What are the types and characteristics of gears?

Spur cylindrical gears (straight gears): the tooth line is parallel to the axis.

　　Spur cylindrical gears (straight‑tooth gears): the tooth profile is parallel to the axis. Characteristics: simple structure, easy manufacturing, and low cost; however, during meshing, the entire tooth width is in contact at once, resulting in high impact and noise, poor smoothness, and suitability for low-speed, light‑load applications.

　　Helical cylindrical gears: their tooth lines are helical. Characteristics: high contact ratio, gradual meshing, smooth operation, low noise, and high load-carrying capacity; however, they generate axial forces, necessitating thrust bearings, and are suitable for high-speed, heavy-load applications.

　　Helical‑toothed cylindrical gears: composed of left‑hand and right‑hand helical teeth. Features: they eliminate axial thrust, while retaining the high load capacity and smooth operation of helical gears; suitable for heavy‑duty machinery, but more complex to manufacture.

　　Internal Gears and Rack Gears: Internal gears have teeth on the inner surface and are commonly used in planetary gearsets; a rack can be regarded as a gear with an infinitely large pitch circle, employed to convert rotary motion into linear motion.

　　Straight bevel gears: their tooth lines are radial straight lines. Characteristics: relatively simple to manufacture and widely used, but they generate significant noise at high speeds.

　　Spiral bevel gears: their tooth profiles follow a helical curve. Characteristics: high contact ratio, extremely smooth operation, strong load-carrying capacity, and low noise; however, they demand exceptionally high design and manufacturing precision and are widely used in automotive final drives, among other applications.

　　Worm and worm gear: Characteristics: A single stage can achieve a very high transmission ratio; compact structure; smooth operation; some models feature self-locking capability. However, the tooth surfaces experience significant sliding friction, resulting in low efficiency and substantial heat generation, and the worm wheel often requires expensive non‑ferrous materials.

　　Helical gears with intersecting axes: point contact, low load-carrying capacity, used only for light loads or in specific spatial configurations.

　　Quasi-hyperboloid gears: their axes neither intersect nor run parallel; the axis of the driving gear may be offset, which helps lower the vehicle’s center of gravity. However, they exhibit significant sliding friction and require special lubricants.