How Gears Work

Rack and pinion gears are used to convert rotation into linear movement. A perfect example of this is the steering system on many vehicles. The steering wheel rotates a gear which engages the rack. As the gear turns, it slides the rack either to the proper or left, depending on which way you convert the wheel.

Rack and pinion gears are also found in some scales to turn the dial that presents 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 part determines the apparatus ratio for the gearset. Let’s take 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 teeth. We can get several different equipment ratios out of this gearset.

Gear Ratio
Sun (S)
Planet Carrier (C)
Ring (R)
1 + R/S
Planet Carrier (C)
Ring (R)
Sun (S)
1 / (1 + S/R)
Sun (S)
Ring (R)
Planet Carrier (C)

Also, locking any two of the three components together will secure the complete device at a 1:1 gear reduction. Notice that the first equipment ratio listed above is a decrease — the output swiftness is slower than the input acceleration. The second reason is an overdrive — the result speed is faster compared to the input rate. The last is usually a reduction again, but the output path is usually reversed. There are several other ratios that can be gotten out of the planetary equipment set, but these are the types that are relevant to our automatic transmission.

So this one set of gears can make all of these different gear ratios without needing 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 reverse equipment our transmission needs. We’ll put the two sets of gears collectively in 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 gear and toward the other. If you were to follow the contact stage, it could 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 one’s teeth engage.

The pitch diameter is the effective contact size. Since the contact diameter isn’t constant, the pitch diameter is really the common contact distance. As the teeth first begin to engage, the top gear tooth contacts the bottom 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 time. As the gears switch, the contact stage slides up onto the thicker part of the top gear tooth. This pushes the top gear ahead, so that it compensates for the somewhat smaller contact size. As the teeth continue steadily to rotate, the contact point moves even more away, going outside the pitch diameter — but the profile of underneath tooth compensates because of this movement. The contact point begins to slide onto the skinny portion of the bottom tooth, subtracting a little bit of velocity from the very best gear to pay for the increased diameter of contact. The end result is that despite the fact that the contact point diameter changes continually, the rate remains the same. Therefore an involute profile gear tooth produces a continuous ratio of rotational rate.