Two important principles in gearing are pitch beval gear china surface area and pitch angle. The pitch surface of a gear is the imaginary toothless surface that you would have 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 gear is the angle between the face of the pitch surface 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 point outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of the two surfaces are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees have teeth that point inward and so 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 points on a crown. That is why this type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equal numbers of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown gear has the teeth that are directly and oblique.