Rack and pinion gears are used to convert rotation into linear motion. A perfect example of this is the steering system on many cars. The tyre rotates a gear which engages the rack. As the gear turns, it slides the rack either to the proper 
or left, based on which way you change the wheel.
Rack and pinion gears are also found in some scales to carefully turn the dial that displays your weight.
Planetary Gearsets & Gear Ratios
Any planetary gearset has 3 main components:
The sun gear
The planet gears and the earth gears’ carrier
The ring gear
Each of these three parts can be the input, the output or can be held stationary. Choosing which piece plays which part determines the gear ratio for the gearset. Let’s have a look at a single planetary gearset.
Among the planetary gearsets from our transmitting includes a ring gear with 72 tooth and a sun gear with 30 teeth. We can get lots of different equipment 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 kind of two of the three components together will secure the complete device at a 1:1 gear reduction. Observe that the first gear ratio in the above list is a decrease — the output speed is slower compared to the input acceleration. The second is an overdrive — the output speed is faster than the input rate. The last is usually a reduction again, but the output direction is certainly reversed. There are several other ratios that can be gotten out of this planetary gear set, but these are the ones that are highly relevant to our automatic transmission.
So this one set of gears can make most of these different equipment ratios without having to engage or disengage any kind of other gears. With two of the gearsets in a row, we can get the four ahead gears and one invert gear our transmission needs. We’ll put the two sets of gears jointly in the next section.
On an involute profile equipment tooth, the contact point starts closer to one gear, and as the gear spins, the contact point moves away from that equipment and toward the other. If you were to follow the contact point, it could describe a straight range that starts near one gear and ends up close to the other. This means that the radius of the get in touch with point gets larger 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 one’s teeth first start to engage, the very best gear tooth contacts underneath gear tooth inside the pitch diameter. But observe that the area of the top equipment tooth that contacts underneath gear tooth is quite skinny at this stage. As the gears switch, the contact point slides up onto the thicker area of the top gear tooth. This pushes the top gear ahead, so it compensates for the Liquid-ring Vacuum Pumps somewhat smaller contact size. As the teeth continue steadily to rotate, the get in touch with point moves even further away, going outside the pitch diameter — but the profile of underneath tooth compensates because of this movement. The get in touch with point starts to slide onto the skinny area of the bottom level tooth, subtracting a small amount of velocity from the top gear to pay for the increased size of contact. The outcome is that even though the contact point size changes continually, the velocity remains the same. So an involute profile equipment tooth produces a constant ratio of rotational swiftness.