What Are Herringbone Gears?
Herringbone gears (also called double helical gears) feature two sets of helical teeth arranged in a V-shape on the same gear. One set has a right-hand helix and the other a left-hand helix, separated by a small gap or meeting at a central apex. This symmetric design cancels out the axial thrust forces that are the main disadvantage of single helical gears.
The Problem They Solve
Single helical gears produce axial thrust forces proportional to the tangential force and the tangent of the helix angle. These thrust forces require thrust bearings to absorb, add complexity, and limit the maximum helix angle that can practically be used. Herringbone gears eliminate this problem entirely — the thrust from one helix is exactly balanced by the opposing helix on the same gear.
Advantages
- Zero net axial thrust: No thrust bearings needed; simpler bearing arrangements
- Large helix angles: Because thrust is cancelled, helix angles of 25-45° are practical, maximizing the benefits of helical tooth engagement
- Very smooth operation: High total contact ratio due to the large helix angle provides extremely smooth, quiet operation
- High load capacity: Combined load sharing of two helical sets plus larger helix angles
- Self-centering: The opposing helix angles tend to center the gear axially, reducing sensitivity to mounting errors
Manufacturing Challenges
The main drawback of herringbone gears is manufacturing complexity:
- Traditional cutting: Requires two separate hobbing or shaping passes, one for each helix direction. A groove between the two halves is needed to allow cutter runout
- Continuous herringbone: Gears without a central groove (teeth meet at an apex) require special machines — Sykes or Maag type generators. These are rare and expensive
- 5-axis CNC: Modern CNC machines can cut herringbone teeth in a single setup, making small-batch production more feasible
- Cost: Typically 2-4 times more expensive than equivalent single helical gears
Applications
Herringbone gears are used where their unique properties justify the higher cost:
- Marine main reduction gears: Ship turbine-to-propeller gearboxes commonly use herringbone gears for their smooth operation and high power capacity at sizes up to 5+ meters in diameter
- Power generation: Large turbine-driven generator sets use herringbone reducers for smooth, reliable power transmission
- Steel mills: Heavy rolling mill drives transmit enormous torques through herringbone gear sets
- Mining: Ball mill and kiln drives use herringbone gears for their high torque capacity and reliability
Design Notes
When designing herringbone gears, each half is designed as a standard helical gear. The total face width is split between the two halves (with a groove if needed). Ensure that the gap between helices is wide enough for cutter clearance during manufacturing, but narrow enough to maintain structural rigidity.