Multiple-disc clutches can be used to hold members to the case instead of using bands but in most transmissions, they couple planetary gear members and shafts together.
A servo-operated band can only hold a member stationary, but multiple-disc clutches can hold or drive individual members.
They have a set of driving plates, and a set of driven plates, collectively called a clutch pack.
Both sets may be spring steel, but one has friction material bonded onto both faces, and since they operate in the transmission fluid they are called wet clutches.
The friction material may be plain or grooved. Grooving allows for fluid to escape from between the plates when the clutch engages.
The steel plates may be dished, or waved, with alternate high and low segments. This design promotes smooth engagement.
The size and number of plates in a clutch pack depends on the maximum torque it is required to transmit.
Three to five plates of each type are commonly used.
The plates are pressed together for engagement by a hydraulically-operated piston. It moves in a short cylinder in a clutch drum, or in a cylinder machined in the casing.
Release can be effected by one large coil spring, or by a pack containing many small springs.
When a diaphragm spring is used, it gives extra leverage to increase the apply force as well as providing the release force.
The outer circumference of the diaphragm spring rests against a step in the clutch drum. The piston pushes against the inner circumference of the spring to apply the clutch.
The force exerted on the clutch pack is multiplied by leverage from the diaphragm, as it pivots against the pressure plate.