Introduction Shifting elements are crucial components of modern drivetrains. In particular, wet disk clutches and synchronizers provide comfortable shifting under differential speeds. In automotive applications, wet disk clutches guarantee fast and reliable shifting under differential Aus Wissenschaft und Forschung 5 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 Nasslaufende Schaltelemente erfüllen sicherheitskritische sowie komfortrelevante Aufgaben in Antriebssträngen aus automobilen, maritimen, industriellen und heavyduty Anwendungen. Deren experimentelle Erprobung ist grundlegend für Auslegung und Validierung entsprechender Antriebssysteme. Durch zahlreiche miteinander in Wechselwirkung stehende Einflussgrößen ist ein standardisiertes methodisches Vorgehen zur Bestimmung der entsprechenden Zielgröße notwendig. In dieser Veröffentlichung wird auf die Bestimmung des Verhaltens nasslaufender Schaltelemente hinsichtlich Reibung, Funktion, Lebensdauer, Verschleiß, thermischem Haushalt und Spontanschädigung eingegangen. Dabei werden insbesondere Lamellenkupplungen und Synchronisierungen betrachtet. Für eine gute Übertragbarkeit der Erkenntnisse auf das jeweilige Praxissystem eignet sich dabei die Untersuchung mittels Komponentenprüfständen, in denen die originalen Reibelemente direkt getestet werden können. Am Beispiel der standardisierten Komponentenprüfstände KLP-260 und SSP-180 wird das methodische Vorgehen zur Untersuchung von Schaltelementen vorgestellt. Dabei wird auf die direkte und indirekte Erfassung der messtechnisch relevanten Zielgrößen eingegangen. Es werden zwei Testmethoden für die Untersuchung von Spontanschäden vorgestellt. Abschließend wird die Nutzung der experimentellen Daten für Forschung und Anwendung durch Verknüpfungen zu weiteren Tools der Antriebsstrangauslegung gegeben. Schlüsselwörter Reibung, nasslaufende Lamellenkupplung, Synchronisierung, Schaltelemente, Tribologie, Experimentelle Untersuchung Wet shifting elements fulfill safety-critical and comfort-relevant tasks in automotive, maritime, industrial and heavy-duty application powertrains. Their experimental testing is fundamental for design and validation of corresponding powertrains. Due to numerous interacting influencing parameters, a standardized methodical procedure for determining related target parameters is necessary. This publication provides insight into the reliable determination of friction behavior, function, durability, wear behavior, thermal stability and spontaneous damage of wet shifting elements. In particular, the focus is on wet disk clutches and synchronizers. For the best transferability of the findings under experimental conditions to the respective system, the investigation is suitable using component test rigs where genuine friction elements can be tested directly. The standardized component test rigs KLP-260 and SSP-180 are used as examples to illustrate the methodical procedure for investigating wet shifting elements. The direct and indirect acquisition of relevant target parameters is discussed. Two test methods for the investigation of spontaneous damage are presented. In the end, the use of the experimental data for research and application is examined by links to further tools of powertrain design. Keywords friction, wet clutch, synchronizer, shifting elements, tribology, experimental testing Kurzfassung Abstract * Patrick Strobl, M.Sc. (federführender Autor) Thomas Schneider, M.Sc., Dr.-Ing. Hermann Pflaum Prof. Dr.-Ing. Karsten Stahl Technical University of Munich (TUM), School of Engineering & Design, Department of Mechanical Engineering, Gear Research Center (FZG) Boltzmannstr. 15, 85748 Garching near Munich, Germany Experimental Setup for the Investigation of Wet Shifting Elements Patrick Strobl, Thomas Schneider, Hermann Pflaum, Karsten Stahl* Eingereicht: 26.10. 2023 Nach Begutachtung angenommen: 15.12. 2023 Dieser Beitrag wurde im Rahmen der 64. Tribologie-Fachtagung 2023 der Gesellschaft für Tribologie (GfT) eingereicht. TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 5 mended to classify four main approaches that vary in the degree of abstraction and complexity to obtain reliable tribological data. These approaches are depicted in Figure 1. The functional behavior of wet shifting elements has been researched and developed for decades. For simplification, some investigations use model testing with, e.g., tribometers (e.g., [Far21]), which guarantee a good basic understanding of the influences in sliding contacts. However, the lack of transferability from simplified model tests to genuine applications, especially in friction and damaging behavior investigations or similar trials, requires application-relevant test methods. On the other