Automatic Gear shifting

Almost all automatic transmissions use a pair of gear groups called epicyclic, or planetary gears. Each group consists of; an outside "ring" gear, a shared "sun" gear in the center, and a set of "planet gears", which mesh in between the sun and the ring gear. Planet gears are so named because each one turns on its own axis as they orbit the sun gear, like planets do. Each group of planet gears is held in a "planet gear carrier". By clamping the ring gears, the sun gear, and the carriers together in various combinations, and by locking some of them in stationary positions, it is possible to achieve three forward gear ratios, and reverse as well. Single epicyclic gear sets work like this: To increase torque: When the ring gear is stopped, and the power is applied to the sun gear, the planet gears are forced to go around the sun gear. This makes the pinion gears revolve more slowly around the inside gears, and drive from the carrier will have lower speed and increased torque. To reverse the torque's direction: If the planet gear carrier is stopped, and torque is applied to the sun gear, the planet gears are forced to turn by the sun gear. This makes the ring gear revolve, but more slowly than the ring gear, which increases the torque, and in the opposite direction as the sun gear, giving reverse. If two members of the gear set are locked together, planetary action is stopped and the gear set turns as one unit. When this happens, there are no increases or decreases in torque transmission. In order to have more than 2 forward speeds, two sets of epicyclic gears are needed. By changing the number of teeth (size) of one set of planetary gears, 4 forward speeds can be produced. The clutches within the transmission are used to connect the input torque, and the brake bands are used to lock the sun gear or the rear planet carrier. One way bearings serve to allow power flow in certain directions only, working as clutches. All of the clutches and brake bands are powered by hydraulic pressure, and regulated by the logic circuit which is connected to the governor and/or directly to a computer-controlled valve assembly. The transmission senses gas pedal position and drive selector position, and engages the proper clutches and bands for you to "Get out of Dodge". The details of automatic transmission functions are vast, and different designs are introduced by the many automakers with great regularity. Some common principles shared by virtually all automatics are: fluid clutches, brake bands, one way bearings (one way clutches), and epicyclic gears. This crazy diagram is a simplified version of but one design among many, and if you think it's hard to understand, don't feel bad. It is! In this type of transmission, to give first gear, the forward drive clutch (C) locks the turbine shaft to the front ring gear. At the same time, the second planet carrier brake band (D) locks the rear planet carrier in place. The power from the turbine shaft flows through the front ring gear, which turns the front carrier, which turns the sun gear. This reduces the RPM's and increases torque one time. The second reduction/multiplication happens when the sun gear turns the rear planet gears, each of which rotate within their stationary carrier. This causes the second ring gear to turn. The second ring gear transfers its torque to the output shaft through the second one-way clutch. (Does your brain hurt yet?) Second gear is accomplished by engaging the sun gear brake band (B) and the forward-drive clutch (C). This gives one reduction in RPM. Third gear (Drive) is engaged by locking the reverse-high clutch (A) and the forward-drive clutch (C). This gives a 1:1 (direct) ratio between the input and output shafts. When reverse is selected, the reverse-high clutch (A) and the second carrier brake band (D) are locked. This reverses the torque direction, and reduces the ratio (twice) for use in backing up the car.