Spur gear

Viewing the gear at 90 degrees from the shaft length (side on) the tooth faces are straight and aligned parallel to the axis of rotation.

Instead of being straight (as in a triangle) the sides of the cross section have a curved form (usually involute and less commonly cycloidal) to achieve a constant drive ratio.

[2] Spur gear can be classified into two pressure angles, 20° being the current industry standard and 14½° being the former (often found in older equipment).

This phenomenon is known as "interference" and occurs when the number of teeth on the smaller of the two meshing gears is less than a required minimum.

[clarification needed] Spur gears can be classified into two main categories: External and Internal.