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 individual 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 encounter of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch angles of significantly less than 90 degrees and they are cone-shaped. This kind of bevel gear is planetary gearbox called external because the gear teeth stage outward. The pitch areas of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of the two areas are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees have teeth that point inward and so are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth that time 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 in right angles.
Skew bevel gears are those for which the corresponding crown equipment has the teeth that are straight and oblique.