Two important ideas in gearing are pitch surface area and pitch angle. The pitch surface area of a gear is the imaginary toothless surface area that you would have by averaging out the peaks and valleys of the average person teeth. The pitch surface area of a typical gear is the form of a cylinder. The pitch angle of a gear is the angle between the face of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch beval gearbox angles of less than 90 degrees and therefore are cone-shaped. This type of bevel gear is named external since the gear teeth stage outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of the two areas are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles in excess of ninety degrees have teeth that point inward and so are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees possess teeth that time outward parallel with the axis and resemble the points on a crown. That’s why this type of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with the same amounts of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown gear has tooth that are straight and oblique.