Automatic Transmission Bands
Automatic Transmission Brake Bands are flexible steel plates lined with friction material. The friction material is either organic (cellulose) or semi-metallic. The lining absorbs transmission fluid to aid in heat dissipation. As the band tightens around the drum, the fluid is squeezed out into grooves cut into the bands surface. The band brings the drum to a stop and holds it there.
The drum can be made of soft or hard metal. The softer iron type is easily distinguished by its dimpled surface. This rough surface aids in band clamping and heat dissipation. The harder drums have a smooth surface.
Bands are either single or double wrapped. Double wrapped bands have more clamping force and require less hydraulic pressure to achieve the same stopping power as a single band. As these bands wear, a gap forms between the band and the drum. This gap will cause a transmission to slip.
The servo is controlled by hydraulic pressure. This pressure is applied in one of two ways. When hydraulic pressure is applied in the same direction as drum rotation, the anchor pin adds to the force from the servo. If hydraulic pressure is applied in the opposite direction of drum rotation, the drum works against the servo's pressure.
A transmission band is considered a reaction device, in the same family as multiple clutch discs and one way overrunning clutches. There is a difference. A brake band is strictly a stopping and holding device. Clutches are used to hold and drive members of the planetary gear set.