All gears can be divided into external gears or internal gears, and external gears are gears with teeth on the outer surface of the gear wheelInternal gears are gears with teeth on the inner surface of gear collars.
A spur cylindrical gear is a cylindrical gear with teeth that are perpendicular to the transverse profile or parallel to the axis of rotation. They are mainly used to transfer rotation between two parallel shafts. Internal gears are straight-toothed cylindrical gears that cut the teeth on the inside of the ring, and are often used to drive or be driven by pinions. However, the main disadvantage of spur cylindrical gears is that the thrust they generate may cause the axis of the meshing gear to not coincide, thus reducing the meshing width and mating surface.
The rack, as shown in Figure 1c, has teeth distributed on a flat surface rather than around the perimeter of the wheel. It is capable of converting circular motion into linear motion instead of circular motion. The function of a rack is similar to a gear with an infinite diameter.
A pinion is a gear with many tints that can be meshed with a rack.
A rack and pinion, as shown in Figure 1c, converts rotational motion into linear motion. When they mesh together, the rotational motion of the pinion can be converted into the reciprocating motion of the rack and vice versa. In some systems, the pinion rotates in a fixed position to drive the rack to move, a design commonly used in vehicle steering. Another way is to fix the rack and let the pinion move up and down, such as the drive mechanism of a funicular railway. Also, on locomotives, the drive gears propel the vehicle uphill by moving the rack on two tracks.