hand, testing whole drivetrains or even vehicles is expensive and complicated to achieve - especially in the early design stages. Thereby, the accessibility of the components is restricted, and cross-effects of the system make the evaluation of individual effects more difficult. In practice, several component test rigs are used to experimentally test wet shifting elements. These test rigs mostly have the common feature of quickly adapting different genuine parts. This guarantees the best possible transferability of the test to the original application. Furthermore, it leads to cost-effective testing without manufacturing particular test parts. Nevertheless, the test method has to be related to the underlying test rig. In the following section, some selected test rig concepts are presented. The ZF Group uses two basic test rig setups for wet disk clutches. The portfolio is detailly described by Frey et al. [Fre98]. Test rig DKA allows the measurement of high differential speeds (up to Δn = 7000 rpm) with the main drive and low differential speeds (Δn = 1 … 20 rpm) with the crawl drive. Test rig GK is restricted to maximum differential speeds of Δn = 1500 rpm. Nevertheless, it uses the drive unit for inertia mass simulation, whereas DKA uses mechanical inertia masses. [Fre98] The component test rig SAE No. 2 of the Society of Automotive Engineers (SAE) can measure functional behavior and is widely used. It allows the measurement of differential speeds up to Δn = 10000 rpm. It enables the investigation of wet disk clutches in different operational modes, from static breakaway, continuous slip over brake shifts. It is available in other models. [Gre23, LIN23] Aus Wissenschaft und Forschung 6 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 speed. Therefore, they are core components of automatic transmissions, dual-clutch transmissions, and limited slip differentials. But they are also crucial components of industrial, maritime, and heavy-duty applications. They can be used for the synchronization of two shafts but also as brakes or torque limiters. In case of a form-fit connection between the shaft and the gear, like at shifting gears in manual transmission systems, synchronizers guarantee a fast synchronization of the rotational speeds for an interlocking connection of shaft and gear. Due to the blocking condition, shifting at differential speed is excluded by design. Furthermore, their compact design allows integration into highly integrated drivetrains. [Sta14, Sto20] The working principle of both systems for synchronizing the shaft speeds is based on sliding friction. Generally, each underlying tribological system mainly consists of a friction lining surface, a counter surface, and a lubricant. These components strongly affect frictional behavior (e.g., [Mei17a]). Nevertheless, their effects underlie interactions (e.g., [Mei17a]), making it hard to separately investigate their influence and predict frictional behavior with the help of simulations. Due to these complex interactions, experimental testing is needed to evaluate functional behavior (e.g. [Str23a]), durability (e.g., [Hen15]), wear behavior (e.g., [Win08]), thermal stability (e.g., [Hen15]), spontaneous damage (e.g., [Gro21]), and running-in (e.g., [Voe20b]). Due to the big number of influencing parameters and their complex tribological interactions, it is only possible to predict the functional behavior of wet-shifting elements using experimental testing. Recent advances in machine learning and computational pattern recognition promise a better understanding of the main influencing factors on functional behavior. Nevertheless, these approaches require high standards for the experimental testing of the components. This underlines the need for application-relevant testing of genuine parts rather than testing of test specimens with different manufacturing or material pairing. According to the withdrawn DIN 50322 [DIN86] and GfT Worksheet Nr. 7 [GFT02], tribological testing can be determined in six categories, from field to model tests. To investigate wet shifting elements, it is recom- Complexity Degree of Abstraction Vehicle Test Transmission Test Component Test Model Test Figure 1: Approaches for the tribological testing of wet shifting elements TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 6 The Institute of Product Engineering (IPEK) at the Karlsruhe Institute of Technology offers an InLine module [Bis22, Ins19b, Ott17] for investigating wet disk clutches with differential speeds of up to 10000 rpm and a maximum axial force F a of nearly 30 kN. In addition, IPEK offers a Continuous Slip Test Rig for investigating clutches at continuous slip. The test rig allows measurements with speeds of up to 