A simple (single row) planetary gear mechanism is the foundation of a transmission mechanism, and usually the transmission mechanism of an automatic transmission consists of two or more rows of planetary gear mechanisms. A simple planetary gear mechanism includes a sun gear, several planetary gears, and a gear ring, where the planetary gears are supported by a fixed shaft of the planet carrier, allowing the planetary gears to rotate on the supporting shaft. Planetary gears are always in constant mesh with adjacent sun gears and gear rings, and helical gears are usually used to improve the smoothness of work.