Two important concepts in gearing are pitch surface and pitch position. The pitch surface area of a gear is the imaginary planetary gearbox toothless surface area that you would possess by averaging out the peaks and valleys of the individual teeth. The pitch surface area of a typical gear is the shape of a cylinder. The pitch angle of a equipment is the angle between your face of the pitch surface area and the axis.
The most familiar types of bevel gears have pitch angles of significantly less than 90 degrees and they are cone-shaped. This type of bevel gear is called external since the gear teeth point outward. The pitch surfaces of meshed exterior bevel gears are coaxial with the gear shafts; the apexes of both surfaces are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles in excess of ninety degrees possess teeth that point inward and so are called internal bevel gears.
Bevel gears which have pitch angles of exactly 90 degrees possess teeth that point outward parallel with the axis and resemble the points 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 numbers of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown gear has tooth that are directly and oblique.