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Tribologie und Schmierungstechnik HERAUSGEGEBEN VON ANDREAS PAUSCHITZ UND MANFRED JUNGK 69. JAHRGANG eOnly SONDERAUSGABE 2 _ 22 Organ der Gesellschaft für Tribologie Organ der Österreichischen Tribologischen Gesellschaft Organ der Swiss Tribology eOnly Sonderausgabe 2 | Dezember 2022 69. Jahrgang Herausgeber: Dr. Manfred Jungk Tel.: +49 (0)6722 500836 eMail: manfred.jungk@mj-tribology.com www.mj-tribology.com Redaktion: Dr. rer. nat. Erich Santner Tel.: +49 (0)2289 616136 / eMail: esantner@arcor.de Ulrich Sandten-Ma Tel.: +49 (0)7071 97 556 56 / eMail: sandten@verlag.expert Beiträge, die mit vollem Namen oder auch mit Kurzzeichen des Autors gezeichnet sind, stellen die Meinung des Autors, nicht unbedingt auch die der Redaktion dar. Unverlangte Zusendungen redaktioneller Beiträge auf eigene Gefahr und ohne Gewähr für die Rücksendung. Die Einholung des Abdruckrechtes für dem Verlag eingesandte Fotos obliegt dem Einsender. 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Aktuelle Informationen über die Fachbücher zum Thema „Tribologie“ und über das Gesamtprogramm des expert verlags finden Sie im Internet unter www.expertverlag.de Ihre Mitarbeit in Tribologie und Schmierungstechnik ist uns sehr willkommen! Impressum Tribologie und Schmierungstechnik Organ der Gesellschaft für Tribologie | Organ der Österreichischen Tribologischen Gesellschaft | Organ der Swiss Tribology Editorial 1 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0031 As spokesperson of the Steering Committee of the 23 rd International Colloquium Tribology 2022, I was involved in the preparations at the TAE from the very beginning. In doing so, I pursued the goal of realising an interesting mixture between very experienced tribologists as well as professional younger tribologists and to bring the latter more clearly to the fore, especially for the participants from industry. The Tribology Colloquium was intended to be an on-site event - but considering the Covid experiences at that time, it was necessary to change to a virtual event. Due to general conditions, the programme had to be focused on about 145 presentations, 6 parallel lecture series and 3 days. I would like to take the opportunity to thank and acknowledge the TAE team for providing the technical basis for the virtual event and for the organisational support during the event. The 23 rd International Colloquium Tribology covered a broad spectrum of current tribological topics in its various disciplines: It focused on new trends in lubricants and additives, test methods and measurement technologies, coatings, surfaces, and underlying mechanisms with the key topics of sustainability, including e-mobility and digitalization in tribology. Five contributions from scientifically active authors on this range of topics were selected and are included in the present edition as a long version. What does the future of the International Colloquium Tribology look like? The preparations for the 24 th event in January 2024 have already begun. We consider that a face-to-face event will be possible again. Exactly these networking opportunities represented the key item of attraction for many participants of the previous events of the TAE. But the virtual participation option will remain, if only to enable participants from regions far away to take part in a timeand cost-efficient way. However, in view of climate change, don't we have to admit that a purely virtual event, which avoids individual journeys to the venue, is the more environmentally friendly solution? Perhaps I am already too old for this or have attended the Tribology Colloquium at TAE too many times in person to be enthusiastic about such a change. Here the hope remains that the next generations of tribologists will find more sustainable solutions, because tribological knowledge is indispensable for sustainability in human activity. Therefore, I am actively involved in ensuring that responsibility is also placed on younger shoulders in the Steering Committee for the coming event and that they hopefully tread new paths, and I will contribute to the Tribology Colloquium 2024 in the second row. The new lecture topics will cover all current aspects of modern tribology. Special attention will be paid to networking between speakers and participants from industry and research organisations in an international atmosphere, exhibitors are invited to display their services and products and you can look forward to an exciting social programme. So don´t miss this leading event on tribology and be a part of it. We look forward to welcoming you at Ostfildern/ Stuttgart in January 2024. Prof. DI Dr. Andreas Pauschitz Steering Committee of the International Colloquium Tribology 2022 Retrospect and outlook Veranstaltungen 2 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 Vor Ort oder online teilnehmen Flex: Präsenz in Ostfildern oder Online-Teilnahme Präsenz in Ostfildern Einstieg jederzeit möglich Einstieg jederzeit möglich Tribologie Experte (TAE) Lehrgang (60160) 14. Mrz - 28. Nov. 2023 Oberflächen Spezialist (TAE) Lehrgang (60163) 15. Mrz - 05. Jul. 2023 Grundlagen der Tribologie - Metalle und Kunststoffe Seminar (35824) 14. Mrz. 2023 Grundlagen der Tribologie - Methodik und Anwendung Seminar (35827) 02. Mai 2023 Getriebedimensionierung Seminar (35390) 27. + 28. Feb. 2023 Weitere Informationen und Anmeldung unter www.tae.de 10 Module à 1 Tag 4 Module à 1 Tag Grundlagen der Oberflächentopographie Seminar (35825) 15. Mrz. 2023 Vertiefung zur Oberflächentopographie Seminar (35826) 16. Mrz. 2023 Ihr Weiterbildungs- Partner in Sachen Tribologie Besuchen Sie unsere Seminare, Lehrgänge und Fachtagungen. Innovative Miniaturlagerlösungen - vom Herz bis zum Mars Seminar (35646) 28.+ 29. Mrz. 2023 Inhalt 3 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 4 Polychronis Dellis Squeeze Film Investigations in a Simulating Piston-Ring Cylinder Liner Experimental Set-up 10 Hans-Martin Eckel, Christian Brecher, Stephan Neus Kugelbewegung in Spindellagern unter dynamischer Belastung Ball motion in spindle bearings under dynamic loads 18 Justus Rüthing, Frank Haupert, Regine Schmitz, Michael Sigrüner, Nicole Strübbe A new approach for the friction and wear characterisation of polymer fibres under dry, mixed, and hydrodynamic sliding 26 Karl Jakob Raddatz, Thomas Tobie, Klaus Michaelis, Karsten Stahl Scientific Evaluation of Investigations on the Load Carrying Capacity of Carbide Cylindrical Gears Lubricated with Water 36 Theodora Tyrovola, Fanourios Zannikos Tribological Assessment of Marine Distillate Fuels under a Variant HFRR Method 1 Editorial Retrospect and outlook Aus Wissenschaft und Forschung Vorab Tribologie und Schmierungstechnik Organ der Gesellschaft für Tribologie Organ der Österreichischen Tribologischen Gesellschaft Organ der Swiss Tribology 69. Jahrgang, eOnly Sonderausgabe 2 Dezember 2022 Veröffentlichungen Die Autoren wissenschaftlicher Beiträge werden gebeten, ihre Manuskripte direkt an den Herausgeber, Dr. Jungk, zu senden (Checkliste und Formatvorgaben siehe Umschlagseite hinten). Authors of scientific contributions are requested to submit their manuscripts directly to the editor, Dr. Jungk (see inside back cover for formatting guidelines). IHR ONLINE-ABONNEMENT DER TuS Ab dem Jahrgang 2019 können Sie die aktuellen Hefte der Tribologie und Schmierungstechnik im Online-Abonnement beziehen. Die Hefte der vergangenen Jahrgänge werden kontinuierlich integriert. Unsere eLibrary bietet Ihnen einen qualitativ hochwertigen und benutzerfreundlichen Zugang zum digitalen Buch- und Zeitschriftenprogramm der Verlage expert, Narr Francke Attempto und UVK. Nutzen Sie mit uns die Chancen der Digitalisierung: https: / / elibrary.narr.digital/ journal/ tus Der Online-Zugang ist in Kombination mit dem Print-Abo oder als e-only-Abo erhältlich. Abo-Service: Tel: +49 (0)7071 97 97 10 Fax: +49 (0)7071 97 97 11 eMail: abo@narr.de Weitere Inhalte zu den Themen Schmierung, Reibung und Verschleiß finden Sie unter www.narr.de/ technik. 46 Nicole Dörr, Andreas Pauschitz Review of the 23 rd International Colloquium Tribology - 25-27 January 2022 Hinweise für Autoren / Checkliste (siehe Umschlag) Bericht Squeeze film is a term denoting a hydrodynamic film that sustains a negative (1) (h is the film thickness, t is time) i.e. when the opposing surfaces squeeze together. An extremely useful characteristic of squeeze films is that they provide increased load capacity (although temporary) when a bearing is suddenly subjected to an abnormally high load, while another characteristic of squeeze films is that the squeeze film force is always opposite in direction to the motion of either bearing surface [1]. Evidence of squeeze film effect can be found in the minimum oil film thickness (MOFT) measurements where one can notice that the measurement profile is not symmetric. As Bolander et al [2] have pointed out, the point of absolute minimum of the oil film thickness measurement is shifted a few degrees from the TDC and BDC. Friction peaks correspond to asperity interaction, contact Aus Wissenschaft und Forschung 4 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0032 1 Introduction The study of overall load carrying capacity of the piston-ring and liner lubricated interface leads to the tribological explanation and evaluation of the different parameters affecting the lubricated conjunction and the surface interaction. The main focus is on optimising lubrication and promote effective lubrication of the surfaces in contact. As part of this, friction reduction, cavitation initiation and development, which in turn limits the load carrying capacity and measurement of the oil film thickness, flow rate and oil film pressure has become a priority that eventually leads to emissions reduction. The saviour in lubrication terms is the squeeze film effect at low velocities where a thick enough film to sustain the load capacity is non-existent. According to Stachowiak and Batchelor [1], an extremely useful characteristic of squeeze films is that they provide increased load capacity (although temporarily) when a bearing is suddenly subjected to an abnormally high load. As regards to the motion of either side of the bearing surface, the squeeze film force is always opposite in direction to their motion. 2 Squeeze Film: Minimum oil film thickness (MOFT) - Friction force measurements - Load capacity of piston-ring Bearings that are subject to transient dynamic loads (such as engine crankshaft bearings) are likely to withstand much larger instantaneous maximum loads than the ones derived from steady state analysis. The reason behind this, is that there is not sufficient time to develop pressure capable of spreading the lubricant layer before total load is reduced. This phenomenon is causing an obvious lubricant film “stiffness” as acting loads impose a continuously thinner film. Squeeze Film Investigations in a Simulating Piston-Ring Cylinder Liner Experimental Set-up Polychronis Dellis* The importance of these investigations lies to the combination of experimental results with cavitation initiation investigations and its development after the dead centers of the stroke as well as rheological behavior of different chemical additives with a view to establishing the likely performance gains in new lubricant formulations. Lubricant formulation plays an important role because at higher temperatures lubricant additives have a different interaction with contacting surfaces and in this manner the resulting effect of asperity contact is either increased or reduced. Keywords squeeze film, piston-ring lubrication, experimental test-rig, friction, oil film thickness, energy losses, lubrication modeling Abstract * Polychronis Dellis School of Mechanical Engineering Educators, ASPETE, Neo Iraklio, 15122, Athens, Greece between the piston-ring and liner surfaces where the boundary lubrication prevails. The lubricant begins to squeeze out of the contact area while the pressure generated through this squeezing motion shifts the profile towards the center line [2]. The interpretation of the minimum oil film thickness measurements is important as it identifies the lubrication rheology phenomena that in turn, affect the load capacity of a certain piston-ring configuration. It was also noted that the profile of the minimum film thickness is not symmetric. The point of absolute minimum is shifted a few degrees from TDC and BDC due to the squeeze film effect. This effect is also seen in the friction signals causing an asymmetry in friction spikes at the ends of the stroke [2]. Theoretical predictions for a Newtonian fluid have determined the effect of a number of independent system parameters on performance for an idealised system involving a Newtonian fluid and perfectly smooth surfaces [3]. The squeeze velocity is studied as a dependent system parameter, derived from the simulation of independent parameters such as contact geometry, angular speed, load and viscosity and surface parameters. The numerical parametric study of the system for the Newtonian fluid was derived with the Swift-Stieber boundary condition imposed on the Reynolds equation. Cavitation delay for specific oils was also considered for a range of boundary conditions and were quantified. With a magnitude of the sinusoidal velocity reaching 0.79, 1.31 and 1.83 m/ sec respectively for rotational speeds of 300, 500 and 700 rpm at the idealised single ring test rig system, the minimum cyclic film thickness is occurring a few degrees crank angle after the reversal due to the squeeze action. The impact of combined effects of speed and dynamic load results is movement of film thickness to be shifted towards BDC [3]. Dynamic effects due to the variation of speed depict the rate of change of film thickness at different speeds. This rate of change represents the squeeze velocity and the finding from the simulation was that increasing the speed from 300 to 500 rpm does not significantly alter the crank angles at which squeeze velocity changes sign, except in the vicinity of 90° CA where the maximum film thickness is reached later in the stroke at 500 rpm. Cross over points at 700 rpm near mid-stroke were found to be clearly shifted towards BDC due to the reduced dynamic load. Power losses due to the squeeze action explain why the power loss at the dead centers are very small but not quite equal to zero [3]. Different modelling conditions showed that in the vicinity and after the reversals, where the squeeze action is dominant, the prediction using the separation boundary condition is not significantly different from that with the Swift-Stieber condition. More knowledge is required on the onset of cavitation in dynamic systems before the mechanism of delayed cavitation that leads to thinner films than predicted when applying the Swift-Stieber condition, can be accurately modelled. Some further understanding can be obtained by comparing the measured friction with the values derived from simulation. The Swift Stieber and separation conditions lead to underestimation of the friction simulated values whereas Coyne and Elrod results is an overprediction. This is due to a corresponding increase of the shear rate in the fluid and the extent of the film over which the lubrication shearing occurs [3]. For the model applied (Greenwood and Tripp), asperity interaction reduces the rate of change of film thickness near the reversals when asperity interaction occurs. Squeeze action is also inhibited as increasing amount of the load is carried by asperities. This is evident in the (2) (h¯ T is the average film thickness (separation) in the mixed lubrication model) sign change to occur very close to the dead centers [3]. It was also stated that application of the quasi-steadystate assumption to an inherently dynamic system should be expected to lead to a degree of error, especially in those parts of the cycle where cavitation regions start to form, i.e. close to the dead centers of the stroke. Frictional losses are increasing due to extensive cavitating regions. The geometry of the piston rings contributes to different cavitation sizes and what needs to be clarified is whether the cavitation development at the beginning of the stroke plays a significant role in the friction peaks at boundary lubrication region [4]. The initiation of cavitation which is affected by speed, load, ring geometry, temperature and lubricant chemistry is affecting the squeeze film as experimental data have shown. Cavitation affects the squeeze film forces by the formation of compressible bubbles in the imcompressible lubricant. The appearance of bubbles is due to a much slower rate of bubble dissolution as compared to the rate of bubble formation. Eventually load capacity is reduced compared to the assumption of no cavitation effects [4]. The lubricant reformation is dependent on squeeze film effect-therefore when this is significant the lubricant reforms earlier. At these low velocities the hydrodynamic action is not sufficient to sustain the thick film and the lubricant begins to squeeze out of the contact as the surfaces move closer. The squeeze pressure is independent of the asperity contact pressure that begins to increase as the film continues to drop [1]. It has been verified that different forms of cavitation appear after the dead centers of the stroke that accompany the squeeze film (which is measured from the capacitance signal and also evidenced by the friction peaks at the same stroke area). Aus Wissenschaft und Forschung 5 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0032 a view to establishing the likely performance gains in new lubricant formulations. 2.2 Results Viscosity variation, physical and chemical properties are correlated to the measured friction force, at the boundary, mixed and hydrodynamic lubrication region of the stroke. Meanwhile, geometrical factors such as pistonring curvature variation, affect total friction, oil film thickness and oil film pressure measurements as well as the squeeze film effect. When the squeeze film effect is studied for lubricants with different HTHS viscosities, it was noticed that the oil with the higher HTHS viscosity produces a thicker film at all temperature testing with the squeeze film moving further down the stroke for the lubricant with the higher HTHS viscosity. For a specific oil, a similar trend was noticed for the squeeze film in oil film thickness curves at all testing temperatures, for the same speed and load test cases. In a set of experiments focused on high temperature testing, high friction peaks were noticed when oil viscosity changed to lower values as lubricant temperature increases and MOFT decreases significantly [7]. At higher temperatures the asperity interaction at the boundarymixed lubrication region is intense giving considerably higher friction results than the ones taken at lower temperatures. As MOFT decreases with high temperature, friction force peaks move closer to the dead centers of the stroke with absolute friction values that are significantly higher. This gives evidence that the squeeze film effect does not have such a strong impact at high lubricant temperatures. In previous publications it was shown that for the MOFT measurements, high load testing is combined with squeeze film movement towards the dead centers of the stroke. High temperature testing showed that the MOFT decreases significantly from ambient temperature (33 °C) to 50 °C and that the squeezing action is getting Aus Wissenschaft und Forschung 6 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0032 2.1 Experimental set-up In a simplified single-ring test rig, a steady piston-ring section is placed under a flat surface used as a reciprocating liner. The idealised simulation test rig benefits from simplified lubrication conditions compared to the real engine taking advantage of the simple design layout. As a result, solid and repeatable results are taken allowing the lubricant film characteristics to be examined in isolation. Sensors that measure oil film pressure, thickness (optical - LIF and electrical - capacitance), friction and imaging, provide the necessary parametric data to study the effect of speed, load, temperature, piston-ring curvature and variable lubricant properties. When the liner decelerates, the interface reaches a state of mixed lubrication and asperity interaction until the liner reaches boundary lubrication close to the dead centers, as the squeeze film prevails. As the liner accelerates away from the dead centers, the lubricated film begins to develop. While being close to the dead center, asperity interaction between the surfaces remains significant with the squeeze film effect also taking place, resulting in beneficial oil support as it is supported partly by the lubricant present in the contact [4]. Increase in temperature in a lubricant model