Two important ideas in gearing are pitch surface area and pitch angle. The pitch surface of a gear is the imaginary toothless surface area that you would have by averaging out the peaks and valleys of the average person teeth. The pitch surface area 
of an ordinary gear is the shape of a cylinder. The pitch angle of a equipment is the angle between the encounter of the pitch surface and the axis.
The most familiar types of bevel gears have pitch beval gear china angles of significantly less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is called external because the gear teeth point outward. The pitch surfaces of meshed exterior 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 which have pitch angles in excess of ninety degrees have teeth that point inward and are called internal bevel gears.
Bevel gears which have pitch angles of exactly 90 degrees possess teeth that time outward parallel with the axis and resemble the factors on a crown. That’s why this type of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equivalent amounts of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown gear has teeth that are straight and oblique.