Two important concepts in gearing are pitch surface and pitch position. The pitch surface of a gear may be the imaginary toothless surface area that you would have 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 and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This type of bevel gear is called external since the gear teeth stage outward. The pitch surfaces of meshed exterior 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 in excess of ninety degrees have teeth that point inward and so 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 type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equivalent amounts of teeth and with axes in right angles.