What You Need to Identify Before Ordering a Replacement Helical Gear
A complete replacement Stirnrad specification requires ten parameters. Some can be measured directly from the worn gear; some are calculated from measurements; a few (material grade, original accuracy class) can only be estimated or confirmed by physical testing of the worn gear:
| Parameter | How to Obtain Without Drawing | Accuracy Available |
|---|---|---|
| Normal module Mn | Calculate from OD measurement and tooth count (see method below) | ±0.05 Mn if OD measured to 0.01 mm |
| Tooth count z | Count directly; every tooth counted with fingernail or scribe | Exact — zero error possible |
| Helix angle β and hand (RH/LH) | Measure from tooth inclination on the face (see method below) | ±0.5° from protractor; ±0.05° from gear analyser |
| Normal pressure angle α_n | Assume 20° (ISO standard for most industrial helical gears) unless gear data plate states otherwise | Correct for 95%+ of industrial gears |
| Face width b | Measure directly with digital caliper | ±0.1 mm |
| Bore diameter | Measure with internal caliper or bore gauge | ±0.01 mm |
| Keyway dimensions | Measure width and depth with caliper; identify ISO key standard from dimensions | ±0.05 mm |
| Overall length / shoulder positions | Measure with caliper and height gauge | ±0.1 mm |
| Material grade | OES spectrometer at a gear manufacturer or certified metallurgical lab (spark test indicates carbon content approximately) | Exact if OES; approximate from spark test |
| Original DIN accuracy class | If the gear is severely worn, cannot be measured accurately. Order replacement at DIN 6 or better as standard; specify original class if gear analyser of a lightly worn area is feasible | Estimated from application type if worn gear cannot be measured |
Step 1 — Count the Teeth and Measure the Outer Diameter Precisely
The most critical measurement for identifying a replacement Stirnrad is the outer diameter (OD, also called addendum circle diameter d_a), measured with a vernier or digital caliper at the tooth tips. For a standard addendum Stirnrad at normal pressure angle 20°, the relationship between module, tooth count, helix angle, and OD is:
d_a = Mn × (z + 2) / cos β
To solve for Mn from the measured d_a and counted z, rearrange to:
Mn = d_a × cos β / (z + 2)
The challenge: both β and Mn are initially unknown. The practical approach is to first estimate β from a direct measurement (see Step 2), then calculate Mn and compare it against the ISO 54 preferred number series. If the calculated Mn is not an exact preferred number, adjust the β estimate slightly until Mn matches a preferred number. The solution that gives a preferred module (M2, M2.5, M3, M4, M5, M6, M8, M10, M12…) is almost certainly correct.
Worked Example — Identifying an Unknown Replacement Helical Gear
Measured OD: d_a = 139.2 mm (measured with caliper at 3 positions, mean value)
Tooth count: z = 24 (counted)
Estimated β from direct measurement: ~20°
Mn = 139.2 × cos 20° / (24 + 2) = 139.2 × 0.9397 / 26 = 130.8 / 26 = 5.03 mm
Result: Mn ≈ 5.03 → rounds to ISO preferred module M5 ✓
Verify: d_a (theoretical) = 5 × (24+2) / cos 20° = 5 × 26 / 0.9397 = 138.3 mm
Difference: 139.2 − 138.3 = 0.9 mm → This is tooth tip wear on the original gear. The replacement helical gear is M5, z=24, β=20°.
Step 2 — Measure the Helix Angle and Determine Left-Hand or Right-Hand
The helix angle β of an existing Stirnrad is measured as the angle between the tooth direction and the shaft axis, visible on the gear face width. A simple digital protractor against the tooth lead gives approximately ±0.5° accuracy — sufficient to identify the nearest standard helix angle (15°, 20°, 25°, 30°)
Direct Protractor Measurement (Workshop Method)
Place the gear on a flat surface with its shaft axis horizontal. Hold a digital angle gauge (inclinometer) against the tooth surface along the tooth lead direction. The angle displayed from the gear face (which is perpendicular to the shaft) is the helix angle β. Typical result: 18–22° for a 20° gear, 23–27° for a 25° gear. Compare against the standard helix angle series (15°, 20°, 25°, 30°) and select the nearest standard value. Confirm with the OD calculation: if the nearest standard β gives a Mn that matches a preferred number, that is the correct helix angle.
Identifying Right-Hand (RH) or Left-Hand (LH) Helix
Stand the Stirnrad upright with one face toward you. If the visible teeth slope from lower-left to upper-right — like the letter “/” — the gear is right-hand (RH). If the teeth slope from lower-right to upper-left — like “” — the gear is left-hand (LH). This must be specified on the replacement order — a RH gear cannot mesh with another RH gear on parallel shafts (both in the same direction of rotation). Korea Ever-Power confirms the helix hand from a photograph of the gear face when the worn gear is sent for reverse engineering.
Step 3 — Identify the Material Grade
Without a material certificate, the material of an existing Stirnrad must be estimated from physical observations and confirmed by testing:
Test with portable HRC tester on the tooth flank. HRC <35 = QT soft flank (45# or 40Cr); HRC 45–55 = induction hardened (42CrMo); HRC 58–62 = carburized (20CrMnTi or 17CrNiMo6). The hardness level immediately narrows the material to the correct tier.
Send the worn gear to Korea Ever-Power — OES spectrometer analysis on the gear body (a small area ground clean on the hub face) gives the exact chemical composition in 60 seconds, identifying the steel grade precisely (e.g. 20CrMnTi confirmed by C=0.19%, Mn=0.95%, Cr=1.05%, Ti=0.08%).
