Two important ideas in gearing are pitch Coupling surface 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 area of an ordinary gear is the shape of a cylinder. The pitch angle of a equipment is the angle between the face of the pitch surface area and the axis.
The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This type of bevel gear is named external since the gear teeth stage outward. The pitch surfaces of meshed exterior 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 that have pitch angles of greater than ninety degrees have teeth that point inward and so are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth that point outward parallel with the axis and resemble the points on a crown. That is why this type of bevel gear is named a crown gear.
Mitre gears are mating bevel gears with equivalent numbers of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown equipment has the teeth that are directly and oblique.