Same as Chain wheel.
Source: Webster's dictionarysprocket-wheel
During continuous contact printing, the raw stock and the negative are placed next to one another around the sprocket wheel of the printer. Source: Internet
The negative, which is the closer of the two to the sprocket wheel (thus creating a slightly shorter path), must have a marginally shorter pitch between perforations (0.1866 in pitch); the raw stock has a long pitch (0.1870 in). Source: Internet