Two important ideas in gearing are pitch surface area and pitch angle. The pitch surface of a gear is the imaginary toothless surface area that you would possess by beval gear china 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 gear 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 type of bevel gear is named external since the gear teeth stage outward. The pitch surfaces of meshed exterior bevel gears are coaxial with the apparatus shafts; the apexes of both surfaces are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles of greater than ninety degrees possess teeth that time inward and are called internal bevel gears.
Bevel gears which have pitch angles of specifically 90 degrees have teeth that time outward parallel with the axis and resemble the factors on a crown. That’s why 
this type of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equal amounts of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown gear has tooth that are straight and oblique.