Multiple Disk Clutch Assembly
Automatic transmission multiple disk clutch assemblies drive and hold members of the planetary gearset. They consist of several parts, as seen in the illustration. Friction disks are splined to one member of the set, and steel plates are splined to another. The friction disks are usually made of cellulose, with grooves cut into their surface to aid in heat dissipation or semi-metallic or metallic materials. These metal friction disks wear their mating steel plates sooner than the paper disks do.
Friction disks are splined on the inside and slide onto the hub of a member of the gearset. The steel separator plates are splined on the outside and slide on the splines located inside of the drum.
The piston is moved by hydraulic pressure. The pressure plate is held in place at the opposite end of the piston with a grooved snap ring. When the piston applies pressure to the disk assembly, the clutch applies, and either drives or holds a member of the planetary gearset.
When a clutch is used as a holding device, the outer splines are fit into slots in the transmission case. When the clutch assembly is applied, the member is locked to the case. This type of clutch is considered case held, grounded, or "locked" to the case. A spring or set of springs is used to move the piston assembly to its parked position when hydraulic pressure is released.
Some residual pressure is left after the venting of the hydraulic pressure. It's released through a vent and check ball or through a controlled orifice located inside of the drum. Typically if the ball doesn't rattle around when the drum is shaken, it's stuck from tarnish. This problem results in excess pressure on the disk friction material and premature wear.
When disassembling a transmission, it's important not to mix the parts of one clutch pack with another. Always inspect and clean the assemblies after removal. Friction disks are soaked in clean transmission fluid for about 20 minutes before reassembly. Review manufacturer specifications for proper clutch pack clearances, TSB's, or special test procedures.