What Is a Transmission Drum?

Automatic transmissions use a gearing design known as a planetary system. The central gear is the sun gear. It meshes with at least two and usually more planet gears. Surrounding the sun and planet gears is a ring gear, which has its teeth on the inside diameter, allowing it to mesh with the planet gears.

The ring gear is connected to the input shaft. The planetary gears are connected to the output shaft. The sun gear is connected to the transmission drum. The input shaft turns the ring gear and the planet gears turn the output shaft. The sun gear is mounted on the inside hollow shaft of the transmission drum. The drum is stationary, with the output shaft passing through the hollow center.

The transmission end of the output shaft of an automatic transmission is shaped something like a multi-arm anchor with a planet gear on each arm. At the end of each arm is one-half of a clutch pack. The clutch pack is a series of disks that alternate between splined steel disks and disks with a friction-generating substance bonded to the surface. The non-steel disks also have splines. The disks on the planet gear half of the clutch pack are of the non-steel type; the other half of the clutch pack, the steel disk half, is part of the inside of the transmission drum.

The transmission drum is constructed with an open seam. This allows the diameter of the drum to be controlled by means of a steel band with a friction-generating substance on the inside surface. The band is anchored at one end and connected to a hydraulic servo mechanism at the other. Tightening and loosening the band affects the engagement of the drum half of the clutch pack with the planet gear the other half of the clutch pack. In this way, an automatic transmission is able to change gears according to engine speed.