A herringbone gear, also known as a double helical gear, is a special type of gear which is a side to side (not face to face) combination of two helical gears of opposite hands. From the top the helical grooves of this gear looks like letter V. Unlike helical gears they do not produce an additional axial load.
Like helical gears, they have the advantage of transferring power smoothly as multiple gear teeth engage and disengage simultaneously. 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.
Where the oppositely angled teeth meet in the middle of a herringbone gear, the alignment may be such that tooth tip meets tooth tip, or the alignment may be staggered, so that tooth tip meets tooth trough. The latter alignment is the unique defining characteristic of a Wuest type herringbone gear, named after its inventor.
A disadvantage of the herringbone gear is that it cannot be cut by simple gear hobbing machines, as the cutter would run into the other half of the gear. Solutions to this have included assembling small gears by stacking two helical gears together, cutting the gears with a central groove to provide clearance, and (particularly in the early days) by casting the gears to an accurate pattern and without further machining. With the older method of fabrication, herringbone gears had a central channel separating the two oppositely-angled courses of teeth. This was necessary to permit the shaving tool to run out of the groove. The development of the Sykes gear shaper made it possible to have continuous teeth, with no central gap. After the W.E. Sykes and Farrel Gear Machine companies dissolved in 1983-84 there are no current production machines that have this ability. It is standard industry practice to obtain an older machine and rebuild it if necessary to create this unique type of gear.
The logo of the car maker Citroën is a graphic representation of a herringbone gear, reflecting André Citroën's earlier involvement in the manufacture of these gears. Early Mors and Citroën cars used a herringbone bevel gear final drive in the rear axle.
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