investigation had the effect of decrease in oil film thickness and advanced the initiation of cavitation and enhanced its intensity [5]. Less viscosity results in less squeezing force from the oil around the dead centers and thus greater asperity contact force is generated to support the radial ring load. Less oil squeezing force is responsible for more asperity contact around the dead centers [6]. For the modelling performed the Swift-Stieber boundary condition imposed on the Reynolds equation resulted in good agreement but for a certain level of high temperature/ high shear (HTHS) viscosity coefficient the measured lubricant films after the reversals were thinner than predicted due to cavitation delay, i.e. the period within the cycle that the lubricant experienced absolute tension. The mixed lubrication models for both Newtonian and non-Newtonian shear thinning fluids were shown to be sensitive to the asperity interaction sub-model [3]. The purpose of this study is to show the effect of squeeze film variation and extract useful parametric results that show how different lubricants and setups impact on friction peaks / losses, correlate and verify them to other measurement techniques for the single ring set-up (such as MOFT measurements). Cavitation initiation and development is another factor that should be taken into account and assess whether cavitation development at the beginning of the stroke together with impeding or aiding factors, play a significant role in friction peaks and MOFT minima. Eventually, a clearer picture will be attained to the aspects of load carrying capacity of the ring and the rheological behaviour of chemical additives with Figure 1: Temperature effect on friction force peaks at 300 rpm, 3371 N/ m load, top dead center [7] marginal. The same action is shown in Figure 2 for the friction peaks [8] at 300 rpm, 971 N/ m and 70 °C. For a set of different lubricants, the properties of which can be found in Table 1, friction force peaks have different behaviour close to the dead centers. Figure 2 shows that for similar speed and load and temperature testing conditions friction force peaks move closer to the dead centers for the lubricant that has the lowest VI, V 40, V 100 and HTHS viscosity. For the same set of lubricants and same testing conditions, the measurements close to top dead center can be seen in Figure 3. Upstroke and downstroke measurements, in terms of cavitation area size and ring geometry (symmetric or non symmetric) can show some discrepancy. As it was shown in [9] the aforementioned factors can have a significant effect. In the case of the simulating single-ring test rig, motion dynamics and location of the liner in relation to the electric driving motor rotating shaft had an effect on the measurements and dynamic load on the liner due to the linear motion. This was quantified with an equation that contained the angular position [7]. For the set of lubricant tested, it was verified that oil 2A had the lowest friction peak at the boundary lubrication region and its location moves further in the stroke compared to the other three lubricants of the test matrix. Oil 2A had the highest V 100 , V 40 and HTHS viscosity according to Table 1. The friction signal for the lubricants tested as in Table 1 can be seen in Figure 4. This figure represents the friction signal for the whole stroke. It can be inferred that in terms of power losses, the most important factor is the behaviour of the lubricants at the dead centers, with the friction peaks’ variation being the major indicator of the friction losses. Hydrodynamic losses also play an important role as they spread throughout the majority of the stroke, but in magnitude are relatively even. Testing conditions are at high temperature 70 °C, 300 rpm and 971 N/ m load. It has been verified that different forms of cavitation appear after the dead centers of the stroke that accompany the squeeze film which is measured in the capacitance and friction signals [5, 7, 8, 10]. The geometry of the piston-ring affects the friction force as well. The flatter the piston-ring, the lower the friction force peak and they also appear earlier in the stroke [7]. In Figure 5, the shift in squeeze film close to BDC for oil 6E, at 400 rpm and 1159 N/ m load can be seen. There is a marginal shift closer to the dead center for 50, 60 and 70 °C, compared to the ambient temperature curve (35 °C). Further testing of oils in Table 1 showed the results in Figure 6. A slight shift towards BDC can be evi- Aus Wissenschaft und Forschung 7 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0032 Figure 2: Lubricant properties effect on friction force peaks at 300 rpm, 971 N/ m load, at bottom dead center Figure 3: Lubricant properties effect on friction force peaks at 300 rpm, 971 N/ m load, at top dead center Figure 4: Friction variation, for the whole stroke length all oils tested, at 300 rpm, 971 N/ m and 70 °C Blend Code 003B 006E/ 02 005A/ 02 002A/ 02 Grade 0W-30 0W-40 0W-20 10W-40 HTHS(mPas) 3.30 3.4 2.14 4.05 V 100 (cSt) 12.16 12.8 6.04 14.97 V 40 (cSt) 68.93 66.8 31 97.8 VI 182 196 146 160 Table 1: Oils tested for temperature-friction investigations boundary, mixed and hydrodynamic lubrication region of the stroke. - Large radius of curvature for the ring profile promotes effective squeeze action at the ends of the stroke, as the flatter ring enhances a stronger squeeze effect than the curved ring at the dead centers [7]. - Different oil blends produce different appearance for the friction peaks in terms of their distance from the dead centers and an obvious absolute friction peak measurement. - For the cavitating region of the lubricant, parameters such as speed, load and temperature affect its initiation and furthermore the number of the specific form of string cavities might accordingly apply to temperature effect results. - Low MOFT decreases the wedging action at the converging profile providing a lower load carrying capacity. - For the set of four lubricants tested, the one with the highest V 100 , V 40 and HTHS viscosity seems to shift the squeeze film further in the stroke. It remains to be seen how is this behaviour established, in a set of lubricants to be tested that have same SAE grade as lubricant 2A and relatively different HTHS, V 100 , V 40 and VI values. The importance of these investigations lies to the combination of previous experimental results with cavitation initiation investigations and its development after the dead centers of the stroke as well as rheological behavior of different chemical additives with a view to establishing the likely performance gains in new lubricant formulations. Lubricant formulation plays an important role because at higher temperatures lubricant additives have a different interaction with contacting surfaces and in this manner the resulting effect of asperity contact is either increased or reduced. References [1] Stachowiak G.W. and Batchelor A.D., “Engineering Tribology”, ELSEVIER, 1993. [2] Bolander, N. W., Steenwyk, B. D., Sadeghi, F. and Gerber, G. R., “Lubrication Regime Transitions at the Piston Ring - Cylinder Liner Interface”, Proceedings of the Institution of Mechanical Engineers Part J: Journal of Engineering Tribology, 219, No 1, 19-31, 2005. [3] Ostovar, P. “Fluid Aspects of Piston Ring Lubrication”, PhD thesis, Imperial College of Science, Technology and Medicine, 1996. [4] Dellis P.S., “Piston-ring performance: limitations from cavitation and friction, International Journal of Structural Integrityˮ, Vol. 10 No. 3, pp. 304-324, 2019. [5] Nouri J. M., Vasilakos I., Yan Y., “Cavitation between cylinder-liner and piston-ring in a new designed optical IC engine”, Int. J. of Engine Research, (on line first) 9 Apr 2021. Aus Wissenschaft und Forschung 8 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0032 denced from the magnification over the area of interest. The oils that the corresponding squeeze film moves towards the dead center are 2A and 3B, with oil 3B being the most prone to alter its squeeze film measurement towards the dead center. 3 Conclusions - The squeeze film effect between the liner surface and the piston ring, shift the friction force peaks, as it forces the lubricant flow to delay compared to the liner movement. - With an increase in load in every lubricant the flow reversal due to the squeeze film effect appears closer to the dead centers. That could be also attributed to the fact that the viscous film impedes the liner’s reciprocation. - Load capacity is affected by the cavitating region. - With an increase in reciprocation speed for constant load, friction force maxima have lower absolute measurements and appear at a greater distance from the dead centers. - Viscosity variation, physical and chemical properties are correlated to the measured friction force, at the Figure 5: Temperature effect on MOFT curves at BDC, for oil 6E, 400 rpm, 1159 N/ m Figure 6: All oils tested-squeeze film behaviour at 600 rpm, 1159 N/ m load 70 °C [6] Tian, T., Wong, V. W., and Heywood, J. B. “A Piston-Ring Pack Film Thickness and Friction Model for Multigrade Oil and rough surfaces”, SAE paper 962032, 1996. [7] Dellis P., “Effect of Friction Force between Piston Rings and Liner: a Parametric Study of Speed, Load, Temperature, Piston-Ring Curvature and High-Temperature, High- Shear Viscosity”, Proc IMechE, Part J: J Engineering Tribology, 224, No 5, 411-426, 2010. [8] Dellis P., “Oil Film Thickness Measurements Combined with High Temperature Friction Investigations in a Simplified Piston-Ring Lubrication Test Rig”, Tribology in Industry, 41 No. 4, 471-483, 2019. [9] Dellis P., “Cavitation development in the lubricant film of a reciprocating piston-ring assembly”, Proc. IMechE, Part J: J. Engineering Tribology, 218 No 3, 157-171, 2004, DOI: 10.1243/ 1350650041323340. [10] Dellis P., “Cavitation initiation and patterns in engine lubricants as a result of different operating conditions and lubricant properties”, STLE Virtual Annual Meeting and Exhibition, May 17-20, 2021, New Orleans, USA. Aus Wissenschaft und Forschung 9 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0032 Käfigtaschen, der so genannte Kugelvor- und -nachlauf (KvKn). Überschreitet dieser Wert den geometrisch möglichen Bewegungsfreiraum der Kugeln, welcher der Summe aus Käfigtaschen- und Käfigführungsspiel entspricht, können erhebliche Kontaktkräfte zwischen Kugeln und Käfig entstehen. Das Risiko eines Käfig- und Lagerausfalls steigt. Entsprechend ist dieser Wert neben den maximalen Pressungen und dem Bohr-Roll-Verhältnis eine zentrale Auslegungsgröße von Spindellagern [2,3]. Beispiele aus der Praxis zeigen hingegen, dass für spezifische Lastfälle hohe KvKn-Werte rechnerisch auftreten, aber keinen unmittelbaren Lagerschaden in Hauptspindeln hervorrufen [4]. Eine mögliche Erklärung ist eine hohe Abweichung zwischen der berechneten und tatsächlichen Kugelkinematik unter dem Einfluss prozessähnlicher Belastungen. Die Kugelbewegung lässt sich nicht eindeutig mit kinematischen Formeln beschreiben, da sie dem lastabhängigen Reibungszustand in den Wälzkontakten unterliegt. Aus Wissenschaft und Forschung 10 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0033 1 Einleitung Spindellager sind in der Anwendung hohen Drehzahlen und Belastungen ausgesetzt. Der Belastungszustand setzt sich häufig aus statischen und überlagerten dynamischen Anteilen zusammen. Typische Anregungsquellen für die dynamischen Belastungen sind Schneideneingriffe bei der Fräsbearbeitung oder die Zahneingriffe in Getrieben. Unter dem Einfluss von Radialkräften und Momentbelastungen entsteht eine Druckwinkeländerung am inneren und äußeren Kontakt entlang des Lagerumfangs. Dies führt zu einer Modulation der Orbitalgeschwindigkeit der Kugeln um die Lagerachse [1]. Die Folge ist ein Vor- und Nachlaufen der Kugeln in den Kugelbewegung in Spindellagern unter dynamischer Belastung Hans-Martin Eckel, Christian Brecher, Stephan Neus* Der Kugelvor- und -nachlauf ist die wesentliche Auslegungsgröße von Spindellagern neben den maximalen Pressungen und dem Bohr-Roll-Verhältnis. Dieser beschreibt das Vor- und Nacheilen der Kugeln in den Käfigtaschen bei radial belasteten Spindellagern. Mit einem neuartigen Messsystem kann die Bewegung der Kugeln präzise entlang des Lagerumfangs erfasst werden. Gleichzeitig bietet der entwickelte Prüfstand die Möglichkeit das Lager mit dynamischen Kräften zu belasten. Die Ergebnisse zeigen eine Erhöhung des Kugelvor- und -nachlaufs bei einer Belastung mit der Käfig- und der technisch relevanten Wellendrehfrequenz im Vergleich zur statischen Belastung. Eine Belastung mit Vielfachen der Wellendrehfrequenz führt zu keiner signifikanten Erhöhung des Kugelvor- und -nachlaufs. Schlüsselwörter Spindellager, Hauptspindel, Kugelkinematik, Lagerlasten, Messtechnik, Prüfstand Ball motion in spindle bearings under dynamic loads The main design parameter of spindle bearings is the ball advance and retardation in addition to the maximum contact pressures and the spin-to-roll ratio. This value describes the leading and trailing motion of the balls in the cage pockets in radially loaded spindle bearings. With a new measuring system, the motion of the balls can be precisely measured along the bearing circumference. At the same time, the developed test rig offers the possibility to load the bearing with dynamic forces. The results show an increase in the ball advance and retardation when dynamically loaded with the cage and the technically relevant shaft rotational frequency compared to the static case. A load with multiples of the shaft rotational frequency does not lead to a significant increase of the ball advance and retardation. Keywords spindle bearing, main spindle, ball kinematics, bearing loads, metrology, test rig Kurzfassung Abstract * Hans-Martin Eckel, M. Sc. (federführender Autor) Prof. Dr. Christian Brecher Dipl.-Ing. Stephan Neus Werkzeugmaschinenlabor WZL der RWTH Aachen Steinbachstraße 19, 52074 Aachen Bereits bei statischer Axiallast zeigen gängige Berechnungsansätze mitunter stark abweichende Geschwindigkeiten der Kugeln [5]. Messergebnisse zum KvKn bei hohen Drehzahlen und radialen Belastungen sind nicht bekannt. Aus diesem Grund ist die messtechnische Erfassung der Kugelbewegung im Betrieb unter statischen und dynamischen Belastungen erforderlich. Die Untersuchungen sollen den Einfluss typischer Belastungen auf den messbaren KvKn aufzeigen. Hierzu wird das Prüflager einerseits mit stationär wirkenden dynamischen Kräften und variierenden Frequenzen belastet. Ergänzend wird der Einfluss umlaufender Belastungen sowie einer Unwucht, die einer mit der Wellendrehfrequenz umlaufenden Kraft entspricht, untersucht. Somit kann der Einfluss typischer Lastzustände erfasst und bewertet werden. 2 Analyse der Kugelkinematik Bei hohen Drehzahlen und Belastungen überlagern sich am inneren und äußeren Wälzkontakt Roll- und Bohrbewegungen. Das Verhältnis dieser Bewegungen definiert den Wälzwinkel, mit dem sich die Kugel um die Lagerachse dreht und damit die Orbitalgeschwindigkeit der Kugel um die Lagerachse. Zur Bestimmung des Wälzwinkels wurden verschiedene kinematischer Hypothesen entwickelt. Die bekanntesten Hypothesen sind die Grenzwertbetrachtungen nach der Innenring- und der Außenringführung [6,7]. Diesen Hypothesen liegt die Annahme zugrunde, dass bei der gewählten Führung die Bohrbewegung nur am anderen Kontakt auftritt. Im Fall der Innenringführung erfährt die Kugel demnach am inneren Kontakt eine reine Rollbewegung und am äußeren Kontakt eine überlagerte Roll- und Bohrbewegung. Im Betrieb stellt sich ein Wälzwinkel zwischen der Innen- und der Außenringführung ein. Detailliertere Berechnungen berücksichtigen die Beanspruchungen in den Wälzkontakten zur Bestimmung des Wälzwinkels [5,8,9]. Die Validierung dieser Berechnungsmethoden erfolgte auf Basis statischer Axialkräfte durch die Messung der Käfigdrehzahl. Ein Abgleich unter radialen Belastungen wurde wissenschaftlich bisher nicht untersucht. In [10] wird der Einfluss verschiedener kinematischer Hypothesen auf die axiale Lagersteifigkeit und den Anstellwinkel berechnet. Bei hohen Drehzahlen unterscheiden sich die berechneten Wälzwinkel und damit die Umlaufgeschwindigkeit der Kugeln deutlich zwischen diesen Hypothesen. In [11] wird das Betriebsverhalten von Wälzlagern unter dynamischer Belastung untersucht. Berechnungen für ein Rillenkugellager der Baugröße 6220 bei 1.000 U/ min zeigen, dass eine signifikante Änderung der Kugeldrehzahl nur bei axialer Belastung auftritt. Die Beanspruchung des Käfigs durch den KvKn bei statischer Belastung für ein Rillenkugellager der Baugröße 6310 im Drehzahlbereich bis 1.600 U/ min wird in [12] experimentell nachgewiesen. Hohe Käfigbelastungen treten insbesondere bei einer Verkippung zwischen den Lagerringen auf. Dies deutet auf einen ausgeprägten KvKn hin. Für höhere Drehzahlen wird in [13] ein System zur Messung der Kugel- und Käfigbewegungen mittels Hochgeschwindigkeits-Videografie vorgestellt. Ergebnisse unter radialer Belastung sind nicht beschriebenen. Erste Ergebnisse zu gemessenen KvKn-Werten in radial und axial (statisch) belasteten Spindellagern mit Drehzahlen bis zu 30.000 1/ min der Baugröße 7014 werden in [14] vorgestellt. Die Ergebnisse bestätigen, dass die Grenzwerte nach der Innen- und Außenringführung prinzipiell eingehalten werden. Die gemessene Kugelbewegung moduliert jedoch geringer, als nach diesen Führungsmethoden berechnet. Zur Analyse der Kugelkinematik unter dynamischen Belastungen sind daher experimentelle Untersuchungen erforderlich. Für die nachfolgenden Untersuchungen wurde das in [14] vorgestellte Prüfsystem um eine hochdynamische Aktorik mit piezoelektrischen Stapelaktoren erweitert. 3 Prüftechnik Die messtechnischen Untersuchungen unter dynamischer Belastung werden auf dem in [14] vorgestellten Spindellagerprüfstand zur Untersuchung des KvKn durchgeführt. Dieser wurde um ein hochdynamisches Belastungssystem erweitert. Das Versuchslager der Baugröße 7014 in Hybridbauweise (Tabelle 1) wird gegen eine Stützlagerung in O-Anordnung mit 1.000 N elastisch vorgespannt (Bild 1). Der geometrisch mögliche KvKn beträgt etwa 1.100 µm. Die Schmierung erfolgt mittels Öl-Luft Schmierung mit einem Öl der Viskosität ISO-VG 32. Die nachfolgenden genannten Belastungen beziehen sich auf die in der Abbildung eingezeichnete Krafteinleitungsposition. Im Betrieb werden die Kugelpositionen, die Belastungskräfte sowie die Verlagerung der Spindelwelle zeitsyn- Aus Wissenschaft und Forschung 11 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0033 Geometrie Wert Einheit Teilkreisdurchmesser Kugeldurchmesser Kugelanzahl Druckwinkel Käfigführungsspiel Käfigtaschenspiel 90 11,9 21 19 600 500 mm mm - ° µm µm Tabelle 1: Eigenschaften des Prüflagers dem Käfig durchleuchten. Gegenüberliegende Fotodetektoren erfassen das Lichtsignal, welches von den Kugeln intermittierend abgeschattet wird. Am Lagerumfang sind 21 dieser Einheiten, die jeweils eine Lichtschranke bilden, angeordnet. Die Signale der Fotodetektoren werden mittels FPGA mit einer Abtastrate von 40 MHz erfasst, vorverarbeitet und mit den weiteren analogen Messgrößen synchronisiert. Die Sensordaten liefern somit den Zeitpunkt, wann sich eine Kugel an einer Sensorposition befindet. Hierzu wird die mittlere Zeit zwischen dem Eintritt und Austritt der Kugeln in den Lichtstrahl ausgewertet. Die hohe Abtastrate ermöglicht eine Auflösung zur Kugeldetektion an den Sensorpositionen von unter 2 µm. Zwei vorgespannte piezoelektrische Aktoren mit einem Winkelabstand von 90° erzeugen die dynamischen Radialkräfte (Bild 3). Die Kräfte werden über eine Belastungseinheit, welche die Rotation der Spindelwelle mit einem weiteren Lagerpaket entkoppelt, in die Welle eingeleitet. Die Aktoren sind über speziell entwickelte Festkörpergelenke mit applizierten Dehnungsmessstreifen an die Belastungseinheit angebunden, sodass Zug- und Druckkräfte spielfrei mit hohen Amplituden aufgebracht werden können. Die Regelung der Belastungskräfte erfolgt über die Prüfstandssteuerung. Hierzu werden die im Soll-Kraftsignal vorkommenden statischen und dynamischen Kraftanteile frequenzdiskret, mit der gemessenen Kraft als Regelwert, geregelt. Dieses Vorgehen ermöglicht es beliebige Belastungen mit dynamischen Kraftamplituden von bis zu 1.000 N präzise einzustellen. Jedem harmonischen Kraftanteil kann eine individuelle Phase vorgegeben werden, sodass durch den Einsatz der zwei Aktoren Belastungen mit veränderlicher Kraftrichtung, wie beispielsweise eine Unwucht, abgebildet werden können. [16] Aus Wissenschaft und Forschung 12 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0033 chron erfasst. Die Wellenverlagerung dient der Bestimmung der wirkenden Lasten beziehungsweise der Lastrichtung auf das Prüflager. Diese wird mittels je drei radial und axiale angeordneten Wirbelstromsensoren vor dem Prüflager erfasst (Bild 1). Die