24th International Colloquium Tribology - January 2024 119 Influence of the Steel Disk on the NVH Behavior of Industrial Wet Disk Clutches Patrick Strobl 1* , Katharina Voelkel 1 , Thomas Schneider 1 , Karsten Stahl 1 1 Technical University of Munich, School of Engineering and Design, Department of Mechanical Engineering, Gear Research Center (FZG), Garching near Munich, Germany * Corresponding author: patrick.strobl@tum.de 1. Introduction Wet disk clutches are critical components of, e.g., modern industrial, maritime, and heavy-duty drivetrains. In these applications, friction systems with sinter-metallic friction linings are standard due to their high robustness and their comparatively low production costs. Nevertheless, these friction systems tend to be more critical regarding NVH due to their characteristic friction behavior. An increase of the Coefficient of Friction (CoF) towards lower sliding velocities is the reason for this phenomenon. Since this friction behavior cannot be simulated yet, it requires accurate experimental measurement. Previous investigations mainly focus on the influence of the friction lining and the lubricant on friction behavior. On the other hand, there is still a lack of knowledge regarding the influence of the steel disk on friction behavior. In our investigation, the friction behavior of industrial clutch systems at application-relevant operational modes is analyzed through experimental studies with three different steel disks in combination with serial friction linings and lubricants. The investigations show differences in the occurrence of shudder depending on the tested steel disk variant. To better understand the observed behavior, the surface roughness of the steel and friction disks is determined. These measurements show different reductions in the surface roughness of steel and friction disks depending on the underlying friction system. The methods are adapted from investigations with paper-based friction systems [1] for the application-relevant needs of industrial clutches. The results give valuable insights regarding the influence of steel disk finishing, which is essential for clutch disk manufacturers for both cost-efficient and functionally relevant surface finishing of the steel disks. 2. Materials and Methods To investigate friction behavior, experimental investigations on well-proven test rigs KLP260 and LK-3 are performed in different operational modes. Among other operating modes, KLP-260 allows the experimental testing of wet disk clutches in brake shift and slip mode. These modes are relevant for industrial clutches and brakes. On the other side, LK-3 allows the measurement of the friction behavior at the transition from static to dynamic friction, according to Voelkel, Meingassner et al. [2]. To characterize the surface of the steel disk and the friction disk, optical measurements of the 3D surface topography with the principle of focus variation are performed. Therefore, the measuring device Alicona-Infinite-Focus-G4 is used. This guarantees reasonable insights into the surface topography of the friction surfaces. For our investigations, steel disks with three surface finishes are tested (beltground (bg), belt-ground and nitrocarburized (bg + nc), and cross-ground (cg)). Figure-1: Investigated steel disk surfaces before run-in (750-x-750-µm, objective lens with 50x magnification) These steel disks are paired with two genuine friction materials and two application-relevant lubricants (one typical industrial lubricant and one bio-degradable lubricant). Figure-2: Investigated friction disks ( ⌀ d m -=-162-mm) 3. Results Besides the influence of the system components, the operating conditions influence the friction behavior of wet clutches. Oil temperature’s effect on CoF is exemplified in Figure-3 for low speeds. With low oil temperatures, the slope of CoF over sliding velocity gets more negative. Figure-3: CoF over sliding velocity at stationary slip for three oil temperatures of one friction system (cg) 120 24th International Colloquium Tribology - January 2024 Influence of the Steel Disk on the NVH Behavior of Industrial Wet Disk Clutches Besides this, also the influence of the steel surface on the friction shown in Figure-4 is strong. In this context, a negative slope of the CoF affects the NVH behavior of the clutch system negatively. Figure-4: CoF over sliding velocity at stationary slip with different steel disks (bg/ bg + nc/ cg) The brake shift investigations confirm this steel disk’s influence even clearer for some friction systems. Figure-5 shows a strong influence of the steel surface on the friction behavior at brake shifts. Figure-5: CoF over sliding velocity of a brake shift with different steel disks (bg/ bg + nc/ cg) According to Figure- 6, the steel disks show significant smoothing due to the run-in procedure compared to the new surfaces shown in Figure- 1. After the run-in, all variants showed a directional surface structure in the sliding direction. In the case of the belt-ground surface, almost no recognizable structure of the initial surface texture is left. The initial surface structure is still identifiable after run-in in the case of additionally nitrocarburized steel surfaces. In contrast, the cross-ground surfaces only show deep surface characteristics from the initial texture. Figure-6: Investigated steel disk surfaces after run-in (750-x-750-µm, objective lens with 50x magnification) 4. Summary In this study, the steel disk for three sinter-metallic friction systems is varied. Low oil inlet temperature and low specific surface pressure leads to a higher shudder tendency for all variants. Compared to belt-ground steel disks with or without nitrocarburization, cross-ground steel disks show increased shudder tendency. At the same time, these variants show the highest roughness in the initial state but the strongest smoothing of the steel surface and the sinter-metallic friction lining. 5. Conclusions The choice of the steel disk can affect the NVH behavior of wet disk clutches. In this context, shudder is observed combined with a cross-ground steel disk. To characterize these influences, the steel friction surface and the friction lining surface is analyzed. Due to higher steel surface roughness in the new state, the smoothing of the friction material is influenced. In this context, the static Coefficient of Friction is higher for systems that show negative NVH behavior. Although the static Coefficient of Friction is lower than the corresponding sliding coefficient of friction, this might indicate an influence on the NVH behavior. This study supports a better understanding of the influence of the surface treatment of the steel disks on the friction behavior of wet disk clutches. Nevertheless, the choice of the surface treatment should not only be decided regarding the frictional behavior but should also consider, e.g., the wear and damaging behavior. Therefore, a combined investigation of these factors for future investigations is proposed. Literature [1] Strobl, P., Schneider, T., Voelkel, K., and Stahl, K. 2023. Influence of the steel plate on the friction behavior of automotive wet disk clutches. Forschung im Ingenieurwesen/ Engineering Research 87, 2, 541-554. [2] Voelkel, K., Meingassner, G. J., Pflaum, H., and Stahl, K. 2021. Friction behavior of wet multi-plate disk clutches at the transition from static to dynamic friction. Forschung im Ingenieurwesen/ Engineering Research. Acknowledgment The presented results are based on the research project FVA no. 343/ V; self-financed by the Research Association for Drive Technology e. V. (FVA). The authors would like to express thanks for the sponsorship and support received from the FVA and the members of the project committee.