When material identity is uncertain, Korea Ever-Power recommends ordering the replacement Stirnrad in 20CrMnTi carburized (if original appears to be hard-flank) or 42CrMo QT (if soft-flank) — either is a safe replacement for most industrial gears in the same tier, with an equal or higher service life than the original.
What Information to Send to the Manufacturer
When ordering a replacement Stirnrad from Korea Ever-Power’s reverse engineering service, provide as much of the following as possible — Korea Ever-Power will identify any missing parameters from the worn gear measurement if the gear is shipped for inspection:
- Measured OD (at tooth tips): 3 measurements at 120° intervals, stated as “mean OD = __ mm (range __ to __ mm)”
- Tooth count z: confirmed by counting
- Face width b: measured with caliper
- Estimated helix angle and hand: “approximately 20°, right-hand helix”
- Bore diameter and tolerance: measured with bore gauge — state keyway dimensions if present
- Hub dimensions: OD, length, shoulder diameter (for pilot bore gears)
- Oberflächenhärte: measured with portable tester, HRC or HB stated
- Photographs: tooth face (showing helix direction), tooth flank (showing wear pattern), and bore area
Like-for-Like vs Upgraded Specification — Which to Choose
Korea Ever-Power gear analyser measuring a worn Stirnrad for reverse engineering — even a heavily worn gear retains its module, helix angle, and pressure angle in the unworn tooth root and tip zones, enabling full parameter extraction and production of an accurate drop-in replacement
When ordering a replacement Stirnrad, the question of whether to order like-for-like or upgrade the specification deserves deliberate consideration:
Identical material, hardness, module, and face width as the original. Correct when the original Stirnrad ran to an acceptable service life and the failure was due to end-of-life wear, contamination, or an isolated event (frozen charge, overload) — not fundamental underspecification. Lead time is typically 2–4 weeks shorter than an upgraded specification because the parameters are confirmed without additional engineering review.
Higher material grade (e.g. QT → carburized), wider face width (if housing allows), or higher DIN accuracy class than the original. Correct when the original failed at less than 60% of the expected service life, when the drive conditions have changed (higher load, more frequent starting), or when the original was specified at minimum cost and the current priority is maximum reliability. Korea Ever-Power identifies upgrade opportunities as part of the reverse engineering review.
Korea Ever-Power — Reverse Engineering and Replacement Helical Gear Service
Korea Ever-Power’s reverse engineering service accepts worn or broken schrägverzahnte Zahnräder for complete parameter extraction — gear analyser measurement (module, helix angle, pressure angle, pitch deviation), OES spectrometer material analysis, CMM dimensional survey, and portable hardness testing — producing a complete specification drawing within 3–5 working days. The replacement Stirnrad is then produced to the confirmed specification with full inspection documentation. As a direct Hersteller von Stirnrädern, Korea Ever-Power handles reverse engineering and production under one roof — eliminating the additional lead time that occurs when reverse engineering is subcontracted separately from manufacture. Browse the Produktpalette an Stirnradgetrieben or send your worn gear for a reverse engineering quote.
Häufig gestellte Fragen
If the tooth tips of the worn Stirnrad are too pitted or broken to give a reliable OD measurement, the module can be estimated from the root circle diameter instead. Measure the distance from a straight edge laid across the tooth tips to the root circle using a depth gauge (this gives the full tooth height h). For standard addendum gears, h ≈ 2.25 × Mn (1.0 × Mn addendum + 1.25 × Mn dedendum). Solving: Mn ≈ h / 2.25. A second method: measure the tooth pitch at the pitch circle using a gear pitch gauge or span measurement (across a known number of teeth) — the pitch p = π × Mn in the transverse plane. Korea Ever-Power uses all available measurements and selects the most reliable method based on the condition of the worn gear received.
Yes — the gear analyser at Korea Ever-Power measures the helix angle directly from the tooth profile across the face width, even on a worn gear, to an accuracy of ±0.01°. The analyser probes the remaining unidamaged tooth surface at multiple face positions and fits the helix angle from the measured data. Even severely edge-worn Schrägverzahnung retain readable helix angle data in the mid-face region. The analyser report states the measured helix angle and the standard value it most closely matches, for confirmation with the customer before production begins.
Standard reverse engineering service (gear analyser + CMM + OES + hardness + drawing): 3–5 working days after the worn gear is received at Korea Ever-Power. The reverse engineering report is provided with a quotation for the replacement Stirnrad. The cost of the reverse engineering service is credited against the first replacement gear order — so the parameter identification is effectively free when the replacement is ordered from Korea Ever-Power. For customers who need to confirm parameters before committing to the order, a fee-only reverse engineering report is available without obligation to order.
A true drop-in replacement Stirnrad fits the existing housing centre distance, bore, and face width without any machining of the housing. For a like-for-like replacement, this is guaranteed if the module and tooth count are correctly identified. If the housing centre distance has changed from bearing wear (the shaft bearing bores are worn oversize), the actual centre distance in the machine is slightly larger than nominal — which increases backlash. Korea Ever-Power can produce the replacement gear with DIN 3967 class d (large backlash tolerance) to accommodate worn housing bore centres, or can calculate the profile shift needed to maintain the correct operating centre distance if the actual bore position is measured and provided.
Send Your Worn Helical Gear for Reverse Engineering
Ship the worn or broken gear to Korea Ever-Power. Within 3–5 working days, receive a complete specification drawing (module, helix angle, material confirmed) and a quotation for the replacement. Reverse engineering cost credited against the first production order.
Gear analyser · OES spectrometer · CMM · Hardness survey · Drawing in 3–5 days · RE cost credited on first order
Herausgeber: Cxm
