Why Helical Gear Drawings Are Different from Standard Machined Part Drawings
A standard machined part drawing specifies geometry through dimensions and geometric tolerances — diameter, length, concentricity, runout, surface finish. A helical gear drawing must specify all of these plus a completely separate set of gear-specific parameters that cannot be expressed as simple linear dimensions: the module (which determines tooth size), the helix angle and hand (which determine the contact geometry), the pressure angle (which determines the tooth profile shape), the accuracy class (which sets tighter tolerances than any standard GD&T tolerance applies to), and the backlash class (which specifies deliberate deviation from nominal tooth thickness).
This is why a helical gear drawing contains a gear data table (also called a Verzahnungstabelle or gear parameter block) alongside the conventional dimensional views. The gear data table contains all gear-specific parameters; the drawing views contain all geometric dimensions (bore, OD, face width, keyway, shoulder positions). Both sections of the drawing are mandatory — neither substitutes for the other. Korea Ever-Power reviews both sections for every incoming helical cut gear drawing to identify missing or conflicting parameters before production begins.
The Gear Data Table — Complete Parameter List
The gear data table for a helical gear must contain at minimum the following parameters. Each is explained below the table:
| Parameter | Symbol | Example Value | Standard Reference | Omission Consequence |
|---|---|---|---|---|
| Normal module | Mn | 5 | ISO 54 | No cutter can be selected; fundamental parameter |
| Number of teeth | z | 24 | — | Gear ratio and pitch diameter cannot be confirmed |
| Helix angle | β | 20° | — | Centre distance and hob tilt angle cannot be set |
| Helix hand | RH or LH | Right (RH) | — | Most common omission. Produces a gear that cannot mesh with its partner or drives axial thrust in the wrong direction |
| Normal pressure angle | α_n | 20° | ISO 53 | Cutter profile angle unknown; assume 20° but verify if non-standard |
| Addendum modification (profile shift) | x_n | 0 (or e.g. +0.25) | ISO 53 | If omitted, standard (x=0) is assumed — wrong if gear has non-standard centre distance |
| Accuracy class | — | DIN 3962 Class 6 | DIN 3962 / ISO 1328 | Tolerance for profile, lead and pitch deviations is undefined; any accuracy is acceptable |
| Tooth thickness / backlash class | — | DIN 3967 − ef | DIN 3967 | Very commonly omitted. Without this, backlash is uncontrolled — gear may fit the housing with too much or too little clearance |
| Tooth root form | — | Full radius root (ISO 53) | ISO 53 | If shallow root or protuberance is required, it must be stated explicitly |
Geometric Dimensions — What the Drawing Views Must Show
The drawing view of a helical gear defines all geometric dimensions — OD, bore, face width, keyway, shoulder, and hub geometry. These are separate from the gear data table but equally mandatory for a complete specification
Bore Tolerance — The Most Critical Dimensional Specification
The bore of a helical gear is its most critical mating dimension — it determines how the gear locates on the shaft and transmits torque. The correct bore tolerance depends on the connection type:
| Connection Type | Bore Tolerance | Shaft Tolerance | Application |
|---|---|---|---|
| Keyed — standard clearance | H7 | k6 or m6 | Most industrial gearboxes; removable for maintenance |
| Keyed — precision | H6 | k6 | High-speed or high-precision drives where minimal radial runout matters |
| Interference fit (press) | H7 | p6 or r6 | Permanent assembly; no keyway needed; high torque without fretting |
| Spline (involute spline) | Per ISO 4156 | Per ISO 4156 | High-torque detachable; automotive and heavy industrial |
Bore-to-OD Concentricity (Radial Runout Tolerance)
The concentricity of the bore relative to the gear’s pitch cylinder OD determines how much radial runout the assembled gear has on the shaft. For standard industrial helical gears: bore-to-OD concentricity tolerance = 0.5 × the pitch deviation tolerance for the specified DIN class. For precision automotive and CNC gears: concentricity tolerance = 0.25 × the DIN class pitch deviation. This must be specified as a GD&T symbol on the drawing view — it cannot be implied by the accuracy class alone.
Face Runout
Face runout (perpendicularity of the gear face to the bore axis) determines the lead deviation contribution from gear mounting. If the gear face is not perpendicular to the bore, the gear tilts on the shaft and adds a systematic lead deviation across the face width. Standard tolerance: face runout = 0.7 × the lead deviation tolerance for the specified DIN class.
Material and Heat Treatment Callout
The material specification on a helical gear drawing must include three elements: the steel grade designation (per national standard), the heat treatment condition, and the resulting hardness requirement. All three are necessary because the steel grade alone does not determine the hardness — 42CrMo can be QT to HB 280 or induction hardened to HRC 52, giving very different fatigue strengths. The three-element callout:
Material: 42CrMo4 (DIN EN 10083) — Induction hardened — HRC 50–55
Case depth min. 1.0 mm at HRC 45
Material: 20CrMnTi — Gas carburized, quenched & tempered — HRC 58–62
Case depth 0.8–1.2 mm effective
For the carburized case depth, specify both the “effective case depth” (depth to HV 550 hardness) and the “total case depth” (depth to core hardness + 50 HV) — these are different measurements and both may be required by the application specification.