10000 rpm and a maximum axial force F a of 36 kN [Ins19a]. Besides the later discussed test rig KLP-260, Gear Research Center (FZG) uses several other test rigs to investigate the functional behavior of wet disk clutches. Test rig LK-1 [Gei03, Häm95] provides measurements in clutch mode with two separately rotating shafts. This enables the possibility to gain insights into influences on the affected functional behavior, for example, due to specific groove patterns at outside disks. As also mentioned by Frey et al. [Fre98], there are test rigs that are optimized for investigating shudder. At the FZG, test rig LK-2 [Lay11, Mos02] is optimized for those investigations. Due to the defined stiffness of the shaft, shudder phenomena can be evoked easily. Other test rigs, like KLP-260, may be modified to allow those measurements. Therefore, the connection between the test rig shaft and the inner carrier is designed for low stiffness. For the investigation of very low sliding speeds, FZG offers a test rig, named LK-3 [Voe21], with static torque application allowing to investigate creep with very low sliding speeds. With this test rig, it is possible to determine the static coefficient of friction. The presented test rigs so far concentrate on determining the functional behavior of wet disk clutches in engaged operation mode. Nevertheless, there are also component test rigs capable of measuring drag torque during the disengaged operation phase. One example is the test rig LK-4 [Drä13, Poi23]. The test rig µ-comp from company SK Hydroautomation is designed for the experimental testing of synchronizers. The test rig is exemplarily shown by Weber et al. [Web10]. In the basis configuration, one synchronizer is accelerated by an asynchronous motor and shifted by a hydraulic actuator. A changeable mechanical flywheel provides a suitable inertia. There are also variants of the test rig with two driven shafts that allow the measurement of combinations of speeds of up to 6000 rpm (hollow shaft) and 8000 rpm (inner shaft) and axial forces up to 5 kN. [Web14] In the following focus will be on component test rigs for wet clutches (KLP-260) and synchronizers (SSP-180). They guarantee application-relevant testing under welldefined conditions without influences from other transmission components. In addition to the publication of Meingassner et al. [Mei15], two test procedures are presented that allow the investigation of the spontaneous damage of wet clutches with carbon friction lining and synchronizers with molybdenum friction lining. Materials and Methods Due to the broad range of applications, suitable testing of shifting elements should provide different operational modes that, on the one hand, meet the drivetrain’s conditions best but also show good reproducibility and stable conditions. Therefore, test rigs were developed at FZG for testing shifting elements. Shifting elements of serial applications can be adapted into both test rigs easily. Three basic categories of friction materials cover most applications for wet shifting elements. Paper-based friction linings are mainly found in automatic transmissions and applications requiring comfortable shifting [Voe20a]. Due to the low thermal conductivity of the friction material, the steel plate experiences high thermal loads during engagement. In friction systems with metallic or sinter-metallic friction material, the friction and steel disks have good thermal conductivity, leading to better heat distribution [Mäk05]. Conversely, sinter-metallic friction lining tends to show a worse tendency to shudder. Metallic friction linings are, therefore, mainly used in industrial or heavy-duty applications. Carbon friction linings are very robust. They are used, e.g., in limited slip differentials [Sch22b]. They combine a comfortable shifting with high robustness, but their manufacturing cost are slightly higher [Ahm19]. Typical friction plates of each category are shown in Figure 2. Besides friction material, the lubricant influences the friction behavior of wet disk clutches strongly. Base oil and additives must be appropriately chosen with the corresponding friction material and application. Aus Wissenschaft und Forschung 7 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 paper-based sinter-metallic carbon Figure 2: Exemplarily friction plates of modern friction systems representing the main types of friction lining paper-based (left), sinter-metallic (middle), and carbon (right) from [Str23b] TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 7 two separate drives. One drive provides high speeds and is connected to the shaft via a belt, enabling the investigations of brake shifts. Therefore, the drive motor is disconnected before