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

Abstract

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.