Clutch units for DCT

A dual clutch unit is the first main part of a DCT transmission. The unit includes two clutches, both driven by the same engine shaft, but drive different input shafts of the gearbox. In order to save space, the input shafts of the gearbox are concentric, one is designed as a solid shaft, the other as a hollow shaft around the solid one.

There are two design principles used in dual clutch units.

Single-disc dry clutches

Dual clutch units with single disc dry clutches apply the same principle as MT / AMT clutches, there are but two clutches build together. The two clutches are arranged axially, one after the other, the first one (closer to the engine) driving the solid input shaft, the other (closer to the gearbox) driving the hollow input shaft.

A dual clutch unit with single disc dry clutches are less complex and generate less drag loss, but are less controllable and are less durable against the heat generated during gear changes. Therefore this type of clutch is applied in  passenger car transmissions only up to 200 – 250 Nm input torque.

Surprisingly, single disc dual clutches are applied also in heavy duty commercial vehicles up to 3500 Nm, since such transmissions are actuated pneumatically and there is no hydraulic circuit for a wet clutch.

Multi-disc wet clutches

For DCT transmissions designed for higher input torque, multi-disc wet clutches are applied, having two advantages over dry clutches.

On one hand, the clutch discs operate in wet environment i.e. run in transmission oil. This enables better dissipation of the heat generated during clutch slipping.

On the other hand, each clutch has a plurality of discs instead of just one clutch disc. The clutch discs are alternatingly fixed to the clutch housing, respectively to the input shaft. The clutch plate pushes the whole clutch pack against the clutch housing, which gives a higher number of friction surfaces, therefore results in higher transmittable torque.

The multi-disc wet clutches might be arranged radially or axially with respect to each other.

The radial arrangement means that there is an inner clutch and an outer clutch. The outer clutch has bigger diameter clutch discs, therefore less discs are sufficient to ensure the same torque capacity as the inner clutch.  Since the radial arrangement has a shorter installation space, this is the most popular layout.

The other option, the axial clutch arrangement is applied if the installation length is not critical, typically for longitudinally oriented transmissions. In this case, both clutches have the same diameter and are located next to each other. The advantage of the axial clutch layout is the optimized clutch cooling, because it is possible to create a separate cooling oil flow for each clutch, which reduces the cooling oil demand and at the end of the day, the fuel consumption of the vehicle.

Unlike the hydrodynamic torque converters, dual clutch units are not inherently capable of creeping. Creeping is emulated by a DCT through engaging the clutch even without pressing the accelerator pedal, However, this comes with mechanical friction and clutch wear.

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