shifting. Another drive is optimized for investigating forced slip modes and directly connects to the shaft via a form-fit clutch. Oil can be injected into the clutch from the inside or the top. Especially for low speeds, a combination of injection from inside and above supports clutch cooling. Wet disk clutches cover a broad range of applications. This leads to the necessity of different application-relevant operational modes that the KLP-260 covers. Due to the test setup with a fixed outer carrier, brake shift mode allows measuring the shifting process occurring in power-shifting or brake-shifting transmissions. Therefore, the differential speed is set before the clutch is closed with defined axial force. Many applications also operate the clutch in continuous slip to reduce the impact of torque oscillations or shocks on the drivetrain, optimizing the comfort of the drivetrain. Continuously slip can also be found in applications where the desired output speed is lower than the engine’s idle speed. For these applications, stationary slip mode is suitable. The axial force is applied in this operational mode before the clutch is teared up at a defined speed. For applications with fast recurring shifting operations, like in limited Aus Wissenschaft und Forschung 8 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 KLP-260, shown in Figure 3, is optimized for investigating wet disk clutches. The basic principle, operational modes, and technical data are described in detail by Meingassner et al. [Mei15]. The test rig operates in brake mode (see Figure 4). Therefore, the outer carrier is connected to the housing over a load cell, allowing the measurement of the friction torque T f at high accuracy. The axial force F a is provided by a hydraulic actuator and measured via a load cell; the Coefficient of Friction (CoF) is calculated based on T f and F a . The test rig offers Figure 4: Schematic sketch of the test rig KLP-260 adapted from Strobl et al. [Str22] according to Meingassner et al. [Mei15] Figure 3: KLP-260 brake shift stationary slip unsteady slip Figure 5: Operational modes with the course of axial force and differential speed according to Meingassner et al. [Mei15] TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 8 slip differentials, unsteady slip mode allows the measurement of defined slip cycles with repeated tearing up of the clutch to a specified maximum speed. The speed is reduced simultaneously with the same time ramp as the tear-up time. For special applications, these basic operational modes can be adapted within the technical limitations of the test rig. In all of these cases, the determination of CoF is crucial for evaluating the friction behavior of the wet disk clutches. Therefore, the friction torque T f and the axial force F a are measured. Together with the number of friction interfaces z and the mean friction radius r m , the calculation of the CoF is done according to Equation (1). (1) The mean friction radius r m can be calculated according to Equation (2). Under consideration of the relative error of < 1 % for the ratio r i / r a = 0.7 … 0.9, the simplified calculation of the mean friction radius shown in Equation (2) is valid. This equation is typical for common clutches. [Win85] Nevertheless, when experimental measurements of different clutch sizes and geometries are compared to each other, not only the exact calculation of the mean friction radius r m but also other influences like further local heat distribution or different manufacturing conditions should be considered. (2) Table 1 shows the primary technical data of the KLP-260. Aus Wissenschaft und Forschung 9 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 A load cell (friction torque) load cell (shifting force) load cell (axial force) tachometer adjustable flywheel hydraulic piston main drive belt drive flywheel (car inertia) B Figure 7: Schematic sketch of the working principle of SSP-180 according to Acuner [Acu16] and Neumueller [Neu01] Figure 6: SSP-180 [Sto23] Max. friction torque T f, max 2000 Nm Max. axial force F a, max 20 kN Inertia (adjusted by actuator) J S 1.0 kg∙m 2 Inertia (manually adjusted) J a 0.1 … 0.75 kg∙m 2 Max. differential speed (shift operation) Δn max 7000 rpm Max. differential speed (slip operation) Δn max 140 rpm Oil temperature ϑ oil 30 … 150 °C Outside disk diameter d o 75 … 260 mm Table 1: Technical data of the test rig KLP-260 SSP-180, shown in Figure 6, is optimized for testing genuine synchronizers of different sizes. For effectiveness reasons, it is designed for the testing of two synchronizers at once. It is described in detail by Hoehn and Pflaum [Höh91]. Tested parts are named A-side and B-side (see principle at Figure 7). On the A-side, the