Two important ideas in gearing are pitch surface area and pitch position. The pitch surface area of a gear 
is the imaginary toothless surface that you would possess by averaging out the peaks and valleys of the average person teeth. The pitch surface area of an ordinary gear is the shape of a cylinder. The pitch angle of a equipment is the angle between your face of the pitch surface and the axis.
The most familiar types of bevel gears have pitch beval gear china angles of significantly less than 90 degrees and they are cone-shaped. This type of bevel gear is called external because the gear teeth point 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 that have pitch angles of greater than ninety degrees have teeth that point 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 points on a crown. That’s why this kind of bevel gear is called a crown gear.
Mitre gears are mating bevel gears with equal numbers of teeth and with axes in right angles.
Skew bevel gears are those for which the corresponding crown gear has teeth that are straight and oblique.