Gears are mechanical elements with teeth on the rim that can continuously mesh to transmit motion and power. They are interlocking teethed mechanical parts, with the diameter of a large gear being twice that of a small gear.

Gears have been in use for transmitting motion since ancient times. Over 300 BCE, the Greek philosopher Aristotle discussed the transmission of rotational motion using bronze or cast iron gears in his work "On the Mechanica." By the end of the 19th century, the principle of forming cutting and the subsequent appearance of machine tools and cutting tools utilizing this principle allowed gear manufacturing to have more complete methods. As production developed, the smoothness of gear operation became a focus.