Three Surface Failure Mechanisms — Overview
Macropitting (Rolling Contact Fatigue)
Mechanism: Cyclic Hertz contact stress exceeds the material’s endurance limit. A fatigue crack initiates at or near the surface and propagates until a fragment spalls out. Timescale: develops over 10⁶–10⁹ load cycles — gives warning before catastrophic failure. Governing condition: σ_H > σ_H lim (material endurance limit).
Micropitting (Grey Staining)
Mechanism: Very shallow fatigue cracks (10–100 µm deep) in asperity contact zones at the tooth flank surface. Produces a grey, mat appearance visible to the naked eye. Timescale: develops over 10⁷–10¹⁰ cycles — slower than macropitting initiation but can progress to macropitting. Governing condition: specific film ratio λ < 2.0.
Scuffing (Adhesive Wear)
Mechanism: Instantaneous adhesive wear as asperity temperatures briefly exceed the lubricant film collapse temperature. Metal-to-metal contact transfers material from one tooth flank to the other. Timescale: can occur on the FIRST contact cycle under extreme conditions. Governing condition: flash temperature T_flash > scuffing temperature T_scuff.
Pitting — Mechanism, Visual Diagnosis and Prevention
How Macropitting Initiates in Helical Gears
Contact fatigue pitting in a kierukkavaihteisto begins at the maximum Hertz shear stress location — either at the tooth flank surface (surface-initiated pitting, more common in boundary lubrication conditions) or just below the surface at the depth of maximum orthogonal shear stress (subsurface-initiated pitting, more common in well-lubricated gears with high contact stress). The Hertz shear stress peak at depth z₀ = 0.786 × b_H (where b_H is the Hertz contact half-width) is approximately 0.30 × σ_H_max — and at this depth, the cyclic stress reversal reaches ±0.30 × σ_H_max with each tooth contact, accumulating fatigue damage until a crack initiates and propagates to the surface.
The depth of subsurface pitting initiation z₀ is important for case depth specification: if the case depth ECD is shallower than z₀, the Hertz stress peak falls below the case in the relatively soft core material — initiating a deep case-crushing failure rather than surface pitting. Korea Ever-Power’s case depth requirement for kierrevaihteet (ECD ≥ 0.15–0.20 × Mn) ensures the case extends beyond the maximum Hertz stress depth for standard tooth contact stresses (see Art53 and Art52 for case depth and ISO 6336 details).
Visual Appearance of Pitting
Macropitting craters on a kierukkavaihteisto tooth flank appear as:
- Sijainti: Concentrated near the pitch line, where the sliding velocity is zero and the EHL film is thinnest for a given contact stress. On the pinion (which sees more fatigue cycles per unit time), pitting typically appears first.
- Shape: Roughly semicircular or fan-shaped craters, 0.5–5 mm diameter, with a smooth, polished inner surface (the spalled fragment left a clean fracture surface).
- Eteneminen: Initial pits are isolated and small. As fatigue progresses, pits coalesce into larger craters (spalling) and eventually cover the pitch line continuously — at which point the gear is clearly in advanced failure and generates distinctive impact noise at the rotation frequency.
EHL Film Ratio λ and Pitting Prevention
The specific film thickness ratio λ governs pitting initiation in a kierukkavaihteisto:
λ ≥ 2.0: Full EHL film — asperities do not contact; only subsurface-initiated pitting from bulk Hertz stress
λ = 1.0–2.0: Mixed lubrication — occasional asperity contact; both surface and subsurface pitting possible
λ < 1.0: Boundary lubrication — frequent asperity contact; surface-initiated pitting accelerated
h_min ≈ 2.65 × η₀^0.7 × v^0.68 × R^0.46 / (E’^0.53 × w^0.13) [Hamrock-Dowson simplified]
where: η₀ = oil dynamic viscosity at inlet [Pa·s]
v = pitch-line velocity [m/s]
R = equivalent radius of curvature [mm]
w = normal contact load per unit width [N/mm]
To improve λ: ↑ oil viscosity grade | ↑ pitch-line velocity (larger gear) | ↑ contact radius (larger module)
| ↓ surface roughness Ra (grind + ISF) | use synthetic PAO with lower traction coefficient
Micropitting — The High-Cycle Surface Failure Mode
Micropitting on a kierukkavaihteisto tooth flank — the grey, mat appearance (“grey staining”) results from thousands of very shallow pits (10–100 µm) formed when the EHL film ratio λ falls below 2.0 at asperity contact zones. The damage zone extends across a larger area than macropitting and can progress to macropitting if not addressed. Distinguishable from scuffing by the absence of directional scoring marks
Micropitting Mechanism and Critical Difference from Macropitting
Micropitting in a kierukkavaihteisto forms when surface asperities contact through an inadequate EHL film (λ < 2.0) and each contact creates a very small fatigue crack in the asperity contact zone — at depths of 10–100 µm, far shallower than macropitting (which can initiate 100–500 µm below the surface). The individual cracks are too small to be visible individually, but the collective damage from millions of asperity contacts creates the grey mat appearance visible to the naked eye across the sliding zones of the kierukkavaihteisto tooth (the areas above and below the pitch line where sliding velocity is highest — the opposite of macropitting, which concentrates near the pitch line where sliding velocity is lowest).
