Double Helical Gear Design, Manufacturing & Applications Guide
Quick Answer
A double helical gear combines two mirrored helical gear sections on one blank so their opposing helix angles cancel axial thrust, and Emitech manufactures these precision power-transmission components by metal injection molding (MIM), CNC hobbing/shaping, and precision grinding for automotive, marine, industrial, and aerospace customers. Because the left-hand and right-hand helix generate equal and opposite axial forces, double helical gears run with very low vibration and no net thrust, making them ideal for high-speed, high-load gearboxes. We produce modules from 0.5 to 6, diameters from 10 mm to 400 mm, and quality levels up to ISO 1328 Grade 5.
When a gearbox must transmit high power quietly without overloading bearings, engineers often choose a double helical gear. This geometry solves the axial-thrust problem of single helical gears by using two opposite helix hands on the same blank. The result is smoother engagement, higher load capacity, and longer bearing life.
What Is a Double Helical Gear?
A double helical gear is a cylindrical gear with two sets of helical teeth of equal helix angle but opposite hand on the same blank. The central gap between the halves is called the groove; it provides tool clearance and prevents the two helixes from merging.
Each helical half produces axial thrust equal to approximately Ft · tan(β). Because the halves have opposite hands, their axial forces are equal and opposite. This axial thrust cancellation means bearings support only radial and moment loads, eliminating the need for thrust bearings required by single helical gears. Common helix angles range from 15° to 35°.
How We Manufacture Double Helical Gears at Emitech
Emitech is an ISO 9001:2015 certified custom gear manufacturer in Nanjing, China. We select the process for each double helical gear based on module, diameter, tolerance, volume, material, and geometry. Our gear manufacturing services integrate MIM, CNC machining, heat treatment, finishing, and inspection under one roof.
CNC Gear Hobbing
CNC hobbing is the most productive way to cut external double helical gears. The hob machines one helix hand, the blank is reversed, and the second helix is cut with the opposite hand. We cut modules from 0.5 to 6 and diameters from 20 mm to 400 mm.
CNC Shaping & Grinding
For internal profiles or shoulders that prevent hob access, CNC shaping is used. After heat treatment, profile or generating grinding corrects distortion and raises quality to ISO 1328 Grade 4–5. Honing further improves surface finish and reduces noise.
Metal Injection Molding
For double helical gears under 50 mm diameter or 50 g mass, metal injection molding offers a near-net-shape route. MIM forms both helix hands, hubs, bores, and undercuts with minimal chip waste, with grinding or honing added only when tighter tolerances are required.
Metal Injection Molding for Double Helical Gears
Metal injection molding is especially attractive for small, complex double helical gears produced in high volume. The MIM process blends fine metal powder with a polymer binder, injects the feedstock into a precision mold, removes the binder by solvent or thermal debinding, and sinters the brown part at high temperature. Because the mold already contains the mirrored helical tooth form, MIM produces net-shape double helical teeth without the multiple setups of conventional cutting.
At Emitech, we use MIM for double helical gears in power tools, automotive actuators, medical devices, robotics, and precision instruments. Typical MIM double helical gears range from module 0.3 to 1.5, with helix angles from 15° to 35°. As-sintered gear tolerance is usually ISO 1328 Grade 7–9; light grinding or honing is added for higher accuracy. Common gear materials include 316L stainless steel, 17-4 PH, MIM 4605, and 8620 low-alloy steel.
The economic crossover for MIM double helical gears typically occurs at 5,000 to 15,000 pieces per year. Once qualified, unit costs are low and repeatability is excellent. For buyers evaluating custom MIM parts, our engineers provide a free DFM review to confirm whether a design is suitable for MIM or should follow a CNC-plus-grinding route.