Methoden zur Berechnung der Wellenverlagerung in radialer und axialer Richtung sowie der Neigung aus den einzelnen Verlagerungssignalen sind in [15] beschrieben. Zur Messung der Kugelpositionen wird das aus [14] bekannte Messsystem verwendet. Das System nutzt Lichtquellen, die das Lager zwischen dem Innenring und Bild 1: Aufbau der Prüfspindel Bild 3: Belastungsprüfstand im aufgebauten Zustand und in der Schnittdarstellung Bild 2: System zur Messung des KvKn [14] 4 Ergebnisse Statische Belastung Messungen bei stationärer, statischer Belastung sind die Grundlage, um das Verhalten der Kugeln unter dynamischer Belastung zu verstehen. Bild 4 zeigt die gemessenen Abweichungen einer Kugel bezogen auf Ihre Soll- Position entlang des Lagerumfangs für verschiedene Radialkräfte und Drehzahlen. Die Belastung wirkt in Richtung 0°. Die Modulation der Kugelbewegung bei 1.000 N ist gering und steigt mit zunehmender Geschwindigkeit an. Nach der Belastungszone bildet sich eine Nachlaufbewegung aus, die kinematisch einer dominierenden Außenringführung entspricht. Bei 2.000 N tritt im mittleren Geschwindigkeitsbereich eine starke Modulation auf, bei der die Kugeln nach der Lastzone eine Vorlaufbewegung zeigen. Dieses Verhalten deutet auf eine dominierende Innenringführung hin. Bild 5 zeigt den Verlauf des gemessenen KvKn für verschiedene Drehzahlen und statische Radialkräfte. Der jeweilige KvKn-Wert beschreibt die Differenz zwischen den Kugeln mit dem höchsten Vor- und Nachlauf, gemittelt über einen Messzeitraum von 0,5 s. Bei diesem Lager treten die maximalen KvKn-Werte im unteren bis mittleren Drehzahlbereich bei hohen Radialkräften auf. Mit steigender Drehzahl reduziert sich der KvKn. Bis zu einer Radialkraft von 1,5 kN tritt im gesamten Drehzahlbereich kein signifikanter KvKn auf. Die in der Abbildung eingezeichneten roten Markierungen sind Betriebspunkte, von denen die dynamischen Belastungen ausgehen. Die Belastung wirkt stationär durch einen Aktor. Die Wahl dieser Punkte berücksichtigt wechselnde Lasten (Radialkraft = 0 N) sowie die stark progressive Erhöhung des KvKn ab 1,5 kN. Dynamisch, stationär wirkende Belastung Bei hohen dynamischen Belastungen können innerhalb einer Kraftperiode Lastzustände auftreten, die entsprechend Bild 5 bei statischer Belastung eine leichte und eine stark erhöhte Modulation der Kugelgeschwindigkeit bewirken. Dies ist insbesondere bei einer Radialkraft von 1,5 kN möglich, wo die Belastung des Lagers zwischen Bereichen mit hohem und geringem KvKn oszilliert. Das Zusammenspiel von dynamischen und statischen Kraftkomponenten kann daher für die Ausprägung des KvKn relevant sein. Bild 6 zeigt die gemessenen KvKn-Werte für die Betriebspunkte aus Bild 5 mit einer überlagerten dynamischen Belastung. Die Belastung erfolgt mit variierenden, diskreten Frequenzen und einer konstanten Kraftamplitude von 500 N (1.000 N Peak/ Peak). Unter reiner Wechselbelastung (F stat = 0 N) tritt bei keiner technisch relevanten Anregungsfrequenz (Drehfrequenz f n und deren Harmonische) eine signifikante Erhöhung des KvKn auf. Demgegenüber können bei einer Belastung im Bereich der Käfigdrehfrequenz f c starke Überhöhungen auftreten. Die Ergebnisse bei 12.000 1/ min und 100 Hz sowie bei 24.000 1/ min und 200 Hz zeigen diesen Effekt eindeutig. Die Anregung im Bereich der Käfigdrehfrequenz führt zu einer gleichbleibenden Belas- Aus Wissenschaft und Forschung 13 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0033 Bild 5: KvKn im gesamten Parameterbereich Bild 4: Gemessene Kugelbewegung für verschiedene Drehzahlen und Radialkräfte Die Modulation der Kugelbewegung und des KvKn infolge der dynamischen Belastung sind in Bild 7 im Frequenzbereich dargestellt. Die Diagramme zeigen die dynamischen Anteile der Kugelbewegung sowie des gesamten KvKn als Differenz zwischen den Kugeln mit dem höchsten Vor- und Nachlauf. Bei allen Belastungen liegt ein signifikanter dynamischer Anteil in der Kugelbewegung bei f c vor. Die Modulation des KvKn hingegen wird mit der Belastungsfrequenz f F moduliert, dessen Amplitude jedoch mit steigender Frequenz abnimmt. Neben diesen beiden charakteristischen Signalanteilen tritt die Schwebungsfrequenz mit f F -f c auf. Bei einem geringen Abstand von f F und f c stellt sich eine niederfrequente, dem statischen Anteil überlagerte, Modulation der Kugelbewegung ein, die zu hohen, statischen KvKn-Werten führen kann. Zusammenfassend zeigen die Ergebnisse, dass bei einer stationär wirkenden dynamischen Belastung die höchsten KvKn-Werte im Bereich von f c auftreten. Die höchsten Werte bei einer technisch relevanten Anregung liegen bei der Drehfrequenz f n vor. Dynamisch, umlaufend Im Betrieb von Spindellagern wirken die dynamischen Belastungen selten in stationärer Richtung, sondern laufen vollständig (Unwucht) oder teilweise (Schneideneingriffe beim Fräsprozess) mit dem Innenring um. Hierbei weisen die Belastung sowie der Käfig den gleichen Drehsinn um die Lagerachse auf. Bei der dynamischen Belastung bestimmt die relative Lage der Kugel zur Kraftrichtung deren Belastung und damit die Kinematik. Aus Wissenschaft und Forschung 14 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0033 tung einzelner Kugeln, sodass sich ein hoher KvKn ausbilden kann. Unter dem Einfluss statischer Kraftkomponenten erreicht der KvKn höhere Werte und bestätigt die Ergebnisse aus Bild 5. Im Vergleich zur statischen Belastung stellen sich höhere KvKn-Werte bei einer Belastung mit f n ein, die mit steigender Anregungsfrequenz wieder abklingen. Bild 6: KvKn unter dynamischer Belastung bei 12.000 1/ min und 24.000 1/ min Bild 7: Spektrum der Kugelbewegung und des KvKn bei 12.000 1/ min und F stat = 1.500 N Eine wiederkehrende Belastung der Kugel über dem Lagerumfang, wie im statischen Fall, tritt aufgrund des ungeraden Verhältnisses aus Käfig- und Innenringdrehzahl nicht auf. Bei einer Belastung durch Unwucht ändert sich der Lastzustand einzelner Kugeln aufgrund der mitrotierenden Kraft langsamer, sodass sich prinzipiell ein erhöhter KvKn aufbauen kann. Den Einfluss umlaufender Belastungen mit f n zeigt Bild 8. Für den Vergleich wurde bei jeder Drehzahlstufe eine reine Wechsellast mit der Drehfrequenz stationär (ein Aktor) sowie gleich- und gegensinnig zur Drehrichtung des Innenrings umlaufend eingeleitet. Die gleichsinnig umlaufende Belastung entspricht einer Unwucht, wobei die gegensinnige Belastung als theoretischer Vergleichswert dient. Die Kraftamplitude beträgt bei allen Drehzahlen 250 N (500 N Peak/ Peak). Dies entspricht einer Unwucht von 40 gmm bei 24.000 1/ min und beträgt damit das Vierfache der zulässigen Unwucht bei G2,5 [17]. Die höchsten KvKn-Werte treten bei der Belastung mit Unwucht auf. Der theoretische Lastfall mit gegensinniger Belastung führt ebenfalls zu erhöhten Werten im Vergleich zum stationären Lastfall. Trotz der hohen Unwucht erreicht der KvKn allgemein keine kritischen Werte. Zur detaillierteren Analyse des Einflusses der Anregungsfrequenz auf die Kugelbewegung wurde das Lager mit gleichsinnig umlaufender Belastung mit und unterhalb von f c sowie als Referenz mit f n (Unwucht) belastet. Die Kraftamplitude beträgt 500 N an der Lasteinheit. Bild 9 zeigt in der linken Spalte jeweils die Position einer Kugel sowie die Lage des Wellenorbits um die Lagerachse. Der Winkel des Wellenorbits entspricht somit der Lastrich- Aus Wissenschaft und Forschung 15 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0033 Bild 8: KvKn bei Belastungen mit der Wellendrehfrequenz Bild 9: KvKn mit umlaufenden Belastungen Danksagung Gefördert durch Bundesministerium für Wirtschaft und Klimaschutz aufgrund eines Beschlusses des Deutschen Bundestages. Die Autoren danken der Arbeitsgemeinschaft industrieller Forschungsvereinigungen (AiF) und dem Verein Deutscher Werkzeugmaschinenfabriken e.V. (VDW) für die finanzielle Unterstützung des Projekts 21640 N/ 1. Literatur [1] C. Brecher, M. Weck, Werkzeugmaschinen Fertigungssysteme 2: Konstruktion, Berechnung und messtechnische Beurteilung, ninth ed., Springer, Heidelberg, 2017. [2] GMN Paul Müller Industrie GmbH @ Co. KG, Hochpräzisionslager. Firmenschrift, Nürnberg, 2010. [3] Schaeffler Technologies AG & Co. KG, Hochgenauigkeitslager. Firmenschrift, Schweinfurt, 2016. [4] J. Falker, Analyse des Betriebsverhaltens von Hochgeschwindigkeits-Wälzlagern unter radialen Lasten. Dissertation, Aachen, 2020. [5] J. Rossaint, Steigerung der Leistungsfähigkeit von Spindellagern durch optimierte Lagergeometrien. Dissertation, Aachen, 2013. [6] A.B. Jones, A General Theory for Elastically Constrained Ball and Radial Roller Bearings Under Arbitrary Load and Speed Conditions, Journal of Basis Engineering 82 (1960) 309-320. [7] T.A. Harris, M.N. Kotzalas, Essential Concepts of Bearing Technology, fifth ed., CRC Press, 2007. [8] U. Tüllmann, Das Verhalten axial verspannter, schnelldrehender Schrägkugellager. Dissertation, Aachen, 1999. [9] C. Ding, F. Zhou, J. Zhu, L. Zhang, Raceway control assumption and the determination of rolling element attitude angle: Changan, D; Fuzhang, Z; Jun, Z; Lei, Z., Chin. J. Mechn. Eng. 37 (2001) 58-61. [10] D. Noel, M. Ritou, B. Furet, S. Le Loch, Complete Analytical Expression of the Stiffness Matrix of Angular Contact Ball Bearings, Journal of Tribology 135 (2013). [11] FVA, Einfluss von Vibrationsanregung auf Wälzlager, 2014. [12] K. Kakuta, The Effects of Misalignment on the Forces Acting on the Retainer of Ball Bearings, Journal of Basis Engineering (1964) 449-456. [13] L. Holland, Analyse des Bewegungsverhaltens der Komponenten in Spindellagern mittels Hochgeschwindigkeitsvideographie. Dissertation, Darmstadt, 2018. [14] C. Brecher, H.-M. Eckel, M. Fey, S. Neus, Measuring the Kinematic Behavior of the Rolling Elements in a Spindle Bearing under Axial and Radial Loads, Bearing World Journal 5 (2020) 159-167. [15] C. Brecher, H.-M. Eckel, M. Fey, F. Butz, Prozesskraftmessung mit spindelintegrierter Sensorik, ZWF Zeitschrift für wirtschaftlichen Fabrikbetrieb 113 (2018) 660- 663. https: / / doi.org/ 10.3139/ 104.111982. [16] M. Fey, „Identifikation der gebrauchsdauerreduzierenden Betriebszustände von Hauptspindellagerungen an Werkzeugmaschinen auf Basis der wirkenden dynamischen Last am Schneideneingriffspunkt - DynaLast“, Schlussbericht zum IGF-Vorhaben 18900 N, Werkzeugmaschinenlabor WZL der RWTH Aachen, 2020. [17] DIN Deutsches Institut für Normung e. V., Mechanische Schwingungen Auswuchten von Rotoren - Teil 11: Verfahren und Toleranzen für Rotoren mit starrem Verhalten, Beuth Verlag, Berlin, 2017. Aus Wissenschaft und Forschung 16 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0033 tung. Die Auslenkung des Wellenorbits wurde mit dem beschriebenen Verlagerungsmesssystem simultan zum Drehgeber des Antriebsmotors erfasst. Die rechte Spalte zeigt die Abweichung von einer Kugel von Ihrer Soll- Bewegung sowie den KvKn zwischen den Kugeln. Im Fall der gezeigten Belastung mit f F ≈ f c bleibt der Belastungszustand jeder Kugel für einen langen Zeitraum konstant, sodass einzelne Kugeln eine hohe Abweichung und damit einen hohen Vor- und Nachlauf aufbauen und beibehalten können. Geringe Abweichungen zwischen f F ≈ f c führen zu einer langsam steigenden Phasenverschiebung zwischen der Belastung und der Kugelposition, sodass eine Umkehrung der Abweichung entsteht. Der KvKn bleibt konstant auf einem hohen Niveau. Bedingt durch die Frequenzauflösung der Kraftregelung von 10 Hz, wurde die Drehzahl auf 12.330 1/ min gesetzt, um eine gute Übereinstimmung zwischen f F und f c einzustellen. Bei einer Reduktion der Drehzahl auf 12.000 1/ min gilt f F < f c . Hierbei kommt es zu einer kontinuierlich steigenden Phasenverschiebung zwischen der Belastung und der Kugelposition, die in einer niederfrequenten Modulation der Kugelbewegung resultiert. Auch bei diesem Lastfall bleibt der KvKn zwischen den Kugeln auf einem konstant hohen Niveau. Bei einer Erhöhung der Anregungsfrequenz mit f F = f n treten stark wechselnde Belastungen der Kugel auf, welche die Ausbildung hoher Abweichungen reduzieren. Der KvKn fällt daher geringer aus. Diese Beispiele bekräftigen, dass dynamische Wechsellasten im Bereich technisch relevanter Frequenzen mit f F ≥ f n keine kritischen Modulationen der Kugelbewegungen und damit KvKn-Werte hervorrufen. 5 Zusammenfassung Entgegen bisherigen Vermutungen zeigen die Messergebnisse für die statische Belastung, dass hohe Drehzahlen und Radialkräfte nicht unmittelbar zu hohen KvKn-Werten führen. Unter dynamischer Belastung mit technisch relevanten Anregungsfrequenzen (Drehfrequenz und deren Harmonische) treten im Vergleich zur mittleren, statischen Kraft keine signifikanten Erhöhungen der KvKn-Werte auf. Eine Verstärkung des KvKn infolge einer dynamischen Belastung wird durch Anregungsfrequenzen im Bereich der Käfigdrehfrequenz verursacht. Eine reine Unwucht, die einer umlaufenden Belastung mit der Drehfrequenz entspricht, führt zu keinen kritischen KvKn-Werten. Erst mit der Überlagerung einer statischen Kraftkomponente treten erhöhte KvKn-Werte auf. Aus Wissenschaft und Forschung 17 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 potential and protect steel from corrosion [11]. As a result of which the total structural mass, and therefore weight, is increased. Substituting the steel for corrosionfree, tensile load carrying polymer fibres would therefore, reduce overall structural weight and construction cost [2, 5], ease construction [5], and reduce CO 2 -emissions [12]. These benefits, however, are still limited as the tensile load carrying ability of the polymer fibres in cured FRC is less that that of steel-reinforced concrete. As such, a total substitution of steel has only been achieved in structures that are non-essential for structural support [2, 5]. To realise the aforementioned benefits of FRC technology, current fibre design focuses on the matrix-bonding of the fibres and their mechanical properties as the paramount design criteria to improve the fibres tensile load carrying ability [8, 9]. To ensure that none of the fibre properties optimised during development are lost during FRC processing, tribological characterisation of the fibres is of interest. Here, information gained about wear characteristics can be Aus Wissenschaft und Forschung 18 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 1 Introduction For several years polymer fibres haven been used as a reinforcement material to improve the mechanical properties of concrete [1-3]. Fibre-reinforced concrete (FRC) is a composite material [2], produced by mixing concrete with reinforcing fibres using a mixing process to ensure homogenous fibre dispersion [3, 4]. Polymer fibres, polypropylene (PP) fibres in particular, are used as reinforcing fibres in FRC to control temperature and shrinking induced cracking, thus improving its material toughness [1, 5-7]. These improved properties of FRC, however, are dependent on the fibres matrix-bonding and mechanical properties which in turn influence the fibres’ tensile load carrying ability [8, 9]. Previous studies [3, 4] have shown that these properties are negatively influenced by the tribological stresses that occur during the concrete mixing process, causing fibre wear by abrasion and material deterioration. The result of which can be seen in the reduction of the materials tensile load carrying ability [3, 4, 7]. With improvements in fibre design, focusing on fibre optimisation as well as tribological characterisation techniques [8-10], further application fields of FRC are opened up. This includes, but is not limited to, the reduction in the overall amount of concrete used. Today, a considerable amount of concrete is used in steelreinforced structures to reinforce its tensile load carrying A new approach for the friction and wear characterisation of polymer fibres under dry, mixed, and hydrodynamic sliding Justus Rüthing, Frank Haupert, Regine Schmitz, Michael Sigrüner, Nicole Strübbe* A new approach for the friction and wear characterisation of polymer fibres under dry, mixed, and hydrodynamic sliding conditions is developed. The production process of the tested polymer fibres is described and an introduction in fibre-reinforced concrete is given. Tribotesting is done on an optimised tribometer capable of measuring the friction and wear behaviour of polymer fibres with diameters of a few 100 µm under lubricated conditions. Three extruded polypropylene macro fibres with varying diameters are characterised under tribological conditions found in an industrial concrete mixing process. It is shown that detailed friction and wear data of polymer fibres can be gathered. Keywords polymer-fibres, fibre reinforced concrete, Pin-on- Disc, abrasive wear, water lubrication, hydrodynamic sliding Abstract * B. Sc. Justus Rüthing; Orcid-ID: https: / / orcid.org/ 0000-0001-7615-4979 Prof. Dr. -Ing. Frank Haupert; Orcid-ID: https: / / orcid.org/ 0000-0002-3312-6844 Dr.-Ing. Regine Schmitz; Orcid-ID: https: / / orcid.org/ 0000-0002-4510-2559 Hamm-Lippstadt University of Applied Sciences, Marker Allee 76 - 78, 59063 Hamm M. Sc. Michael Sigrüner; Orcid-ID: https: / / orcid.org/ 0000-0002-0644-023X Prof. Dr.-Ing. Nicole Strübbe; Orcid-ID: https: / / orcid.org/ 0000-0002-2084-9031 Rosenheim Technical University of Applied Sciences, Hochschulstraße 1, 83024 Rosenheim used to predict fibre wear and, as a result, positively influence fibre design. In order to aid this design process, tribological characterisation of polymer fibres under conditions found in industrial concrete mixing processes has been undertaken, with the development of a tribological characterisation process capable of modelling these conditions being the overall aim of this study. Continuing the development described by Schmitz et al. [10], a new approach for the friction and wear characterisation of polymer fibres is developed using an optimised Pin-on-Disc test rig. Using this test rig, three different types of single extruded macro PP-fibres, with varying cross-sections of a few 100 µm 2 , are characterised under conditions found in industrial concrete mixing processes. The results gathered from this characterisation are then used to compare the fibres’ tribological properties using the coefficient of friction (COF) and the steadystate wear rate. 2 Materials 2.1 Pin-on-Disc test rig used for tribological fibre characterisation For the characterisation of the polymer fibres an optimised, in-house designed and built, Pin-on-Disc test rig is used (Figure 1). Normal force is applied by a mechatronic controlled load and test unit. The load and test unit is able to apply precise force to the specimen in the range from 1 to 1000 N. During measurement a precise normal force application (± 0.3 N) is realised using a stepper motor and corrected if any deviations of the target normal force are detected. Utilising a load cell friction force is measured. To measure wear, a µm-accurate laser sensor is used to measure specimen wear depth. Counterpart rotation is realised by a servomotor. The temperature sensors and heating cartridges are located underneath the counterpart. Using the heating cartridges, heating of the counterpart is possible while the temperature sensors are used to monitor the counterpart’s overall temperature. To conduct measurements under lubricated conditions, a programmable peristaltic pump (Ismatec ® Reglo ICC) is used. Using this set up, lubricants can be applied directly onto the counterpart’s surface with rates in the range of 0.01 to 5.70 ml/ min over the water injection attachment, attached to the peristaltic pump using a tube with an inner diameter of 1.02 mm. Surrounding the counterpart, there is a liquid enclosure protecting the tribometers fragile electronics. All of the features of the described test rig are controlled by a MATLab based in-house designed operating system which is operated over a graphical user interface (GUI). During the conduced tests, applied normal force [F N ], friction force [F x ], coefficient of friction [µ], friction speed [v], specimen wear distance [s] and temperature [T] are measured and displayed as a function over time using said GUI. 2.2 Specimen-holder Preparing the fibres for tribotesting, the extruded fibres are individually fixated onto a sample holder, designed for the tribological characterisation of fibres (Figure 1 and 2). During testing, the fibre is held in place using screw-in mounts on each side of the specimen holder. Using the specimen holder described, a fibre-counterpart contact-length of 20 mm is realised. Aus Wissenschaft und Forschung 19 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 Figure 1: Optimised Pin-on-Disc test rig for the tribological characterisation of single polymer fibres Figure 2: Specimen-holder with PP-fibre for tribological fibre characterisation 2.3 Specimen - Extruded PP-fibres 2.3.1 Fibre Production and Draw Down Fibre production was executed on an HAAKE Polydrive single screw extruder with a length/ diameter ratio of 25 and a screw diameter of 19 mm (Figure 3). All polymer granulates were pre-dried and a round shaped strand of approximately 3 mm diameter was extruded. The strand was cooled through a combination of water bath and air cooling and continued into a Dr. Collin GmbH MDO laboratory drawing machine with a take up speed of 3 m/ min (v 1 ). The drawing machine resembles a onestep drawing process, consisting of two roll packages and a heating oven. The oven temperature was set at 157 °C for all compounds. A second roll package with higher speed (v 2 ) stretched the polypropylene to its individual maximum draw ratio to accomplish high mecha- Figure 4. Additionally, the mechanical properties of the fibres are given in Table 2. 