# Experimental Study of Drag Torque between Wet Clutch Discs
## 3 Results and Discussion  
Drag torque measured on the rig consists of the drag induced by the discs and caused by the bearings and sealing. Although, the scope of investigation covered the speed between discs ranging from 300 to 7000 rpm, four disc gaps: 0.5, 0.25, 0.15 and 0.05 mm, four oil flow rates: 0, 1, 3, 6 L/min, three oil temperatures 25, 55, 80 \({}^{\circ}\)C and tests for an extended gap of 0.75 mm were carried out additionally to determine the drag torque produced by bearings and sealing. In all cases, the air was delivered between discs at a constant pressure of 0.05 MPa apart from tests for 0.75 mm which were performed for all temperatures but without any oil and airflow. The test program is shown in Table 1. All presented results are the average of measurements repeated twice and are the net discs drag torque with subtracted torque of bearings and sealing. The measurements were conducted according to the following procedure. For each of the four gaps, three oil temperatures were investigated. At each temperature, four oil flows rates were applied and for each of them, the series of 15 measurement points were captured for different rotational speeds. The proposed concept of analysis of one pair of discs appears to be easier in implementation, compared to the multidisc clutch configuration reported elsewhere [5; 8; 9]. In multidisc conception, the layout of oil flow rates and slippage between specific disc pairs can be noticeably different which makes the research more complex. Figure 1: The scheme of the test rig: (**a**) test module; (**b**) general overview. Description in text above. Figure 2: Photos of the test bench: (**a**) view of the test module; (**b**) general view of the test rig Description in text above. The solution when one disc is stationary and the second rotates is popular in the literature [3; 6; 7], as the control of the rotational speed of one electric motor is easier and more accurate, compared to the solution with two discs rotating in opposite directions [8]. Figure 3 depicts the results for disc drag torque as a function of rotational speed for the gap of 0.5 mm, the oil flow rate of 6 L/min and 25, 55 and 85 \({}^{\circ}\)C of oil temperature. The influence of temperature is clearly seen. Due to viscosity change, the higher temperature gives the lower drag torque. At each temperature, the torque increases proportionally to the viscosity change up to about 5000 rpm. Then, between 6000 and 6500 rpm, it achieves the maximum and drops about 10% at the highest speed of 7000 rpm. [PERSON] and [PERSON] [5] explain this run by the change of oil flow mode between the discs: from one-phase at low speed to two-phase with gas bubbles at high speed. In Figure 4, the results for the same gap between the discs and oil temperature of 80 \({}^{\circ}\)C but various oil flow rates are presented. It can be noticed that the increase in the oil flow rate causes higher disc drag torque. For oil flow equal to 3 and 6 L/min, the torque runs have a clear maximum. For lower flow rates, the curves become flatter, and the position of maximum shifts to the left towards lower speed. It seems that at speeds over 6000-6500 rpm there is a lack of oil in the oil-air mixture and the space between discs is filled by the less viscous air. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Conditions** & & & & \\ \hline speed between discs & gap between plates & & & \\ (rpm) & (mm) & oil flow rate (L/min) & oil temperature (\({}^{\circ}\)C) \\ \hline
300 & & & & \\
660 & & & & \\
1000 & & & & \\
1500 & (0.75), & & & \\
2000 & 0.5, 0.25, 0.15, 0.05 & 0, 1, 3, 6 & 25, 55, 80 \\ \(\ldots\) & & & & \\
6500 & & & & \\
7000 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Test schedule. Figure 3: Influence of oil temperature on drag torque characteristics—gap between plates 0.5 mm, oil flow rate 6 L/min. In Figure 5, the results for the 0.25 mm gap are presented.