Spur gears are the most commonly used gear type. They are characterized by teeth which are perpendicular to the face of the gear. Spur gears are by far the most commonly available, and are generally the least expensive. The basic descriptive geometry for a spur gear is shown in the figure below. Spur gears generally cannot be used when a direction change between the two shafts is required.
Spur gears are easy to find, inexpensive, and efficient.
The resulting teeth are longer than the teeth on a spur gear of equivalent pitch diameter. Bevel gears are primarily used to transfer power between intersecting shafts. The teeth of these gears are formed on a conical surface. Standard bevel gears have teeth which are cut straight and are all parallel to the line pointing the the apex of the cone on which the teeth are based. Spiral bevel gears are also available which have teeth that form arcs.