Two important principles in gearing are pitch surface and pitch position. The pitch surface area of a gear is the imaginary toothless surface area that you would possess by averaging out the peaks and valleys of the individual teeth. The pitch surface area of an ordinary gear is the form 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 kinds of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This kind of bevel gear is called beval gear china external since the gear teeth stage 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 of greater than ninety degrees have teeth that time inward and so are called internal bevel gears.
Bevel gears that have pitch angles of precisely 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 the same amounts of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding 
crown gear has the teeth that are straight and oblique.