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A spiral bevel gear is a gear with a truncated cone profile and helical or curved teeth. Bevel gears are typically used where the direction of a drive input has to be turned 90 degrees. The most common example of the use of bevel gears to achieve this 90 degree displacement is the differential of an automobile. A spiral bevel gear features helical or curved teeth because this tooth design produces less vibration and noise and can withstand higher rotational speeds than spur or straight cut teeth. Spiral bevel gears are cut with either right or left handed tooth orientation with their corresponding pinion or driven gear being of opposite orientation. The spiral tooth design is specifically used for this sort of application due to its low noise and vibration characteristics and high speed capabilities. The teeth of straight or spur cut bevel gears engage each other in an abrupt manner which produces a considerable amount of noise and vibration. If this type of gear arrangement is subjected to high rotational speeds, the gear teeth are inclined to break due to this abrupt engagement. The teeth of a spiral bevel gear engage in a progressive manner due to the curved surfaces. This makes for quieter, vibration free running at high speeds. Spiral bevel gears are produced with teeth which curve in either a left or right hand orientation when the gear is looked at face on. The gears they drive or are driven by are always of the opposite orientation. When spiral bevel gear sets in a differential are replaced due to wear, they should always be replaced in pairs even if one gear is still serviceable. This precludes differences in tolerances in left and right side gears which may lead to backlash problems.
source: wiseGeek.com: Clear Answers for Common Questions
