Two important ideas in gearing are pitch surface area and pitch angle. 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 average person teeth. The pitch surface area of an ordinary gear is the form of a cylinder. The pitch angle of a gear is the angle between the face of the pitch surface area and the axis.
The most familiar kinds of bevel gears have pitch gear box for greenhouse angles of less than 90 degrees and they are cone-shaped. This type of bevel gear is called external because the gear teeth point 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 which have pitch angles in excess of ninety degrees possess teeth that time inward and are called internal bevel gears.
Bevel gears that have pitch angles of specifically 90 degrees possess teeth that point outward parallel with the axis and resemble the points on a crown. That is why this type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with the same amounts of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown gear has tooth that are directly and oblique.