Two important principles in gearing are pitch surface area and pitch angle. The pitch surface area of a gear may be the imaginary toothless surface that you would possess by averaging out the peaks and valleys of the average person teeth. The pitch surface of a typical gear is the form of a 
cylinder. The pitch angle of a gear is the angle between your encounter of the pitch surface and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and they are cone-shaped. This kind of bevel gear is called external since the gear teeth point outward. The pitch areas of meshed external bevel gears are coaxial with the gear beval gearbox shafts; the apexes of both surfaces are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees possess teeth that time inward and are called internal bevel gears.
Bevel gears which have pitch angles of precisely 90 degrees possess teeth that time outward parallel with the axis and resemble the factors on a crown. That’s why this type of bevel gear is called 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 that the corresponding crown gear has tooth that are straight and oblique.