Two important China Pulley principles in gearing are pitch surface area and pitch position. The pitch surface area of a gear is 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 form of a cylinder. The pitch angle of a equipment is the angle between your face of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch angles of significantly less than 90 degrees and they are cone-shaped. This kind of bevel gear is called external since the gear teeth stage outward. The pitch areas of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of both areas are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles of greater than ninety degrees have teeth that time inward and are called internal bevel gears.
Bevel gears which have pitch angles of precisely 90 degrees possess teeth that point outward parallel with the axis and resemble the factors on a crown. That is why this kind of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equivalent 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.