The ISO 54 Preferred Number Series for Helical Gear Modules
Module Mn in a helical gear is standardised by ISO 54 into two series of preferred numbers. Using a preferred module ensures that standard hobs, grinding wheels, and inspection arbors are available for production — specifying a non-standard module (e.g. Mn = 3.7) requires custom tooling that adds lead time and cost. The ISO 54 series:
| First Choice (mandatory for new designs) | Second Choice (only when first choice does not work) | ||
|---|---|---|---|
| Module | Typical Application Range | Module | Where Second Choice Is Used |
| M1 | Instruments, precision mechanisms | M1.125 | Rarely; non-standard tooling required |
| M1.25 | Small mechanism drives, electronics | M1.375 | Space constraints only |
| M1.5 | Precision instruments, fine mechanism | M1.75 | Centre distance constraints |
| M2 | Light automation, food machine, servo | M2.25 | Replacement of imperial 10 DP gears |
| M2.5 | Light industrial, small gearboxes | M2.75 | Space constraints |
| M3 | General light industrial, packaging | M3.5 | — |
| M4 | General industrial, medium servo | M4.5 | — |
| M5 | Standard industrial — the most common module for enclosed gearboxes | M5.5 | — |
| M6 | Medium industrial, pump and fan drives | M7 | — |
| M8 | Heavy industrial, crane, conveyor | M9 | — |
| M10 | Heavy industrial, rolling mill auxiliaries | M11 | — |
| M12 | Large industrial, marine enclosed gearboxes | M14 | — |
| M16 | Heavy-duty drives, kiln enclosed stages | M18 | — |
| M20 | Very heavy duty, cement mill pinions | M22 | — |
| M25 | Large open gear drives | M28 | — |
| M32, M40, M50 | Very large open gear bull and ring gears | M36, M45 | — |
Pitch Diameter Reference — Module × Tooth Count at β = 20°
For a helical gear with normal module Mn at helix angle β = 20°, pitch diameter d = Mn × z / cos 20° = Mn × z / 0.9397 = Mn × z × 1.064. The table below covers the most common module and tooth count combinations:
| z (teeth) | M3 d (mm) | M4 d (mm) | M5 d (mm) | M6 d (mm) | M8 d (mm) | M10 d (mm) |
|---|---|---|---|---|---|---|
| 18 | 57.5 | 76.7 | 95.9 | 115.0 | 153.4 | 191.7 |
| 20 | 63.9 | 85.2 | 106.5 | 127.8 | 170.3 | 212.9 |
| 24 | 76.7 | 102.2 | 127.8 | 153.3 | 204.4 | 255.5 |
| 30 | 95.9 | 127.8 | 159.8 | 191.7 | 255.5 | 319.4 |
| 36 | 115.0 | 153.4 | 191.7 | 230.0 | 306.7 | 383.4 |
| 48 | 153.4 | 204.4 | 255.5 | 306.7 | 409.0 | 511.0 |
| 60 | 191.7 | 255.5 | 319.4 | 383.4 | 511.0 | 638.8 |
| 80 | 255.5 | 340.7 | 425.9 | 511.0 | 681.4 | 851.7 |
d = Mn × z / cos β at β = 20°. All values in mm.
Face Width Guidelines — b/d and b/Mn Rules
The face width b of a helical gear must satisfy two independent requirements: (1) it must provide adequate overlap contact ratio ε_β for the full noise and load capacity benefit of the helical form; (2) it must not exceed the practical ratio b/d₁ that shaft deflection and gear body stiffness can support without unacceptable edge loading.
| Rule | Formula | Typical Range | Why This Limit |
|---|---|---|---|
| Minimum face width for ε_β ≥ 1 | b_min = π × Mn / sin β | At β=20°: b_min ≈ 9.2 × Mn | Below this, ε_β < 1 and the full helical advantage is not realised |
| Recommended face width | b = 10–15 × Mn | ε_β = 1.1–1.6 at β=20° | Comfortable margin above ε_β = 1; practical for most gearbox proportions |
| Maximum b/d₁ ratio | b ≤ 1.5 × d₁ (soft flank); b ≤ 1.1 × d₁ (hard flank) | b/d₁ = 0.5–1.0 typical | Above this, shaft deflection and gear body twist cause severe edge loading even with crowning |
| Maximum face width for standard housing | b ≤ 0.6 × a (centre distance) typically | Set by housing width and shaft bearing span | Wider face width requires longer shaft span with proportionally more shaft deflection |
Example: M5, z₁=24, β=20° → d₁ = 127.8 mm
b_min (ε_β ≥ 1) = π × 5 / sin 20° = 46.1 mm
Recommended b = 12 × Mn = 60 mm (ε_β = 1.30)
Max b = 1.1 × d₁ = 140.6 mm (hard flank, with crowning)
Practical standard: b = 60–80 mm for this gear
Standard Bore Diameter Guidelines for Helical Gear Blanks
The bore diameter of a helical gear is determined by the transmitted torque and the shaft material strength. A practical starting point for steel shafts transmitting the full rated torque:
d_bore_min ≈ 4.5 × ∛T [mm, where T is in N·m, for standard alloy steel shaft]
The bore must also be large enough to maintain adequate wall thickness between bore and root circle of the gear teeth. Korea Ever-Power requires a minimum wall thickness of 2.5 × Mn between the bore and the root circle for structural integrity — ensuring the gear blank does not crack at the root-to-bore junction during heat treatment or under service loading. The bore tolerance for keyed connections is H7; for interference-fit connections, H7 or H6 depending on the required fit.
