The epicyclic gearbox
While the actual automatic gearbox unit is quite daunting, the principles upon which it operates are also fairly simple. The heart of the unit is the epicyclic gear set, which creates two inputs and one output. At least several of these gear sets are installed within a typical automatic transmission. Named after ancient astronomy theory, a so-called sun gear is mounted in the centre and meshing with its outer teeth is, usually, a trio of planetary gears that are supported by a planetary carrier frame. The ring gear (or annulus) sits on the outer edge and its internal teeth run on those of the planetary gears. Thus, the planetary carrier acts as the output and interacts with both the sun and ring gears as the inputs (see illustration below). Theoretically, any one of these three gears can be locked, which allows the others to handle the output. By having several interlinked epicyclic gear sets and a set of brakes, or clutches, a gearbox can be designed with a number of different ratios.
The clutch consists of a series of friction plates, each of which bears against metal plates (called ‘steels’) and this arrangement provides a large working surface area. The friction plates and their steels are housed within a clutch drum that contains a piston, activated by oil pressure, which squeezes the clutch pack together, locking the relevant component and permitting it to turn. Older transmissions tended to use adjustable brake bands, which are wrapped around the ring gear, literally holding it in position. In most cases, modern wet clutch plates have replaced them. The oil pressure is provided by the valve body, which receives commands from the transmission control module (TCM), based on feedback from the various sensors on the transmission and vehicle.