# Image-Based Analysis of Plate Separation and Clearance Distribution in Wet Clutches
## 1 Introduction  
Wet clutches are essential components of modern drive systems, offering comfortable shifting, dynamic torque control, and the ability to shift even under load and differential speed. However, when disengaged, parasitic losses arise from fluid shearing in the sub-millimeter gaps between adjacent plates. This noticeably reduces the overall efficiency of the drivetrain [1, 2]. Since the 1970s, extensive research has focused on understanding and optimizing the drag loss behavior of wet clutches [3, 4]. Drag loss behavior generally refers to the change in drag torque over the differential speed. Figure 1 illustrates a typical drag torque curve and its division into three characteristic phases. The characteristics of the drag torque curve can be seen with injection and dip lubrication, despite differences in the flow's development [5, 6]. The drag torque initially increases to a peak value in Phase 1a and decreases to a low level in Phase 1b. In Phase 2, the drag torque remains relatively steady. In Phase 3, the drag torque may increase suddenly due to speed-induced phenomena [7, 8]. Previous studies investigated the effects of various design and operating parameters on drag loss behavior, revealing several ways to reduce drag losses. These include reducing the facing area [10], oil viscosity [4; 11; 12], flow rate during injection lubrication [4; 10; 11; 12], or oil level during dip lubrication [5; 11; 13], and optimizing groove design [10; 12; 14; 15]. Additionally, increasing the set total clearance or nominal clearance can significantly reduce drag losses [4; 11; 12; 13]. However, increasing the total clearance comes along with a reduced clutch reaction time and an increased likelihood of a sudden torque re-increase in Phase 3 [7]. The disengagement process, meaning the transition from boundary lubrication in the engaged state to hydrodynamic lubrication in the disengaged state, takes only a few split seconds [16]. It is generally known that rapid plate separation and uniform clearance distribution reduce drag losses [17; 18; 19]. Plate separation can be enhanced by inducing mechanical restoring forces from separation springs or waved plates [18]. Separation springs simultaneously ensure uniform clearance distribution. However, the consequences of using separation springs are increased space requirements, installation effort, and higher costs. Also, the engagement behavior is affected [20; 21]. Using waved plates also supports plate separation due to the spring-like effect during disengagement of the clutch and, thus, improves the clearance distribution [11]. The separation process can also be supported by axial hydrodynamic forces generated by the oil flow, which can be influenced by the groove design [18]. Experimental investigations conducted on a single-plate test rig with fixed clearance have shown that an asymmetrical groove edge design, e.g., increases the axial hydrodynamic force [18], which theoretically improves plate separation. A recent study [22] investigated the plate separation behavior under real-world operating conditions by applying external forces resulting from factors such as vehicle dynamics and inclination. The study found that the plates even separate under external forces, but the utilization of the set total clearance decreases as external forces increase. While the influence of plate separation and clearance distribution on drag loss behavior is generally known, it has not yet been systematically investigated for relevant operating conditions and modes. Hence, there is a lack of knowledge on which design and operating parameters support rapid plate separation and uniform clearance distribution. Figure 2 shows different clearance distribution scenarios of a clutch pack in the radial view. Once the clutch is disengaged, the plates are free to move axially within the set total clearance \(h_{\text{tot}}\) (see Figure 2a). Immediately after disengaging the clutch, the plates separate, and the gaps form.