test rig measures the friction torque T R and axial force F a , which allows the cal- TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 9 sented component test rigs KLP-260 and SSP-180 to evaluate the spontaneous damage behavior of wet disk clutches with carbon friction lining and synchronizers with molybdenum friction lining. Friction behavior and its change over time are the focus of many investigations, giving information about NVH, function, and torque transmission behavior. In Figure 8 the measurement of a brake shift with a paper-based friction lining (left) and one with a sinter-metallic friction lining (right) are shown. First, the shifting behavior of the friction system with paper-based friction lining is discussed. Initially, inner plates rotate at a defined speed n and are lubricated with oil at a specified temperature ϑ oil , which is measured at the oil inlet. The differential speed n is reduced due to the applied axial force F a , resulting in a friction torque T fr and a rise of the clutch temperature ϑ clutch . This temperature is measured inside one outer steel plate by a Ni-Cr-Ni thermocouple. The CoF µ allows the characterization of the friction behavior. Generally speaking, the paper-based friction system and the sinter-metallic friction lining represent the typical friction brake shift behavior of the underlying friction system. Nevertheless, the shown brake shifts cannot be compared directly due to the use of different genuine clutch sizes, lubricants, and slightly different test conditions. The friction system with the paper-based friction lining uses a typical automatic transmission fluid, whereas the sinter-metallic friction uses a -typical lubricant for industrial application. In the case of the friction system with paper-based friction lining, a positive slope of the CoF over sliding velocity indicates good NVH behavior with low shudder tendency (e.g., [Mos02]). The fluctuations of the CoF indicate thickness deviations (e.g., [Mei17a]). Among others, typical characteristic CoF values like µ avg for evaluating the CoF level or µ2/ µ5 for evaluating the Aus Wissenschaft und Forschung 10 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 culation of the CoF at high accuracy. The A-side is blocked with the housing, whereas the B-side is connected to the flywheel, representing the vehicle’s inertia. A hydraulic cylinder providing the shifting force F S realizes shifting from A-to B-side and vice versa. A tachometer enables the measurement of the differential speed Δn measured on the shaft connecting the sliding sleeve and the test inertia. Oil is provided from above. Compared to the calculation of the CoF of wet disk clutches in(1), the analysis of the CoF for synchronizers now considers the average cone angle α m . (3) CoF= T ∙ sin α m F a ∙ z ∙ r m Max. friction torque T R,max 400 Nm Max. axial force F a,max 4 kN Inertia (car) J 0 1.75 kg∙m 2 Inertia (synchronized) J K 0.01 … 0.4 kg∙m 2 Max. differential speed Δn max 3000 (5000) rpm Oil temperature ϑ oil 30 … 150 °C Table 2: Technical Data of the test rig SSP-180 According to Baumgartner [Bau20], the uncertainty of measurement is also applied to the test rig SSP-180 in the dissertation of Stockinger [Sto23]. For a probability of 95.45 %, the relative uncertainty of the CoF can be derived for typical circumstances of a dual cone synchronizer as approximately 2.9 %. Exemplarily Results First, the possibilities of measurements and evaluations with exemplarily measured signals of different friction systems are described to give a general overview. After that, novel test procedures are introduced using the pre- Figure 8: Measured signals during an exemplary brake shift with paper-based friction lining (left) and sintermetallic friction lining (right) TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 10 shudder tendency can be obtained. The value µ avg is the arithmetic mean of CoF in the range 0 % to 60 % of the maximum sliding velocity v s,max for brake shifts [Hen14]. The value µ2/ µ5 [Häm95, Hen14] describes the ratio of the CoF at 50 % of the maximum sliding velocity v s,max and the maximum of the CoF between 0 % and 10 % of the maximum sliding velocity vs,max. Conversely, the friction system with sinter-metallic friction lining shows a steep rise of the CoF over sliding velocity at lower sliding speeds. This increase, known as “rooster tail,” indicates bad NVH behavior. The negative slope may lead to frictional vibrations. Figure 9 shows the CoF over time for a stationary slip at different sliding velocities vs for the friction system with sinter-metallic friction lining of Figure 8 (right). A drop of the CoF is visible during the unsteady begin and end of the slip phase. For two