Two important concepts in gearing are pitch surface area and pitch angle. The pitch surface of a gear may be the imaginary toothless surface area that you would have got by averaging out the peaks and valleys of the individual teeth. The pitch surface area of an ordinary gear is the form of a cylinder. The pitch angle of a gear is the angle between your encounter of the pitch surface and the axis.

The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and they are cone-shaped. This kind of bevel gear is named planetary gearbox external since the gear teeth stage outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear 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 are called internal bevel gears.

Bevel gears which have pitch angles of exactly 90 degrees possess teeth that time outward parallel with the axis and resemble the factors on a crown. That is why this kind of bevel gear is named a crown gear.

Mitre gears are mating bevel gears with equivalent numbers of teeth and with axes in right angles.

Skew bevel gears are those that the corresponding crown gear has teeth that are directly and oblique.