Two important ideas in gearing are pitch surface and pitch angle. The pitch surface 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 a typical gear is the form of a cylinder. The pitch angle of a gear is the angle between the encounter of the pitch surface area and the axis.
The most familiar types of bevel gears have pitch angles of significantly less than 90 planetary gearbox degrees and therefore are cone-shaped. This kind of bevel gear is named external since 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 possess teeth that time inward and so are called internal bevel gears.
Bevel gears which have pitch angles of specifically 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 equal amounts of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown equipment has teeth that are straight and oblique.
