Two important principles in gearing are pitch surface area and pitch angle. The pitch surface of a gear is the imaginary toothless surface area that you would planetary gearbox 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 equipment is the angle between your encounter of the pitch surface area and the axis.
The most familiar kinds 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 apparatus shafts; the apexes of the two areas are at the idea of intersection of the shaft axes.
Bevel gears which have pitch angles of greater than ninety degrees possess teeth that point inward and so are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth that point outward parallel with the axis and resemble the factors on a crown. That is why this type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equal amounts of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown gear has the teeth that are directly and oblique.