# Image-Based Analysis of Plate Separation and Clearance Distribution in Wet Clutches
## 1 Introduction
The distance between two adjacent plates is generally referred to as the clearance \(h\). Generally, a distinction must be made between nominal clearance \(h_{\text{nom}}\) and actual clearance \(h(t)\). The theoretical case of equidistantly distributed clutch plates shows the nominal clearance \(h_{\text{nom}}\) (see Figure 2b). The nominal clearance \(h_{\text{nom}}\) is typically used to describe the clearance setting of a clutch system [4; 10; 13]. However, in field applications, the positions of the plates and, thus, the clearances \(h(t)\) usually vary continuously due to forces induced by, e.g., vibrations or acceleration. Consequently, during operation, the distribution of the set total clearance \(h_{\text{tot}}\) between the gaps is usually non-uniform and varies over time (see Figure 2c) [17]. Figure 1: Typical drag torque curve and its division into three characteristic phases [9]. Numerical simulations and experimental testing have been widely applied to investigate the drag loss behavior of wet clutches. Most of the existing CFD (computational fluid dynamics) models [23; 24; 25] are limited to only a single gap with constant clearance. Thus, those models do not allow for investigation of the plate separation process or time-dependent clearance distribution. In contrast, modeling the fluid-structure interaction of the oil and plates allows for investigation of the plate movements [26]. A mathematical model [16] which combines several sub-models (i.e., axial kinetic model, lubrication model, contact model, sliding model, thermal model, spline resistance model) allows for studying the disengagement process. It has been shown that the clearances increase depending on the prevailing lubrication stages (i.e., boundary lubrication, mixed lubrication, and hydrodynamic lubrication stage) during the disengagement process [16]. A statistical method has been developed to determine the clearance distribution by back calculation from experimental and analytical drag torques [17]. In contrast, experimental testing allows investigations on real clutch systems under application-oriented operating conditions. Axial hydrodynamic forces can be experimentally determined from pressure distribution measured with multiple pressure sensors [18]. This enables drawing conclusions on the separation behavior and optimizing operating conditions and groove design [18]. In contrast, a newly developed image-based methodology [27] allows direct measurement of the plate movements and, thus, determination of the plate separation and clearance distribution on a real clutch system. The main goal of this study was to investigate the plate separation process and the resulting clearance distribution under various operating conditions and modes. Based on this, the effects on drag loss behavior were evaluated. An image-based measurement methodology was applied to investigate plate separation and clearance distribution based on the measured plate movements. Superimposed highly dynamic movements of smaller amplitudes were not analyzed. The investigation of plate wobbling at very high speeds was also not part of this study. The findings of this research extend the knowledge of wet clutches' separation and drag loss behavior and can contribute to the development of efficient clutch systems.