Two important principles in gearing are pitch surface area and pitch position. The pitch surface area of a gear may be the imaginary toothless surface area that you would have got by averaging out the peaks and valleys of the individual teeth. The pitch surface of a typical gear is the shape of a cylinder. The pitch angle of a gear is the angle between your face of the pitch surface and the axis.
The most familiar types of bevel gears have pitch angles of less than 90 degrees and they are cone-shaped. This type of bevel gear is called external since the gear teeth point outward. The pitch areas of meshed exterior bevel gears are coaxial with the apparatus shafts; the apexes of both surfaces are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees beval gear china possess teeth that point inward and so are called internal bevel gears.
Bevel gears which have pitch angles of exactly 90 degrees possess teeth that point 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 equivalent 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.