Spur gears must have their shafts always parallel to the axis. Bevel gears may be designed to run properly at any angle from parallel to 150 degrees. Bevel gears with shafts at 90 degrees are the more common.
To facilitate proper cutting, the drawing should give the face angle and the cutting angle for each gear, and the depth of the teeth at the outer end. When shafts are at right angles, the sum of the edge angles will equal 90 degrees, and the sum of the face angles and edge angles will be equal.
Just as with a hand drill, bevel gears change the rotation of the chuck to a horizontal rotation and can transmit increased or reduced speed of rotation. They are efficient for transferring high torque.
The teeth can be straight, spiral or hypoid. Straight bevel gear teeth actually have the same problem as straight spur gear teeth, as each tooth engages, it impacts the corresponding tooth all at once. Smooth, quiet, wear and life span are all largely determined by how gear and teeth make contact. As with spur gears, the solution to this problem is to curve the teeth, so that the contact starts at one end of the gear and progressively spreads across the whole tooth.
Spiral bevel gears are machined by special types of multiple-tooth machine tool (hobs) in a process known as gear hobbing. CNC gear hobbing cuts the gear surface ensuring that the ratio of the pitch, depth, and angle of the teeth are uniform and will work perfectly together.
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