sliding velocities vs this leads to the occurrence of shudder. In steady slip mode, the friction behavior is typically characterized using the average coefficient of friction over a defined slip duration. Therefore, the characteristic CoF value µ stat is used. Regularly, the transient slip phase at the beginning and end of the slip cycle is not subject to investigation. The unsteady slip mode is preferred for investigating transient slip phases. In this operational mode, the clutch is torn to a defined maximum sliding velocity and decelerated back to standstill. One cycle can consist of multiple slip phases. Figure 10 exemplarily shows one measurement with five slip phases. Here, typical characteristic CoF values are µ top [Voe20a], which is the CoF at maximum sliding velocity v s,max and m µ [Voe20a], which describes the gradient of the CoF over sliding velocity. The presented operational modes allow application-relevant testing for a big variety of drivetrains. By these tests detailed insight into the functional behavior of the underlying friction systems is obtained. The clutch can undergo different collectives or repetitions to investigate the running-in, lifetime, wear, or long-term damaging behavior. Depending on the operational mode, the change of characteristic CoF values gives information about the changes in the friction behavior of wet disk clutches. Therefore, trend plots to analyze changes in the friction behavior are used. One example is shown in Figure 11. Measuring the position of the hydraulic piston is also done to measure wear during the run. Scientific test procedures are specified with respect to failure mode and used friction material. Therefore, two application-relevant test procedures are used to investigate the spontaneous damage behavior of wet disk clutches with carbon friction lining and the occurrence of clashing of synchronizers with molybdenum friction lining. Both procedures are well-proven in characterizing shifting elements due to spontaneous damage. Aus Wissenschaft und Forschung 11 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 v s ↑ v s / m∙s -1 μ / time in s . . . . . . . . . . . . . . . Figure 10: Measured signals in unsteady slip mode Figure 9: CoF over time in stationary slip for a friction system with high shudder tendency at different sliding velocities (sinter-metallic friction lining) TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 11 clutch in terms of friction behavior quantified by µ top or m µ . After each reference load stage, the clutch is again dismounted, inspected, and documented. The number of slip phases is then increased for every following test load stage until the function of the clutch is no longer guaranteed due to detachment of the friction lining pads or substantial buckling of the clutch. This marks the end of test (EoT). The procedure is illustrated in Figure 12. Exemplarily, the test procedure is applied for two friction systems with different friction linings. The test conditions are chosen, such as both systems failing at load stage 4. Figure 13 shows graphical documentation of the steel and lining plate from new to damaged condition. In this case, the measured mass temperature of one middle steel during the corresponding test load stage is shown. The clutches show a color change during the operation in increasing load stages. After load stage 3, the steel Aus Wissenschaft und Forschung 12 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 Test procedure for spontaneous damage of clutches with carbon lining The following test procedure under unsteady slip operation to investigate the spontaneous damage of wet disk clutches from limited slip differentials is well proven. Each run is defined by a set of oil inlet temperature, specific surface pressure and maximum sliding velocity. At the start of test (SoT), the clutch follows 200 cycles of the run-in collective proposed by Voelkel [Voe20a], consisting of six load stages of unsteady slip with five slip phases. After that, the clutch is dismounted, inspected, and documented. Then, the clutch is mounted again, and the primary investigation starts. Therefore, the clutch follows ten cycles at the test load stage with one slip phase, and after that, ten cycles at a reference load stage follow. On the one side, in the test load stage, the clutch is operated to damage it, and the reference load stage aims to see changes in the functional behavior of the Figure 11: Trend plot of running-in of a friction system with sinter-metallic friction lining Figure 12: Test procedure for the testing of spontaneous damage of wet disk clutches with carbon friction lining TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 12 plate of the upper clutch shows