Double Helical Gear – Steel, OD up to 1370mm, Custom OEM

A combination of left-handed and right-handed helical gears with the same twist angle is called a double helical gear. Double helical gears transmit rotational motion between two parallel shafts. They have all the advantages of helical gears — strong and quiet — plus the benefit of cancelling axial thrust forces by combining helical gears with both right- and left-hand twists.

Double helical gears and herringbone gears are the same types of gear, but those with a groove in the centre are called double helical gears, while those without a groove where the left and right teeth meet are called herringbone gears. Both transmit motion exclusively between parallel shafts.

Power Generation & Turbines

Steam turbine and gas turbine reduction gearboxes at very high continuous power. Double helical gears eliminate the thrust bearing stage, reducing losses and maintenance. 20CrNiMo carburised, tooth-ground to DIN 6 accuracy is the standard specification.

Marine Propulsion

Ship propulsion reduction gearboxes — among the largest applications for double helical gears. Marine drives require classification society documentation (DNV, ABS, Lloyd's Register). Nitrided surfaces improve corrosion resistance in marine atmospheres.

Rolling Mills

Steel and aluminium rolling mills with frequent load reversals and shock events. Through-hardened 42CrMo at 280–320 HB provides core ductility needed to absorb impact without case spalling.

Mining Equipment

Bucket wheel excavator drives, conveyor head drives, and ball mill drives requiring continuous high torque with limited maintenance access. Highest available torque density per unit of centre distance.

Oil & Gas Compressors

Integrally geared centrifugal compressors for gas pipeline and process plant. Zero axial thrust is critical — the compressor impeller already generates axial thrust, and any additional thrust from gear stages compounds the impeller bearing load.

Cranes & Heavy Industrial

Large crane hoisting gearboxes, cement mill drives, sugar mill drives, and pump station reduction gears. The axial self-centering property is particularly valuable in long, heavy gearbox shafts subject to thermal expansion.

If the axial thrust cancels internally, why do double helical gears still require precise alignment?

The thrust cancellation depends on both helical faces being equally and correctly loaded. If misaligned, one face carries more load than the other — cancellation is incomplete and a net axial force develops. More critically, misalignment shifts the tooth contact zone toward one edge of each face, concentrating stress and initiating pitting at a fraction of the design torque. Alignment requirements are therefore more stringent for double helical gears, not less, because optimal performance depends on symmetric loading of both tooth faces simultaneously.

What are the consequences of incorrect central groove dimensions?

The groove provides runout space for the cutting tool when machining each helical face. If too narrow, the cutter cannot complete the tooth form near the groove. If too wide, it reduces the effective face width of each helical section below the design value, lowering load-carrying capacity and contact ratio. Korea Ever-Power's dimensional inspection report includes groove width and depth verification for every double helical gear.

Can you produce a double helical gear from a worn sample without a drawing?

Yes. Send us the gear with outer diameter, tooth count, face width, bore, hub diameter, and hub length. Our engineering team measures the helix angle directly from the sample — a one-degree error in helix angle causes offset tooth contact and reduces thrust cancellation. A production drawing is prepared and submitted for customer approval before cutting begins.

How does lubrication work for high-power double helical gears?

High-power double helical gears use splash or pressure-fed oil lubrication. The central groove creates a potential drainage path — oil flung by centrifugal force tends to drain from the central section. Lubrication systems must deliver oil to the mesh zone on both tooth faces, typically via spray nozzles aimed at both incoming and outgoing mesh zones, positioned to compensate for centrifugal drainage.

What information is needed to get a quotation?

For a new design: module, tooth count, helix angle and hand on each face, pressure angle, face width, central groove width, bore type, material, heat treatment, accuracy class, and documentation requirements. For replacement from a worn sample: the measurements above, plus whether the original drawing is available. Contact our engineering team and we will return a feasibility review and price within two working days.

"We replaced double helical gear sets in a 12 MW steam turbine reduction gearbox — the original OEM had ceased support for this gear size. Korea Ever-Power reverse-engineered from our worn sample and produced to DIN 6 accuracy in 20CrNiMo carburised. The UT report, heat treatment curve, and dimensional inspection package met our power station operator's maintenance documentation requirements. Gearbox has run 18 months since overhaul without issue."

Kim Hyeong-jun  |  Chief Mechanical Engineer, Gyeongnam Power Generation Co.  ·  Q1 2026

"We switched from single helical to double helical gears in our latest marine propulsion reduction gearbox generation. Eliminating the thrust bearing stage reduced gearbox length by 220 mm and weight by 380 kg at the same rated power. DNV material certificates and dimensional reports accepted without query on every delivery."

Park Seung-min  |  Gearbox Design Manager, Busan Marine Propulsion Systems  ·  Q3 2025

"We needed replacement double helical gear sets in 42CrMo through-hardened for a hot strip rolling mill main reduction drive. Shock loading during strip entry was fracturing carburised replacement gears from our previous supplier. Ever-Power's team correctly identified through-hardening as the appropriate route. No tooth fractures in 14 months of operation across two rolling lines."

Lee Jae-won  |  Plant Maintenance Manager, Incheon Steel Processing  ·  Q4 2025

"We supply integrally geared compressors for natural gas pipeline service. The double helical bull gear and pinion sets from Ever-Power in 20CrNiMo carburised to our DIN 5 drawing have passed incoming CMM inspection on all three batches received. Central groove width consistent to within 0.05 mm batch to batch."

Choi Byeong-soo  |  Component Procurement Lead, Daejeon Compressor Technology  ·  Q2 2025

"We needed nitrided double helical gears for an offshore platform reduction gearbox — salt-air environment was causing corrosion pitting on through-hardened tooth flanks. Ever-Power specified 31CrMoV9 nitriding steel to our case depth requirement. Post-nitriding dimensions were within drawing tolerance without grinding. Corrosion pitting eliminated in two years since installation."

Ryu Chang-hyun  |  Reliability Engineer, Ulsan Offshore Plant Services  ·  Q1 2026