Two important concepts in gearing are pitch surface area and pitch angle. The pitch surface area of a gear may be the imaginary toothless surface area that you would have got 
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 and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is named external since the gear teeth point outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of the two surfaces are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles in planetary gearbox excess of ninety degrees possess teeth that time inward and are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth that time 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 at right angles.
Skew bevel gears are those for which the corresponding crown gear has the teeth that are straight and oblique.
