Two important concepts in gearing are pitch surface area and pitch angle. 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 of an ordinary gear is the shape of a cylinder. The pitch angle of a gear is the angle between your encounter 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 point outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of both areas are at the point 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 that have pitch angles of precisely 90 degrees have 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.
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