| Process | Best Volume | Typical Size | Typical Quality | Relative Cost |
|---|---|---|---|---|
| Metal Injection Molding (MIM) | 5,000+ / year | Module 0.3–1.5 | ISO 1328 Grade 7–9 | Low–Medium at volume |
| CNC Gear Hobbing | 500–100,000 / year | Module 0.5–6 | ISO 1328 Grade 6–8 | Medium |
| CNC Gear Shaping | 500–50,000 / year | Module 0.5–6 | ISO 1328 Grade 6–8 | Medium–High |
| Precision Grinding | 100–10,000 / year | Module 0.5–6 | ISO 1328 Grade 4–5 | High |
Materials & Heat Treatment
Material selection for double helical gears balances strength, wear resistance, corrosion resistance, and cost. Because the two helix hands share the load, surface durability remains critical due to sliding contact.
| Material | Typical Hardness | Key Properties | Common Applications |
|---|---|---|---|
| AISI 1045 Carbon Steel | 170–250 HB | Economical, good strength | General machinery, low-speed gearboxes |
| AISI 4140 Alloy Steel | 28–34 HRC (Q&T) | High strength, fatigue resistant | Industrial gearboxes, power transmission |
| AISI 8620 / 20MnCr5 | 58–64 HRC case | Hard surface, tough core | Automotive transmissions, high-load gears |
| 316L Stainless Steel | 180–220 HB | Corrosion resistant, biocompatible | Medical devices, marine, food equipment |
| 17-4 PH Stainless | 32–44 HRC (H900) | High strength + corrosion resistance | Aerospace, firearms, precision instruments |
Heat treatment is matched to load condition. Carburizing 8620 and 20MnCr5 creates a hard wear surface with a tough core for heavily loaded gears. Induction hardening suits moderate loads, while nitriding offers wear resistance with minimal distortion. Stainless grades such as 17-4 PH are precipitation hardened to H900 or H1150.
Quality Standards & Tolerances
Double helical gear quality is specified by standards that define allowable deviations in tooth profile, lead, pitch, runout, backlash, and symmetry between the two helix halves. At Emitech, we manufacture and inspect to ISO 1328, AGMA 2000, DIN 867, and customer-specific standards.
| Quality Level | ISO 1328 Grade | AGMA Quality | Typical Application | Process Required |
|---|---|---|---|---|
| Commercial | 8–10 | 6–8 | Appliances, hand tools, low-speed machinery | Hobbing, shaping, MIM |
| Precision | 6–7 | 9–11 | Automotive, industrial gearboxes, pumps | CNC hobbing, shaving, grinding |
| High Precision | 4–5 | 12–14 | Aerospace, high-speed drives, medical instruments | Gear grinding, honing, CMM verification |
Our metrology lab uses a CNC gear measuring center to inspect tooth profile, lead, pitch deviation, radial runout, and helix-hand symmetry. A CMM verifies bore position, hub concentricity, groove width, and overall dimensions. Production orders include first-article reports, material certificates, and SPC data.
Double Helical Gear Applications Across Industries
The combination of smooth engagement, high load capacity, and axial thrust cancellation makes double helical gears valuable across many industries. Below are the sectors where Emitech most often supplies them.
Marine & Offshore Propulsion
Large marine gearboxes use double helical gears to transmit megawatts of power from engines to propellers. Eliminating net axial thrust simplifies bearing and housing design in ship engine rooms.
Industrial Gearboxes & Rolling Mills
High-power parallel-shaft gearboxes and rolling-mill drives rely on double helical gearing for durability. The split torque path between the two helix hands improves reliability under shock loads.
Automotive & Electric Vehicles
Compact double helical gears are used in EV reduction gearboxes, transfer cases, and actuator systems where low noise and long bearing life are required. MIM is often the most cost-effective process for these smaller gears at volume.
Aerospace, Defense & Robotics
Actuator systems, flight-control mechanisms, and robotic joints use lightweight double helical gears in 17-4 PH, titanium, or Inconel. Precision grinding and CMM verification ensure tight backlash and runout, while proper backlash control keeps noise low during rapid direction changes.
Double Helical Gears vs Herringbone Gears
The terms double helical gear and herringbone gear are often used interchangeably, but there is a technical distinction. A herringbone gear has two opposite-hand helical teeth that meet at a sharp central apex with no groove. A double helical gear has the same geometry but includes a central groove between the two halves.
