Herringbone Gear: Double-Helix Teeth That Cancel Axial Thrust

What a Herringbone Gear Is and Where It Runs

A herringbone gear is a double-helical gear: a single wheel carrying two rows of helical teeth of opposite hand, meeting at the center in a chevron. It is the workhorse of high-power, parallel-shaft transmission where net thrust must be zero and load capacity must be enormous.

• Marine propulsion: reduction gears between steam/gas turbines (running at 3,000–10,000 rpm) and propeller shafts (80–120 rpm), transmitting tens of MW.
• Heavy industry: rolling mills, cement and ore mills, extruders, sugar mills — drives from hundreds of kW to >20 MW.
• Power generation: some turbine and pump gearboxes.

Its most famous cultural footprint is the Citroën logo — the double chevron directly depicts herringbone teeth, commemorating André Citroën's early gear-manufacturing business. The gear stays quiet and smooth because, like all helical gearing, teeth engage gradually rather than all at once — but unlike single-helical, it does so without the thrust penalty.

The Mechanism: How Two Helices Cancel Thrust

When a helical tooth transmits torque, the normal force on the tooth flank has three components: a tangential force Ft (does the useful work), a radial force Fr (pushes gears apart), and an axial force Fa (shoves the gear along its shaft). The axial component arises because the tooth is skewed by the helix angle β:

• Fa = Ft·tan(β)
• Fr = Ft·tan(αₙ)/cos(β), where αₙ is the normal pressure angle (usually 20°)

In a single helical gear this Fa reacts entirely against a thrust bearing. In a herringbone gear the wheel is effectively two helical gears back-to-back with opposite hand. The left half generates +Fa; the right half generates −Fa of equal magnitude. The two are transmitted through the same rigid gear body, so they cancel internally — the shaft and its bearings feel zero net axial load. The radial and tangential forces still exist and are carried by radial bearings as normal.

Key Quantities and a Worked Example

Consider a herringbone pinion transmitting P = 500 kW at n = 1,500 rpm on a pitch diameter d = 200 mm, helix angle β = 30°.

• Torque: T = P/ω = 500,000 / (2π·1500/60) ≈ 3,183 N·m
• Tangential force: Ft = 2T/d = 2·3183 / 0.20 ≈ 31.8 kN
• Per-half axial force (what a single helical would send to a bearing): Fa = Ft·tan 30° ≈ 18.4 kN
• Net axial thrust on the herringbone shaft: ≈ 0 kN (each half's 18.4 kN opposes the other)

That eliminated 18.4 kN is why herringbone gears exist. A 30° helix converts about 58% of tangential load into thrust (tan 30° = 0.577); a 20° helix, about 36% (tan 20° = 0.364). Because thrust cancels, herringbone designers can use aggressive helix angles (30°–45°) that yield very high transverse contact ratios (often >2), spreading load over more teeth and lowering bending stress, typically kept below ~300–400 MPa for hardened alloy steel.

Design, Cutting, and Operation in Practice

Two construction styles exist. The true (continuous) herringbone has the two helices meeting at an apex with no gap — hard to machine because a hob cannot run out. The double-helical with a center groove leaves a narrow relief channel (typically 10–20% of face width) so a hob or shaper cutter can clear; this is by far the most common industrial form.

• Cutting: historically the Sykes generating machine cut true herringbones with twin reciprocating cutters; today most are shaped or hobbed as two opposed helices with a runout groove.
• Apex alignment: the two half-apexes on mating gears must line up. Any mismatch reintroduces thrust and unequal load sharing.
• Axial float: one shaft is deliberately left free to float axially. This is critical — it lets the gear self-center so both halves share torque equally. Locking both shafts causes one helix to carry most of the load.
• Standards: rating per AGMA 2001 / ISO 6336 (bending + pitting), geometry per ISO 53; lubrication and quality per AGMA 6011 for high-speed units.

How It Compares to Spur, Helical, and Double-Reduction Gears

Versus spur gears: spur teeth engage abruptly along the full face, producing more noise, vibration, and lower contact ratio; they carry no thrust but far less load per unit width. Herringbone gives spur-like zero thrust plus helical smoothness.

Versus single helical: single-helical is cheaper and easier to cut, and is fine wherever a thrust bearing (or an opposing helical stage) can absorb Fa — most automotive gearboxes do exactly this. But at high power the thrust bearing becomes a bulky, lossy, failure-prone component. Herringbone deletes it.

• Versus back-to-back helical pairs: you can mount two opposite-hand single-helical gears on one shaft to cancel thrust — this is essentially a fabricated herringbone and is common in large gearboxes. It trades manufacturing simplicity for extra assembly and alignment work.
• Versus double-reduction trains: often a herringbone stage is the high-speed first reduction in a two-stage marine or mill gearbox, precisely because its thrust-free running suits fast pinions.

Failure Modes, Trade-offs, and Significance

Herringbone gearing is not free lunch. Its trade-offs:

• Cost & manufacturability: two helices, a center groove, and tight apex alignment make it the most expensive parallel-axis gear to cut and inspect.
• Uneven load sharing: if the floating shaft binds or the apexes are mismatched, one helix overloads — leading to accelerated pitting and tooth-bending fatigue on that side. Axial float and precise apex phasing are mandatory.
• Trapped lubricant: the center groove and V-geometry can trap oil, causing churning losses and localized heating at high pitch-line velocities (often >100 m/s in turbine gears).
• Standard failure limits: contact (Hertzian) stress typically kept below ~1,500 MPa for carburized steel; bending stress below ~350 MPa; both governed by AGMA/ISO safety factors.

Significance: herringbone gearing solved the central problem that limited early high-power reduction drives — thrust — and made compact multi-megawatt turbine and mill transmissions practical. Over a century after Citroën's 1905/1910 patents, it remains the default where power is large and thrust must be nil.

Herringbone vs. other parallel-axis gear types: thrust, load capacity, and typical use

Property	Spur	Single Helical	Herringbone (Double Helical)
Net axial thrust	None	High (Fa = Ft·tan β)	None (self-cancelling)
Practical helix angle β	0°	8°–30°	20°–45°
Contact ratio / smoothness	Low, noisier	High, quiet	High, quiet
Relative load capacity	Baseline (1.0)	1.3–1.5×	1.5–2×
Manufacturing cost	Low	Moderate	High (V-cut, center groove)
Typical use	Low-speed, low-load	Autos, general drives	Turbines, mills, marine