Two important concepts in gearing are pitch surface and pitch angle. The pitch surface area of a gear may be the imaginary toothless surface that you would have got by averaging out the peaks and valleys of the individual teeth. The pitch surface of a typical gear is the shape of a cylinder. The pitch angle of a equipment 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 less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is called external because the gear teeth stage outward. The pitch areas of meshed exterior bevel gears are coaxial with the gear shafts; the apexes of both surfaces are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles of greater than ninety degrees have teeth that time inward and 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 factors on a crown. That’s why this type of bevel gear is named a crown gear.
Mitre 
gears are beval gearbox mating bevel gears with equal numbers of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown equipment has teeth that are straight and oblique.