The construction of this type of clutch is similar to that of The dry type, except that here the clutch plates are always wetted by oil circulation. 

Many types of wet clutches are used in trucks. 

The simplest type out of this is the spray type as shown in figure. 

Its construction is similar to that of a dry clutch, except the different type of friction material is used and the construction of the clutch plate is also different. 

The oil is sprayed through holes in the clutch plate by a nozzle. 

This type of clutch is suitable only for small trucks where sufficient torque can be obtained with a single plate unit, since the value of friction Coefficient decreases due to the presence of oil on the friction surfaces. 

Thermal limitation because of the poor oil distribution to the more remote friction surfaces makes this type unsuitable for higher number of clutch plates.

Another type of wet clutch which is suitable for heavy duty trucks is shown in figure. 

It differs from the spray clutch in that a disc pack and a different method of oil supply are employed. 

An oil collector is provided with the hub of the disc pack and is supplied by oil passing between the transmission input shaft and the clutch throwout bearing support tube. 

With this arrangement, the oil is not diverted due to rotation of the clutch oil levers and even oil distribution to all the frictional surfaces is provided, thus making it suitable for heavy duty 3 or 4 plate design.

In multi plate wet clutches, the clutch brake has to be provided to overcome viscous drag and permit shifting from neutral into first or reverse when the vehicle is standing stills. 

Figure shows a multi plate wet clutch with clutch brake actuator. 

This design utilizes 'pull type' released so that the clutch brake can be applied at the end of the release stroke. 

A gear type pump is provided in the release sleeve which operates when the engine is running, to supply the oil. 

The oil is delivered through internal passages to the rim of the clutch plate hub. 

This provides even oil distribution so desirable for multi plate design. 

The oil flow path in this type of clutch has been shown in figure.

The oil requirement for the wet clutch varies considerably, depending upon the application and efficiency of the grooving pattern on the clutch plate, but generally falls within a range of 1 gallon (4.54 litres) per minute for each 10-20 HP rating of the engine.