Science and Research 42 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0018 Introduction The vision of “Tribometry 4.0” is an application-oriented, data-driven tribology that, beyond individual parameters, enables a deep understanding of dynamic interactions in the tribological system [GREB23, GREB23a]. Test rigs are evolving into digital platforms that combine flexible test planning, networked data acquisition, and simulation-supported analyses [HEINL25]. The goal of modern “Tribometry 4.0” is to reproduce tribological systems in the laboratory in an applicationoriented manner so as to understand relationships and interactions - and not, as was often customary in the past, to generate simple key figures for glossy brochures. Background The need for new test methods and evaluation methods arises from the numerous challenges currently facing technology in general and mechanical engineering in particular: a) Replacement of well-known and proven raw materials (lead, SAPS, metal-containing additives, PFAS); use of recycled materials and raw materials This results in the necessity of numerous tests to evaluate new materials/ chemicals in order to find substitutes during the development phase and, ultimately, to release them prior to market launch. Rapid, cost-effective, yet still meaningful tests are therefore required (so-called “high-throughput testing”). b) Focus on energy efficiency and friction reduction If one wishes to measure low friction down to super-lubricity, highly accurate and reproducible friction measu- Modern Application-Oriented Tribometry: Understanding Tribology Instead of Producing Characteristic Values - How Modern Measurement Technology Can Enable a View into the Hidden Tribological Contact Markus Grebe, Henrik Buse, Richard Heinlein, Andreas Keller* Presented at GfT Conference 2025 Application-oriented, modern tribological testing technology (“Tribometry 4.0”) is a decisive development tool on the way to a high-performance and reliable product. Based on tribological system analysis and the testing strategy derived from it, the potential of various optimization approaches can only be investigated and evaluated within an acceptable framework in terms of both time and cost by using meaningful laboratory tests. However, the current disruptive developments in many areas of automotive and mechanical engineering require completely new test rigs, test methods, and evaluation approaches adapted to the new requirements. This paper presents these new trends and the resulting requirements and testing technology in detail. Using the example of four laboratory tests for the high-speed suitability of lubricating greases, the evaluation of the performance of plain bearings, and investigations into the presence of grease during long sliding movements, it is shown how additional measurement variables and video documentation with automatic image evaluation help to better understand the processes involved in tribological contact. Keywords Tribometry 4.0, Advanced Measurement Techniques, Machine Learning, Lubrication States, Electric Drives, Image Analysis Abstract * Dr. Markus Grebe Orcid-ID: https: / / orcid.org/ 0000-0002-7048-2645 Dr. Henrik Buse, Andreas Keller Kompetenzzentrum Tribologie Mannheim (KTM) at Technische Hochschule Mannheim, 68163 Mannheim Richard Heinlein Tribologie - Engineering Mannheim GmbH (TEMa) Auf dem Hochschulcampus, 68163 Mannheim predestines them for thermal management (keyword “one-fluid” for thermal management and lubrication of mechanical components). c) Electric and alternative drives A disruptive development for automobile construction is the transition from combustion engines to electric drive systems. In this context, it is initially irrelevant whether the vehicles are battery-electric or fuel-cellpowered. This transition results in new focus areas in the mass market that previously only played a role in special-purpose machine building. Figure 2 provides a conrements are required. This poses major challenges for mechanics and metrology, because high normal forces have to be applied and measured while the acting friction forces (tangential forces) are only a fraction of these (lubricated systems in mixed lubrication approx. f = 0.12; hydrodynamics or super-lubricity f < 0.01 [BMWE25]). Friction measurement results must be critically reviewed with other measured variables. Under this heading, the increasing use of water-based lubricants can also be addressed, as these have low internal friction and thus allow low shear and churning losses. In addition, their heat capacity is high, which Science and Research 43 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0018 Figure 2: Challenges in the context of electric drive concepts Figure 1: Our definition of Tribometry 1.0 to 3.0 - 1.0: First described tribological test rig; 2.0: First commercially available tribometer distributed worldwide; 3.0: Standard friction, wear, and temperature measurement