Two important 
ideas in gearing are pitch beval gearbox surface area and pitch position. The pitch surface area of a gear is the imaginary toothless surface that you would have by averaging out the peaks and valleys of the average person teeth. The pitch surface area of a typical 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 therefore are cone-shaped. This type of bevel gear is named external since the gear teeth point outward. The pitch areas of meshed external bevel gears are coaxial with the apparatus shafts; the apexes of the two areas are at the point of intersection of the shaft axes.
Bevel gears which have pitch angles in excess of ninety degrees possess teeth that time inward and so 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’s why this kind of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equal numbers of teeth and with axes at right angles.
Skew bevel gears are those that the corresponding crown equipment has teeth that are straight and oblique.