Custom Helical Gears
CNC Hobbed & Ground

A helical gear is a cylindrical gear whose teeth are cut at an angle — the helix angle — to the gear's rotational axis, rather than parallel as in a spur gear. This angled tooth form means multiple teeth engage simultaneously, producing a gradual, overlapping contact pattern. The result is quieter operation, higher load capacity, and smoother transmission compared to straight-cut spur gears, at the cost of an axial thrust force that must be handled by thrust bearings. Helical gears are the dominant gear type in automotive gearboxes, industrial gear reducers, and high-speed power transmission.

An internal helical gear (also called a helical ring gear or annular helical gear) has helical teeth cut on the inside of a hollow ring rather than the outside of a cylinder. It meshes with an external helical pinion that sits inside the ring. The pinion and ring rotate in the same direction, allowing very compact reduction designs — which is why internal helical gears are the foundation of planetary gearboxes, integrated wheel hubs, and high-ratio inline helical gear reducers. Internal helical gears cannot be hobbed and must be manufactured by gear shaping, broaching, or wire EDM, which makes them more expensive than external helical gears but irreplaceable for compact, high-ratio systems.

Helical gears are used wherever smooth, quiet, high-speed power transmission is needed between parallel shafts. Key applications include: automotive gearboxes and EPS systems; inline helical gear reducers for conveyors and industrial drives; helical gear motors and helical geared motors; helical gear pumps for hydraulic and lubrication systems; aerospace actuators; robotic servo gearheads (helical servo gears); and helical gear rack and pinion systems for machine tool linear axes. Crossed helical gears transmit motion between non-parallel, non-intersecting shafts.

Helical gears transmit power between parallel shafts using angled teeth on a cylindrical body. Bevel gears — including helical bevel gears — transmit power between intersecting shafts, typically at 90°, using teeth on a conical body. A helical bevel gear combines both: conical body for shaft angle change plus helical (curved) teeth for smoother engagement. In gearbox terminology, a bevel helical gear reducer or helical bevel gear unit uses a bevel stage for the right-angle turn and a helical stage for the main speed reduction.

The classic helical gear vs spur gear (or spur gear vs helical gear) comparison: Helical gears are better for high-speed operation, low noise, high load, and smooth transmission — which is why automotive gearboxes and industrial inline helical gear reducers use them. Spur gears are better for simplicity, zero axial thrust, lower cost, and precise positioning at moderate speeds. For most continuous-duty power transmission at speeds above ~3 m/s pitch velocity, choose helical. For instruments, indexing drives, and low-speed machinery, spur gears are the practical choice.

For the same module and face width, helical gears are 20–40% stronger in bending and surface fatigue than spur (straight-cut) gears. This is because helical teeth have a longer contact line (the diagonal mesh distributes load over more tooth area) and a higher contact ratio. In the straight cut vs helical gears debate: helical wins on strength and noise; straight cut wins on simplicity and no axial thrust. Are straight cut gears stronger than helical? No — helical gears carry more load at the same tooth size.

Helical gears are better than spur gears in most continuous-duty power transmission applications: quieter (the biggest advantage of helical gears over spur gears), stronger, smoother, and capable of higher speeds. The advantages of helical gears over spur gears also include higher contact ratio and the ability to carry more torque per unit size. However, the one disadvantage of helical gears is the axial thrust force, which requires thrust-capable bearings and adds to system cost. For low-speed, low-load, or precision indexing applications, spur gears remain the simpler and cheaper choice.

Helical gears cost 10–30% more than equivalent spur gears of the same module and material. The premium comes from: more complex hobbing setup (angled hob head), higher tooling wear due to the helical cutting path, and the need for thrust bearings in the assembly. However, for high-volume production using CNC helical gear hobbing, the per-piece cost difference narrows significantly. Custom helical gears for low volumes cost more due to setup time. At HXC, we provide competitive helical gear pricing and helical gear quotes from prototype (1 pc) to wholesale production with no tooling fee.

Yes — helical gears typically outlast spur gears in comparable applications for two reasons: first, the distributed contact load reduces contact stress (Hertzian pressure) on the tooth flanks, slowing pitting fatigue; second, the gradual tooth engagement reduces dynamic loads and impact stresses that cause fatigue cracks in spur gear roots. In automotive gearboxes, the shift from spur to helical gears in the 20th century was driven partly by durability as well as noise. With proper lubrication and a correct helix angle, helical gears in good working conditions can last the lifetime of the machine.

