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. Unlike helical gears they can sustain axial load smoothly. From the top the helical grooves of this gear looks like letter V.
Like the helical gear, they have the advantage of transferring power smoothly as multiple gear teeth engage and disengage simultaneously. Their advantage over the simple helical gear is that the side-thrust of one half is counter-balanced by that of the other half, so that they can be used in a high-power gearbox without requiring an equally substantial thrust bearing. Herringbone gears were thus an important step in the
introduction of the steam turbine to marine propulsion.
For: Hera and Bauo
introduction of the steam turbine to marine propulsion.
They are used in heavy machinery. Herringbone gears are carefully made and of high precision, so they are more expensive than spur gears or helical gears.
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.
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 now makes it possible to have continuous teeth, with no central gap. 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.
From:Sajjad Ahamad Mughal
(source:wikipedia)
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