Two important concepts in gearing are pitch surface area and pitch position. The pitch surface area of a gear is 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 of an ordinary gear is the shape of a cylinder. The pitch angle of a equipment is the angle between your face of the pitch surface 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 named external because the gear teeth point outward. The pitch areas of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of 
both areas are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees have teeth that point inward and are called internal bevel gears.
Bevel gears that have pitch angles of precisely 90 degrees possess teeth that time outward planetary gearbox parallel with the axis and resemble the factors on a crown. That’s why this type of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equivalent amounts of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown equipment has tooth that are directly and oblique.