Found in varying sizes throughout File Island, the Black Gears were most prominently used by Devimon to control various Digimon. Infected Digimon became more hostile and attacked the DigiDestined. By destroying the gears, the DigiDestined were able to return infected Digimon to their normal personality. The light of a Digivice destroys a Black Gear.
As they were built into File Island itself, the black gears can also be used to reformat the geography of island itself.
Devimon's attack, 'touch of evil' is similarly used to directly control a Digimon
Hagurumon is also known to make 'dark gears'.
List of Digimon that were under the control of a Black Gear (by order of appearance)