regions with a substantial color change. In this case, the steel plate indicated significant buckling, which ended the test run. In the second example, the woven carbon pads detached after load stage 4. The steel plate also showed a substantial color change. The measured steel plate temperature for both instances rises with each load stage. By repeating this procedure for different test conditions, it is possible to determine operation limits based on the maximum friction work and friction power for each friction system itself. Test procedure for spontaneous damage of synchronizers with molybdenum lining For synchronizers, the blocking condition strongly depends on the frictional behavior [Sta14]. If the CoF falls below a specific value, the blocking requirement is not fulfilled anymore, leading to system failure in the form of clashing. For molybdenum friction lining, scuffing is a typical damaging mode. Figure 14 shows a procedure for the application-relevant testing of synchronizers with molybdenum friction lining. The synchronizers are new at the start of the test (SoT). Run-in guarantees the generation of a tribological boundary layer. Therefore, 950 cycles are run in moderate load (p = 1.5 N·mm -2 , vs = 5.2 m·s -1 ). After that, the main procedure starts. In this procedure, reference and test load stages are alternated. As the reference load stage, 50 cycles with application-relevant loads (p = 2,4 N mm -2 , v s = 5,2 m·s -1 ) are used. After each reference load stage, 950 cycles of the test load stage are run. Here, sliding velocity is set to 10.4 m s -1 in combination with a specific surface pressure p that is increased in every test load stage sequence according to the values shown in Figure 14. If clashing occurs, the test is completed - early end of the test (eEoT). If no clashing occurs until load stage 11 the test is finished regularly (rEoT) by1000 cycles of the reference load stage. The whole test procedure is depicted in Figure 14. Aus Wissenschaft und Forschung 13 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 Figure 13: Change of the friction partners using the test procedure in Figure 12 with measured steel-plate mass temperature Run-In Load Stage 950 cycles p = 1,5 N∙mm -2 v s = 5,2 m∙s -1 Reference Load Stage 50 cycles p = 2,4 N∙mm -2 v s = 5,2 m∙s -1 Test Load Stage i 950 cycles p = acc. to LS i v s = 10,4 m∙s -1 p / N∙mm -2 for each LS i = 1: 1,0 2: 1,2 3: 1,4 4: 1,6 5: 1,8 6: 1,9 7: 2,1 8: 2,3 9: 2,5 10: 2,7 11: 2,9 Clashing SoT eEoT rEoT yes no Reference Load Stage 1000 cycles p = 2,4 N∙mm -2 v s = 5,2 m∙s -1 i < 12 yes no i = i+1 i = 1 Figure 14: Test procedure for the testing of spontaneous damage of synchronizers with molybdenum friction lining TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 13 rEoT. Variation of eEoT load stages of candidate underlines the necessity for sufficient number of tests for statistical validation of the result on spontaneous damaging behavior. Current Research Outlook Due to the progressing trend of the electrification of automotive drivetrains, the research regarding synchronizations is currently decreasing. Nevertheless, wet shifting elements are widely used in all drivetrains that fulfill safety-relevant tasks. Simultaneously, lightweight and cost-effective drivetrains with adequate reliability require detailed knowledge of the underlying friction behavior. Therefore, ongoing research deals with the influences on the friction behavior of wet disk clutches (e.g., [Str23a]). Here, the transition from static to dynamic friction is also crucial for the reliability of wet disk clutches. But also, a sound understanding of the influence of the oil damage through water or iron on the frictional and damaging be- Aus Wissenschaft und Forschung 14 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0035 Due to heavy smoothing, scuffing changes the surfaces of the molybdenum friction layer so that the coefficient of friction falls below the coefficient of friction required for the blocking condition, resulting in failure due to clashing. In this test procedure, the occurrence of scuffing is interpreted as an indicator of an imminent failure of the synchronizer but is not interpreted as a failure itself. Scuffing can be observed by a substantial and sudden increase of the friction torque or coefficient of friction due to the spontaneous, local welding of the friction partners (see Figure 15). The presented method allows the objective evaluation of the performance of synchronizers with molybdenum friction lining. It is adaptable for application-relevant loads and operating conditions. Figure 16 shows the incremental sum of cycles showing scuffing as a percentage of cycles per load stage. In this case, candidate