What is a gear having a pitch circle as a straight line known as?

A gear having a pitch circle represented as a straight line is specifically known as a rack gear. Rack gears are used in applications where linear motion is required, converting rotary motion into straight-line movement. The pitch line of a rack is a straight line that extends infinitely, and it meshes with pinion gears, which are essentially small spur gears.

The unique design of a rack gear allows it to create continuous linear motion, making it ideal for operations in machinery such as milling machines and CNC machines. This is different from other types of gears, like helical, spur, and bevel gears, which are typically used to transmit rotary motion between shafts. Thus, the characteristics of rack gears make them distinct and important components in various mechanical systems.