Two important principles in gearing are pitch surface area and pitch angle. The pitch surface of a gear is the imaginary toothless surface that you would possess by averaging out the peaks and valleys of the average person teeth. The pitch surface area of an ordinary gear is the shape of a planetary gearbox cylinder. The pitch angle of a gear is the angle between the face of the pitch surface area 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 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 both areas are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees possess teeth that point inward and are called internal bevel gears.
Bevel gears which have pitch angles of precisely 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 named a crown gear.
Mitre gears are mating bevel gears with equivalent numbers of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown equipment has teeth that are directly and oblique.