The Black Gear (黒い歯車 Kuroi Haguruma?) is a dark gear of varying size that appears in Digimon Adventure. Devimon did not invent the Black Gears, but can summon them and transform other Digimon into them. They are built into the very structure of File Island, so that the island can be split or united by operating them, and can also be used to cause Digimon to go berserk, or further empower evil Digimon. However, the light of the Digivice can expel Black Gears from one of their victims.
The Black Gears appeared to be flying around at random during the series. It hit a Digimon, and drove it insane. This could even work on Ultimate level Digimon like Andromon and Monzaemon; however, in Andromon's case the effect was a malfunction that caused him to believe everyone was an enemy.