Rack and Pinion

A rack is a type of special gear with teeth distributed along a strip shape. Racks come in two types: straight-tooth racks and helical racks, which are paired with straight-tooth cylindrical gears and helical cylindrical gears, respectively; the rack's tooth profile is a straight line rather than a involute (for the tooth face, it is a plane), equivalent to a cylindrical gear with an infinite pitch circle radius.