Mathematical Model and Experimental Evaluation of Drag Torque in Disengaged Wet Clutches

When the clutch is in disengaged condition, ideally no torque should be transmitted. However, in reality, the relative motion between the disks causes viscous shearing of fluids in the gap. This results in a drag torque which is considered as a loss. The objective of the present study is to formulate a drag torque model as well as to experimentally evaluate the effect of several parameters on the drag torque. A model based on continuity and Navier-Stokes equations, considering laminar flow, is deduced. The drag torque estimated by the model is the sum of drag torque due to shearing of the automatic transmission fluid (ATF) and mist (suspension of ATF in air) film. In order to validate the model and characterize the drag torque, experiments are performed using an SAE no. 2 test setup under real conditions of variable ATF flow rate and disks' rotational states for higher clutch speed range. The drag torque predicted by the model is in good agreement with the experimental results obtained by varying the flow properties and disks' rotational states. By analyzing the experimental results, a factor by which, the variation in parameters such as ATF flow rate, ATF temperature, disk size, and disk rotational state influencing the drag torque is determined. 1. Introduction When an automatic transmission (AT) is in operation, some of the clutches are engaged to transmit power, while the rest of the clutches remain disengaged. During engagement and disengagement processes, frictional heat is generated which causes wear and deterioration of the disks. Therefore, the disks must be cooled in order to increase the service life of the clutch. For this purpose, ATF is continuously supplied to the disks, irrespective of the state of the clutch. In addition, both the friction disk (FD) and the separator disk (SD) sets are always rotating. The relative motion between the disks causes viscous shearing of fluids film in the gap between the disks. Fluids in the present study collectively refer to the ATF and the mist. This results in a drag torque on both the disks. Depending upon the configuration of the clutches in an AT, the drag torque transmitted by the open clutches is considered as a loss. This unwanted drag torque reduces efficiency and increases temperature of the transmission. The increased temperature promotes the wear of the gears, bearings, and the aging of the ATF. Currently, AT designers are concerned about how to reduce the drag torque in disengaged wet clutches in order to overcome its undesirable effects. To achieve this reduction, a good understanding of the