A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. A gear can mesh a non-rotating toothed part, called a rack, thereby producing translation instead of rotation. However, gears hardly ever work alone. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio. Geared devices can change the speed, magnitude, and direction of a power source.
The majority of gears are composed of carbon and low-alloy steels, including carburised steels. Among the carburised steels used in gears are 1018, 1524, 4026, 4118, 4320, 4620, 4820, 8620 and 9310. Typical gear manufacturing processes include gear forming, cutting, and finishing. Gear forming processes include Plastic Injection Molding, Powder Metal Sintering, Forging, Casting, Roll-Forming. Gear cutting processes include Broaching, Hobbing, Shaping, Lapping, Milling. Gear finishing processes include Grinding, Honing, Shaving.