Editing Automatic transmission (section)

====Gearsets and shifting mechanism====
A hydraulic automatic transmission uses planetary gearsets instead of the manual transmission's design of gears lined up along input, output and intermediate shafts. To change gears, the hydraulic automatic uses a combination of internal clutches, friction bands or brake packs. These devices are used to lock certain gears, thus setting which gear ratio is in use at a given time.

A [[sprag clutch]] (a ratchet-like device which can [[freewheel]] and transmits torque in only one direction) is often used for routine gear shifts. The advantage of a sprag clutch is that it eliminates the sensitivity of timing a simultaneous clutch release/apply on two planetary gearsets, simply "taking up" the drivetrain load when actuated, and releasing automatically when the next gear's sprag clutch assumes the torque transfer.

The friction bands are often used for manually selected gears (such as low range or reverse) and operate on the planetary drum's circumference. Bands are not applied when the drive/overdrive range is selected, the torque being transmitted by the sprag clutches instead.