Two important concepts in gearing are pitch surface area and pitch angle. The pitch surface of a gear may be the imaginary toothless surface that you would beval gearbox possess 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 the encounter of the pitch surface area and the axis.
The most familiar kinds of bevel gears have pitch angles of significantly less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is named external because the gear teeth point outward. The pitch areas of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of the two areas are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees possess teeth that point 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 is why this kind of bevel gear is called 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 directly and oblique.