Two important concepts in gearing are pitch surface and pitch position. The pitch surface of a gear is the imaginary toothless surface that you would have got by averaging out the peaks and valleys of the individual 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 significantly less than 90 degrees and they are cone-shaped. This kind of bevel gear is called external since the gear teeth stage outward. The pitch areas of meshed exterior bevel gears are coaxial with the apparatus shafts; the beval gearbox 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 possess teeth that time inward and are called internal bevel gears.
Bevel gears which have pitch angles of specifically 90 degrees have teeth that 
point outward parallel with the axis and resemble the factors on a crown. That is why this kind 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 that the corresponding crown equipment has the teeth that are directly and oblique.