2.4 Counterpart - Alumina-Disc To recreate the abrasive conditions found in industrial concrete mixing processes, a counterpart investigation was undertaken in conjunction with the aforementioned experiments to model the abrasive conditions of concrete mixing. As multiple experiments using counterparts with varying average surface roughness’s and topographical properties were conducted, the Aluminacounterpart was chosen to be the most suitable to model Aus Wissenschaft und Forschung 20 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 nical properties. The draw ratio (DR) was defined as the ratio of roll speed differences. (1) 2.3.2 Dimensions and Mechanical Properties Using the production and draw down process described in 2.3.1, three oval-shaped PP-fibres with draw ratios of 1: 10, 1: 14 and 1: 17 were produced. The exact vertical and across diameters of the fibres are given in Table 1. A schematic cross section of the PP-fibres can be seen in ed PP-fibres and Draw Down = nical Properties Figure 3: Schematic description of the fibre draw down process PP-fibres raw Down anical Properties Figure 4: Schematic cross section of an extruded oval-shaped PP-fibre Figure 5: 3D surface topography image of the alumina counterpart used (white light interferometry, section: 1x1 mm) image taken after the grinding procedure Table 1: Dimensions for the across and vertical diameter of the fibre draw ratios 1: 10, 1: 14 and 1: 17 the abrasive conditions of fresh concrete during mixing. The counterpart selected consists of an Alumina (Al 2 O 3 ) disc with an average surface roughness (R a ) of 1.6 ± 0.1 µm (Figure 5). To ensure the same surface properties apply for each measurement, the disc surface is grinded using a diamond grinding disc (Schmitz Metallography, stated grain size 0080) before each tribotest. The average surface roughness of the ceramic disc is measured using a white light interferometer (FRT Mirco-Prof ® ) and is controlled in defined intervals. Table 2: Mechanical Properties: tensile strength, E modulus and elastic strain for PP-fibres 1: 10, 1: 14. and 1: 17 3 Methods 3.1 Run-in-period and data acquisition As the polypropylene fibre is abraded during tribological characterisation, a frequent geometry dependent reduction in the applied surface pressure of the oval PP-fibre can be measured. This frequent change is present within fibre wear depths of up to 100 µm, with its magnitude changing for each fibre-diameter characterised. To account for this, a run-in-period was defined as t start = 0 µm to t 100 = 100 µm under dry sliding conditions. After the wear depth of 100 µm is reached, the data acquisition of the friction and wear data is initiated with the change in surface pressure now being a marginal factor during fibre characterisation (Figure 6). A more detailed description of the run-in-period and its causes are outlined in Schmitz et al. [10]. 3.2 Testing Parameters In order to ensure consistency across experiments under dry, mixed and hydrodynamic sliding conditions, the pvproduct was controlled and kept at 0.16 MPa m/ s across all tests conducted. To account for the varying fibre diameters, the applied normal force is adapted and three lubrication rates are used to model the sliding conditions, see Table 3. The tests were conducted under temperatures between 24 °C and 25 °C to which no adjustments were made during the experiments. 3.3 Experimental procedure To model the varying sliding conditions, similar to those found in industrial concrete mixing processes, an experimental procedure is developed (Figure 7). As the starting phase is predetermined by the run-in-period, as defined in 3.1, characterisation of the three sliding conditions is as follows. The dry sliding phase is modelled using a lubrication rate of 0.00 ml/ min. The end of the tribological characterisation under dry sliding conditions was defined as the wear depth (s) at t 150 = 150 µm. The friction and wear data are thus taken in the wear range of 50 µm, as this was found to contain sufficient enough data to characterise the fibres by previous experiments. As t 150 is reached, water as a lubricant is added in the rate of 0.50 ml/ min to model the mixed sliding phase for 25 min. The mixed sliding test phase, in which data is acquired, is defined as t 150 + 15 min to t 150 + 25 min, to ensure sufficient lubrication conditions. As the test time of t 150 + 25 min is reached, the experiment proceeds to the hydrodynamic sliding condition by increasing the lubrication rate to 4.00 ml/ min. Equivalent to the mixed sliding phase, the relevant fibre characterisation data is taken in the last 10 min, of the 25 min test time, at t 150 + 40 min to t 150 + 50 min. As the test time of t 150 + 50 min is reached, the tribological characterisation of the polymer fibre under dry, mixed and hydrodynamic sliding conditions is complete. As absolute test time is predetermined by the run-in-period, test time varies between each experiment conducted. Aus Wissenschaft und Forschung 21 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 Table 3: Applied tribological testing parameters: normal force, sliding speed and lubrication rate 3 Methods 3.1 Run-in-period and data acquisition 3.2 Testing Parameters 3.3 Experimental procedure Figure 6: Schematic image of the cross section of an extruded PP-fibre with the first 100 µm marked as the run-in period (green line) and the range from 100 µm to the fibres’ maximum diameter (horizontal dotted line) marked as the data acquisition period Figure 7: Flow diagram of the experimental procedure for the tribological fibre characterisation over test time the lubrication rate is increased to 4.00 ml/ min, another reduction in the COF is visible. This transition point is marked by a dotted line at t = 30 min. During the modelling of the hydrodynamic sliding phase, an overall reduction of the COF by approx. 15 % is recorded. In the example case shown in Figure 8, the total test time ends at t = 55 min as the predefined endpoint of t 150 + 50 min is reached. Figure 9 shows the COF results of PP-fibre 1: 10 (in black), PP-fibre 1: 14 (in blue) and PP-fibre 1: 17 (in red) under dry, mixed and hydrodynamic sliding conditions gathered in this study. The average values as well as the standard deviation (in brackets) taken from five experiments each are displayed in Table 4. Examining the three curves of Figure 9, an overall trend can be seen in a reduction in the COF that occurs with each increase in lubrication rate. Thus, the modelling of the dry sliding phase results in the highest COF and the modelling of the hydrodynamic sliding phase results in the lowest COF per fibre tested. Comparing the fibres on an individual level, PP-fibre 1: 10 records the highest COF across all fibres tested, across all modelled sliding conditions, with a COF of 0.39 being the maximum recorded during the experiments. This can Aus Wissenschaft und Forschung 22 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 4 Findings 4.1 Friction behaviour Figure 8 provides an example of the typical curved relationship between the COF relative to test time (t) of PP-fibre 1: 14 under dry, mixed and hydrodynamic sliding conditions. The curve is created using the experimental procedure described in 3.3 using the parameters shown in Table 3. The COF is given as the ratio between friction and normal force. Overall, five experiments were carried out for each of the thee fibres tested. The curve shows a COF of about 0.36 in the first five minutes of test time. Here the run-in-period is completed and the dry sliding phase is being modelled. Fluctuation in the recorded COF-curve can be seen during the run-in-period, although this decreases and remains at a steady level throughout modelling of the dry sliding phase. As the predefined wear-depth is reached at t 150 , water as a lubricant is added in the rate of 0.50 ml/ min resulting in an immediate reduction of the COF by approx. 30 %. During the time frame in which the COF data is taken for tribological characterisation, the COF decreased by approx. 47 % compared to the COF data taken under dry sliding. At the end of t 150 + 25 min, as Figure 8: Coefficient of friction over test time of PP-fibre 1: 14 under dry, mixed and hydrodynamic sliding further be seen by the comparison of the COF under hydrodynamic sliding of PPfibre 1: 10 with the COF taken under mixed sliding conditions of PP-fibres 1: 14 and 1: 17 as all three recorded COF record an equal value of 0.19 despite the lower relative lubrication rate applied to PP-fibre 1: 10. While PP-fibres 1: 14 and 1: 17 show a lower overall COF than fibre 1: 10, a distinction between these two fibres cannot be made due to the overlap in their standard deviation. Table 4: Coefficient of friction data of PP-fibres 1: 10, 1: 14 and 1: 17 under dry, mixed and hydrodynamic sliding conditions. COF value is given as an average of 5 with standard deviation in brackets 4.2 Wear behaviour Figure 10 shows an example wear curve of PP-fibre 1: 14 with the wear distance (s) on the y-axis and the test time (t) on the x-axis. The coefficient chosen to characterise the fibres is the steady-state wear rate (w const. ), which is calculated using the wear curve gradient given in µm/ h. Overall, five experiments were carried out for each of the three fibres tested. Starting with high wear PP-fibre 1: 14, represented by a steep rise in the wear distance curve, the run-in-period of the experimental procedure is displayed. During this run-in-period, in particular the initial 50 µm wear depth, that the fibre is subjected to its maximum wear rate. This, however, reduces by a noticeable margin in the latter 50 µm of the run-in-period. A further reduction in wear distance over time can be seen during the period under dry sliding. This reduction can be explained by the increase in fibre-counterpart contact area as a direct result of the increase in wear depth displayed in Figure 3. As the wear-depth of 150 µm is reached at t 150 and the mixed sliding condition is modelled, a decrease in w const. by approx. 97 %, moving from 1555 µm/ h under dry sliding to 57 µm/ h (under mixed sliding) is recorded. As the lubrication rate is increased to 4.00 ml/ min moving from t 150 + 25 min onwards, no further change in w const. is apparent form the recorded data. As the wear distance and the COF are recorded simultaneously during the experiments, the total test ends at t = 55 min when the predetermined terminus of t 150 + 50 min is reached. Figure 11 shows the steady-state wear rate results of PPfibre 1: 10 (in black), PP-fibre 1: 14 (in blue) and PPfibre 1: 17 (in red) under dry, mixed and hydrodynamic sliding conditions gathered in this study. The average w const. is taken out of five experiments conducted for each fibre and is presented in Table 5 alongside the standard deviation of each measurement. Aus Wissenschaft und Forschung 23 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 Figure 9: Comparison of the coefficient of friction of PP-fibres 1: 10 (black), 1: 14 (blue) and 1: 17 (red) over the variation of lubrication rate Figure 10: Wear distance over test time of PP-fibre 1: 14 under dry, mixed and hydrodynamic sliding gathered using the optimised Pin-on-Disc test rig under varying lubrication rates and sliding conditions. During the experiments, the PPfibre with the draw ratio of 1: 10 exhibits the lowest steady-state wear rate and thus provides better wear resistance under dry sliding conditions, while PP-fibres 1: 14 and 1: 17 exhibit increased wear resistance under mixed and hydrodynamic sliding conditions relative to fibre 1: 10. A reduction in the COF was examined across all fibres tested as water was added in modelling of mixed and hydrodynamic sliding conditions, with PP-fibres 1: 14 and 1: 17 showing the lower overall COF across all modelled sliding conditions. While a compression of the tribological characteristics of PP-fibres 1: 10, 1: 14, and 1: 17 was described in section 4, no significant distinction between fibres 1: 14 and 1: 17 could be made in this study due to the overlap in the standard deviation of these fibres. Using the optimised Pin-on-Disc test rig in the configuration described, it has been shown that detailed friction and wear information of single polymer fibres can be gathered under tribological conditions similar to those found in industrial concrete mixing processes. Applying the information gathered from this suit of experiments, it has been shown that the selection of polymer fibres for particular construction and end-use scenarios can be made on the bases of their tribological properties, allowing for further development of future design processes. 6 Acknowledgements The authors thank the German federal ministry of education and research (BMBF) for the funding of this study as part of the FHprofUnt project ConPlasite - 13FH068PB6. Further, the authors would like to show their gratitude to Mr. S. S. Fellows for comments that greatly improved the manuscript. Aus Wissenschaft und Forschung 24 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 As well as with the COF results described in 4.1, a reduction of the steady-state wear rate occurs with each increase in lubrication rate. This decrease is most apparent when progressing from dry to mixed sliding conditions across all tested fibres. The dry sliding phase records the highest steady-state wear rates while the hydrodynamic sliding phase records the lowest wear rates overall per fibre tested. Comparing the experimental results of each fibre under dry sliding conditions, PP-fibre 1: 14 experiences the highest overall wear rate (1555 µm/ h) and PP-fibre 1: 10 experiences the lowest overall wear rate with PP-fibre 1: 17 in between. Moving to the mixed sliding conditions, the correlation of wear rate properties of the fibres is changed, with PP-fibre 1: 10 experiencing the highest overall wear rate; PP-fibres 1: 14 and 1: 17 on the other hand experience a significant decrease in their wear rate and the curves intersect. This overlap continues into the hydrodynamic sliding conditions, however, whereas PP-fibre 1: 10 experiences notable wear in this phase PP-fibres 1: 14 and 1: 17 show no significant change in wear rate. 5 Summary The results show that reliable friction and wear data of three 500 - 700 µm thick single polymer fibres could be Figure 11: Comparison of the steady-state wear rate of PP-fibres 1: 10 (black), 1: 14 (blue) and 1: 17 (red) over the variation of lubrication rate Table 5: Steady-state wear rate data of PP-fibres 1: 10, 1: 14 and 1: 17 under dry, mixed and hydrodynamic sliding conditions. Steady-state wear rate value is given as an average of 5 with standard deviation in brackets Literature [1] Z. Zheng, „Synthetic fibre-reinforced concrete“, Progress in Polymer Science, 20, No. 2, p. 185-210, 1995. [2] M. Di Prisco, G. Plizzari und L. Vandewalle, „Fibre reinforced concrete: new design perspectives“, Mater Struct, 42, No. 9, p. 1261-1281, 2009. [3] O. Czoboly und G. L. Balázs, „Possible mechanical deterioration of fibres influenced by mixing in concrete“, Concrete Structures, 16, p. 18-23, 2015. [4] O. Czoboly und G. L. Balázs, „Are fibers sensitive to mixing? “, Structural Concrete, 18, p. 19-28, 2017. [5] S. Yin, R. Tuladhar, F. Shi, M. Combe, T. Collister und N. Sivakugan, „Use of macro plastic fibres in concrete: A review“, Construction and Building Materials, 93, p. 180- 188, 2015. [6] N. Thibodeaux, D. E. Guerrero, J. L. Lopez, M. J. Bandelt und M. P. Adams, „Effect of Cold Plasma Treatment of Polymer Fibers on the Mechanical Behavior of Fiber- Reinforced Cementitious Composites“, Fibers, 9, No. 10, 2021. [7] J. O. Lerch, H. L. Bester, A. S. van Rooyen, R. Combrinck, W. I. de Villiers und W. P. Boshoff, „The effect of mixing on the performance of macro synthetic fibre reinforced concrete“, Cement and Concrete Research, 103, p. 130-139, 2018. [8] M. Sigrüner, D. Muscat und N. Strübbe, „Investigation on pull-out behavior and interface critical parameters of polymer fibers embedded in concrete and their correlation with particular fiber properties“, J. Appl. Polym. Sci., 138, No. 28, 2021. [9] Sigrüner M., Muscat D., Strübbe N., „Investigations of the bonding behavior of modified polypropylene to a concrete matrix by single fiber pull out tests“, SAMPE Europe Conference 2020 Amsterdam - Netherlands, 2020. [10] R. Schmitz, F. Haupert, J. Rüthing, M. Sigrüner und N. Strübbe, „Tribologische Charakterisierung von Polymerfasern unter Trockenreibung, Mischreibung und Hydrodynamik mittels einer optimierten Pin-on-Disc-Prüfmethode“, TuS, 68, 2021. [11] DIN EN 1992-1-1: 2011-01, Eurocode_2: Bemessung und Konstruktion von Stahlbeton- und Spannbetontragwerken_- Teil_1-1: Allgemeine Bemessungsregeln und Regeln für den Hochbau; Deutsche Fassung EN_1992-1- 1: 2004_+ AC: 2010, Berlin. [12] C. Camille, D. K. Hewage, O. Mirza, F. Mashiri, B. Kirkland und T. Clarke, „Evaluation of Macro-Synthetic Fibre Reinforced Concrete as a Sustainable Alternative for Railway Sleepers“ in Lecture Notes in Civil Engineering, CIGOS 2019, Innovation for Sustainable Infrastructure, C. Ha-Minh et al., Hg., Singapore: Springer Singapore, 2020, p. 471-476. Aus Wissenschaft und Forschung 25 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0034 The results of the gear investigations were made available by Reintrieb GmbH (Vienna) for scientific evaluation and publication. 