How Automatic Transmissions Work
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:
[SIZE=-1]From left to right: the ring gear, planet carrier, and two sun gears[/SIZE]
Any planetary gearset has three main components:
*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
[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
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yadavundertaker mohitPlanetary 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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