Epicyclic Gear Trains Gear trains are used to transmit motion and/or power from one rotating shaft to another. An enormous number of gear trains are used in machinery such as automobile transmissions, machine tool gear boxes, robot manipulators, and so on. Perhaps, the earliest known application of gear trains is the South Pointing Chariot invented by the Chinese around 2600 . The chariot employed an ingenious differential gear train such that a figure mounted on top of the chariot always pointed to the south as the chariot was towed from one place to another. It is believed that the ancient Chinese. | Chapter 7 Epicyclic Gear Trains Introduction Gear trains are used to transmit motion and or power from one rotating shaft to another. An enormous number of gear trains are used in machinery such as automobile transmissions machine tool gear boxes robot manipulators and so on. Perhaps the earliest known application of gear trains is the South Pointing Chariot invented by the Chinese around 2600 . The chariot employed an ingenious differential gear train such that a figure mounted on top of the chariot always pointed to the south as the chariot was towed from one place to another. It is believed that the ancient Chinese used this device to help them navigate in the Gobi desert. Other early applications include clocks cord winding and rope laying machines steam engines etc. A historical review of gear trains from 3000 . to the 1960s can be found in Dudley 3 . A gear train is called an ordinary gear train if all the rotating shafts are mounted on a common stationary frame and a planetary gear train PGT or epicyclic gear train EGT if some gears not only rotate about their own joint axes but also revolve around some other gears. A gear that rotates about a central stationary axis is called the sun or ring gear depending on whether it is an external or internal gear and those gears whose joint axes revolve about the central axis are called the planet gears. Each meshing gear pair has a supporting link called the carrier or arm which keeps the center distance between the two meshing gears constant. Figure shows a compound planetary gear train commonly known as the Simpson gear set. The Simpson gear set consists of two basic PGTs each having a sun gear a ring gear a carrier and four planets. The two sun gears are connected to each other by a common shaft whereas the carrier of one basic PGT is connected to the ring gear of the other PGT by a spline shaft. Overall it forms a one-dof mechanism. This gear set is used in most three-speed automotive automatic .
