Two important ideas in gearing are pitch surface and pitch position. The pitch surface area of a gear may be 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 of an ordinary 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 types 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 because the gear teeth stage outward. The pitch areas of meshed external bevel gears are coaxial with the gear shafts; the 
apexes of the two areas are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles of beval gear china greater than ninety degrees possess 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 factors on a crown. That’s why this kind of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with the same numbers of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown gear has the teeth that are directly and oblique.