A chain sprocket or sprocket-wheel is a profiled wheel with teeth, or cogs, that mesh with a chain, track, or other perforated or indented material. It is distinguished from a gear in that sprockets are never meshed together directly and differs from a pulley in that sprockets have teeth and pulleys are smooth. The name ‘sprocket’ generally applies to any wheel upon which radial projections engage a chain passing over it.
Sprockets are used in bicycles, motorcycles, cars, tracked vehicles, and other machinery to transmit rotary motion between two shafts where gears are unsuitable or impart linear motion to a track, tape, etc. Perhaps the most common form of sprocket may be found in the bicycle, in which the pedal shaft carries a large sprocket wheel, which drives a chain, which, in turn, causes a small sprocket on the axle of the rear wheel. Early automobiles were also primarily driven by sprocket and chain mechanisms; a practice essentially copied from bicycles.