# Experimental Study of Drag Torque between Wet Clutch Discs
## 3 Results and Discussion
Compared to the 0.5 mm gap size, the results are similar, however, the drag torque decreases about 5% for the two highest oil flow rates (6 L/min and 3 L/min). For lower flow rates, the oil supply is not sufficient to refill the space between the discs due to centrifugal force. Thus, in the oil-air mixture, the air replaces oil and disc friction loss is lower. Figure 6 presents the results for a 0.15 mm gap between plates and the same value of oil temperature. When the oil flow rate equals 6 L/min and 3 L/min, the drag torque decreases about 50% in comparison to the results from Figure 5. The maximum value is gained at about 6000 rpm and 6500 rpm and the drop at the highest speed of 7000 rpm is still observed but only for the oil flow rate of 6 L/min. Figure 4: Influence of oil flow rate on drag torque characteristics—gap between plates 0.5 mm, oil temperature 80 \({}^{\circ}\)C.  
Figure 5: Drag torque characteristics for gap between plates 0.25 mm, oil temperature 80 \({}^{\circ}\)C and various values of oil flow rate. For 3, 1 and 0 L/min the drag torque curves are close to each other in the whole range of speed. Even for 0 L/min when there is no oil flow and only air is between the discs, the torque is the highest when the speed does not exceed 2000 rpm. Probably the speed is too low then to produce the air pad between the discs, and with such a small gap, accidental contact of discs may occur. In Figure 7, the results for the smallest gap of 0.05 mm are presented. It corresponds to the clutch engagement or disengagement point and for such a small gap between the discs, it is not possible to avoid their accidental contact. Therefore, at low and medium speeds up to 3000 rpm, the drag torque is the highest when the oil is not delivered (0 L/min). In this range, the oil is necessary for the lubrication of discs when they are in contact. For higher speed, the air forms a pad separating both discs and the drag torque decreases. It appears that 1 L/mm of the oil flow rate is sufficient for lubrication even at 7000 rpm. For higher oil flow, the drag torque rises due to oil viscosity, for 6 L/min it reaches the maximum, and then it drops a little near the highest speed like in the cases of the larger gap. Figure 6: Drag torque characteristics for gap between plates 0.15 mm, oil temperature 80 \({}^{\circ}\)C and various values of oil flow rate. Figure 7: Drag torque characteristics for gap between plates 0.05 mm, oil temperature 80 \({}^{\circ}\)C and various values of oil flow rate. All results for 0 L/min of oil flow are gathered in Figure 8. The drag torque generally decreases with the growing distance between the discs. The exception is observed for the 0.05 mm gap, near the engagement or disengagement point, when the maximum appears at 3000 rpm. Around the gap equal to 0.25 mm, the torque stabilizes and only slightly depends on the discs speed up to 7000 rpm. For 0.5 mm, the character of torque runs is similar to that for 0.25 mm, only the values are about 0.03 Nm lower. In summary, it may be concluded that in the investigated discs setup, the air without any oil flow produces the lower drag torque when the gap is at least 0.25 mm. For lower gap values, the discs touch each other temporarily and 1 L/min of oil flow rate lubricates the contact area sufficiently which improves the interaction between rotating and immovable discs.