State of the art and motivation In industrial gear systems, a balanced lubricant system is essential for ensuring the longest possible service life of the components. Radial shaft seals (RSS) can be used to reliably seal the gear system. Made of an elastomer material, the RSS’s are subjected to thermal, chemical, and mechanical stress during operation, which can lead to different wear and aging characteristics at the sealing edges. Depending on their severity, these wear and aging characteristics can impair the sealing ability, making safe operation of the transmission system no longer possible. In order to investigate the compatibility of a sealing system - consisting of RSS, lubricant, and shaft - dynamic sealing ring tests are often carried out in industry (SEW63190052.07, 2024). The wear and aging characteristics are evaluated visually and haptically according to the state of the art in order to assess the compatibility of the lubricant and elastomer. The damage characteristics evaluated can be divided into chemical, physical-thermal, and thermal primary damage (Bauer, 2021). This paper examines the physical-thermal damage characteristics of “hardening” and “hard deposits”. A high seal edge temperature can lead to post-crosslinking of the elastomer and thus to the aging characteristic of “hardening” (Bauer, 2021). On the model test bench for dynamic testing of RSS, the lubricants can degenerate due to a high oil sump temperature and test duration. This can cause “hard deposits” to form on the sealing edge surface, consisting of aged lubricant and elastomer residues. The aging characteristics of an RSS, as well as the “hard deposits” on the sealing edge, are evaluated according to (SEW63190052.07, 2024) with a rating system. As already shown in the studies (Alt, et al., 2023) and (Wilbs, et al., 2023), nanoindentation can be used to characterize the wear and aging of the elastomer at the sealing edge due to µ-mechanical material changes. By determining suitable parameters for characterizing the damage, the analysis can be carried out in a measurable and reproducible manner in the future. Furthermore, the evaluation of the stiffness of the tribologically stressed elastomer via the indentation depth according to (Wilbs, et al., 2023) enables a more detailed analysis of the properties of the deposits on the sealing edge, which are collectively referred to as “hard deposits” according to (SEW63190052.07, 2024). The aim of the present study is to calculate the stiffness discretely over the indentation path and to examine the characteristics of the stiffness gradient. Mechanical characterization The aim of mechanical characterization is to determine mechanical parameters for stressed RSS’s that can be correlated with the previous haptic characterization. For Science and Research 31 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0016 Systematic investigation of the µ-mechanical material change of the sealing edge of radial shaft seals Felix Bernhardt, Katrin Alt, Markus Wöppermann* Presented at GfT Conference 2025 The stability of sealing systems is tested on dynamic model tests. The wear and aging characteristic are evaluated haptically and optically according to the state of the art to rate the lubricant-elastomer compatibility. A nanoindenter can be used to investigate µ-mechanical material changes because of tribological stress in radial shaft seals. The results of new measurement methods are correlated with the haptic evaluations of the sealing edges and allow for distinction in the tribologically induced layer on the sealing edge. Keywords radial shaft seal, dynamic test, µ-mechanical characterization, nanoindentation, elastomer-lubricant compatibility Abstract * M.Sc. Felix Bernhardt M.Sc. Katrin Alt Dr.-Ing. Markus Wöppermann SEW-EURODRIVE GmbH & Co KG Ernst-Blickle-Str. 42 76646 Bruchsal/ Germany the sealing edge as standard, and the mean value and standard deviation are then calculated. During post-processing, the dissipated work W diss and the reversible work W rev are calculated from the hysteresis when the force is applied over the indentation path, as shown in Figure 1. The dissipated work W diss describes the proportion of material damping, while the reversible work W rev describes the proportion of reversible deformation work during deformation reversal due to unloading. The relative dissipated work rel.W diss is calculated as the relative magnitude of material damping compared to elasticity, which represents the ratio of dissipated work W diss to total work W ges = W diss + W rev . The maximum force F max is recorded at the maximum indentation depth. The stiffness curve over the indentation depth is evaluated for each measuring point up to the maximum indentation depth. Results When plotting the relative dissipated work over the maximum force according to (Wilbs, et al., 2023) in Figure 2, clusters are formed from an internal data set of haptically evaluated RSS’s that were tribologically stressed. The color coding of the clusters indicates the corresponding degree of hardness based on the tactile assessment. The tribologically non-stressed RSS’s form a cluster at the lowest relative dissipated work rel.W diss and maximum force F max . The maximum force F max increases proportionally to the degree of hardening from the haptic evaluation. As hardening increases, the relatively dissipated work also increases up to the point of slight hardening, and then decreases with increasing maximum force F max up to a point of significant hardening. If the stiffness is evaluated incrementally over the indentation depth up to the maximum indentation according to (Wilbs, et al., 2023), two different characteristic stiffness curves result Science and Research 32 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0016 the tests with the nanoindenter from the study (Alt, et al., 2023), a probe tip with a radius of 40 µm at the spherical hemisphere of 30° is used. In the study (Alt et al., 2023), the sealing lips of the RSS are turned inside out for vertical indentation on the sealing edge. One measurement method is distance-controlled quasi-static