Rack and pinion gears are used to convert rotation into linear motion. An ideal example of this is the steering program on many vehicles. The steering wheel rotates a gear which engages the rack. As the apparatus turns, it slides the rack either to the right or left, depending on which way you switch the wheel.
Rack 
and pinion gears are also found in some scales to turn the dial that displays your weight.
Planetary Gearsets & Gear Ratios
Any planetary gearset has three main components:
The sun gear
The earth gears and the planet 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 takes on which role determines the gear ratio for the gearset. Let’s check out a single planetary gearset.
One of the planetary gearsets from our transmission includes a ring gear with 72 tooth and a sun gear with 30 teeth. We can get lots of different equipment ratios out of this 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 kind of two of the three components together will lock up the complete device at a 1:1 gear reduction. Observe that the first equipment ratio listed above is a reduction — the output velocity is slower compared to the input speed. The second is an overdrive — the result speed is faster than the input speed. The last is normally a reduction again, but the output path is certainly reversed. There are many other ratios which 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 make most of these different equipment ratios without needing to engage or disengage any other gears. With two of these gearsets in a row, we can get the four forwards gears and one invert equipment our transmission requirements. We’ll put both sets of gears collectively within the next section.
On an involute profile gear tooth, the contact point starts nearer to one gear, and as the gear spins, the contact stage moves away from that gear and toward the other. If you were to follow the contact stage, it would describe a straight collection that begins near one gear and ends up close to the other. This implies that the radius of the get in touch with point gets bigger as the teeth engage.
The pitch diameter may be the effective contact diameter. Since the contact diameter isn’t constant, the pitch diameter is really the common contact distance. As one’s teeth first start to engage, the very best gear tooth contacts the bottom gear tooth within the pitch size. But notice that the part of the top equipment tooth that contacts the 
bottom gear tooth is very skinny at this time. As the gears turn, the contact stage slides up onto the thicker portion of the top equipment tooth. This pushes the very best gear ahead, so it compensates for the somewhat smaller contact size. As the teeth continue steadily to rotate, the contact point moves even more away, going beyond your pitch diameter — however the profile of underneath tooth compensates for this movement. The get in touch with point starts to slide onto the skinny area of the bottom level tooth, subtracting a little bit of velocity from the very best gear to pay for the increased diameter of contact. The outcome is that despite the fact that the contact point diameter changes Cast Iron continually, the speed remains the same. Therefore an involute profile gear tooth produces a continuous ratio of rotational acceleration.