How Automatic Transmissions Work

yadavundertaker mohit
yadavundertaker mohit
@yadavundertaker-0p3HBs • Oct 26, 2024

The Planetary Gearset

*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 #-Link-Snipped-# do its thing. This amazing piece of gearing has appeared on HowStuffWorks before. You may recognize it from the #-Link-Snipped-# article. An automatic transmission contains two complete planetary gearsets folded together into one component. See #-Link-Snipped-# for an introduction to planetary gearsets.

[​IMG]
[SIZE=-1]From left to right: the ring gear, planet carrier, and two sun gears[/SIZE]
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.

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  • yadavundertaker mohit
    yadavundertaker mohit

    @yadavundertaker-0p3HBs • Sep 23, 2009

    Planetary Gearset Ratios

    One of the planetary gearsets from our transmission has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gearset.


    [FONT=arial,helvetica][SIZE=-1] Input [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Output [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Stationary [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Calculation [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Gear Ratio [/SIZE][/FONT] [FONT=arial,helvetica][SIZE=-1] A [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Sun (S) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Planet Carrier (C) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Ring (R) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] 1 + R/S [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] 3.4:1 [/SIZE][/FONT] [FONT=arial,helvetica][SIZE=-1] B [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Planet Carrier (C) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Ring (R) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Sun (S) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] 1 / (1 + S/R) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] 0.71:1 [/SIZE][/FONT] [FONT=arial,helvetica][SIZE=-1] C [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Sun (S) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Ring (R) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] Planet Carrier (C) [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] -R/S [/SIZE][/FONT][FONT=arial,helvetica][SIZE=-1] -2.4:1 [/SIZE][/FONT]
    Also, locking any two of the three components together will lock up the whole device at a 1:1 gear reduction. Notice that the first gear ratio listed above is a reduction -- the output speed is slower than the input speed. The second is an overdrive -- the output speed is faster than the input speed. The last is a reduction again, but the output direction is reversed. There are several other ratios that can be gotten out of this planetary gear set, but these are the ones that are relevant to our automatic transmission.



    So this one set of gears can produce all of these different gear ratios without having to engage or disengage any other gears. With two of these gearsets in a row, we can get the four forward gears and one reverse gear our transmission needs. We'll put the two sets of gears together in the next section.

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