Two important principles in gearing are pitch surface 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 average person teeth. The pitch surface of an ordinary gear is the shape of a cylinder. The pitch angle of a gear is the angle between the encounter of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch beval gear china angles of less than 90 degrees and they are cone-shaped. This kind of bevel gear is called external because the gear teeth stage outward. The pitch areas of meshed exterior bevel gears are coaxial with the apparatus 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 have teeth that point inward and so 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 points 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 in right angles.
Skew bevel gears are those for which the corresponding crown gear has tooth that are straight and oblique.