Rack and pinion gears are accustomed to convert rotation into linear movement. An ideal example of this is actually the steering system on many cars. The steering wheel rotates a gear which engages the rack. As the gear turns, it slides the rack either to the right or left, based on which way you switch 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 three main components:
The sun gear
The planet gears and the earth gears' carrier
The ring gear
Each one of these three parts can be the insight, the output or could be held stationary. Choosing which piece takes on which function determines the gear ratio for the gearset. Let's check out an individual planetary gearset.
One of the planetary gearsets from our transmission includes a ring gear with 72 teeth and a sun gear with 30 teeth. We can get several 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 parts together will secure the whole device at a 1:1 gear reduction. Observe that the first equipment ratio in the above list is a decrease — the output velocity is slower compared to the input velocity. The second is an overdrive — the result speed is faster than the input acceleration. The last is certainly a reduction again, but the output path is reversed. There are many other ratios which can be gotten out of this planetary gear set, but they are the ones that are relevant to our automatic transmission.
So this one set of gears can produce most of these different equipment ratios without needing to engage or disengage any kind of other gears. With two of these gearsets in a row, we can get the four ahead gears and one invert gear our transmission requirements. We'll put both sets of gears jointly within the next section.
On an involute profile gear tooth, the contact stage starts closer to one equipment, and as the gear spins, the contact point moves from that equipment and toward the other. If you were to check out the contact stage, it would describe a straight line that begins near one gear and ends up close to the other. This implies that the radius of the contact point gets larger as the teeth engage.
The pitch diameter may be the effective contact diameter. Because the contact diameter is not constant, the pitch size is really the average contact distance. As one's teeth first start to engage, the very best gear tooth contacts the bottom gear tooth in the pitch diameter. But notice that the part of the top gear tooth that contacts the bottom gear tooth 
is quite skinny at this stage. As the gears turn, the contact point slides up onto the thicker section of the top gear tooth. This pushes the very best gear ahead, so that it compensates for the slightly smaller contact diameter. As the teeth continue to rotate, the contact point moves even more away, going beyond your pitch diameter — however the profile of the bottom tooth compensates for this movement. The contact point starts to slide onto the skinny part of the bottom level tooth
, subtracting a small amount of velocity from the top gear to compensate for the increased size of contact. The outcome is that even though the contact point diameter changes continually, the speed remains the same. Therefore an involute profile gear tooth Taper Pulleys produces a continuous ratio of rotational acceleration.