The Planetary Gearset

ring gear, planet carrier, sun gears
From left to right: the ring gear, planet carrier, and two sun gears

­When you take apart and look inside an automatic transmission, you find a huge assortment of parts in a fairly small space. Among other things, you see:

  • An ingenious planetary gearset
  • A set of bands to lock parts of a gearset
  • A set of three wet-plate clutches to lock other parts of the gearset
  • An incredibly odd hydraulic system that controls the clutches and bands
  • A large gear pump to move transmission fluid around

­The center of attention is the planetary gearset. About the size of a cantaloupe, this one part creates all of the different gear ratios that the transmission can produce. Everything else in the transmission is there to help the planetary gearset do its thing. This amazing piece of gearing has appeared on HowStuffWorks before. You may recognize it from the electric screwdriver article. An automatic transmission contains two complete planetary gearsets folded together into one component. See How Gear Ratios Work for an introduction to planetary gearsets.

Any planetary gearset has three main components:

  • The sun gear
  • The planet gears and the planet gears' carrier
  • The ring gear

Each of these three components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset. Let's take a look at a single planetary gearset.