Annulus Gear

• The planetary gears intermesh with the teeth of a gear, known as an annulus or hub gear, on the inside of the gearbox casing or hub. Note that, unlike the sun gear, the annulus gear in a planetary gear chain cannot be replaced, so beware of chipped teeth in planetary gears.
  
  

Gear Ratio

• Determining the gear ratio of a planetary gear train is more difficult than other types of gear train because the centers of the planetary gears rotate around the sun gear as they turn. The combination of rotation and revolution means that the calculation must be split into two stages, with the sun gear or planetary held fixed in each stage.
  
  

Reduction Ratio

• The main advantage of a planetary gear train is a high ratio of the rotational speeds of the first and final gears in the train, otherwise known as reduction ratio. Reduction ratios between 25:1 and 100:1 are typical and can be increased still further by performing reduction in multiple stages.