2 Test Methods, Test Rigs and Test Gears Test Methods The test methods presented in the following cover different gear failure modes, such as wear, scuffing, pitting and tooth root breakage [12]. Tooth root breakage and Aus Wissenschaft und Forschung 26 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 1 Introduction According to the current state of the art power gearboxes must be lubricated, typically with a mineral or synthetic based oil. Since mineral or synthetic oils can lead to the pollution of the environment due to leakage, damages or improper disposal the application of such lubricants can be limited. If water would be suitable for lubrication, significantly environmental-friendlier gearboxes could be created. Figure 1 shows a possible application of gear boxes with water lubrication, e.g. a cruise ship entering endangered environments such as a fjord. Investigations on water and water-based gear lubrication show, that the lubricating film thickness is significantly lower compared to mineral or synthetic oil [2-6]. Due to the thin lubricating film thickness, impermissible scuffing and wear is expected. To cope with such a thin film thickness, extraordinary high surface hardnesses would have to be achieved. Tungsten carbide compositions are characterized by such extraordinary high surface hardness [7-10]. As tungsten carbide is not soluble in tungsten carbide, scuffing characterized by a welding of the surfaces is expected to be avoided. The usage of the tungsten carbide composition as gear material is equally aimed at achieving the necessary resistance against water corrosion which conventional steel does not provide. In this paper, the material-lubricant-systems of water lubrication combined with gears made from different tungsten carbide compositions are investigated regarding their behavior concerning wear as well as further gear failures such as scuffing, pitting and tooth root breakage. The innovative combination of tungsten carbide composite gears with water lubrication is a pioneering step towards sustainable gear sets and has been patented [11]. Scientific Evaluation of Investigations on the Load Carrying Capacity of Carbide Cylindrical Gears Lubricated with Water Karl Jakob Raddatz, Thomas Tobie, Klaus Michaelis, Karsten Stahl* In this paper, the material-lubricant-systems of water lubrication combined with gears made from different tungsten carbide compositions are investigated regarding their behavior concerning wear as well as further gear failures such as scuffing, pitting and tooth root breakage. Keywords gears, boxes, synthetic based oil, mineral based oil, water-based, tungsten carbide, lubrication, scuffing, wear, pitting, tooth root breakage Abstract * Karl Jakob Raddatz (corresponding author) Thomas Tobie Klaus Michaelis Karsten Stahl Gear Research Center (FZG) Technical University of Munich, Garching, Germany Figure 1: Possible application of sustainable gear boxes with water lubrication, e.g. a cruise ship entering an endangered environment such as a fjord [1] pitting are fatigue failures, while scuffing and wear are mainly tribological failures. Figure 2 shows the investigated gear failures as well as their respective standardized test methods. Since wear is the critical gear failure mode for water lubricated gears, the wear amount was additionally measured during the pitting tests. This specific wear of the test gears was named ”high-speed wear” due to its higher circumferential speed compared to the conventional “slow-speed wear”. The experimental investigations were performed in the following order and are based on established and standardized test methods: ■ Tests based on the standardized test procedure DIN ISO 14635-1 [13] regarding the scuffing load carrying capacity. ■ Tests based on the standard test procedure FVA-Information 0/ 5 [14] regarding the tooth root bending strength. ■ Tests based on the standard test procedure C- PT/ 8,3/ 90/ 9: 10 according to FVA 2/ IV [15] regarding the pitting and high-speed wear (wheel rotational speed of n 2 = 1500 min -1 ) load carrying capacity. ■ Tests based on the standard test procedure C/ 0,05/ 90: 120/ 12 according to DGMK 377 [16] regarding the low-speed wear (wheel rotational speed of n 2 < 500 min -1 ) load carrying capacity. The standard test procedures C-PT/ 8,3/ 90/ 9: 10 and C/ 0,05/ 90: 120/ 12 are designations for the following test conditions: test gear type / circumferential velocity / temperature of the lubricant / load stage (KS). Table 1 shows the test conditions for the different test methods. Test Gears For the experimental investigations regarding scuffing, pitting and wear, a test gear set with pinion and wheel of the FZG test gear geometry Type C is used. For the experimental investigations regarding tooth root breakage, the wheel of the Type C test gear set is used. Figure 3 Aus Wissenschaft und Forschung 27 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 Figure 2: Investigated gear failures and their respective standardized test methods Figure 3: Exemplary photographic documentation of the test gear set Type C Test Speed Temperature Load Stages Torque Hertzian Pressure [17] Nominal Contact Stress [18] Symbol v t in m/ s T in °C KS T 1 in Nm p c in N/ mm 2 H0 in N/ mm 2 scuffing 8,3 30 - 75 (no cooling) KS 1 - KS 10 3,3 Nm - 266 Nm Material A KS 10: 2545 Material A KS 10: 2421 pitting and highspeed wear at n 2 = 1500 min -1 8,3 40 - 50 (active cooling) KS 4 KS 5 KS 6 KS 7 KS 8 KS 9 38 70 99 133 172 216 Mat. C / Mat. D 886 / 849 1202 / 1152 1430 / 1370 1657 / 1588 1884 / 1805 2112 / 2024 Mat. C / Mat. D 843 / 808 1144 / 1096 1360 / 1304 1579 / 1511 1793 / 1718 2010 / 1926 low-speed wear at n 2 < 500 min -1 0,05 0,57 2,76 25 - 30 (active cooling) Note: The Hertzian pressure p c according to Niemann [17] as well as the nominal contact stress H0 according to ISO 6336 [18] differ in their values due to the different modu lus of elasticity E documented in Table 3. Table 1: Test conditions for the different test methods Test Rigs The investigations for the evaluation of the gear failure tooth root breakage were conducted on a pulsator test rig and for evaluation of the gear failures wear, scuffing and pitting on a standard FZG back-to-back gear test rig. On the pulsator test rig a pulsating load is introduced on the gear teeth by clamping jaws. The pulsating load leads to a defined and calculable tooth root stress. Detailed information on the test rig and its function can be taken from literature [20, 21]. The test is performed until a tooth root breakage occurs or until the maximum load cycle number is reached. For most of the presented tests the maximum load cycle number usually was N max = 10 5 load cycles, only for selected tests the maximum load cycle number was increased up to N max = 6 · 10 6 load cycles. Figure 4 shows a schematic drawing of a pulsator test rig. On the FZG back to back test rig, a load is applied to the test gears by a closed power-loop implemented by a loading clutch and a load lever with weights. Detailed information on the test rig and its function can be taken Aus Wissenschaft und Forschung 28 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 shows an exemplary photographic documentation of the test gear set Type C. Table 2 shows the corresponding material and lubricant used for the conducted tests. All gears are made out of tungsten carbide composite material with the different compositions A, B, C and D of tungsten, carbon and further alloying elements. The exact material compositions are proprietary knowledge of Reintrieb GmbH. Different kinds of water such as salt water, distilled water and tap water without salt were used for lubrication to investigate the behavior of the different materials with different water lubricants. The different kinds of water were not modified with additional chemicals or additives. Dedicated and comprehensive corrosion tests were conducted at an external laboratory on behalf of Reintrieb GmbH. Table 3 lists the main geometry, quality and material data of the applied test gear sets of Type C. The macrogeometry of the gear is created by sintering, the microgeometry of the gear flanks is realized by grinding. Profile modifications were specified in the form of tip reliefs for the pinion and wheel. Test Material Water scuffing A salt water tooth root breakage A, B, C, D no lubrication needed pitting and high-speed wear at n 2 = 1500 min -1 C, D distilled water low-speed wear at n 2 < 500 min -1 C, D tap water (without salt) Table 2: Materials and lubricants for the tests Description Symbol Value center distance a 91,5 mm normal module m n 4,5 mm number of teeth (pinion / wheel) z 1/ 2 16 / 24 face width b 14 mm pressure angle 20° helix angle 0° profile shift coefficient x 1/ 2 0,182 / 0,172 profile modifications — tip relief at pinion and wheel quality acc. DIN 3961 - 3967 [19] Q 5 - 10 roughness Ra Material A: Ra A = 0,6 - Material B: Ra B = 0,4 - Material C: Ra C = 0,2 - Material D: Ra D = 0,3 - modulus of elasticity E Material A: E A = 580 GPa Material B: E B = 580 GPa Material C: E C = 490 GPa Material D: E D = 450 GPa Table 3: Main geometry, quality and material data of the test gear sets of Type C 3 Test Results 3.1 Scuffing Scuffing was examined in an initial screening test with a test gear set made from the material A and lubricated with salt water. The temperature during the test reached up to 75 °C without active cooling or heating. During the test no scuffing occurred up to the load stage KS 10, but the test was terminated by tooth root breakage during the test cycle at load stage KS 10. Figure 6 shows the photographic documentation of the scuffing test. The examination of the broken surfaces revealed that there was one smooth fatigue fracture and several rough forced ruptures. Obviously, the fatigue tooth root fracture occurred first, followed by consecutive forced ruptures. Due to the fact, that not the scuffing load carrying capacity, but the tooth root bending strength was the limiting gear failure, subsequent investigations regarding the tooth root breakage were performed with the help of a pulsator test rig. Further tests specifically aimed at the scuffing load carrying capacity were not conducted. However, for all following tests with cri- Aus Wissenschaft und Forschung 29 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 from literature [13, 22, 23]. The tests are performed until a load stage defined by the test method is reached or until scuffing respectively pitting occurs. The test method for wear does not define a maximum wear amount but compares the wear amounts after the different test cycles. The wear amounts correspond to the weight change measured with a precision scale. Figure 5 shows a schematic drawing of a FZG back-to-back test rig used for the scuffing, pitting and high-speed as well as lowspeed wear tests. Figure 4: Schematic drawing of a pulsator test rig Figure 5: Schematic drawing of a FZG back-to-back test rig Figure 6: Photographic documentation of the scuffing test, no scuffing occurred until test termination by tooth root breakage during load stage KS 10 ing capacity against tooth root breakage ranging from low to high values compared to conventional steel. It must be noted that the values are based on a limited number of tests and are not statistically validated. However the difference between the materials were very distinct and thus fulfil the screening function sufficiently. For the carbide composite materials A and B, the measured endurable nominal tooth root stress is 30 % lower compared with the reference case carburized steel. For material A, the nominal tooth root stress at load stage 10 of the scuffing test correlates with the endurable nominal tooth root stress stated in Figure 7, thus confirming the plausibility of the test results as well as the reproducibility between pulsator and FZG back to back test rig. The carbide composite materials C and D show endurable nominal tooth root stresses above the reference case carburized steel. It was thus decided to use materials C and D for further investigations regarding pitting and highspeed wear as well as low-speed wear. 3.3 Pitting and High-Speed Wear The tests regarding pitting and high-speed wear were conducted with the materials C and D. Distilled water was used as a lubricant for the test gear set Type C. All tests were performed at a circumferential speed at the pitch point of v = 8,3 m/ s, while the load stages (KS) were varied between the test cycles. After each test cycle, the gear flanks were photographically documented, the tooth flank profile 3D-measured, and the entire gear was weighed on a precision scale. The weight-measured amount of wear was used to derive the linear wear coefficient c lt [24]. The important and often used linear wear coefficient c lt is directly proportional to the amount of wear and is shown on the ordinate axis of the following graphs. Figure 8 shows the results of the pitting and high-speed wear test for the material C. Figure 9 shows the results of the pitting and high-speed wear test for the material D. Figure 10 shows the photographic documentation and 3D- Aus Wissenschaft und Forschung 30 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 tical scuffing conditions, especially during pitting and high-speed wear tests, scuffing was not observed. 3.2 Tooth root breakage The tests regarding tooth root breakage were conducted as a screening of the tooth root bending strength for four different tungsten carbide composition materials. The wheel of the Type C test gear was chosen for testing on the pulsator test rig, where no lubrication is necessary. Figure 7 shows the results of the tests regarding the tooth root bending strength of the different tungsten carbide composite materials (blue) and a commonly used case carburized steel 18CrNiMo7-6 as comparative reference (green). The moduli of elasticity of the tungsten carbide composite materials are significantly higher compared to the conventional steel. The higher moduli of elasticity lead to an increased notch effect and sensitivity for the tungsten carbide composite materials. In consideration of the increased notch sensitivity, the tungsten carbide composite materials show a load carry- 0 200 400 600 800 1000 1200 1400 1600 1800 Material A Material B Material C Material D 18CrNiMo7-6 Values 713 734 1207 1666 1109 long life endurable nominal tooth root stress 2 Figure 8: Results of a pitting and high-speed wear test for material C Figure 7: Results of the tests regarding the tooth root bending strength of different tungsten carbide composite materials (long life) measurements of the tooth flank profile after the respective test cycles for material D. The flanks optically show typical running marks as well as an increasing profile form deviation in the area below the pitch point. This profile form deviation is likely caused by wear due to unfavorable tribological conditions in the area below the pitch point. Wear equally affects the area above the pitch point, even though the impacts observed are not as noticeable as below the pitch point. The reduced wear above the pitch point is likely supported by the applied tip reliefs. During all tests, no pittings were observed. The results show that the linear wear coefficient does not correlate well with the load. There is a tendency that for gears made from the material C a lower wear is measured compared to the gears made out of the material D. This result correlates with and is supported by the findings of the external laboratory that material C shows a reduced corrosion compared to material D. The other tendency showing is, that a longer test cycle duration leads to lower wear. This is observed for the last test cycle run of material D, where the wear is reduced significantly even though the load stage is increased from KS 7 to KS 8. This observation might correlate with an increased running-in of the tooth flanks with an increased time of the test cycle. In general, the linear wear coefficients for the water lubricated gears made from tungsten carbide composite are increased compared to experience-based values for conventional steel gears with oil lubrication. 3.4 Slow-Speed Wear The slow-speed wear tests were conducted with the material C and D. Tap water without additional salt was used as a lubricant for the test gear set Type C. All tests were performed at load stage KS 7, while circumferential speeds at the pitch point varied between the test cycles. After each test cycle, the gear flanks were photographically documented, 3D-measured regarding the tooth flank profile and the entire gears weighed on a precision scale. The weightmeasured amount of wear was used to derive the linear wear coefficient c lt . Figure 11 shows the results of the slow-speed wear test for the material C. Figure 12 shows the photographic documentation of the gear Aus Wissenschaft und Forschung 31 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 Figure 10: Photographic documentation and 3D-measurements of the tooth flank profile (material on the left side of the line) after the respective test cycles for material D Figure 11: Results of the slow-speed wear test for material C Figure 9: Results of a pitting and high-speed wear test for material D ■ Wear is the main limiting factor for the gear endurance. The amount of wear can vary with the operating conditions such as load and rotational speed as well as the composition of the tungsten carbide material. ■ Further investigations are required for the optimization of the material-lubricant-system. The scientific conclusions show the great potential regarding the water lubrication of tungsten carbide composite gears: ■ The prove of concept for the innovative combination of tungsten carbide composite gears and water lubrication has been brought forward with the presented investigations. ■ Tooth root breakage, scuffing and pitting do not limit the industrial application of the presented material-lubricant-system in gear boxes, when appropriate technical boundary conditions regarding wear are defined. ■ Gear boxes for industrial applications with appropriate wear allowances and frequent maintenance services (e.g. harbor tugboats) can be designed and further investigated regarding the behavior of the material-lubricant-system in practical usage. Aus Wissenschaft und Forschung 32 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 flanks and 3D-measurements of the tooth flank profile after the respective test cycles for material C. The flanks show typical running marks as well as an increasing profile form deviation in the area above and below the pitch point. This profile form deviation is likely caused by an increased wear due to the slow circumferential speed of the gears. The slow circumferential speed is leading to increased unfavorable tribological conditions and increased wear of the slow-speed wear tests compared to the high-speed wear tests. Figure 13 shows the results of the slow-speed wear test for the material D. The results show that the linear wear coefficient is significantly higher compared to the values of the pitting and highspeed wear test from Figure 8 and Figure 9. The linear wear coefficient decreases with higher rotational speeds, indicating certain film formation properties for water as a lubricant. Material C shows a lower wear compared to material D, which is consistent with the results from the high-speed wear tests as well as the increased corrosive behavior of material D. As observed from the high-speed wear tests, a tendency of lower wear caused by a longer tests cycle is observed. In general, the linear wear coefficients for the water lubricated gears made from tungsten carbide composite are increased compared to experience-based values for conventional steel gears with oil lubrication 4 Conclusions The presented investigations on water lubricated gears made from tungsten carbide composite lead to the following scientific conclusions: ■ Gears made from tungsten carbide composite material can be operated in practical transmissions with pure water lubrication if a certain amount of wear can be tolerated. ■ The test results regarding tooth root bending, scuffing, and pitting are comparable to conventionally manufactured and oil lubricated steel gears. Figure 13: Results of the slow-speed wear test for material D Figure 12: Photographic documentation and 3D-measurements of the tooth flank profile (material on the left side of the line) after the respective test cycles for material C ■ When more experience is gathered and further optimizations are performed, a much broader scope of industrial applications (e.g. cruise ships) can be realized. The benefits of these research results are: ■ Water as lubricant is abundantly available and does not have to be refined in energy consumptive processes. ■ Water as a lubricant is completely biodegradable and sustainable. ■ The challenges of corrosion and wear were effectively counteracted by the usage of tungsten carbide composite material. ■ The findings motivate further optimizations of the material (e.g. composition and alloying elements) as well as the lubricant (e.g. biodegradable additives or thickener). Summarizing the aforementioned results and conclusions, it can be stated, that the sustainable lubrication of gears with water has been technically proven and that further developments regarding optimized lubricant-material-systems are promising. Acknowledgement This work was funded by the “Austrian COMET-Program” (project InTribology1, no. 872176) via the Austrian Research Promotion Agency (FFG) and the federal states of Niederösterreich and Vorarlberg and was carried out within the “Excellence Centre of Tribology” (AC2T research GmbH). References [1] Armygov, A., 2022, Cruise Liners On Geiranger Fjord. Article-ID: 552192805. www.shutterstock.com [2] Jeng, Y.-R., Huang, Y.-H., Tsai, P.-C., and Hwang, G.-L., Tribological Properties of Carbon Nanocapsule Particles as Lubricant Additive, Journal of tribology, 136 4, pp. 418011 - 418019, 2014. DOI: 10.1115/ 1.4027994 [3] Chen, W., Amann, T., Kailer, A., and Rühe, J., Thin-Film Lubrication in the Water/ Octyl β-d-Glucopyranoside System: Macroscopic and Nanoscopic Tribological Behavior, Langmuir : the ACS journal of surfaces and colloids, 35 22, 2019. DOI: 10.1021/ acs.langmuir.9b00383 [4] Sagraloff, N., Winkler, K. J., Tobie, T., Stahl, K., Folland, C., and Asam, T., Investigations on the Scuffing and Wear Characteristic Performance of an Oil Free Water-Based Lubricant for Gear Applications, Lubricants, 9 3, 2021. DOI: 10.3390/ lubricants9030024 [5] Yilmaz, M., Lohner, T., Michaelis, K., and Stahl, K., Bearing Power Losses with Water-Containing Gear Fluids, Lubricants, 8 1, 2020. DOI: 10.3390/ lubricants8010005 [6] Sagraloff, N., Dobler, A., Tobie, T., Stahl, K., and Ostrowski, J., Development of an Oil Free Water-Based Lubricant for Gear Applications, Lubricants, 7 4, 2019. DOI: 10.3390/ lubricants7040033 [7] Ahmed, R., Ali, O., Faisal, N. H., Al-Anazi, N. M., Al- Mutairi, S., Toma, F.-L., Berger, L.