Automatic Transmission Brake Bands
A brake band consists of a flexible steel plate lined with friction material. This friction material is either organic (cellulose) or semi-metallic. The lining of a brake band absorbs the 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 the band is wrapped around is either made of soft or hard metal. The softer iron type is easily distinguished by the dimpled surface. This rough surface aids in band clamping and heat dissipation. The harder drums have a smooth surface tha tends not to wear the friction material down as quickly.
These bands are either single or double wrapped. The 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.
Some bands need adjustment; this is typical with older transmissions. One end of the band is held with an anchor pin and the other a servo piston. The anchor pin is where the adjusting screw will be found. If there is no adjustment the anchor is fastened to the case.
The servo is controlled by hydraulic pressure. This pressure is applied in one of two ways. If the pressure is applied in the same direction as drum rotation the rotating drum against the anchor pin adds to the force from the servo. If the pressure is applied in the opposite direction of drum rotation the drum works against the servos pressure requiring more force.
This band is considered a reaction device in the same family as the multiple clutch discs and one way overrunning clutches. What separates the band from the clutches is that the band is strictly a stopping and holding device. Clutches are used to hold and drive members of the planetary gear set.
