Two important principles in gearing are pitch surface and pitch position. The pitch surface area of a gear may be the imaginary toothless surface that you would have got by averaging out the peaks and valleys of the individual teeth. The pitch surface of a typical gear is the shape 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 types of bevel gears have pitch angles of less than 90 degrees and they are cone-shaped. This kind of bevel gear is called external since the gear teeth point outward. The pitch areas of meshed external bevel gears are coaxial with the gear shafts; the beval gear china apexes of the two surfaces are at the idea of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees possess teeth that point inward and so are called internal bevel gears.
Bevel gears which have pitch angles of precisely 90 degrees possess teeth that point 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 equal amounts of teeth and with axes at right angles.
Skew bevel gears are those that the corresponding crown gear has tooth that are directly and oblique.