A herringbone gear, a specific type of double helical gear, is a special type of gear that is a side to side (not face to face) combination of two helical gears of opposite hands. From the top, each helical groove of this gear looks like the letter V, and many together form a herringbone pattern (resembling the bones of a fish such as a herring). Unlike helical gears, herringbone gears do not produce an additional axial load.
Like helical gears, they have the advantage of transferring power smoothly because more than two teeth will be in mesh at any moment in time. Their advantage over the helical gears is that the side-thrust of one half is balanced by that of the other half. This means that herringbone gears can be used in torque gearboxes without requiring a substantial thrust bearing. Because of this, herringbone gears were an important step in the introduction of the steam turbine to marine propulsion.
Precision herringbone gears are more difficult to manufacture than equivalent spur or helical gears and consequently are more expensive. They are used in heavy machinery.