Decreased tooth loading is a chief advantage of helical gears. On a spur cut gear, each tooth has to carry the torque. In a helical gear design, four or five teeth are in constant contact, so the torque is spread out. This is why helical cut gears are used in automotive transmissions. For example, suppose 100 horsepower from the engine has to be transferred between two shafts. If the gears were straight cut, the full 100 horsepower would be applied to just one tooth, which would shear it off. On a helical cut system, the 100 horsepower is spread out over five teeth, and dividing 100 by five yields 20 horsepower per tooth. The tooth will not break off in this scenario because it can handle 20 horsepower.
"Backlash" is another name for the gap in between the teeth of adjoining gears. The gears can move back and forth because of the gap, hence the name. Spur gears have a lot of backlash. When the gears are running, vibration can set in because of the backlash. This vibration is called "chattering," because the gears pound against each other, making a chattering noise. Chattering due to backlash can cause self destruction of the gears by breaking off teeth. Because more than one tooth is in contact with its adjoining gear at all times, helical cut gears have reduced or no backlash.
The shafts must be perfectly parallel for the straight cut gears to mesh smoothly. If a machine design calls for perpendicular shafts, special straight cut gears, called bevel gears, must be used. For the same perpendicular shaft configuration, helical cut gears will work directly, since adjoining gears mesh equally well if the shafts are parallel or perpendicular to each other.