Prevention of Micropitting in Helical Gears
Four interventions reduce micropitting risk in kierukkavaihteisto drives, in order of effectiveness:
1. ISF surface finishing
ISF reduces kierukkavaihteisto Ra from 0.3 µm to 0.05 µm, doubling λ. For EV and wind turbine gears where micropitting is the primary life limiter, ISF is the single most cost-effective intervention.
2. Micropitting-resistant oil
FVA 54/7 test rating MLS ≥ 10 (polysulfide EP package in PAO base) prevents micropitting at λ below 2.0 by forming a protective tribochemical film. The standard mineral oil GL-4 achieves only MLS 6–8 — insufficient for high-cycle drives above 10⁸ cycles.
3. Higher precision class
DIN Class 4–5 ground kierrevaihteet have lower profile waviness and finer surface texture than DIN Class 7–8, providing higher λ at the asperity scale even at the same Ra measurement. Tip relief further reduces the contact pressure at tooth entry, where λ drops transiently during the stiffness transition.
4. Increased helix angle
Higher β increases ε_β on a kierukkavaihteisto — more tooth pairs share the load, reducing contact stress σ_H and increasing λ to reduce micropitting risk at high cycle counts.
Scuffing — Instantaneous Adhesive Failure
The Blok Flash Temperature Model
Scuffing in a kierukkavaihteisto occurs when the asperity contact temperature — the “flash temperature” — briefly exceeds the temperature at which the lubricant film collapses and metal-to-metal adhesive contact occurs. The Blok flash temperature model (the basis of AGMA 925 and ISO TR 15144 scuffing risk assessment) calculates the flash temperature rise at the tooth contact:
T_flash = T_bulk + ΔT_flash
ΔT_flash = f × w_n × |v_s| / (b_H × √(ρ₁ × c₁ × k₁ × v_r1) + √(ρ₂ × c₂ × k₂ × v_r2))
where: f = friction coefficient at contact (≈ 0.04–0.08 for EHL; higher in mixed film)
w_n = normal contact load per unit width [N/mm]
v_s = sliding velocity at the contact point [m/s] — highest at tooth tip and root
b_H = Hertz contact half-width [mm]
ρ, c, k = density, specific heat, thermal conductivity of gear material
v_r = rolling velocity component of each gear surface
Scuffing initiates when T_flash > T_scuff (the scuffing temperature)
For mineral oil: T_scuff ≈ T_oil_bulk + 100–150°C
For PAO with anti-scuff additive: T_scuff ≈ T_oil_bulk + 150–200°C
Visual Appearance of Scuffing — Distinctive from Pitting
Scuffing damage on a kierukkavaihteisto is distinguishable from pitting by its directional scoring:
- Sijainti: Tooth tips (addendum — recess zone) and tooth roots (dedendum — approach zone) where the sliding velocity is maximum. The pitch line itself is typically undamaged or minimally affected. This is the OPPOSITE of macropitting location.
- Directionality: Deep scratches or scoring marks running in the direction of tooth sliding — radially across the tooth from root to tip (for gear) or tip to root (for pinion) at each scoring mark. The marks are not random as in abrasive contamination wear, but oriented consistently with the sliding direction.
- Material transfer: Microscopic examination reveals material transferred from one tooth flank surface to the mating flank — the defining characteristic of adhesive wear. The “receiving” surface (typically the slower-moving gear) shows welded lumps of transferred material alongside the scoring grooves.
Rapid Three-Way Diagnosis — Which Failure Mode?