Korea Ever-Power Supply Range — M1 to M50
Korea Ever-Power’s production range covers Mn = M1 to M50, OD 20 mm to 2500 mm — from fine-pitch instrument helical gears to large-module open drive components. All sizes use ISO 54 preferred modules as standard; non-standard modules accepted with custom tooling lead time.
Korea Ever-Power manufactures helical cut gears from Mn = M1 through M50, OD 20–2500 mm, DIN Class 3–9, in all standard materials from 45# QT through 17CrNiMo6 carburized and SS316L stainless. All modules from the ISO 54 first-choice series are standard production items with available tooling; second-choice modules and non-standard modules (e.g. converted imperial DP sizes) are available with a tooling lead time of 2–4 weeks. As a direct helical gear manufacturer, Korea Ever-Power holds master hobs and grinding dressing profiles for all first-choice ISO 54 modules from M1 to M32 on permanent tooling stock.
Frequently Asked Questions
Module Mn and diametral pitch DP (US/imperial) are inversely proportional: Mn = 25.4 / DP. Common equivalents: DP 8 = Mn 3.175 (close to M3); DP 6 = Mn 4.233 (close to M4); DP 5 = Mn 5.08 (close to M5); DP 4 = Mn 6.35 (close to M6); DP 3 = Mn 8.47 (close to M8). For replacement helical gears from US-designed machinery, Korea Ever-Power can produce to the exact imperial DP module (e.g. DP 6 exactly, not M4.233 rounded) using custom tooling, or can produce the nearest standard ISO module (M4) with a small centre distance or profile shift adjustment. Korea Ever-Power confirms which approach is correct after reviewing the housing centre distance and mating gear specification.
The overlap contact ratio ε_β = b × sin β / (π × Mn) must be ≥ 1.0 for the full helical advantage to be realised — meaning at least one additional full tooth pitch is simultaneously in contact at all times. Solving for b_min at ε_β = 1.0: b_min = π × Mn / sin β. At β = 20°: b_min = π × Mn / 0.342 = 9.19 × Mn. Below this face width, there are instants in the mesh cycle when a single tooth pair carries the full transmitted load — the same condition as a spur gear. All the noise and load capacity benefits of the helical form depend on ε_β ≥ 1. This is the most frequently violated face width rule in preliminary gear designs where the face width is set by packaging constraints without checking ε_β.
Standard CNC gear hobbing machines handle Mn = M1–M50+ without a fundamental limitation — the hob dimensions and machine power scale with module. CNC gear grinding (HÖFLER-type generating grinding) is practically limited to approximately Mn = M32–M40 by grinding wheel size and profile stiffness; very large modules (M40–M50) are typically finish-ground using single-point grinding or profile milling rather than generating grinding. Korea Ever-Power’s in-house grinding capability covers Mn = M1 to M36 in generating grinding and M36 to M50 in form grinding, covering the full module range for enclosed gearbox helical gears.
Non-standard modules are necessary in two situations: (1) replacement of an existing helical gear that was originally designed to an imperial DP (common in US and UK machinery), or to a non-ISO module specification from older equipment; (2) a very specific centre distance constraint that cannot be met with standard modules and profile shift. Korea Ever-Power handles non-standard modules by producing a custom hob (2–4 week additional lead time and tooling cost of 800–3000 USD depending on module) or by using profile shift on the nearest standard module to achieve the required centre distance. For the second option, Korea Ever-Power calculates the required profile shift coefficient and confirms that tooth undercut and tip narrowing remain within acceptable limits.
Confirm Your Helical Gear Module and Size
Provide your power, speed, centre distance, and ratio. Korea Ever-Power selects the correct ISO 54 module, calculates pitch diameter and face width, checks ε_β ≥ 1 compliance, and confirms tooling availability — at the quotation stage before any production commitment.
M1–M50 · ISO 54 preferred series · ε_β verification · Face width calculation · DP-to-module conversion · MOQ 1 piece
Editor: Cxm