investigations show clashing leading to an early end of the test in different load stages while reference runs up to 0 100 80 60 40 20 F A / % of max. F A F A / % of max. F A regular shifting occurrence of scuffing sleeve position sleeve position 0 100 80 60 40 20 μ / μ / - Figure 15: Comparison of regular shift (left) and a shift with the occurrence of scuffing (right) Figure 16: Incremental sum of cycles showing scuffing for reference and candidate investigations with early failure of candidates havior could be an essential step. Generally, a general criterion of damage is yet to be defined. Modern electric drivetrains tend to have only one speed. Nevertheless, multi-speed drivetrains promise advantages regarding power density [Sch22c]. In this context, wet shifting elements could guarantee smooth and reliable shifting. The compatibility of lubricants for e-drives with wet shifting elements has yet to be the object of public research. The experimental data can be used as input for several tools to design drivetrains like KUPSIM [Woh12] or SYNTEM [Win05], which enable the calculation of the thermal budget of wet disk clutches or synchronizers. But also multibody simulations [Mei17b, Str21] or thermo-mechanical simulations [Sch22a] require the CoF as input parameters. These tools support the design of modern drivetrains. TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 14 Conversely, innovation regarding AI and machine learning improves the evaluation methods substantially. Up to now, a series of measurements aim for a specific objective. This leads to a good understanding of individual influences on the functional or damaging behavior. Therefore, AI and machine learning could support a more general knowledge of tribological processes and a better recognition of patterns over the boundaries of specific series of measurements. It is considered that a large, high-quality experimental data basis is required. For this, it is beneficial using component test rigs like the presented KLP-260 and SSP-180 to evaluate different friction systems with application-relevant parts. In this context, all test procedure data (run-in, main load stages, and reference load stages) should be considered and structured in a suitable database to understand complex relationships using AI. Combined with real-application sensor data in context with condition monitoring of the drivetrains, this could support predictive maintenance and extend the operating time of clutches. This impacts the sustainability of modern drivetrains. Conclusion Modern drive systems aim for higher power density and reliability standards, and they strive to meet stricter NVH standards, leading to increased requirements for the shifting elements. Current developments in the electrification of drivetrains will result in additional requirements like compatibility with e-drive fluids. This might especially be necessary for multi-speed e-gearboxes, which will be relevant for commercial vehicles or highly powered cars. Innovative shifting elements like synchroclutches combine the advantages of clutches and synchronizers and might be helpful for distinctive challenges. For product development and quality assurance of shifting elements, component test rigs like KLP-260 and SSP-180 are essential tools for fast and reliable determination of product performance. Due to their cost-effective design and operation combined with good transferability of the results to the drivetrains in practical application, component test rigs support engineers in developing modern drive systems. The presented test setup and exemplary procedures allow the objective evaluation of the performance and reliability of wet shifting elements. These procedures can be adapted to application-relevant needs and provide a solid basis for implementing new testing procedures. Abbreviations and Symbols α m ° cone angle CoF - Coefficient of Friction d o mm outer clutch diameter eEoT early end of test EOT end of test F a N axial force J kg·m 2 Inertia LS Load Stage µ 2 - CoF at 50 % v s,max µ 5 max. CoF at 0-10 % v s,max µ avg average CoF (0-60 % v s,max ) µ stat average, stationary CoF µ top - CoF at v s,max m µ ms·m -1 friction gradient n rpm speed NVH Noise, Vibration, Harshness p N·mm -2 specific surface pressure rEoT regular end of test r i mm inner friction radius r o mm outer friction radius r m mm mean friction radius SoT start of test ϑ °C temperature T R ,T f Nm friction torque v s m·s -1 sliding velocity z number of friction interfaces References [Acu16] Acuner, R.: Synchronisierungen mit Carbon-Reibwerkstoffen unter hohen und extremen Beanspruchungen, Dissertation, Technische Universität München (2016). 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