Rack and pinion gears are accustomed to convert rotation into linear motion. A perfect example of this is actually the steering system on many vehicles. The tyre rotates a equipment which engages the rack. As the apparatus turns, it slides the rack either to the right or left, depending on which way you change the wheel.
Rack and pinion gears are also used in some scales to turn the dial that displays your weight.
Planetary Gearsets & Gear Ratios
Any planetary gearset has 3 main components:
The sun gear
The earth gears and the earth gears' carrier
The ring gear
Each one of these three components can be the input, the output or can be held stationary. Choosing which piece has which part determines the apparatus ratio for the gearset. Let's have a look at an individual planetary gearset.
Among the planetary gearsets from our transmitting has a ring gear with 72 teeth and a sun gear with 30 tooth. We can get lots of different gear ratios out of the gearset.
Input
Output
Stationary
Calculation
Gear 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 secure the whole device at a 1:1 gear reduction. Observe that the first gear ratio listed above is a decrease -- the output rate is slower compared to the input acceleration. The second reason is an overdrive -- the result speed is faster than the input swiftness. The last is usually a reduction again, but the output path is usually reversed. There are many other ratios which can be gotten out of this planetary equipment set, but these are the types that are highly relevant to our automatic transmission.
So this one set of gears can produce most of these different gear ratios without having to engage or disengage any other gears. With two of these gearsets in a row, we are able to get the four forwards gears and one invert equipment our transmission needs. We'll put both sets of gears collectively within the next section.
On an involute profile gear tooth, the contact stage starts nearer to one equipment, and as the apparatus spins, the contact point moves from that gear and toward the other. If you were to follow the contact point, it could describe a straight series that starts near one equipment and ends up close to the other. This means that the radius of the contact point gets bigger as the teeth engage.
The pitch diameter may be the effective contact size. Since the contact diameter is not constant, the pitch diameter is really the average contact distance. As the teeth first begin to engage, the top gear tooth contacts underneath gear tooth inside the pitch size. But notice that the area of the top gear tooth that contacts underneath 
gear tooth is very skinny at this point. As the gears change, the contact point slides up onto the thicker section of the top equipment tooth. This pushes the very best gear ahead, so it compensates for the slightly smaller contact diameter. As the teeth continue to rotate, the get in touch with point moves even more away, going outside the pitch diameter -- but the profile of the bottom tooth compensates for this movement. The get in touch with point begins 
to slide onto the skinny area of the bottom tooth, subtracting a little bit of velocity from the top gear to compensate for the increased diameter of contact. The outcome is that despite the fact that the contact point diameter changes Electric Motors continually, the rate remains the same. Therefore an involute profile equipment tooth produces a constant ratio of rotational speed.