Two important principles in gearing are pitch surface and pitch angle. The pitch surface of a gear may be the imaginary toothless surface area that you 
would possess by averaging out the peaks and valleys of the average person teeth. The pitch surface of an ordinary gear is the shape of a cylinder. The pitch angle of a equipment is the angle between your encounter of the pitch surface area and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This type of bevel gear is named external because the gear teeth point outward. The pitch surfaces of meshed exterior bevel gears are coaxial with the apparatus shafts; the apexes of the two areas 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 beval gear china internal bevel gears.
Bevel gears which have pitch angles of precisely 90 degrees possess teeth that time outward parallel with the axis and resemble the factors on a crown. That is why this type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with the same numbers of teeth and with axes at right angles.
Skew bevel gears are those that the corresponding crown gear has tooth that are straight and oblique.