-M., Potthoff, A., and Goosen, M.F.A., Sliding wear investigation of suspension sprayed WC-Co nanocomposite coatings, Wear, 322-323, pp. 133 - 150, 2015. DOI: 10.1016/ j.wear.2014.10.021 [8] Fang, Z. Z., Wang, X., Ryu, T., Hwang, K. S., and Sohn, H. Y., Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide - A review, International Journal of Refractory Metals and Hard Materials, 27 2, pp. 288 - 299, 2009. DOI: 10.1016/ j.ijrmhm.2008.07.011 [9] Liu, X., Liang, Z., Wang, H., Zhao, Z., Liu, C., Lu, H., and Song, X., Wear resistance of ultra-coarse WC-WCoB-Co cemented carbide at different oxidation stages, International Journal of Refractory Metals and Hard Materials, 105, pp. 105827, 2022. DOI: 10.1016/ j.ijrmhm.2022.105827 [10] García, J., Collado Ciprés, V., Blomqvist, A., and Kaplan, B., Cemented carbide microstructures: a review, International Journal of Refractory Metals and Hard Materials, 80, pp. 40 - 68, 2019. DOI: 10.1016/ j.ijrmhm.2018.12.004 [11] Lais, S., Getriebe. Patentschrift EP2614000, 2011. [12] International Organization for Standardization (ISO), Gears - Wear and damage to gear teeth - Terminology, ISO 10825: 1995. Beuth Verlag GmbH, Berlin, 1995. [13] International Organization for Standardization (ISO), Gears - FZG test procedures - Part 1: FZG test method A/ 8,3/ 90 for relative scuffing load-carrying capacity of oils, ISO 14635-1: 2000. Beuth Verlag GmbH, Berlin, 2000. [14] Matt, P., Tobie, T., and Stahl, K., FVA Guideline 0/ 5 - Recommendations for the Standardisation of Load Capacity Tests on Hardened and Tempered Cylindrical Gears. Forschungsvereinigung Antriebstechnik e. V., Frankfurt/ Main, 2012. [15] Schedl, U., Oster, P., and Höhn, B.-R., Influence of Lubricant on the Pitting Capacity of Case Carburized Gears in Load-Spectra and Single-Stage-Investigations. FVA 2 IV, Pittingtest. FVA-Informationsblatt 2/ IV. Forschungsvereinigung Antriebstechnik e. V., Frankfurt/ Main, 2010. [16] Bayerdörfer, I., Michaelis, K., and Höhn, B.-R., Method to Assess the Wear Characteristics of Lubricants FZG Test Method C/ 0,05/ 90: 120/ 12. DGMK 377. DGMK Informationsblatt 05/ 97. Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e.V. (DGMK), Hamburg, 1997. [17] Niemann, G., Winter, H., and Höhn, B.-R., 2005, Maschinenelemente - Band 1: Konstruktion und Berechnung von Verbindungen, Lagern, Wellen. Springer Berlin Heidelberg [18] International Organization for Standardization (ISO), Calculation of load capacity of spur and helical gears (including all current standards, technical specifications and technical reports), ISO 6336. International Standard. Beuth Verlag GmbH, Berlin, 2016, 2019. Aus Wissenschaft und Forschung 33 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 totype Testing. Konferenzbeitrag, AGMA Fall Technical Meeting [22] Güntner, C., Tobie, T., and Stahl, K., Alternative microstructures and their influence on mechanical properties of case-hardened gears, Forsch Ingenieurwes, 81 2-3, pp. 245 - 251, 2017. DOI: 10.1007/ s10010-017-0222-4 [23] König, J., Tobie, T., and Stahl, K., Nitriding of Heavily Loaded Gears - Potentials and Challenges, European Conference on Heat Treatment (ECHT) [24] Plewe, H.-J., Untersuchungen über den Abriebverschleiß von geschmierten, langsam laufenden Zahnrädern, Dissertation, Forschungsstelle für Zahnräder und Getriebebau, Technische Universität München, 1980. Aus Wissenschaft und Forschung 34 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 [19] Deutsches Institut für Normung e.V. (DIN), Geometrie von einzelnen außen- oder innenverzahnten Stirnrädern, DIN 3961 - 3967. Deutsche Norm, 1978 - 1986. [20] Fuchs, D., Schurer, S., Tobie, T., and Stahl, K., A model approach for considering nonmetallic inclusions in the calculation of the local tooth root load-carrying capacity of high-strength gears made of high-quality steels, Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 233 21-22, pp. 7309 - 7317, 2019. DOI: 10.1177/ 0954406219840676 [21] Weber, C., Tobie, T., and Stahl, K., 2019, Rapid and Precise Manufacturing of Special Involute Gears for Pro- Aus Wissenschaft und Forschung 35 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 More information and registration www.tae.de/ tribology 24 th International Colloquium Tribology Industrial and Automotive Lubrication Join the leading event on lubrication, additives and tribology in Europe: 3 intense days with 145 lectures from research, industry and practice in 6 parallel sessions. Ostfildern/ Stuttgart, Germany Save the Date 23 th - 25 th January 2024 by the California Air Resource Board, which also now requires the maximum sulphur content of marine gas oil and marine diesel oil to be 0,1 % m/ m within 24 nautical miles of the Californian coast. According to the 78 th session of the Marine Environment Protection Committee (MEPC 78) of IMO, the Mediterranean Sea will be included in the Emission Control Area for Sulphur Oxides and Particulate Matter (Med SOx ECA) in 2025 [2]. After its entry into force, no ship entering the Mediterranean Sea can use fuel with a sulphur content exceeding 0,10 % m/ m. The recently reduced limits on sulphur in fuel oil brought about a 70 % cut in total sulphur oxide (SOx) emissions from shipping, ushering in a new era of cleaner air in ports and coastal areas, by using fewer polluting fuels. Aus Wissenschaft und Forschung 36 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 1 Introduction: Maritime Industry in The Green Era Maritime transport consists the lifeblood of global economic vitality and the conduit of world trade in a costeffective and reliable manner. Given the large capacity of the fleet, shipping industry involves important economies of scale, making it a rather economic mode of transport. Seaborne shipping is one of the most important transport activities, since it generates benefits for consumers across the world through competitive freight costs. Nevertheless, it is a growing source of greenhouse gas (GHG) emissions and a major source of air pollution. Shipping industry aims to meet the International Maritime Organization’s (IMO) initial goals to reduce carbon dioxide (CO 2 ) emissions by at least 40 % by 2030 (pursuing efforts of a 70 % reduction by 2050) and total annual GHG emissions by at least 50 % by 2050, compared to 2008 levels. On January 1st 2020, a new limit in the sulphur content of the fuels used on board ships came into force, marking it a significant milestone to improve air quality, preserve the environment and protect human health. The reduction of sulphur emissions is prescribed in the sixth (IV) Annex of the Marine Pollution (MARPOL) Convention of the International Maritime Organisation (IMO) [1]. At a global level the highest permissible sulphur content of fuels is limited to 0,5 % m/ m and in some very fragile ecosystems known as SECAs (Sulphur Emission Control Areas) is already being reduced to 0,1 % m/ m. The SECAs include the Baltic Sea area, the North Sea area, The United States, Canada and the United States Caribbean Sea area. The required maximum sulphur content of 0,10 % m/ m for marine gasoils used in ships sailing or operating in the SECAs reveals that it is practically impossible to mix residual fuel with a distillate and still meet the highest permissible sulphur content. Therefore, only marine distillates that meet the environmental requirements of the fragile SECAs will be available. The shift from residual fuels to low sulphur distillates is driven not only by the EU and IMO regulations, but also Tribological Assessment of Marine Distillate Fuels under a Variant HFRR Method Theodora Tyrovola, Fanourios Zannikos* Maritime transport has a vital role in world economy. Its efficiency depends on the effective trade, transport facilitation, low cost of customs and the integration of new technologies for sustainable operation. However, the con-temporary demands have turned shipping industry into an emerging air pollutant with significant share to the global climate change problem. The industry is growing rapidly and it needs to lower greenhouse gas emissions in order to contribute towards the valuable effort for net zero emissions by 2050. A milestone to the ambitious strategy of decarbonization is the use of low or zero sulphur fuels that will contribute to the development of viable zeroemission vessels by 2030. Netherless the introduction of low-sulphur marine gasoils in the global fuel supply chain is accompanied by a huge range of side effects related to their storage, combustion, ignition and lubricity. The objective of the study is the evaluation of the lubricity of different marine distillate fuels with the High Frequency Reciprocating Rig (HFRR) test, either by following the primary conditions defined by ISO 12156-1 standard or by modifying them. The ultimate goal is the accurate and reliable assessment of their lubricating capacity so as to identify the challenges related to it, on time. Keywords Zero Emissions, Low Sulphur, Lubricity, HFRR Abstract * Theodora Tyrovola (corresponding author) Fanourios Zannikos Laboratory of Fuels and Lubricants, Chemical Engineering Department, National Technical University of Athens The global marine fuel market is steaming towards a major upheaval, as the industry has already entered the low-sulphur era. From year 2020 and on, people around the world will be able to breathe cleaner air at last because of the implementation of the International Maritime Organization’s Sulphur Cap. Under the severe pressure of IMO for immediate SOx emissions reduction, shipping enterprises take effective measures so as to meet the emission limitation requirements of relevant international organizations, regions and countries. Ship owners and operators must comply with the imperative environmental provisions, reduce their vessels’ emitted pollutants and switch to low or zero sulphur fuels that are more environmentally friendly. The path to decarbonization is paved and it requires significant changes as how power and propulsion is generated on board. 2 Major Shipping Emitted Pollutants Over 90 % of world trade is carried across the world’s oceans by some 90.000 marine vessels. Shipping has become an essential mode of transportation between trading countries due to the globalization of trade and the rapid development of the world economy, but has leaded to adhere air pollution in ocean and coastal areas due to its emitted pollutants [3]. Like all modes of transportation that use fossil fuels, ships produce hazardous emissions that significantly contribute to global climate change and acidification. It is estimated that almost 70 % of ship air pollutant emissions in global routes are emitted within 400 km of the coast. While the emitted pollutants from landbased sources are gradually reduced, the ones attributed to shipping industry are already experiencing a significant increase. The main fuels used in international shipping are HFO (Heavy Fuel Oil) and MGO / MDO (Marine Gas Oil / Marine Diesel Oil) [4]. Domestic shipping shows a large variety of fuels with the most important being MGO/ MDO (60 %), HFO (31 %) and motor gasoline 9 %). Both the complete and incomplete combustion of conventional fuels inside a naval engine result in the formation of a complex mixture of exhaust gases and particles. Ship-source pollutants most closely linked to climate change and public health impacts, include carbon dioxide (CO 2 ), nitrogen oxides (NOx), sulphur oxides (SOx) and particulate matter, as a result of the fuel used to power them. It is assumed that maritime transport emits around 940 million tons of CO 2 annually. Shipping is the lowest carbon form of transport per tonne of goods moved, but yet is responsible for more than 2,2 % of global GHG emissions. Over the last three decades, the shipping industry has grown by an average of 5 % per year. Ships generate approximately 13 % of SOx and 15 % of NOx emissions per year at a global level. Sulphur dioxide (SO 2 ) emissions can travel long distances, are responsible for the formation of acid rain and when combined with diverse pollutants they generate fine particles. Particulate matter (PM) contributes to the overall PM 2,5 air pollution burden in the European and form “black carbon”, the second largest contributor to climate change after CO 2 . The emitted CO 2 contributes to the widespread climate change by trapping the sun’s heat. Extreme climate changes include increased average temperatures, shifting rainfall patterns, thawing permafrost, and increases in hazardous weather. Sulphur oxide emissions resulting from the burning of fuel oil are proven to be a significant source of air pollution. Sulphur is a naturally occurring element, present in all fossil fuels. Its presence in the atmosphere in the form of SOx has a cooling effect on global warming but at high concentration can cause many serious health and environmental problems. Various combinations of nitrogen and oxygen can cause lung inflammation when breathed. NOx may enter the bloodstream and with longterm exposure could lead to eventual heart and lung failures. Both NOx and SOx emissions are responsible for the acidification of soil and water, causing the disastrous acid rain [5]. The IMO predicts that without establishing and implementing immediate measures and barriers to reduce emissions from shipping, CO 2 emissions from the industry could rise to 1,48 billion metric tons by 2022, equivalent to putting 65 million new cars on the road. Given the urgency of the climate crisis, and the technological advances since 2018, shipping industry can and must move faster, with a goal of reaching zeroemissions by 2050. 3 The Lubricity of Marine Diesel Fuels Lubricity of a fluid is the indicative measure of the protection efficiency of two mating surfaces, from wear or scarring, due to the relative motion between them. Fuel lubrication is absolutely necessary to diesel engine components in order to reduce the friction between the mating surfaces. In order to avoid the undesirable phenomenon of wear, it is necessary to insert a third layer that has good lubricating capacity between two rubbing surfaces in order to reduce friction. The role of this intervening layer in many metal components of a diesel engine distribution system is played by the fuel itself. Diesel fuels like all liquid hydrocarbon fuels need to possess a modicum of lubricating ability to protect sliding surfaces in fuel pumps, injection valves and other moving parts. The recognition of this requirement is originated in the mid 1960’s, when improvements in the refining and treatment processes led to the production of Aus Wissenschaft und Forschung 37 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 diesel fuel is the boundary lubrication regime. In the hydrodynamic regime, a film of fluid prevents contact between the sliding surfaces. The sulphur content of marine diesel fuels drops significantly leading to an increase in the number of fuel pump failures regardless of manufacturer and country of origin, which in many cases have been catastrophic for the pumps. As the operation of the fuel injection pump relies on the lubricity of the fuel, the failures are attributed to the sharp reduction in the lubricity level of the new refined fuels. A fuel’s ability to keep the surfaces separated is governed by its viscosity. When two liquids have the same viscosity and one gives lower friction, wear or scuffing, then is said to have better lubricity. If one fuel does not contain enough lubricating ingredients, it is considered as a “dry fuel” for its incapacity of lubricating the metal engine components. In boundary lubrication, asperities (rough spots) on the sliding surfaces are just touching, but the lubricating means still supports most of the load. The fuel’s effectiveness as a boundary lubricant is dictated by its chemistry. Diesel fuel molecules with polar groups will adhere to the metal surfaces, while the non-polar portion of these molecules will occupy space between the surfaces. These non-polar tails effectively trap additional lubricating means to reduce the degree of contact, thereby protecting the surfaces from wear. Friction and wear are the obvious requirements when one substance is moving over another substance. In engine fuel system, the relevant components experience the friction with the fuel flow activities. The effective work is obtained from the engines only when the produced energy can overcome the friction of these moving parts. 4 Reasons for the Reduction of Marine Gasoil’s Lubricating Capacity The new 0,50 % m/ m global sulphur limit consists a milestone for marine activities, as it is expected to be one of the first important steps to enter a new green season that leads to the decarbonization of the shipping industry. The global marine fuel market is steaming towards an unprecedented upgrade, as the industry is already experiencing the low-sulphur era. The growing tendency to find biological energy sources along with the global commitment for further reduction of exhaust gas emissions, are leading to the use of alternative fuels or very low sulphur fuels in ships. Low and zero sulphur fuels are proven to have positive impact both on the marine environment and coastal health, but they are accompanied by a huge range of side effects related to their storage, combustion, ignition and lubricity. Harmful gaseous emissions are obligingly being reduced since January 2020, and refineries are constantly developing new technologies in order to mi- Aus Wissenschaft und Forschung 38 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 very pure aviation fuels [6]. Later though the converging trends of increasingly rigorous fuel treatment and of higher fuel injection pressures have focused attention on the less severely refined automotive diesel fuels. In recent years there has been a particular concern regarding the reduction of unwanted emissions from diesel engines, which in turn has led to the establishment of new specifications for automotive and marine diesel fuels. The fuel quality and potential environmental impact due to sulphur and other components are dependent on the process of production of marine bunker fuels. Whereas MGO and MDO are the result of distillation processes in oil refineries, HFO is a residual product of the oil refinery process. The imperative need for reduction of harmful gaseous emissions form shipping industry, has led to the establishment of new standards for marine fuels which are included in the recently revised ISO 8217: 2017 [7]. The International Standard EN ISO 8217 specifies the requirements of petroleum fuels for use in marine diesel engines. ISO 8217: 2017 standard specifies three different distillate grades (DMA, DMB, DMZ) and a number of residual grades (RM). The changes in the specifications of marine gasoils concern the gradual reduction in their sulphur content. The peremptory demand for use of distillate fuels with extremely low sulphur content in marine engines has led to further research and data collection that allowed the incorporation of fatty acid methyl esters (FAME) in specific marine distillate grades (DFA, DFB, DFZ). A diesel fuel’s lubricity is the measure of its ability to prevent or minimize wear that the sliding components are subjected to and is a function of the way it has been refined and blended. Components with the greatest dependence on the fuel for lubrication, demand fuels with superior lubricating capacity. In -line fuel injec-tion pumps that are lubricated by a combination of the engine’s crankcase oil and the fuel are far less sensitive to the diesel fuel’s lubricity than rotary/ distributor type fuel pumps that rely solely on the fuel for lubrication. In order to perform under acceptable ranges, different components experience various lubrication regimes, e.g., hydrodynamic (HDL), elastohydrodynamic (EDHL), boundary (BL) and Mixed (ML) lubrication. The fuel guards the metallic parts away from rapid wear by forming HDL films (function of fuel viscosity) or BL films (function of diaromatic constituents) in between the solid surfaces. The sliding surfaces in fuel injection pumps are protected from wear by hydrodynamic and boundary lubrication mechanisms. Although hydrodynamic as well as boundary lubrication occur in several components of the fuel delivery system, the lubrication regime that is affected most by the removal of the sulphur and aromatics in nimize the sulphur content of fuels in an effort to comply with the strict emission limitations. The extensive interest in diesel fuel lubricity has escalated since the early 90 s, right after the commercialization of the low sulphur fuels. Their prolonged use in diesel engines revealed remarkable drivability problems and pump failures. These problems were soon linked with excessive wear on critical components of fuel injection pumps. Fuel lubricity is in correlation with the chemical composition of the fuel. Sulphur is one of the compounds in the fuel that imparts lubricity characteristics The lubricating capacity of marine diesel fuels is directly related to the polarity of its molecules [8]. Wear is increased by poly-cyclic aromatics at low concentration but it is redacted at high concentration. Therefore, polar impurities and poly-cyclic aromatics are considered as the most valuable natural lubricity additives in diesel fuels. The currently established refining processes for the production of low (LSD) and very low sulphur diesel fuel (ULSD) offer an extended engine life with significant reduced wear but also a rapid decline in the lubricating capacity of such fuels. Over the past few years, a range of hydrogenation treatments of varying severity have become common. The most efficient and least costly refining method in order to remove sulphur from fuels is the chemical process of Hydrodesulfurization. Hydrotreating or hydrodesulfurization refers to a set of operations that remove sulphur and other impurities from petroleum products, which increase the efficiency of the fuels and reduce the production of harmful combustion by-products such as NOx and SOx. During hydrotreating, crude oil cuts are selectively reacted with hydrogen in the presence of a catalyst at relatively high temperatures and moderate pressures [9]. The process converts undesirable aromatics, olefins, nitrogen, metals, and organosulphur compounds into stabilized products. Some hydrotreated cuts may require additional processing to meet final product specifications. Hydrodesulphurisation is not a selective process it removes nitrogen (N) and oxygen (O) as well as sulphur (S). The removal of N and O from heterocompounds destroys their ability to perform as lubricity agents. Thus, low S fuels have lower levels of active N and O compounds and thus worse lubricity. According to hydrotreatment technology, the sulphur content in fuel is removed and is replaced with hydrogen, delivering as a final product a cleaner fuel with extremely improved efficiency [10]. As hydrogen reacts with specific components of the fuel it removes the polar and aromatic compounds that provide the conventional diesel fuel with sufficient lubrication capacity. The loss of these polar compounds is considered to be responsible for the reduced lubricating capacity of marine diesel fuel. It is proven that the polar fraction contains compounds that adsorb on a metal surface, thus providing a protective layer. Sulphur content is usually correlated with the lubricating capacity of MGO as it reflects the refining intensity of the fuel and therefore the levels of polar and polyaromatic compounds. In particular, the more intense the desulphurization processes, the more noticeable is the reduction in the lubricating capacity of the diesel fuel. Poor lubricity significantly affects engine performance, by provoking accelerated wear and insufficient engine power. It also shortens the lifespan of the marine engine and causes energy dissipation by friction and failure of engine parts such as fuel injectors and pumps. Diesel fuel injection pumps are lubricated primarily by the fuel itself. A sustainable fuel needs a critical threshold of polar compounds in order to have a good lubricity performance. Poor lubricity fuels can still contain measurable levels of the key agents, but just not enough. Poor quality fuels or insufficient can be responsible for problems in handling and/ or combustion. In addition, higher maintenance requirements, shorter service intervals and possibly shorter service life of various components of the marine engine equipment will be required. 