Sprockets and gears

A sprocket drives a chain and is defined by the chain series and tooth count. A gear meshes directly with another gear and is defined by its module and tooth count. The two are not interchangeable.

Module is the pitch circle diameter divided by the tooth count, and it describes the tooth size. A larger module means coarser teeth. Two gears mesh only if the module is the same.

Pitch circle diameter equals the chain pitch divided by sin(180 / tooth count). For 08B at 12.7 mm pitch with 15 teeth that comes to about 61 mm. The calculator on this site gives the same figure along with the outside diameter and tooth width.

For continuous duty at speed, 17 teeth or more is the usual recommendation. Fewer is possible, but the chain then runs more polygonally on the wheel, with more noise and wear.

A gear with an infinite radius, which is to say teeth cut along a straight bar. Paired with a pinion it turns rotation into linear motion. You see it most often on sliding gates and machinery with a reciprocating stroke.