| Diagnostic Question | Macropitting | Micropitting | Scuffing |
|---|---|---|---|
| Tooth flank appearance | Smooth-sided craters, 0.5–5 mm, shiny inner surface | Grey mat/dull coating; fine texture; must look carefully | Deep scratches/scoring; rough torn surface; directional marks |
| Location on tooth | Near pitch line (sliding zone minimum) | Away from pitch line (addendum and dedendum, high sliding zone) | Tooth tips and roots (maximum sliding velocity zone) |
| Time to develop | 10⁶–10⁹ cycles — months to years | 10⁷–10¹⁰ cycles — may take years; progresses slowly | Minutes to hours — can occur at first operation |
| Oil particle count signal | Increasing large particles (50–200 µm), high L/W ratio | Increasing fine particles (1–15 µm) | Sudden sharp rise in large metallic particles; ferrous concentration spike |
| Primary cause | σ_H > σ_H lim (material or load) | λ < 2.0 (oil, speed, surface roughness) | T_flash > T_scuff (oil, speed, contact pressure) |
| Primary fix | Better material (carburized), reduce load, increase module | Better oil (MLS 10), ISF surface finish, tip relief | Anti-scuff oil additives, reduce pitch-line velocity, reduce load per tooth |
Korea Ever-Power — Surface Failure Analysis and Material Recommendation
Kova hampaan kylki hiiletetty kierukkavaihteisto — the combination of HRC 58–62 surface hardness (σ_H lim 1500–1800 MPa), Ra ≤ 0.2 µm HÖFLER ground tooth flank, and correctly specified EHL oil viscosity provides λ ≥ 2.0 at rated load speed — the threshold for preventing both macropitting and micropitting initiation
Korea Ever-Power offers surface failure analysis: send the failed kierrehammaspyörä (or high-quality photographs showing location, size, and character of the damage) to Korea Ever-Power’s engineering team. Within 5 working days, Korea Ever-Power identifies the failure mode (macropitting, micropitting, or scuffing), estimates the λ ratio at the time of failure from the operating conditions, and recommends the corrective specification for the replacement gear — material upgrade, accuracy class change, surface finish improvement, or oil specification change. As a direct kierrevaihteiden valmistaja, Korea Ever-Power produces the replacement kierukkavaihteisto to the corrected specification with the same delivery schedule as a standard order. Browse the kierukkavaihteiden tuotevalikoima for all material and surface finish options.
Usein kysytyt kysymykset
Yes — micropitting in a kierukkavaihteisto can arrest and stabilise in specific conditions. As the micropitted surface gradually smooths (the asperity peaks are worn down by the micropitting process itself), the combined composite roughness R_q decreases, which increases λ above the micropitting threshold of 2.0. This self-limiting mechanism is sometimes observed in the initial running-in period of new gears — a period of micropitting followed by stabilisation at a new, slightly rougher but stable surface. However, self-limiting behaviour cannot be relied upon for design purposes: if the operating λ is significantly below 2.0 (e.g. λ = 1.0–1.3), the micropitting will progress to macropitting rather than stabilise. Korea Ever-Power’s recommendation: if the gear analyser of a service-life kierukkavaihteisto shows micropitting texture but no macropits, conduct an oil analysis and λ calculation — if λ < 1.5, intervene with oil upgrade before the next maintenance window.
Even after precision grinding, a new kierukkavaihteisto has surface asperity heights that produce λ below the full-film threshold in the first hours of operation — before running-in smooths the surface. The asperity flash temperatures during this initial period can exceed T_scuff if: (1) the oil does not yet contain adequate anti-scuff additive activation products from the running-in contacts; (2) the kierukkavaihteisto is operated at full load immediately without a break-in period; or (3) the gear and oil are not pre-warmed before load application. Korea Ever-Power recommends a 4-hour graduated break-in for all new kierukkavaihteisto installations in high-speed drives (v > 20 m/s): start at 25% rated load for 1 hour, then 50% for 1 hour, 75% for 1 hour, then full load — allowing progressive surface conditioning and additive activation before the full-load flash temperature is reached.
They overlap but are not identical. Polysulfide extreme pressure (EP) additives provide both anti-scuff protection (by forming a sacrificial iron sulphide tribofilm that prevents adhesive contact at flash temperature) and anti-micropitting protection (by reducing the friction coefficient at asperity contacts below the micropitting initiation threshold). Borate EP additives provide excellent micropitting protection (FVA 54/7 MLS 10) but somewhat lower anti-scuff performance than polysulphide. Conventional sulphur-phosphorus (S/P) EP additives provide moderate anti-scuff but generally poor anti-micropitting (MLS 6–8) in kierukkavaihteisto applications. For high-cycle applications (wind turbines, EV reducers) where both risks are present: specify PAO base oil + polysulfide EP, which is the only common additive type that achieves MLS 10 (micropitting) AND adequate anti-scuff performance in the same package.
Not significantly — scuffing is governed by the flash temperature and oil film behaviour, not by the bulk material hardness. A carburized HRC 60 kierukkavaihteisto scuffs at approximately the same flash temperature as a QT HB 280 gear if both have the same surface roughness and oil. However, carburized gears are routinely ground to Ra ≤ 0.2 µm while soft-flank QT gears are typically only hobbed to Ra ≈ 1.5–2.5 µm. This roughness difference means the carburized gear has much higher λ and therefore operates further from the scuffing threshold, even though the scuffing temperature threshold itself is similar. The practical result: carburized and ground kierrevaihteet are significantly less susceptible to scuffing not because of their higher hardness per se, but because the grinding process that follows carburizing dramatically reduces surface roughness.
Submit a Failed Helical Gear for Surface Failure Analysis
Send the failed gear (or photos showing damage location, scale, and character) with the operating conditions (power, speed, oil grade, ambient temperature). Korea Ever-Power identifies the failure mode — pitting, micropitting, or scuffing — and recommends the corrective specification within 5 working days.
Pitting · Micropitting · Scuffing · λ calculation · Oil recommendation · Corrective specification · 5 working days
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