5 The High Frequency Reciprocating Rig Test 5.1 The History of its Development Lubricity is among the most important factors to ensure the best quality of any diesel fuel. The everchanging alterations in fuel specifications and the tighter tolerances in modern engines, render the understanding of marine gasoil’s lubricity more important now, than ever. Fuel lubricity is the most important aspect to consider when it comes to the durability of diesel engine components. Naval engine manufactures consider marine diesel fuel lubricity as a critical parameter because certain parts of the fuel injection equipment (FIE) depend entirely on the fuel for lubrication [11]. The lubricating capacity of a diesel fuel is determined either by measuring the wear prevention characteristics using the High Frequency Reciprocating Rig (HFRR) test according to ISO 12156-1 and ASTM D 6079 test methods and/ or by applying the Scuffing Load Ball on Cylinder Lubricity Evaluator (SLBOCLE) procedure based on ASTM D 6078 test method [12]. The HFRR test was first pioneered by Imperial College London. In 1994 Rinaldo Caprotti and one of his colleagues at Exxon Chemical’s PARAMINS additive division joined with Imperial College, London and Robert Bosch GmbH to develop the technique into a highly re- Aus Wissenschaft und Forschung 39 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 dary friction measurements of engine oils, greases and other compounds. It has become the industry standard test for diesel fuel lubricity and conforms to ASTM D 6079, CEC F-06-A, ISO 12156, EN 590, JPI-5S-50 and IP 450, SH/ T0765 standards. HFRR PCS instrument’s ability to collect continuous friction measurements, the wide range of specimen materials available and the ability to customise test parameters, make it the ideal choice for the evaluation of diesel fuel’s tribological properties. The High Frequency Reciprocating Rig (HFRR) test is the predominant method on a macroscopic scale and is widely used for the engineering of fuels, lubricants and engine parts [14]. It can provide macrotribological information, since the test is performed on objects of relatively large mass and under high load. Wear is inevitable and the mass properties of the components in contact dominate the tribological performance. The parameters of the HFRR test method simulate boundary lubrication conditions. The result given is the corrected - with respect to the standard water vapor pressure Aus Wissenschaft und Forschung 40 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 producible and accurate measurement of lubricity in diesel fuels [13]. The HFRR test was developed over almost 30 years ago and has stood up to all of the changes made to fuels and engines since its development. Nevertheless, since then, diesel fuel world has been through a period of significant change. Fuel sulphur has fallen dramatically, crude sources have changed and the use of biofuels and highly paraffinic diesel has increased. The specifications governing diesel fuel quality have gradually tightened in response to all qualitative changes so as to ensure the fuel in the pumps is fit for purpose and offers adequate protection to the advanced hardware which is used in today’s naval engines. 5.2 The HFRR Apparatus The High Frequency Reciprocating Rig (HFRR) of PCS Instruments is a microprocessorcontrolled reciprocating friction and wear test system which provides a fast, repeatable assessment of the performance of fuels and lubricants. It is particularly suitable for wear testing relatively poor lubricants such as diesel fuels and for boun- Scheme 1: Scheme of HFRR device Scheme 2: HFRR device of PCS Instruments / mechanical unit at 1,4 kPa - wear diameter (WS1,4) expressed in micrometers (μm) and constitutes the lubricating capacity of the fuel. The specification for the lubricating capacity of marine distillate fuels (with a sulphur content of less than 500 ppm) is specified in ISO 8217 standard, and the maximum permissible wear scar diameter is 520μm. The HFRR apparatus is based a spherical specimen of 6mm diameter which is subject to reciprocating motion in a frequency of 50Hz and an oscillation amplitude of 1mm with the help of an electromagnetic oscillator. The spherical sample is in contact with a flat sample under the application of a weight of 200 g while the contact point is immersed in a quantity of 2 ml of the fuel under examination throughout the test. The fuel is preheated to a temperature of 60 °C and the test lasts 75 minutes. During the test, humidity and ambient room temperature are controlled and noted. The results of the measurement can be observed online by means of a computer program. This software enables the operator to follow the course of the film formation as well as the changes of the friction coefficient over the testing time. The formation of a lubricating film is indicated by the electric potential existing between the steel ball and the plate. The built up lubricating film acts as an insulator and separates the test specimen, steel ball and the plate by a thin layer. After the test the abraded area on the ball is evaluated by means of an optical microscope. The observed dimensions are converted into a defined wear scar value by a formula. The WS1,4 is the standardized measure for the lubricating capacity of the marine distillate fuel. Additionally, to the wear scar dimensions, the temperature and humidity at the beginning and the end of each testing procedure are also included into the evaluation of the WS1,4. The device has also the ability to calculate the frictional force developed between the metal samples as well as the electrical contact potential (ECP). Based on these measurements there can easily be calculated the average coefficient of friction and the percentage thickness of the boundary oil layer (oil film). The measurement of the mean wear diameter is performed in a stereoscope (Leica M165C) and at 120 x magnification. 6 Detection of Wear 6.1 In Lab Analysis Lubricity is a parameter that is still not thoroughly understandable in the Marine Sector. Since the implementation of IMO’s cut down regulations, more and more misunderstandings related to lubricity have arisen. Monitoring fuel’s lubricity couldn’t be more imperative than now. By far there has been no better method than the HFRR test to accurately determine fuels’ lubricity levels. Most everyday marine diesel owners don’t need to worry about owning or operating an HFRR test by themselves, however, it is important that they understand how to interpret a lubricity rating and why it is important to the longevity of their marine engines. In order to increase the sensitivity and accurateness of HFRR over marine distillates, we rely firstly on the basic parameters of ISO 12156-1 standard and subsequently on the modification of them. The modification of the basic parameters of ISO 12156-1 is made so as to identify the poor lubricating capabilities of low-sulphur marine gasoils and to track wear on the metallic parts of the engine’s equipment that cannot be detected by the original method. Base Fuels: 1. Distillate marine DM 1 (grade DMA): Fuel’s properties comply with EN ISO 8217: 2017. 2. Distillate marine DM 2 (grade DMA): Fuel’s properties comply with EN ISO 8217: 2017. The properties of both fuels are given in Table 1. DM 1 has a slightly greenish hue, while DM 2 has a dark blue colour. The sulphur content in DM 1 is 900 ppm and in DM 2 is 850 ppm. Aus Wissenschaft und Forschung 41 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 Scheme 3: Leica Stereoscope DM1 DM2 Properties Unit Test Method Reference Kinematic viscosity at 40oC mm2/ s ISO 3104 2,7124 3,5851 Density at 15oC kg/ m3 ISO 3675 or ISO 12185 830,8 845,5 Cetane Index _ ISO 4246 38 31 Sulphur Content mass% ISO 8754 , ISO 14596, D 4294 0,0900 0,0850 oC oC -3 oC oC -24 -16 oC oC -19 -11 Lubricity, WS1,4 at 60oC μm ISO 12156-1 408 420 Fuel Type Pour point ISO 3016 CFPP IP 309 or IP 612 Cloud point ISO 3015 Result Table 1: Basic Properties of Marine Distillate Fuels Aus Wissenschaft und Forschung 42 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 Case 1: In case one the test conditions of HFRR method are followed as they are determined in ISO 12156- 1. An upper spherical specimen (test ball) with 6 mm diameter is subject to reciprocating motion, with frequency of 50 Hz and oscillation width of 1 mm, with the help of an electromagnetic oscillator. The spherical specimen is tangent to a flat specimen under a weight of 200 g while the point of contact is immersed into 2 ml of the tested fuel. The fuel is preheated to 60 °C and the test lasts 75 minutes. • The test plate is steel ISO 683-17-100Cr6 machined from annealed rod, having a Vickers hardness “HV 30” scale number of 190 to 210 (according to ISO 6507- 1. It shall be lapped and polished to a surface finish of Ra < 0,02 μm. • The test ball is grade 28 (G28) according to ISO 3290 of steel ISO 683- 17-100Cr6. It has a Rockwell Hardness “C” scale (HRC) number of 58 to 66 according to ISO 6508-1[2]. Case 2: In case two only the load imposed on the spherical specimen is altered and all the rest parameters are kept unchanged. The metal ball is firmly fixed in the vertically positioned holder and pressed with different loads on a horizontally reinforced metal plate. The ball reciprocates with the same certain frequency and stroke length as in case one. The per- Temperature Load DM1 DM2 200 443 543 300 513 569 400 509 575 500 463 522 600 437 549 700 434 564 800 461 559 900 401 612 1000 461 677 60 Fuel Type (oC) g WSD μm Modified Rockwell Hardness Scale Table 3: Wear Scar Diameter in Case 3 Temperature Load DM1 DM2 200 408 420 300 466 466 400 487 487 500 448 448 600 441 441 700 411 458 800 494 453 900 497 497 1000 463 464 Fuel Type WSD μm Test Ball Hardness ISO 12156-1 60 (oC) g Table 2: Wear Scar Diameter in Case 2 Figure 1: The graphic histogram of the variant Load & Wear Scar Diameter. (*mod: modified test ball type) missible load that HFRR PCS Instruments device (mechanical unit) can bear is 1000 g. Wear scar diameter is measured per 100 grams added, starting from the base load (200 g) and ending to 1000 g. Case 3: In case three both the imposed load and the type of the upper specimen (test ball) are changed. • The modified test ball is grade 25 (G25). It has a Rockwell Hardness “C” scale (HRC) number of 55 to 60. The spherical specimen meets the requirements of ASTM D7688, ISO 12156-1, ASTM D6079, CEC F- 06-A, EN 590, JPI-5S-50 & IP 450. • The test plate is steel ISO 683-17-100Cr6, the same as in case one. Wear scar diameter is measured per 100 grams added, starting from the base load (200 g). 6.2 Scientific Findings Under normal conditions of ISO 12156-1 standard, DM 1 and DM 2 are marine gasoils of an excellent lubricating capacity, approved for on board use. DM 1 Marine distillate has a WS1,4 of 408 μm and DM 2 marine distillate has a WS1,4 of 420 μm. They differ in their kinematic viscosity, density and cold flow properties, while they have relative sulphur content. Under no modification both fuels have excellent tribological properties, since their wear scar diameter is far less than the permissible limit. By keeping temperature stable and changing only the imposed load there is a significant limitation in fuel’s lubricity, which in fact degrades considerably. In DM 1 fuel the effect of the increasing load on its lubricating capacity is greater than in DM 2 . The imposed load can challenge the efficacy of marine distillates. By keeping all parameters immutable - only changing the load - and always taking into account the repeatability and reproducibility of the method, there is a significant limitation in fuel’s lubricity from the load of 800 grams and above. As the load gradually increases, the lubricating capacity of the fuel decreases and eventually both distillate marine fuels are marginally converted into dry fuels. Determining lubricity by using more vulnerable HFRR specimens than the ones dictated by ISO 12156-1 standard, an increased wear on their surfaces is observed, which consequently leads to excessive friction. Lubricity of DM 2 is highly affected compared to the correspondent of DM 1 , when using a softer metal ball. Τhe most significant increase in wear scar diameter of both fuels occurs when simultaneously load and upper specimen are altered. When using the more vulnerable and less hard upper specimens, as soon as the imposed load increases, wear scar diameter is maximized in both marine gasoils. Thereafter, when the load increases, given that the more fragile specimens are used, wear in DM 2 is remarkably substantial than the corresponding one in DM 1 . Through the results of the research, it can be drawn that as sulphur content diminishes it induces lack of lubricity, increase of wear and it definitely consists a prominent factor for the malfunction of fuel’s injection system. Lubricity is a complex mechanism that has a molecular structure component. Polyaromatics and oxygen constituents are likely to be the most important natural contributors to diesel fuel lubricity and the more they are limited, the more the lubricity of the fuels decrease. Based on HFRR’s repeatability (r = 70 μm), precision, and correlation with field data it is clearly demonstrated that based on the original method conditions it doesn’t consist an accurate method for determining the lubricity of marine gasoils. The considerable change in the marine diesel fuel market, and the substantial advances in fuel injection equipment are questioning the suitability of the HFRR laboratory test. The conduction of great and thorough research is imperative nowadays, in order to establish an exclusive control standard for the ship’s fuel pumps and be able to deter engine’s future breakdowns. Τhe replacement of standard specimens with mοre sensitive and vulnerable Aus Wissenschaft und Forschung 43 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 Figure 2: Wear Scar Diameter vs Imposed Load (*mod: modified test ball type) avoid future breakdowns. The need to establish an innovative perspective on the evaluation of the lubricating capacity of such fuels is imperative, since the already existing equipment might be unsatisfactory for protecting naval fuel systems. The scientific research is ongoing in the great effort to identify the weak spots of new technologies when using very thin oils. The implementation of the optimal technology is based on whether it is a costeffective means of complying, on the infrastructure of the existing fleet and ports, on the feasibility of modifications in ship’s engines and on the complexity of crew’s training. As the world moves to a lower emissions future, the shipping industry will follow the current stream of great changes. Most of the potential low-carbon technologies are still in the early stages of development with limited commercial application, meaning the majority of new orders are still for vessels powered by fuel oil and other fossil fuels. The efficient and effective transition depends on the promising and successful collaboration of governments, energy companies and shipping firms. It will take great effort by all to ensure that the industry continues to grow in a sustainable manner. Shipping is the beating heart of global trade but its pulse is progressively getting slower. Faced with uncertainty about which fuels to use in the long term to cut greenhouse gas emissions, many shipping firms are sticking with ageing fleets and older vessels will have to start sailing slower in order to comply with the new environmental rules [17]. At the moment, only about 5 % of the world’s fleet can run on lesspolluting alternatives to fuel oil, even though more than 40 % of new ship orders will have that option. The decarbonisation of the sector and eventually its sustainability, can be achieved firstly with the improvement of energy efficiency and secondly with the corporate use of renewable fuels. 8 References [1] MARPOL 73/ 78. 2015. “International Convention for the Prevention of Pollution from Ships” Practical Guide 38: 1-57. [2] UNEP - The Mediterranean is making strides in tackling air pollution from ships, September 2022.https: / / www.unep.org/ unepmap/ news/ news/ medi terranean-making-strides-tackling-air-pollution-ships . [3] The impact of international shipping on European air quality and climate forcing / EEA Technical report No 4/ 2013. [4] Ferguson, C. R., 1986. Internal combustion engines: Applied thermosciences. New York: John Wiley & sons. [5] Eyring, V., Koehler, H. W., van Aardenne, J. and Lauer, A., 2005, “Emissions from international shipping: The last 50 years”, J. Geophys. Res., 110 (D17), D17305. [6] Danping, Wei D., H.A. Spikes, The lubricity of Diesel Fuels, Wear Vol 11, Issue 2, September 1896, Pages 217- 235. Aus Wissenschaft und Forschung 44 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 to reciprocating motion, reveals an increased wear on their surfaces, which eventually leads to excessive friction. The continued drive for improved fuel efficiency is changing the materials that engine oils interact with. A variety of analytical methods are required to adequately explore different theories regarding marine gasoils lubricity. 