing Conference, Stuttgart, 2010. [6] M. Klaiber, „Additivverträglichkeit: Einfluss verschiedener Additive auf Elastomere und die tribologischen Eigenschaften im System Radial-Wellendichtung,“ Forschungskuratorium Maschinenbau e.V. (FKM), Forschungsvorhaben 290, IGF-Nr.: 15903 N, Abschlussbericht, Heft Nr. 318, 2012. [7] T. Gastauer, „Vergleichende Reibungs- und Verschleißuntersuchungen durch Experimente und Simulation an