Two planetary gearbox important principles in gearing are pitch surface and pitch angle. The pitch surface of a gear is 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 area of an ordinary gear is the form 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 kind of bevel 
gear is named external because the gear teeth stage outward. The pitch surfaces of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of both surfaces are at the idea of intersection of the shaft axes.
Bevel gears which have pitch angles of greater than ninety degrees have teeth that time 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 points on a crown. That is why this kind of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with the same amounts of teeth and with axes at right angles.
Skew bevel gears are those that the corresponding crown equipment has the teeth that are straight and oblique.