The main disadvantages of helical gears: (1) Axial thrust — the helix angle generates a force component along the shaft axis, requiring thrust bearings or a double helical (herringbone) arrangement to cancel it. (2) Higher manufacturing cost vs spur gears. (3) More complex design — helix angle, lead, and hand must all be specified and matched between mating gears. (4) Efficiency slightly lower than spur gears due to the sliding component in helical tooth contact (though the difference is small — typically 0.5–1% per mesh). The double helical gear advantages include eliminating axial thrust at the cost of even higher manufacturing complexity.

A right hand helical gear has teeth that spiral up and away from the viewer in a clockwise direction (like a standard screw thread); a left hand helical gear spirals counter-clockwise. For external helical gears meshing on parallel shafts, one must be left hand and the other right hand — same hand will not mesh. For crossed helical gears (non-parallel shafts), both gears use the same hand. The hand must always be specified on the drawing alongside the helix angle. HXC supplies left and right hand helical gears as matched pairs, with hand verification on every CMM inspection report. Hand also affects the direction of axial thrust — useful when designing thrust bearing arrangements.

Helical gears are significantly quieter than spur gears, especially at high speeds. The angled teeth create a gradual, overlapping engagement rather than the simultaneous full-tooth impact of spur gears. Noise increases with: helix angle accuracy errors (grind to DIN 5–6 for quiet operation), bearing misalignment, inadequate lubrication, and worn flanks. At very high speeds or with large helix angles, axial vibration can become a noise source. Double helical gears are quietest because the opposing helix forces cancel, eliminating axial oscillation.

Yes — helical gears can be back-driven just like spur gears, because their tooth normal angles create low friction relative to the driving force. This distinguishes helical gears from worm gears, which are self-locking at low lead angles. The helical worm gear (crossed helical gear) is a special case: at very high reduction ratios and low lead angles on the worm element, it may become self-locking. Standard parallel-axis helical gear sets — in inline helical gear reducers or gearboxes — are always reversible and can be back-driven freely.

Yes — steel helical gears require lubrication for efficient, long-lasting operation. The sliding action between helical teeth (from the helix angle) is greater than in spur gears, generating more heat. Gear oil viscosity should be selected based on pitch-line velocity, load, and operating temperature — typically ISO VG 150–320 for industrial helical gear reducers. The exception is plastic helical gears (POM, Nylon): these are often run dry because the low-friction polymer surface and lower contact loads make lubrication unnecessary or even harmful. In miniature helical gears for instruments, a light grease application is often sufficient.

The best material depends on the application. For automotive and industrial helical gear reducers: 20CrMnTi (case-hardened, HRC 58–62) is the strongest material for helical gears under high cyclic load. For heavy-duty industrial: 42CrMo4 through-hardened. For lightweight aerospace: Al 7075-T6 or Grade 5 titanium. For helical gear pumps and food/medical: SUS316L stainless. For low-noise automation: POM plastic helical gears. For high-temperature environments: PEEK miniature helical gears. We stock and certify all these materials with full mill certificates.

For maximum surface and bending fatigue strength, case-hardened 20CrMnTi steel (carburized to 0.8–1.2 mm case depth, surface HRC 58–62, core HRC 30–45) is the standard choice for high-performance helical gears in automotive gearboxes. M2 tool steel ground to DIN 4 achieves the highest surface hardness but is more brittle. For aerospace helical gears where weight matters, Grade 5 titanium offers the best specific strength. For plastic helical gears, PEEK GF30 gives the highest strength-to-weight among engineering polymers.

The optimal helix angle for helical gears balances noise reduction, load capacity, and axial thrust. In practice: 15°–25° is the most common range for industrial and automotive gearboxes — this gives a significant improvement in contact ratio and noise over spur gears while keeping axial thrust manageable. 30°–45° helix angles (used in high-speed turbine gearboxes) maximise smoothness but require heavy-duty thrust bearings or a double helical arrangement. Standard helical gears from catalogue suppliers typically use 15° or 19.5° helix angles for interchangeability. For custom helical gears, our engineers recommend 20° as the default unless your application demands otherwise.

The 17-tooth rule states that a standard 20° pressure-angle spur gear needs at least 17 teeth to avoid undercutting. For helical gears, the effective tooth count is the virtual number of teeth (actual teeth ÷ cos³ψ, where ψ is the helix angle), which is always higher than the actual count. This means a helical gear can have fewer physical teeth than 17 without undercutting — one of the advantages of helical gears over spur gears for compact, high-ratio single-stage reductions. For example, a helical pinion with 14 actual teeth at 20° helix has a virtual tooth count of ~16.8, safely above the undercut limit. For very small tooth counts, our engineers apply profile shifting to eliminate undercutting entirely.