Two important principles in planetary gearbox gearing are pitch surface and pitch angle. The pitch surface area of a gear is the imaginary toothless surface that you would have got by averaging out the peaks and valleys of the individual teeth. The pitch surface area of an ordinary gear is the form of a cylinder. The pitch angle of a gear 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 significantly less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is called external because the gear teeth point outward. The pitch surfaces of meshed exterior bevel gears are coaxial with the apparatus shafts; the apexes of the two surfaces are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles in excess of ninety degrees have teeth that point inward and so 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 kind of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equivalent amounts 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.