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 possess by Gearbox averaging out the peaks and valleys of the average person 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 and the axis.
The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This type of bevel gear is named external because the gear teeth point outward. The pitch areas of meshed exterior bevel gears are coaxial with the apparatus shafts; the apexes of the two surfaces are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles in excess of ninety degrees have teeth that point inward and so are called internal bevel gears.
Bevel gears which 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 type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equal numbers of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown gear has teeth that are directly and oblique.