7 Goals to be Achieved in the Near Future Deepsea transport is yet the most trustful and reliable means for the safe exchange and transfer of the vast majority of goods between the European Union and overseas. The shipping gaseous emissions can be effectively reduced as a result of various governments’ policy formulations and implementations [15]. Maritime decarbonization and reduction of greenhouse gas emissions from ships have become a priority case for the policymakers and the industry to be achieved, through the adoption of energy-efficient technologies, the optimization of ship operations and use of low and zerosulphur fuels. The creation of a ship emission inventory is a critical step in developing ship emission control measures and related regulations. Public institutions as the European Environment Agency (EEA) and US Environmental Protection Agency (EPA) recognize the seriousness of air pollution emission and enhance the engine development in order to meet emissions and fuel economy requirements is creating new durability challenges. As a result of the enforcement of IMO’s Sulphur Cap regulations, there are a number of operational factors that must be considered in order to ensure efficient and safe vessel operation on low sulphur distillates and whenever changeovers of fuels are made. Downsizing of engines usually means that bearings are narrower and are therefore subject to increased stresses for the same load. A great number of engine operational procedures arise and they need to be followed precisely in order to ensure that short - and long - term operation on distillate fuels does not lead to wear or malfunctioning of the fuel system and engine’s components. It is a fact that fuel quality remains a major concern for ship operators. The move to thinner oils for fuel economy is gaining momentum, with several Original Equipment Manufacturers (OEMs) developing new, even lower viscosity specifications down at dynamic viscosities of 2.6 cP or less [16]. Netherless their poor lubricating capacity may result in blocking fuel lines, damaging fuel pumps, injectors and even contribute to the loss of engine power (LOP), but it is not the only factor that provokes a failure. Great and thorough research must be done so as to identify sources of variability in the HFRR test method and to improve its precision to marine distillates, in order to [7] Petroleum products - Fuels (Class F) - Specifications of Marine Fuels, International Standard ISO/ FDIS 8217: 2017, Final Draft. [8] Brewer, T.L., Regulating international maritime Ing., verifying and enforcing regulatory compliance. J. Int. Marit. Saf. Environ. Aff. Shipp. 2021, 5, 196-207. [9] Marine Fuel Oil Advisory, December 2019, American Bureau of Shipping. [10] Hydrodesulfurization, H 2 S, hydrodesulfurization reaction resultant is a kind of typical polar molecule, Advances in Chemical Engineering, 2015. [11] Maragkogianni, A.; Papaefthimiou, S.; Zopounidis, C., Mitigating Shipping Emissions in European Ports: Social and Environmental Benefits; Springer International Publishing: Berlin, Germany, 2016. [12] Diesel Fuel - Assessment of lubricity using the high frequency reciprocating rig (HFRR), Draft International Standard ISO/ DIS 12156-1. [13] The lubricity requirement of low sulphur diesels, SAE 942015. [14] Wie D., The lubricity of Fuels II, Wear Studies using model compounds, J. of Petrol. (Pet. Processing.) 1988, Vol 4, No.1, p90. [15] https: / / www.hellenicshippingnews.com/ imo-study-shipping-emissions-rose-by-almost-10-during2012-2018-period/ IMO available online, (accessed on 25 May 2022). [16] Bosch, P., Coenen, P., Fridell, E. et al., 2009, Cost Benefit Analysis to Support the Impact Assessment accompanying the revision of Directive 1999/ 32/ EC on the Sulphur Content of certain Liquid Fuels. [17] European Commission - Reducing emission from the shipping sector https: / / climate.ec.europa.eu/ eu-action/ transport-emissions/ reducing-emissions-shipping-sector _en. Aus Wissenschaft und Forschung 45 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0036 Bericht 46 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 Abstract The 23 rd International Colloquium Tribology in January 2022 was held for the first time as an online conference only, with about 300 participants. The organising team, speakers and participants therefore made the best of this traditional event in Germany in view of the Covid-19 pandemic, even though the “online conference is not a substitute for a conference on site”, as one participant stated aptly. In almost 140 talks, 8 of which were plenary talks, new findings were reported on the topics of lubricants and additives, measuring techniques, digitalisation, coatings and surfaces, transport and industry, and sustainability. Several presentations were dedicated to the EU project “i-Tribomat”, which will soon be launched as “The European Tribology Centre” that offers services for the tribological characterisation of materials and lubricants. Virtual event for the first time While 2020 was mainly characterised by cancellations of face-to-face conferences due to the outbreak of the Covid-19 pandemic, organisers moved to set up online conferences from 2021 on. We have also seen the emergence of webinars and YouTube channels to provide scientists with specific knowledge in tribology and to enable the exchange of new knowledge, such as Web Seminar Series on Tribology (WeSST) [1] and from SKF Group [2] to name a few. The Technische Akademie Esslingen (TAE) made considerable efforts to enable a smooth running of an online conference. Eventually, the 23 rd International Colloquium Tribology took place for the first time as a completely virtual event in January 2022. The conference covered 3 days with more than 300 attendees and about 140 contributions provided in 6 parallel live streams (see Figure 1). Needless to say, that such an event required a lot of organisational talent and technical support comprising the upload of the programme, speaker list and attendee data to a brand-new platform that was established in just 6 weeks. Issues that arose before and during the event were fixed in a competent and calm manner, just as we have come to appreciate from the TAE team during face-to-face conferences. Plenary talks The 8 plenary talks reflected the versatility of tribology by its nature as well as developments in view of global trends, among others electrification, sustainability, and digitisation. Review of the 23 rd International Colloquium Tribology - 25-27 January 2022 Nicole Dörr, Andreas Pauschitz* * Nicole Dörr Andreas Pauschitz AC2T research GmbH, Viktor-Kaplan-Straße 2/ C, 2700 Wiener Neustadt, Austria Figure 1: Screenshot of the virtual participant’s view showing the plenary talk of Lutz Lindemann, Fuchs Petrolub SE on 25 January 2022 (with permission of Technische Akademie Esslingen, 2022) DOI 10.24053/ TuS-2022-0037 Bericht 47 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 On the first conference day, Lutz Lindemann from Fuchs Petrolub SE has provided an overview of the global lubricant market in a proven manner, at this conference with a focus on e-mobility and raw materials. Exemplarily, the increasing use of electrical vehicles will heavily change the petrochemical (refinery) landscape. In view of changes in the world order (from global to regional), circular economy and sustainable raw materials, Lindemann emphasised on the importance of a shift in a coordinated way. Inga Herrmann from VSI Verband Schmierstoff-Industrie e.V. provided valuable insight into the dimensions of sustainability, i.e., ecology, economy and social. Interesting detail was that by far the largest portion of the CO 2 footprint is done before the lubricant industry comes into play. Herrmann therefore highlighted the cradle-to-grave approach that leads to the product carbon footprint (PCF). It covers the total greenhouse gas emissions of a product from the extraction of the raw materials to the product’s end-of-life, i.e., waste treatment, recycling, or reuse. The second plenary session was opened by Volker Weihnacht from Fraunhofer Institut für Werkstoff- und Strahltechnik (IWS) who reported on vacuum tribology of superhard ta-C coatings. Unidirectional ball-on-disc experiments in vacuum of ta-C against various materials revealed good friction and wear properties when brass and aluminium oxide were used for the counter-bodies. Pairings of ta-C: X against steel offer slight advantages with Band Si-doping. Susanne Beyer-Faiß, CEO of Dr. Tillwich GmbH Werner Stehr, presented a holistic approach of using ionic liquids both in polymers and lubricants to achieve electrical conductivity in radial plain bearings for reliable electrical discharge. The combination of ionic liquid, graphene and carbon nanotubes significantly improved electrical properties of a plain bearing made of PA66 running against a steel shaft. In view of the vast results of the study, 2 more presentations were given focusing on a new apparatus to simulate lubricant ageing under current flow and modern evaluation methods of tribotest results, respectively. Franz Pirker from AC2T research GmbH provided an update on the EU project “i-TRIBOMAT” [3] dealing with the establishment of an open test bed for materials tribological characterisation. This one-stop shop for tribological characterisation will be operated by partners from all over Europe, which is dedicated to validating and up-scaling new materials. 6 more individual presentations were brought to “i-TRIBOMAT” illustrating digital tribological services together with 2 use cases related to friction control by surface texturing and novel journal bearing materials. The third plenary session was held as the closing session and was a dignified conclusion of the 23 rd International Colloquium Tribology. Steffen Glänzer from Clariant shared the findings of a benchmark of polyglycols (PAG) of various chemical structures against a polyalphaolefin (PAO) for the use in electrical vehicle drivetrains. Lower friction than with PAO could be demonstrated in a unidirectional pin-on-disc contact. Energy efficiency was further stated by a low viscosity of 3-4 mm 2 / s at 100 °C. Most crucial for electrical vehicles is thermal management in the battery and powertrain. Here, better performance of some PAG than the chosen PAO was shown expressed by higher Mouromtseff numbers as indicator for heat transfer by a fluid. Ken Hope from Chevron Phillips Chemical Company, STLE President 2021-2022, provided details on the perspective of STLE regarding emerging trends and lubrication challenges currently arising from them. The report is based on a global survey of tribologists and lubrication engineers in 2020 [4]. 20 interviews with experts and 591 members from 48 countries participated in the survey to evaluate the effects of new technologies in the application sectors transportation, energy, manufacturing, and medical & health. As to transportation, electric vehicles, fuel cells and autonomous driving are considered the drivers of the anticipated changes of the future. Regarding energy, wind and solar energy have been predicted as the major game changers in terms of clean energy, provided that progress goes hand in hand with advances in energy storage. Concerning manufacturing, it was found that electric vehicles and additive manufacturing (3D printing) will cause a significant decline in metalworking and thus the use of metalworking fluids. In medical & health, 3D printing was deemed to have potential to produce customised tissues and prosthetics. Denis Mazuyer from Ecole Centrale de Lyon concluded the conference’s talks by showing an innovative approach to reduce friction in transport from hydrodynamics to boundary regime, based on the IMOTEP research platform. This platform allows for the investigation of lubrication mechanisms over 11 orders of magnitude of sliding velocity and 3 orders of magnitude of contact pressure. Mazuyer’s approach was illustrated by a number of fundamental experiments with a polymer additive in a base oil spanning the range from boundary via elastohydrodynamic to hydrodynamic lubrication regime. Scientific contributions in parallel sessions 129 talks were organised in 8 main topics and presented during 6 parallel sessions, i.e., parallel live streams (see Table 1). As expected, topics concerning lubricants and lubrication dominated the conference. It is impossible to adequately address all the presentations. Therefore, some highlights from the multitude of talks and topics are discussed in the following. In trends in lubricants and additives, Kevin Duncan from Croda Europe Ltd. has proposed liquid amines as novel high-performance base oils. Dominic Linsler from Fraunhofer IWM showed that reversible tuning of viscosity using UV light can be realised by functionalisation of polydimethylsiloxane with anthracene groups. As to stability and lifetime behaviour of lubricants, a number of talks have reported on laboratory and engine bench tests to account for the operating conditions lubricants are exposed to in the field, e.g., Markus Grebe from Hochschule Mannheim on grease thickener degradation and Markus Matzke from Robert Bosch GmbH on antioxidants in greases. Talks on grease components formulation showed a focus on applications in electrical vehicles, e.g., low viscosity low volatility (LVLV) PAO base oil for bearing grease presented by Sven Meinhardt from Exxon-Mobil Chemical Central Europe. Automotive lubricants were mainly represented by engine oils and fuel economy, e.g., with a floating liner single-cylinder engine tests as demonstrated by Abdullah Alenezi from University of Leeds. White etching layers (WEC) on industrial machine elements, especially wind turbines, were addressed in a couple of talks. Daniel Cornel from RWTH Aachen illustrated the impact of lubricant formulation on WEC formation under sliding, mixed friction and current passage conditions. Although electric vehicles are pushed by technological and legal means (European Green Deal), reports on advances have remained moderate at this year‘s International Colloquium Tribology. The focus was on reports on efficiency increases through suitable synthetic base oils as well as defoamers and polymers to improve functio- Bericht 48 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 Table 1: Overview of topics, sub-topics and corresponding number of talks DOI 10.24053/ TuS-2022-0037 Bericht 49 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0037 nality, e.g., polymeric additives presented by Dmitry Shakhvorostov from Evonik Operations GmbH. Fewer than 10 talks on this topic were contrasted with more than 20 presentations at the STLE‘s 76 th Annual Meeting & Exhibition in May 2022. In view of the recently announced EU phase-out of vehicles with internal combustion engine by 2035, it can be expected that significantly more R&D work in the field of e-mobility will be reported in the future. This is also in line with the results of the survey conducted after the 23 rd International Colloquium Tribology, according to which the topic of e-mobility was seen as essential for the next conference, alongside sustainability, lubricants based on recycled materials, CO 2 footprint, and green tribology. Several aspects were related to sustainable lubrication, among others: Sustainability by design criteria using tribology and lifecycle assessment as proposed by Amaya Igartua from Fundación TEKNIKER and novel bio-based base oils as presented by Arthur Coen from Oleon NV. One aspect in coatings, surfaces and underlying mechanisms was materials tribology. Julia Rau from KIT provided insight into tribo-oxidation of high-purity copper the mechanism of which was revealed by use of an atom probe. Another presentation from KIT given by Carina Morstein was about the humidity influence on graphite lubrication. Structural changes of graphite in the wear scar were elucidated by transmission emission microscopy (TEM) and electron energy loss spectroscopy (EELS). Digitisation in tribology covered traditional topics such as modelling and simulation of rolling and sliding contacts as well as molecular dynamics of additive adsorption on surfaces and viscosity modifiers in oil. Stefan Mitterer from Oilcheck GmbH explained how digitisation can support lubricant analysis. Shin Ho Kim Lee from CIB Margarita Salas reported on the use of artificial intelligence tools for the design of new dispersants. In test methodologies and measurement technologies, well known tribometer manufacturers reported on use cases of tribometry: Dirk Drees from Falex Tribology on optimization of testing, Matthew Smeeth from PCS Instruments on mini-traction machine (MTM) rig for wear testing, Ameneh Schneider from Optimol Instruments Prüftechnik on material compatibility, among others. Continuous wear measurement by radioactive tracers was presented by Peter Lee from Southwest Research Institute and Manuel Zellhofer from AC2T research GmbH, the latter for diamond-like carbon (DLC) coatings. Heat maps for visualizing friction coefficient curves were presented many times at this conference. In this way, the friction coefficient of the entire tribometric experiment can be displayed and evaluated, i.e., for each cycle, for each position along the wear track and the friction coefficient level displayed in false colours. This method of visualisation will certainly become common in tribology. Exhibitions The general and unfortunately disappointing resume of the exhibitors was that there was little interaction with the conference participants. Apparently, there is still too much reluctance to make an enquiry via an online tool than to start a conversation with a spontaneous question while walking through the exhibition. Attendance A total of about 300 people participated in the conference. Presenters and “only” participants were about 150 each. As expected, Germany accounted for the largest share of attendance, at around 40 %. The conference provided a platform for participants from all over Europe as well as the United States, Japan, India, China, Korea, and Mexico. Countries represented with about 10 % each of the attendees were United Kingdom and France. Austria, Spain, Sweden and the United States were represented by about 5 % of the attendees each. Closing remarks The pandemic undoubtedly pushed the use of online communication tools both in our daily professional life and conference activities. It has become common to record the presentations and make them available as videos on demand. Besides the availability of the videos on the TAE’s event platform, participants were able to contact speakers, exhibitors, and other participants directly via the platform after the conference for 3 months. In view of the number of parallel sessions, making it impossible to listen to all presentations of interest, videos on demand can complement the offer of face-to-face conferences. This way, networking to establish new contacts and maintain existing ones can be intensified. Indeed, personal discussions with conference participants form the backbone of conferences together with presentations and exhibitions. Virtual events will certainly permanently complement our means of information exchange. In addition, we are able to optimize our business-driven travels, or more precisely, reduce them. However, that is what we also learned in the past 2 years, talking face-toface to someone cannot be replaced (yet) by modern technology. The Steering Committee of the conference, Ksenija Topolovec-Miklozic and Andreas Pauschitz, would like to express their sincere thanks to all presenters, exhibitors, participants, program committee members, and of cour- Bericht 50 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 se the organisational and technical team at TAE, all of them have made this virtual event possible. We look forward to seeing you again - in person - at the 24 th International Colloquium Tribology in January 2024. References [1] Web Seminar Series on Tribology (WeSST), YouTube Channel, https: / / www.youtube.com/ channel/ UCN0Jlz Er-iPAcWNb2JBnwMw, accessed 31 October 2022 [2] SKF Stronger, YouTube Channel, https: / / bit.ly/ SKF stronger_playlist, accessed 31 October 2022 [3] i-TRIBOMAT - THE European Tribology Centre, http: / / i-tribomat.com/ project.html, accessed 31 October 2022 [4] STLE 2020 Report on Emerging Issues and Trends in Tribology and Lubrication Engineering, White Papers in Technical Library, https: / / www.stle.org/ files/ , accessed 31 October 2022 Acknowlegdement This review was supported by the Austrian COMET Program (project InTribology1, FFG no. 872176). 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ISSN 0724-3472 Aus Wissenschaft und Forschung Science and Research www.expertverlag.de Polychronis Dellis Squeeze Film Investigations in a Simulating Piston- Ring Cylinder Liner Experimental Set-up Hans-Martin Eckel, Christian Brecher, Stephan Neus Kugelbewegung in Spindellagern unter dynamischer Belastung Justus Rüthing, Frank Haupert, Regine Schmitz, Michael Sigrüner, Nicole Strübbe A new approach for the friction and wear characterisation of polymer fibres under dry, mixed, and hydrodynamic sliding Karl Jakob Raddatz, Thomas Tobie, Klaus Michaelis, Karsten Stahl Scientific Evaluation of Investigations on the Load Carrying Capacity of Carbide Cylindrical Gears Lubricated with Water Theodora Tyrovola, Fanourios Zannikos Tribological Assessment of Marine Distillate Fuels under a Variant HFRR Method