Surface Finish Requirements
Surface finish on a helical gear drawing appears in two places: on the tooth flank surfaces (Ra specification) and on the bore, OD, and face (standard machined finish Ra). The tooth flank Ra is determined by the manufacturing process and accuracy class:
| Process | DIN Class Achievable | Tooth Flank Ra | Drawing Callout |
|---|---|---|---|
| Precision hobbing (Class AA hob) | DIN 6–7 | Ra 1.6–2.5 µm | Ra ≤ 2.5 µm on tooth flanks |
| HÖFLER generating grinding | DIN 4–6 | Ra 0.2–0.6 µm | Ra ≤ 0.4 µm on tooth flanks (standard) or Ra ≤ 0.2 µm (automotive) |
| Gas nitriding (no grinding) | DIN 6–7 | Ra 0.4–0.8 µm | Ra ≤ 0.8 µm on tooth flanks |
Five Most Common Helical Gear Drawing Errors
The drawing specifies β = 20° but omits RH/LH. The manufacturer produces a gear with the correct angle but the wrong hand — it physically cannot mesh with the partner gear. Always state helix hand explicitly, even when it seems obvious from context.
Only DIN 3962 accuracy class is stated. Without DIN 3967, the manufacturer may produce a correctly profiled gear with any tooth thickness from nominal to badly undersize — resulting in uncontrolled backlash from near-zero to excessive.
The gear was designed with a profile shift to achieve a non-standard centre distance, but x_n = 0 is assumed when not stated. The manufactured gear fits the housing with the wrong backlash or causes interference with the mating gear.
“Case depth 1.0 mm” without specifying whether this is effective case depth (to HV 550) or total case depth (to core + 50 HV). These differ by 30–60%. The gear is produced with the correct number but wrong metallurgical interpretation.
Bore diameter given as “Ø50” without H7, H6, or other tolerance. Manufacturer applies ISO H11 (very loose general tolerance) rather than H7 (standard gear bore fit). Gear fits shaft with excessive radial play, causing fretting and fatigue at the bore.
Korea Ever-Power — Drawing Review Service
Korea Ever-Power reviews every helical gear drawing for completeness and consistency before production begins — identifying missing parameters, conflicting specifications, and non-standard combinations that would produce an incorrect part
Korea Ever-Power’s engineering team reviews every submitted drawing for completeness before production begins. Missing parameters are flagged and confirmed with the customer; conflicting specifications (e.g. a face width that gives ε_β < 0.5 for the specified helix angle, or a bore tolerance incompatible with the specified fit) are identified and resolved. As a direct helical gear manufacturer, Korea Ever-Power’s engineers review hundreds of helical cut gear drawings annually — the five errors above are the most frequently encountered, and catching them before production begins saves weeks of remake time.
Frequently Asked Questions
DIN 3962 and ISO 1328 define the same accuracy parameters (profile deviation, lead deviation, pitch deviation) but with different tolerance band values and notation. DIN 3962 uses classes 1–12 (lower number = tighter tolerance). ISO 1328-1 uses grades 0–12 (lower number = tighter). DIN Class 6 is approximately equivalent to ISO Grade 6 for most practical gear sizes and modules, but the exact tolerance values differ slightly. For international procurement of helical gears, specify both standards when possible: “DIN 3962 Class 6 / ISO 1328-1 Grade 6” — or specify the actual measured tolerance values rather than just the class designation.
A complete written specification containing all parameters in the gear data table (Mn, z, β, helix hand, α_n, x_n, accuracy class, backlash class, material, heat treatment, bore diameter with tolerance, face width, OD, keyway dimensions) is sufficient for Korea Ever-Power to produce the gear without a formal engineering drawing. However, complex geometries — integral flanges, multiple shaft steps, non-standard hub profiles — must be dimensioned on a drawing because a written specification cannot unambiguously describe 3D geometry. For simple cylindrical helical gears (hub or disk form without complex features), a complete parameter specification is accepted.
Yes — this is Korea Ever-Power’s reverse engineering service. The worn helical gear is measured by gear analyser (profile, lead, pitch, helix angle), OES spectrometer (material grade), CMM (bore, OD, face width, keyway, shoulder dimensions), and surface profilometer (tooth flank Ra). From these measurements, Korea Ever-Power produces a complete engineering drawing with all parameters in the gear data table, verified against the measured dimensions. The drawing is reviewed with the customer before production begins.
The gear OD (addendum circle diameter, da) is a derived parameter — it equals Mn × (z + 2) / cos β for a standard addendum gear. It is therefore shown on the drawing view as a reference or verification dimension rather than in the gear data table as a defining parameter. If the OD on the drawing view conflicts with the calculated value from Mn, z, and β, the gear data table parameters take precedence — the OD view dimension is for inspection reference only. This hierarchy is important for helical gear reverse engineering: if the OD is slightly worn, the exact OD measurement cannot be used to back-calculate Mn and z — the gear analyser measurement of Mn, z, and β is the reliable source.
Submit Your Helical Gear Drawing for Review
Korea Ever-Power reviews every drawing for completeness before production begins — confirming all gear data table parameters, geometric tolerances, and material specification. Submit as PDF, DWG, or STEP. Drawing review and production quotation returned within 24 working hours.
PDF · DWG · STEP · Written spec accepted · Drawing review included · Reverse engineering available · MOQ 1 piece
