Two important ideas in gearing are pitch beval gear china surface and pitch angle. The pitch surface of a gear is the imaginary toothless surface 
that you would have got by averaging out the peaks and valleys of the average person teeth. The pitch surface 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 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 named external because the gear teeth point outward. The pitch areas of meshed external bevel gears are coaxial with the gear shafts; the apexes of both surfaces are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles in excess of ninety degrees possess teeth that point inward and so 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’s why this kind of bevel gear is called 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 for which the corresponding crown equipment has tooth that are directly and oblique.