Two important concepts in gearing are pitch surface and pitch angle. The pitch surface area of a gear may be the imaginary toothless surface area that you would possess by averaging out the peaks and valleys of the average person teeth. The pitch surface area of a typical gear is the form of a beval gear china 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 less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is named 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 which have pitch angles in excess of ninety degrees have teeth that point inward and 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 factors on a 
crown. That is why this kind of bevel gear is named a crown gear.
Mitre gears are 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 the teeth that are straight and oblique.