Two important ideas in beval gear china gearing are pitch surface and pitch position. The pitch surface area of a gear is the imaginary toothless surface area that you would have got 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 cylinder. The pitch angle of a equipment 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 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 areas are at the idea of intersection of the shaft axes.
Bevel gears that 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 precisely 90 degrees possess teeth that point outward parallel with the axis and resemble the 
points on a crown. That’s why this kind of bevel gear is called 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 gear has teeth that are straight and oblique.