Two important ideas in beval gearbox gearing are pitch surface area and pitch angle. The pitch surface of a gear may be the imaginary toothless surface that you would have got 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 your face of the pitch surface area and the axis.
The most familiar kinds of bevel gears have pitch angles of significantly less than 90 degrees and therefore are cone-shaped. This type of bevel gear is called external because 
the gear teeth stage outward. The pitch areas of meshed external bevel gears are coaxial with the gear shafts; the apexes of the two areas are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees possess teeth that point inward and are called 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 kind 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 that the corresponding crown equipment has teeth that are directly and oblique.