Most vehicles today have an automatic transmission equipped with a torque converter. A planetary gear is used in this system in order to control the speed of the transmission. In the center of a planetary gear is a part called a sun gear, or helical gear. Rotating around the sun gear are pinion gears, and around the pinion gears is ring gear.AIKOKU ALPHA’s cold-formed sun gears (helical gears) are renowned for their smooth forging surface and high tooth surface fatigue strength, due to the cold formed tooth finish (no finish operation is conducted after coldforming).