b) Digital Twin A major future topic will be the use of digital twins of tribometers. There is still great potential here. However, as is generally the case in tribometry, the challenge will remain in reconciling simulation with real application. c) Machine Learning The practical use of artificial intelligence - and specifically machine learning - in tribometry is still in its infancy. Online analysis of measurement signals (categorical recognition of operating states such as boundary friction, mixed friction, hydrodynamics, sudden failure/ seizure), prediction of the further course of tests, and improved interpretation of tests through multidimensional linking of measured variables are major topics for the near future. Numerous challenges must also be overcome here, such as generating “good” and FAIR data [GARA25], selecting suitable algorithms, and objectively validating the approaches and solutions. Concrete Application Examples for the Use of Additional Sensors, Variables, or Video Analysis Below, we present four concrete application examples from ongoing projects. a) Vibration Measurement on a High-Speed Rolling Bearing Test Rig As part of a laboratory study on the suitability of lubricating greases for high-speed applications (within FAM Science and Research 44 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0018 cise overview of this challenge. Widely discussed are problems with current passage and electrical discharge, which are being addressed in the FAM standards working group AA653 [NAK53]. High rotational speeds also lead to lubrication issues, which will be discussed in more detail later (application example a), as well as lifetime issues due to the resulting high number of cycles. In the context of electric drive systems, investigations into hydrogen as an energy carrier can also be considered, which likewise poses major challenges for tribology. From these challenges arise concrete requirements for new test rigs and the associated measurement technology. These are illustrated schematically in Figure 3. Digitalization in Tribology and Tribometry In addition to test rigs, digitalization in the field of tribology and tribometry will play an increasingly decisive role in the future. This affects three major areas: a) Metrology In order to increase the informative value of measurements, high-frequency data acquisition of as many variables as possible is necessary. Structure-borne sound or vibration measurements in particular require high sampling rates in the range of several kilohertz. Challenges then arise in data reduction and data storage. Current research is focusing more and more on automatic data analysis and feature engineering rather than simple reduction via mean values [HEINL24]. Figure 3: Requirements for test rigs and measurement technology NA 062-06-52 - Lubricating Greases), comparative investigations were carried out on four sample greases on various high-speed rolling bearing test rigs (including spindle bearing test rig WS22) [NAK52]. KTM took part in this study together with various industrial partners. A newly developed high-speed rolling bearing test rig of the institute was used (Test rig details in [GREB22]). In contrast to the other laboratories, KTM uses, in addition to classical friction torque and temperature measurement, high-resolution vibration measurements to detect signs of starvation and disturbances in the rolling kinematics at an early stage (Figure 4). The results show that, by means of vibration measurement and acoustic emissions (Figure 5, bottom row), critical lubrication states can be detected significantly earlier than with the classical variables, which often only respond when total failure has occurred. Science and Research 45 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0018 Figure 5: Speed ramp in the framework of the laboratory study within NA 062-06-52; top: speed ramps (red) and friction torque evolution (blue); middle: temperature trends at various locations of the bearings; bottom from left to right: acceleration, noise, and peak signals (noise) over load steps (grey) Figure 4: Temperature, acceleration, friction force and sound as measurable values c) Timeand Position-Resolved Contact Voltage Measurement to Determine Lubrication State In the FVA research project “Grease Presence” (FVA- 987), the friction and lubrication behavior in grease-lubricated sliding contacts with long stroke is being investigated [KELL25, KELL25a]. In the field of rolling bearings, important new findings on the lubrication behavior of greases are currently being obtained using optical tribometers and computer simulations [POLL19; WAND21; ZANG23]. Similar knowledge is still lacking for sliding contacts with long stroke, as found, for example, in linear guides or in screw drives. In the project, model tests are carried out in a pin-on-plate configuration (PoP) as well as original-component tests on trapezoidal screw drives of a rear-axle steering