Two important principles in gearing are pitch surface area and pitch position. 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 individual teeth. The pitch surface of an ordinary gear is the form of a cylinder. The pitch angle of a equipment is the angle between the face of the pitch surface and the axis.
The most familiar types of bevel gears have pitch angles of significantly less than 90 degrees and therefore are cone-shaped. This type of bevel gear is named external because the gear teeth point outward. The pitch surfaces of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of the two surfaces are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles in excess of 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 point Scroll Vacuum Pumps outward parallel with the axis and resemble the points on a crown. That is why this type of bevel gear is named a crown gear.
Mitre gears 
are mating bevel gears with the same numbers of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown gear has the teeth that are directly and oblique.