Clutches and Bands

Automatic transmission clutches and bands are friction devices that drive or lock planetary gearsets members. They are used to cause the gearset to transfer power.

Multiple-Disc Clutch

The multiple-disc clutch is used to transmit torque by locking elements of the planetary gearsets to rotating members within the transmission. In some cases, the multiple-disc clutch is also used to lock a planetary gearset element to the transmission case so it can act as a reactionary member. The multiple-disc clutch is made up of the following components (Figure 10-20):

• Discs and plates—The active components of the multiple-disc clutch are the discs and the plates. The discs are made of steel and are faced with a friction material. They have teeth cut into their inner circumference to key them positively to the clutch hub. The plates are made of steel with no lining. They have teeth cut into their outer circumference to key them positively with the inside of a clutch drum or to the inside of the transmission case. Because the discs and plates are alternately stacked, they are locked together or released by simply squeezing them.

• Clutch drum and hub—The clutch drum holds the stack of discs and plates and is attached to the planetary gearset element that is being driven. The clutch hub attaches to the driving member and fits inside the clutch discs and plates.

• Pressure plate—The pressure plates are thick clutch plates that are placed on either end of the stack. Their purpose is to distribute the application pressure equally on the surfaces of the clutch discs and plates.

• Clutch piston—The clutch piston uses hydraulic pressure to apply the clutch. Hydraulic pressure is supplied to the clutch piston through the center of the rotating member.

• Clutch piston seals—The clutch piston seals serve to prevent the leakage of hydraulic pressure around the inner and outer circumferences of the clutch piston.

• Clutch springs—The clutch springs ensure rapid release of the clutch when hydraulic pressure to the clutch piston is released. The clutch springs may be in the form of several coil springs equally spaced around the piston or one large coil spring that fits in the center of the clutch drum. Some models use a diaphragmtype clutch spring.

The operation of the multiple-disc clutch is as follows (Figure 10-21):

Released—When the clutch is released, there is no hydraulic pressure on the clutch piston, and the clutch discs and plates are free to rotate within each other. The result is that the clutch hub rotates freely and does not drive the clutch drum.

Figure 10-21 — Multiple-disc clutch operation.

Applied—When the clutch is applied, hydraulic pressure is applied to the clutch piston that in turn applies pressure to the clutch discs and plates, causing them to lock together. The result is that the clutch hub drives the clutch drum through the clutch.