system. In the PoP tests, decreasing friction values are frequently observed over the sliding cycle (so-called “bathtub curve”), which could not be evaluated and explained with confidence [KELL24]. With the aid of a self-developed position-resolved contact voltage measurement, the lubrication state can now be correlated with the friction coefficient [KELL25a]. It is evident that hydrodynamic states are reached at times despite the comparatively low sliding speed of an oscillating motion and the lack of an ideal wedge geometry (Figure 7). These changes of the lubrication regime explain the low friction values and the favorable wear behavior. In Figure 8 one can see how the model system pin-onplate, initially lubricated with grease, first runs in (graphic at bottom right: hydrodynamic shares at mixed friction increase) and then relatively abruptly fails as a result of the sudden onset of starved lubrication (Figure 8, Science and Research 46 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0018 b) Vibration Measurement with ML Evaluation on a Sliding Bearing Test Rig Within the industrial working group “Machine Learning in Tribology”, in which five industrial companies joined forces, the question was investigated how the friction and wear behavior of a radial plain bearing can be examined more precisely, i.e., more informatively. For this purpose, in addition to the classical variables friction torque and temperature, a highly accurate online wear measurement, contact voltage measurement, and structure-borne sound measurements were used (Figure 6). It was again shown that the information content of highfrequency vibration measurements clearly exceeds that of the classical parameters and that the measurements are considerably more sensitive when it comes to describing system states [HEINL24]. First mixed-lubrication contacts could be detected early, repeatably, and with clear separation, whereas no excursions were yet visible in the friction and temperature trends. In combination with contact voltage measurement, this metrology allows reliable prediction of when the running-in of a bearing is complete and when critical operating states occur, for example as a result of overload or starved lubrication. The transferability of trained anomaly-detection models was also investigated with regard to bearing material and lubricant. The algorithm-based combination of run-in detection, definition of a baseline state, and detection of deviations from this state (anomaly detection) is a strategy that can also be applied to other test rigs. A current doctoral project is investigating whether the features and ML algorithms generated in the laboratory allow direct predictions for original components and aggregates in the field. Figure 6: Measurement data of a test with a run-in radial plain bearing and average temperature; third from top: wear signal (unfiltered and mean); bottom: acoustic emission signal (blue) and contact voltage signal (red) single graphic at bottom left: sudden rise in friction; collapse of the lubricant film at bottom right visible in the contact voltage signal). d) Grease Presence Assessment Using a Camera System and ML-Based Evaluation In the same project (FVA - Grease Presence), the influence of different lubricant depots that form on the moving friction partner and on the counterbody was to be in- Science and Research 47 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0018 Figure 8: Measurement and video data of a model test of oscillating sliding with long stroke; top left: friction coefficient over stroke cycle (friction hysteresis) (red) and contact voltage signal (black); top right: image documentation of the left and right end position (captured from time-lapse video); bottom left: friction trend over test time (peak (orange) and mean values (blue)); bottom right: temperature (blue) and change in the contact voltage signal over the test duration Figure 7: Friction coefficient (red) and contact voltage signal (black) over one friction cycle (hysteresis, forward and backward stroke ±15 mm) vestigated. For this purpose, a camera system was applied that observes the moving friction partner during the test. Using automated image evaluation, the grease quantities at the end positions and on the moving friction partner can now be determined automatically and user independent (Figure 8). In addition, it is possible to detect a color change of the grease. This generally correlates very well with the onset of wear due to starved lubrication, since even the In summary, the following can be stated once again: • Test rigs, test methods - and later also standards - are required that can simulate the new practical questions in the laboratory in an application-oriented manner. • Digital solutions such as “digital twin” and “machine learning” must be more strongly linked to test technology. • The goal of “Tribometry 4.0” must be to understand tribology and not to generate key