Three Hardness Scales and When Each Is Used for Helical Gears
Three hardness scales are used in helical gear inspection, each suited to a different hardness range and measurement context. Using the wrong scale — attempting to measure a carburized HRC 60 surface with a Brinell tester, for example — produces unreliable results and can damage the test equipment:
Range: HB 80–650 (reliable up to HB 450 for gears). Tested with a 10 mm tungsten carbide ball at 3000 kg load. The large indenter (2–6 mm diameter impression) averages over a wide area, making HB appropriate for QT soft-flank helical gears (HB 220–350). HB cannot be used on carburized surfaces — the indenter is too large and penetrates through the thin case into the softer core, giving a mixed reading that underestimates the true surface hardness.
Range: HRC 20–70 (reliable for gear surfaces HRC 30–65). Tested with a diamond Brale cone at 150 kg load. The smaller indenter (approximately 0.2 mm impression diameter) measures the surface case hardness reliably on carburized (HRC 58–62) and induction-hardened (HRC 50–55) helical gears. The minimum case depth for a valid HRC reading is approximately 0.5 mm — below this, the test overestimates case hardness because the indenter senses the softer core beneath the thin case.
Range: any — 50–3000 HV. Tested with a pyramid diamond under loads from 0.01 to 30 kg. The micro-hardness indenter (0.01–0.1 mm impression) allows measurement at closely spaced depths from the surface — every 0.1 mm through the case — providing the full hardness-depth profile needed to determine effective case depth. This is the only scale that can measure case depth in a helical gear.
Where to Test Hardness on a Helical Gear Tooth
Helical gear tooth detail — the three correct hardness test locations: (1) tooth flank at the pitch circle for HRC surface hardness; (2) tooth end face for Brinell (soft-flank) verification; (3) metallographic cross-section at the tooth mid-face for Vickers micro-hardness case depth traverse. Each location serves a different inspection purpose
Surface Hardness Test Location — On the Tooth Flank at Pitch Circle
The correct location for HRC surface hardness testing on a carburized or induction-hardened helical gear is on the tooth flank surface, at the pitch circle (the mid-height of the active tooth profile). Do not test on the tooth tip (the gear OD) or the tooth root — these areas have different carbon profiles from the active flank and may give misleading hardness readings. For a hard-tooth-flank helical gear with M5, the pitch circle is approximately 3.5 mm above the root circle, well accessible for a portable HRC tester.
Korea Ever-Power tests HRC hardness on at least 3 teeth per gear, at the pitch circle on the tooth flank surface, and reports the minimum and maximum values alongside the average. The specified range (e.g. HRC 58–62) must be met by all individual measurements, not just the average.
Brinell Test Location — Gear End Face or Bored Hub Face
For QT soft-flank helical gears (HB 220–320), Brinell hardness is tested on the gear end face or the bored hub face — not on the tooth flank (the tooth is too narrow for a reliable 10 mm ball impression on the curved flank surface). The end face is machined flat to Ra ≤ 1.6 µm before testing to ensure a representative flat area for the indenter. The Brinell hardness of the end face is taken as representative of the through-section hardness of the soft-flank helical gear body.
Case Depth Measurement by Vickers Micro-Hardness Traverse
Case depth verification is the most important and most frequently questioned item in a carburized helical gear inspection certificate. Two case depth measurements are defined in ISO 6336-5 and DIN 50190:
| Term | Definition | Test Method | Why It Matters |
|---|---|---|---|
| Effective Case Depth (ECD, CHD) | Depth from the surface to the point where hardness falls to HV 550 (≈ HRC 52) | Vickers micro-hardness traverse at 0.1 mm intervals from surface inward; interpolate depth at HV 550 crossing | Directly related to pitting resistance — the depth of the fatigue-resistant layer. Must equal or exceed the minimum specified in the drawing. |
| Total Case Depth (TCD, CD) | Depth from surface to the point where hardness equals core hardness + 50 HV | Same Vickers traverse continued until hardness stabilises at core level; depth at (core + 50 HV) reading | Indicates total carbon-enriched zone depth. TCD is always greater than ECD — by 25–60% depending on alloy and quench. Drawing must specify which measurement is required. |
How to Read a Case Depth Traverse
A Vickers micro-hardness traverse for a carburized helical gear shows hardness on the Y-axis and depth from surface on the X-axis. The characteristic shape: high surface hardness (HV 700–850, corresponding to HRC 60–65) at the surface; gradual decrease through the case; transition zone where hardness drops from HV 550 to core hardness over 0.2–0.5 mm; flat core hardness (HV 350–450, HRC 35–45). The effective case depth ECD is read at the HV 550 crossing on the hardness axis. For a specification of “ECD min. 0.8 mm”, the traverse curve must cross HV 550 at a depth of 0.8 mm or greater from the surface.
