A combination of left-handed and right-handed helical gears with the same twist angle is called a double helical gear. Double helical gears transmit rotational motion between two parallel shafts.
Double helical gears have all of the advantages of helical gears, which are strong and quiet, plus have the advantage of being able to cancel the thrust forces by combining helical gears with both right and left hand twists. On the other hand, they have the disadvantage of being difficult to manufacture.
Double helical gears and herringbone gears are the same type of gear, but those with a groove in the center are called double helical gears, while those without a groove where the left and right teeth meet in the center, are called herringbone gears.