figures for advertising brochures. The challenges offer great potential for further development and an increase in the significance of test technology. References [BMWE25] Information des Bundesministeriums für Wirtschaft und Energie; https: / / www.energiefor schung.de/ de/ glossar/ Supraschmierung+%28eng l%3A+superlubricity%29; Download 20.8.2025 [GARA25] Garabedian, N.T., Schreiber, P.J., Brandt, N. et al. Generating FAIR research data in experimental tribology. Sci Data 9, 315 (2022). https: / / doi.org/ 10.1038/ s41597-022-01429-9 [GREB22] M. Grebe: Influence of a vibration load on the service life of rolling bearings in e-drives; 7 th World Tribology Conference 2022, Lyon 10-15 2022 [GREB23] M. Grebe: Tribometry 4.0; NextLub Conference, Presentation and Digital Proceedings, Uniti/ FVA/ GfT, Düsseldorf, 2023. [GREB23a] M. Grebe: Tribology 4.0: From Da Vinci to digitalisation; Lube Magazine - The European Lubricants Industry Magazine; Issue 178, p. 15-17; December 2023 Science and Research 48 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0018 smallest amounts of abrasion lead to darkening (Figure 9). Within that project it was shown that the combination of contact voltage measurement and automated visual evaluation of grease presence and grease condition provides significant added value compared to the classical variables friction force and temperature. The question of how individual grease deposits contribute to lubricating contact could thus be answered for the tribosystem in question. Summary This publication has shown that tribometry is continuously evolving. Current topics such as electromobility or sustainability issues bring new challenges for tribological test technology. The combination with new technical possibilities forms the basis for the next stage in tribometry, which we refer to as “Tribometry 4.0”. Using four concrete application examples from ongoing research projects, it was demonstrated how additional measurement technology and AI-based evaluation methods help to provide insights into the tribological contact, which is usually hidden. They clearly show the added value of new variables or features and of image analysis compared to the classical friction and wear parameters. The methods presented, as well as further approaches, for example based on impedance measurement technology, are being continuously developed in further projects (e.g. [TIDO25]) and in the context of industrial research in order to understand tribology ever better in the future and to advance developments in a targeted and thus timeand cost-efficient manner. Figure 9: Video documentation with automated pixel evaluation [HEINL24] R. Heinlein, D. Glowania, G. Tidona, M. Grebe: Einsatz von maschinellem Lernen in der Schmierfett-Evaluierung; Vortrag GfT Jahrestagung 2024; Tagungsband Vortrag 69, S. 426-435, ISBN 978- 3-9817451-9-1; 2024. [HEINL25] R. Heinlein, M. Grebe: Expertensystem zur Erkennung des Schmierungszustandes auf Basis dynamischer Versuchsführung; Vortrag anlässlich des DVM-Workshops „Zuverlässigkeit tribologischer Systeme“, Mannheim, 13. März 2025 [KELL24] A. Keller, M. Grebe: Verbesserung des Verständnisses der Fettrückhalte- und Schmierungsmechanismen von oszillierenden Gleitkontakten mit großem Hub; Vortrag GfT Jahrestagung 2024; Tagungsband Vortrag 15, S. 95-102, ISBN 978-3- 9817451-9-1; 2024. [KELL25] A. Keller, M. Enhuber, M. Grebe, K.-H. Jacob: Abschlussbericht zum FVA-Eigenmittelvorhaben FVA 987-I: „(Vor-)Entwicklung einer Prüfstrategie zur Qualifizierung von Schmierfetten für lebensdauergeschmierte Gleitanwendungen mit langem Hub“; 2025 [KELL25a] A. Keller, D. Kursawe, M. Grebe, K.-H. Jacob: Zwischenbericht zum FVA-Eigenmittelvorhaben FVA 987-II: „Einfluss der Fettdegradation durch thermo-oxidative Alterung auf die Schmierung von Gleitanwendungen mit langem Hub“; 2025 [NAK52] Protokoll „NAK Drehzahlkennwertbestimmung“ im FAM NA 062-06-52 - Schmierfette; 05. Mai 2025 (WebCon) [NAK53] Protokollentwurf AA653_RV3 im „NAK Elektrische Kennwerte“ vom 17.7.2025 im FAM NA 062-06-53; (WebCon) [POLL19] G. Poll; X. Li; N. Bader; F. Guo: Starved Lubrication in Rolling Contacts - A Review; Bearing World Journal Vol. 4 (2019), pp. 69-81 [TIDO25] G. Tidona, M. Grebe: Zwischenbericht zum IGF- Forschungsprojekt „Einsatz von maschinellem Lernen in der Schmierfettentwicklung“ (DGMK 871), Industrielle Gemeinschaftsforschung (IGF); 2025 [WAND21] S. Wandel; N. Bader; F. Schwack; J. Glodowski; B. Lehnhardt; G. Poll: Starvation and Relubrication Mechanisms in Grease Lubricated Oscillating Bearings; Tribology International; 09/ 2021; https: / / doi.org/ 10.1016/ j.triboint.2021.107276 [ZANG23] S. Zhang; G. Jacobs; S. Vafaei; S. von Goeldel; F. König: CFD investigation of starvation behaviors in a grease lubricated EHL rolling contact; Forschung im Ingenieurwesen, 2023; https: / / doi.org/ 10.1007/ s10010-023-00633-2 Science and Research 49 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0018