Portable Field Hardness Testing — Leeb and UCI Methods
Workshop HRC and Brinell testers require the gear to be brought to the instrument. For large helical gears (OD > 400 mm, mass > 500 kg) that cannot be moved to a laboratory, portable hardness testers allow in-situ verification at the installation site or on the machine shop floor:
| Method | Working Principle | Accuracy vs Lab | Best For |
|---|---|---|---|
| Leeb rebound (HL) | A tungsten carbide ball is spring-launched against the surface; ratio of rebound to impact velocity is measured electronically and converted to HRC or HB via empirical table | ±3–5 HRC vs lab; reliable for QT gears; less reliable on thin carburized cases (<1 mm) | Large soft-flank helical gears in the field; bulk verification that heat treatment was applied at all |
| UCI (Ultrasonic Contact Impedance) | A Vickers diamond-tipped rod vibrates ultrasonically; contact with the test surface changes the resonant frequency proportionally to surface hardness | ±2–3 HRC; more accurate than Leeb for carburized surfaces > 0.3 mm case depth | Hard tooth flank helical gears on installation; curved surfaces where Brinell or Rockwell testers cannot be positioned |
Portable hardness testers should always be calibrated against a certified reference test block (HRC 60 ± 0.5 and HRC 40 ± 0.5 brackets) before each measurement session. For helical gear tooth flank surfaces, the tester must be oriented perpendicular to the curved flank surface — using a holding fixture for the probe is recommended to ensure consistent probe angle.
What to Check on a Helical Gear Hardness Certificate
Hard tooth flank helical gear — the hardness survey certificate accompanying this gear records HRC on 3+ teeth (minimum and maximum values alongside mean), Vickers traverse curve, and clearly stated ECD to HV 550 with depth in mm. Any certificate that lacks individual tooth measurements or case depth definition should be queried before accepting the shipment
A complete hardness certificate for a carburized and ground helical gear must contain:
- Surface hardness: HRC values for at least 3 individual teeth (not the average alone), with gear serial number and test location clearly stated. Specification range (e.g. HRC 58–62) must be met by every individual reading.
- Core hardness: HRC value at the tooth cross-section centre (typically HRC 30–40 for carburized grades). Confirmed by testing a test coupon from the same batch processed through the same furnace cycle.
- Case depth: ECD value in mm at HV 550, with the hardness traverse table or graph attached. The definition “ECD to HV 550 per DIN 50190 Part 1” should be stated explicitly.
- Furnace batch identification: Furnace load number, temperature chart reference, and date — enabling the specific heat treatment conditions to be traced if a quality question arises after installation.
Korea Ever-Power — Hardness Documentation on Every Carburized Gear Order
Korea Ever-Power supplies hardness survey documentation as a standard component of the inspection pack for every carburized or induction-hardened helical cut gear order. The standard documentation includes: HRC readings on minimum 3 teeth (individual values, not averaged); core hardness from a test coupon in the same batch; Vickers traverse table with hardness at each 0.1 mm depth interval; effective case depth (ECD) stated explicitly at HV 550 per DIN 50190; and furnace chart reference for full traceability. As a direct helical gear manufacturer with in-house heat treatment, Korea Ever-Power’s hardness documentation covers the entire process from blank to finished gear without gaps. Browse the helical gear product range for all material and heat treatment options.
Frequently Asked Questions
Yes — with a portable UCI or Leeb tester, hardness can be measured on the tooth flank of an installed helical gear without removal, provided the gear is accessible and stationary. The tooth flank surface must be clean (oil removed with solvent) and the probe oriented perpendicular to the flank. For gears in enclosed gearboxes, an inspection cover must be opened. Portable hardness testing at the installation site is particularly valuable for verifying that an installed replacement helical gear has the correct hardness before the machine is returned to service, without waiting for laboratory test results.
HRC variation of more than ±2 HRC between teeth on the same gear indicates either inconsistent quenching (some teeth cooled more slowly due to their position in the furnace load) or inconsistent case carbon from uneven gas flow in the furnace. Variation of more than ±4 HRC across a helical gear is generally grounds for rejection — the teeth with lower hardness will reach their contact fatigue limit (σ_H lim) before the higher-hardness teeth, creating a scatter in the pitting initiation time that makes service life unpredictable. Korea Ever-Power reports the minimum, maximum, and mean HRC per gear body to give the customer full visibility of the hardness distribution.
ISO 6336-5 provides minimum effective case depth guidelines as a function of normal module Mn and required contact stress level. A simplified rule: ECD_min ≈ 0.15 × Mn for carburized gears at MQ material quality and moderate contact stress (σ_H ≤ 1200 MPa). For higher contact stress (σ_H = 1300–1500 MPa), ECD_min ≈ 0.2 × Mn. At M5: ECD_min ≈ 0.75–1.0 mm at MQ quality level. Insufficient case depth causes case crushing — a failure mode where the subsurface shear stress peak (from Hertzian contact) is located below the case-core boundary rather than within the hard case, initiating a deep spalling fracture. Case crushing gives less warning than surface pitting and is therefore a more dangerous failure mode for helical gears under heavy load.
The tooth tip (addendum circle) has a different case carbon profile than the tooth flank active surface. In gas carburizing, the tooth tip carburizes faster than the flank because it has three exposed surfaces (tip and both flanks converging at the tip) while the flank has only one exposed surface. The tip therefore develops a slightly higher carbon content and potentially higher surface hardness than the active flank — making OD measurements slightly optimistic and non-representative of the flank hardness where contact fatigue actually initiates. Korea Ever-Power tests on the tooth flank at the pitch circle and reports this as the representative surface hardness for helical gear pitting resistance assessment.
Hardness Survey and Case Depth Documentation with Every Order
Korea Ever-Power supplies HRC readings on 3+ teeth, core hardness, Vickers traverse table, and explicit ECD at HV 550 — as standard documentation on every carburized or induction-hardened helical gear order. No certificates without individual measurements. No ambiguous “total case depth” when ECD is specified.
HRC on 3+ teeth · Core hardness · Vickers traverse · ECD to HV 550 · DIN 50190 · Furnace batch traceability
Editor: Cxm
