# Experimental Study of Drag Torque between Wet Clutch Discs
###### Abstract  
The development of automatic gearbox control systems contributes to their increasing popularity. By raising the number of gears and sophisticated algorithms, the reduction of fuel consumption is feasible regarding traditional manual transmissions. The continuously variable transmissions (CVT) are also growing their share of the market. Wet clutches are employed in all types of automatic transmissions, but because of oil viscosity, when such a type of clutch is disengaged, the drag torque phenomenon occurs. This deteriorates the global efficiency of the transmission system. To recognize the nature of this drag torque and how it is influenced by the main parameters (distance between plates, value of oil flow rate lubricating clutch, oil temperature, rotational speed between plates), a new test stand has been designed and assembled. The value of rotational speed has been studied up to 7000 rpm which is novel in literature. Due to strongly nonlinear dependences, the achieved results can contribute to designing more efficient clutch systems due to the strongly nonlinear nature of observed phenomena. transmission efficiency; automatic transmission losses; oil share strain; wet clutch drag torque +
Footnote †: journal: Applied sciences  
These clutches are usually multidisc and compressed against each other to be able to transmit sufficient torque by friction. Frequent engaging and disengaging of the clutches, and therefore, the necessity to equalize the rotational speeds of the individual friction pairs causes the generation of heat which must be extracted. For this reason, oil is used by which these clutches are cooled. On the other hand, when the clutches are disengaged, the oil film between the rotating friction plates is sheared, resulting in the drag torque which influences the fuel economy. This problem and the accompanying phenomena are discussed in the literature. [PERSON] et al. [1] investigated losses in a wet brake centrally supplied with oil at different temperatures. It appears that at low temperatures, the influence of cavitation is significant due to the micro-hydrodynamic phenomenon, which is additionally intensified by higher fluid viscosity. Moreover, the oil film between the friction pairs breaks in some working circumstances. Comparable conclusions have been previously described by [PERSON] [2] based on his research on wet clutches that are centrally lubricated. He also noticed that the drag torque strictly depends on the relative rotational speed of discs. [PERSON] et al. [3] presented an experimental study of a single-plate wet clutch with different design factors and lubrication conditions. It was shown that for low speed, the oil totally overlays the disc surface and for high speed, the gap is filled with a mixture of oil and air. Therefore, the observed drag torque at low speeds increases linearly and slowly decreases at higher speeds. [PERSON] et al. [4] also noticed that the drag torque depends on the clutch disc's rotational speed. He observed that small air bubbles appear in the oil at low speed. As the rotational speed increased, the air bubbles also grew. The outcomes from the developed gas cavitation model are almost coincident with the experimental test results. [PERSON] and [PERSON] [5], based on their research, have made the following conclusion: the rotating discs of the disengaged wet clutch create drag torque which is changing with relative rotational speed. Three stages from the operating range of such a clutch can be distinguished: (a) low speed, the clearance between the discs is fully filled with single-phase fluid, drag torque increases linearly with increasing the relative speed; (b) medium speed, the fluid film becomes discontinuous, the clearance between the discs is filled with two-phase fluid, drag torque decreases with increasing speed; (c) high speed, the rotating discs are getting closer to each other due to the negative value of fluid pressure in the gap, drag torque increases again with increasing speed.