indentation followed by de-indentation (retraction) of the probe tip. The distance control sets the appropriate indentation depth to reduce friction of the elastomer material if the indentation depth is too great. The maximum indentation depth is -60 µm and both the indentation rate and the unloading rate are 5 µm / s. The preload force is 1,91 mN. For each RSS, 20 measurement points are recorded on Figure 1: Calculation of dissipated work W diss , reversible work W rev , and maximum force F max . Figure 2: Cluster of the correlation between the haptic evaluation and the relative dissipated work over the maximum force. from the examination of RSS with haptically evaluated deposits. An example of a sealing edge with the two characteristic stiffness curves per measuring point and as an average value can be seen in Figure 3. The upper sealing edge shows a deposit with a stiffness curve that rises and then falls to the maximum indentation depth at the individual measuring points. The sealing edge shown below exhibits high initial stiffness at the start of the indentation depth, which initially decreases rapidly over a short indentation path. Subsequently, the mean value shows an approximately constant stiffness curve up to the maximum indentation depth. The measurement points in this area show a spread between small amounts of positive and negative gradients. Discussion Figure 2 shows a clear cluster structure of the degrees of hardening based on the haptic evaluation and the measured values of the relatively dissipated work rel.W diss over the maximum force F max . This allows initial limit values in the relative dissipated work rel.W diss and the maximum force F max , which correlate with the haptic evaluation and thus enable the hardening to be characterized on the basis of measureable parameters. It is also clear that the dispersion per haptic evaluation category is high and that future aging characterization based on measured values is necessary. The examples in Figure 3 show that there are two different characteristic curves of stiffness over the indentation depth, which, according to (SEW63190052.07, 2024), can only be evaluated with a defined damage characteristic of “hard deposits”. This indicates the need for differentiation in the characteristic “hard deposits”. The generic term “tribologically induced layer formation” is introduced, which can manifest itself in a “deposit” or “hard layer”. Figure 4 shows the schematic layer structures of both tribologically induced layers based on optical observation in connection with the measured stiffness curves over the indentation depth. Science and Research 33 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0016 Figure 3: Stiffness curves per measuring point and average value for two sample sealing edges with the same rating of a significant “hard deposit” with the grade 3 according to (SEW63190052.07, 2024). Figure 4: Schematic representation of the structure of tribologically induced layers and the corresponding stiffness curve over the indentation depth. Conclusion As part of this work, a fixed number of RSS examples are examined using a nanoindenter to define limit values in mechanical characteristics that correlate with previous haptic assessments and could replace them in the future, enabling characterization based purely on measured values. The previous term “hard deposit” according to (SEW63190052.07, 2024) is generally understood. Based on the findings of the different prevailing stiffness curves, the tribologically induced layers as a general term can be differentiated into “hard layers” or “deposits” and evaluated measurably. References Alt Katrin [et al.] Measurement device and automation solution for analysing tribologically damaged radial shaft seals / / Reibung, Schmierung und Verschleiß, GfT e.V.. - 2023. Bauer Frank Federvorgespannte-Elastomer-Radial-Wellendichtungen: Grundlagen der Tribologie & Dichtungstechnik, Funktion und Schadensanalyse. - Wiesbaden, Deutschland : Springer-Verlag, 2021. SEW63190052.07 SEW 63190052.07 Prüfvorschrift: Statische und dynamische Prüfungen von Radialwellendichtringen (RWDR). - [s.l.] : SEW-EURODRIVE GmbH & Co KG, 2024. Wilbs Christian [et al.] µ-Mechanical characterization of tribologically stressed elastomer surfaces with respect to radial shaft sealing systems / / Reibung, Schmierung und Verschleiß, GfT e.V.. - 2023. Science and Research 34 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0016 On the left side of Figure 4, is the stiffness curve of the “deposit” with the increase and subsequent decrease in stiffness over the indentation depth. When looking at the associated sealing edge, a viscous mass consisting of lubricant and elastomer components can be seen. After indentation, a probe tip impression clearly shows that this deposit is irreversibly deformed. The stiffness is therefore low at the beginning of the indentation. Underneath this is intact elastomer material, which is usually hardened in the upper layer due to the high temperature. When the probe tip hits the hardened elastomer material, the stiffness is higher. This explains the maximum stiffness. As the indentation depth increases further and more surrounding material is deformed, the stiffness decreases up to the maximum indentation depth and the hysteresis shows a degressive stiffness curve. This degressive stiffness curve is typical for hardened elastomer material. The right-hand side of Figure 4 shows the stiffness curve of the “hard layer” with high stiffness at the beginning and a rapid decrease followed by slight positive gradients up to the maximum indentation depth at a sample measuring point. Visual and tactile examination of the associated sealing edge reveals a thin, brittle, and cracked layer. This explains the very high stiffness at the beginning of the indentation. The rapid drop in stiffness is related to the probe tip breaking through the hard layer. The subsequent slight increase, which accounts for a maximum of 10 % of the change in stiffness of the stiffness decrease, is due to the indentation of elastomeric material without hardening under the layer.