Two important concepts in gearing are pitch surface and pitch angle. The pitch surface area of a 齒輪 may be 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 
of an ordinary gear is the shape of a cylinder. The pitch angle of a gear is the angle between your face of the pitch surface area and the axis.
The most familiar types of bevel gears have pitch angles of significantly less than 90 degrees and they are cone-shaped. This type of bevel gear is named external because the gear teeth stage outward. The pitch areas of meshed exterior bevel gears are coaxial with the apparatus shafts; the apexes of both areas are at the idea of intersection of the shaft axes.
Bevel gears that 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 have teeth that time 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 amounts of teeth and with axes in right angles.
Skew bevel gears are those that the corresponding crown equipment has the teeth that are straight and beval gear china oblique.
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