Two important principles 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 got by averaging out the peaks and valleys of the average person teeth. The pitch surface area of a typical gear is the form 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 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 because the gear teeth stage outward. The pitch surfaces of meshed exterior 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 that have pitch angles of greater than ninety degrees possess teeth that point inward and are called internal bevel gears.
Bevel gears which have pitch angles of precisely 90 degrees have teeth that time 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 equivalent amounts of teeth and with axes in right angles.
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