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| Know Your Car : Automatic Transmission |
When you take apart and look inside an automatic transmission, you find an amazing assortment of parts in a fairly small space. Among other things you see :
- An extremely ingenious planetary gearset
- A set of bands that lock parts of a gearset
- A set of 3 wet-plate clutches to lock other parts of the gearset
- An incredibly odd hydraulic control 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 cantelope, this one part creates all the different gear ratios that the transmission can produce. Everything else in the transmission is there to help the planetary gearset do its job. An automatic transmission contains two complete planetary gearsets folded together into one component.
From L to R : ring gear, planet carrier, and two sun gears
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.
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.
| |
Input |
Output |
Stationary |
Calculation |
Ratio |
| A |
Sun (S) |
Planet Carrier (C) |
Ring (R) |
1 + R/S |
3.4 : 1 |
| B |
Planet Carrier (C) |
Ring (R) |
Sun (S) |
1 / (1 + S/R) |
0.71 : 1 |
| C |
Sun (S) |
Ring (R) |
Planet Carrier (C) |
- R/S |
-2.4 : 1 |
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 extracted of this planetary gear set, but these are the ones that are relevant to our automatic transmission.
Different gear ratios can be generated out of these
So this one set of gears can produce all 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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