Two important concepts in gearing are pitch surface area and pitch angle. The pitch surface area 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 the face of the pitch surface area and the axis.
The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and they are cone-shaped. This kind of bevel gear is called external because the gear teeth point outward. The pitch areas of beval gearbox meshed external bevel gears are coaxial with the apparatus shafts; the apexes of the two surfaces are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees have teeth that time inward and are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees possess teeth that time outward parallel with the axis and resemble the factors on a crown. That’s why this kind of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equivalent numbers of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown equipment has the teeth that are straight and oblique.