The groove makes double helical gears easier to machine because the cutting tool has clearance to exit between halves. Herringbone gears require specialized tooling and are more common in very large gearboxes where space is at a premium. Both forms deliver axial thrust cancellation and allow higher helix angles than single helical gears without thrust bearings.
Design Tips & Common Failure Modes
Good double helical gear design anticipates load sharing, bearing selection, lubrication, and heat-treatment response. Below are practical guidelines Emitech engineers apply during DFM reviews.
Symmetry & Groove Width
The two helix halves should be geometrically symmetrical so axial forces cancel evenly. The central groove must be wide enough for tool clearance, but not so wide that it weakens the web.
Helix Angle Selection
Higher helix angles reduce noise and increase overlap ratio, but also raise sliding velocity and heat generation. For general industrial use, 15°–25° is typical; for noise-critical applications, 25°–35°. Angles above 35° are rare because they demand extreme manufacturing accuracy.
Bearing Selection
Because net axial thrust is theoretically zero, double helical gears can use deep-groove or cylindrical roller bearings rather than angular-contact bearings. However, transient axial loads during engagement still require adequate bearing face support.
| Failure Mode | Typical Cause | Prevention Strategy |
|---|---|---|
| Pitting / Contact Fatigue | High contact stress, soft tooth surface | Increase hardness, improve finish, profile grinding |
| Tooth Bending Fatigue | Overload, small root fillet, poor material | Increase module, enlarge root fillet, select stronger alloy |
| Scuffing / Scoring | High sliding speed, poor lubrication | Use EP lubricant, reduce load, nitride or coat surfaces |
| Uneven Load Sharing | Asymmetric helixes, misalignment, thermal gradient | Tighten symmetry, improve alignment, balance housing stiffness |
| Axial Thrust Imbalance | Unequal helix angles, worn bearings, assembly offset | Match helix angles, pre-load bearings, control axial stack-up |
Frequently Asked Questions
Q: What is a double helical gear?
A cylindrical gear with two sets of helical teeth of equal angle but opposite hand on the same blank. The opposing helixes cancel axial thrust, making it ideal for high-power, low-noise gearboxes.
Q: Why are double helical gears used instead of single helical gears?
Double helical gears eliminate net axial thrust, so bearings and housings do not react large axial loads. This allows smoother, quieter operation and longer bearing life.
Q: What is the difference between a double helical gear and a herringbone gear?
A herringbone gear has two opposite-hand helical teeth meeting at a sharp apex with no groove. A double helical gear has the same geometry but includes a central groove for tool clearance.
Q: How does axial thrust cancellation work in a double helical gear?
Each helical half produces axial force equal to Ft · tan(β). Because the two halves have opposite hands, the forces point in opposite directions and cancel each other, leaving no net axial load on the shaft.
Q: Can double helical gears be made by metal injection molding?
Yes. MIM is excellent for small, complex double helical gears in high volume. It forms both helix hands, hubs, and bores in one or two molding actions, with grinding or honing added for tighter tolerances.
Q: What materials are best for double helical gears?
Carburizing steels such as 8620 suit high-load gears. 4140 is common for moderate loads. 316L or 17-4 PH stainless steel works for medical and corrosion-resistant applications. Brass and bronze suit low-load instruments.
Q: What quality level can Emitech hold on double helical gears?
We manufacture to ISO 1328 Grade 7–9 as-hobbed or as-sintered, Grade 6–7 after finishing, and Grade 4–5 after precision grinding or honing.
Q: What are common failure modes of double helical gears?
Common gear failure modes include pitting, tooth bending fatigue, scuffing, uneven load sharing between helix halves, and axial thrust imbalance caused by misalignment or worn bearings.
Get a Quote for Custom Double Helical Gears
Whether you need a single prototype double helical gear or a high-volume MIM double helical gear program, Emitech's engineering team can recommend the right material, process, and quality level for your application. Upload your drawing, 3D model, or sample photo and receive a detailed quotation with DFM feedback within 48 hours.
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