The relative motion between the friction and separate plates in a disengaged wet clutch causes viscous drag torque when the lubrication fluid flows through the clearance. Reduction of the drag torque is one of the important potentials for the improvement of transmission efficiency. The objective of this study is to set up an experimental rig to measure drag torque for a single-plate wet clutch. Visualization of the flow pattern in the clearance through transparent quartz was presented. Design factors and lubrication conditions were tested to evaluate the effects on drag torque. A comparison between the nongrooved plate and grooved plate was made. Plates made up of different materials were also tested to reveal the effects caused by the contact angle. Drag torque increases linearly at low rotating speeds and gradually decreases at high rotating speeds. It is confirmed that fluid completely covers the plate surface at a low rotating speed and air mixes with the fluid at a high rotating speed. A low feeding flow rate is useful to reduce drag torque. The reduction of the drag torque benefits from radial and deep grooves compared to a flat plate. A small contact angle near the stationary plate plays an important role in maintaining the oil film, however, it has little effect on the drag torque at the rotating side because the hydrodynamic force becomes dominant compared to the surface tension force. The test results help to build an accurate mathematical model based on two-phase flow lubrication.

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