The external tooth gears
This is the most common type used which features a tooth cut on the outside perimeter of the mating cylindrical wheels. In its typical figure, it has a bigger wheel which is the gear and a smaller wheel which is the pinion. The simplest arrangement of spur gears is a single pair of gears called a single reduction stage. In this arrangement, the output rotation works in direct opposite of the input.
The internal ring gears
As their name suggests, internal ring spur gears have teeth that are cut on the inside surface. A set of external-tooth spur gears usually consists of three to four bigger gears or called the planets that surround the smaller pinion or the sun. The ring gear is often fixed and it is the planets that move around the sun, and thus, these gears are also hailed as the planetary system. The orbiting movement of the planets is transferred to the output shaft through a planet carrier.
The rack and pinion gears
Essentially, this class of spur gear has a straight bar with teeth cut straight across it. It is considered a special case among spur gears because the rack is unrolled and laid out fiat. It is applicable in converting rotary motion to linear movement and the other way around. The rotation of the pinion creates the linear travel of the rack. Then it is the rack’s motion that makes the pinion spin. It is often used in power shovels, lift trucks, machine tools, and other heavy-duty machines where the rotation of the pinion is needed.
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