Why Failure Analysis Matters Before Ordering a Replacement

Installing a replacement spiralni zupčanik into a gearbox that failed without first diagnosing why it failed almost guarantees a repeat failure at the same or shorter service interval. The replacement gear is identical to the original — if the original failed at 18 months due to gear oil contamination with water, the replacement will also fail at 18 months. If the original failed due to insufficient face width (ε_β < 1) causing edge loading, a like-for-like replacement will fail identically. Correct failure analysis identifies whether the replacement gear needs a different specification, whether the lubrication must be changed, whether a seal must be repaired, or whether a machine alignment issue must be corrected before the new gear is installed.

Korea Ever-Power offers a failure analysis service alongside its reverse engineering capability: send the failed spiralno rezani zupčanik for visual and metallographic examination, and receive a failure mode report with root cause and specification recommendation before a replacement is ordered. Browse the Asortiman proizvoda sa spiralnim zupčanicima for replacement specifications.

Pitting — Surface Contact Fatigue, the Most Common Helical Gear Failure

Tvrda bočna strana zuba cementirana spiralni zupčanik — the HRC 58–62 surface resists pitting when lubrication film is adequate. Pitting initiates at the pitch line where the EHL film is thinnest and sliding velocity is lowest, typically appearing as small hemispherical craters 0.1–1.0 mm diameter in early stages

Pitting: Visual Characteristics and Location

Pitting is surface fatigue — tiny cracks initiate at or just below the tooth flank surface and propagate under cyclic Hertzian contact stress until a small fragment detaches, leaving a roughly hemispherical crater (the “pit”). Pitting in a spiralni zupčanik has three distinctive location and appearance characteristics:

• Lokacija: Pitting initiates predominantly at or slightly below the pitch line — the position on the tooth flank where the EHL film is thinnest (lowest sliding velocity, minimum film thickness) and Hertzian contact stress is highest. In a single spiralni zupčanik, pits concentrate near the centre of the face width where the contact pattern is densest; at one face end if lead deviation (mis-alignment) is causing edge loading.
• Napredak: Initial pits (0.1–0.3 mm diameter) are hemispherical, with a smooth surface from the fatigue crack propagation. As pitting continues, adjacent pits coalesce into spalling — larger, irregular craters with rougher surfaces. Spalling accelerates rapidly once started because the rough spalled area disrupts the EHL film for neighbouring teeth.
• Distinguishing progressive vs destructive pitting: Progressive (acceptable) pitting produces pits that arrest after an initial running-in period — the rough surface redistributes load to a slightly wider contact zone, and the EHL film re-establishes. Destructive pitting continues to spread, with spalled areas growing. If pitting is still growing at the next inspection, it will not arrest without a root cause change.

Root Causes of Pitting in Helical Gears

Micropitting — The Fine-Scale Fatigue Preceding Full Pitting

Micropitting (also called grey staining, frosting, or corrosion fatigue in different industry standards) is a fine-scale surface damage mode that precedes full pitting in many spiralni zupčanik failures. Micropits are 10–100 µm diameter craters — too small to see individually with the naked eye, but producing a characteristic matte grey appearance on the tooth flank in the pitch line region. Under a ×20 optical microscope or SEM, the micropitting zone shows a dense array of tiny craters with angular edges, clearly distinct from the polished adjacent surface.

Micropitting forms when the specific film thickness ratio λ is in the range 0.4–1.0 — the mixed lubrication regime where occasional asperity contact occurs but full metal-metal contact has not yet developed. The micropits gradually merge and deepen, transitioning to macro-pitting after typically 10⁷–10⁸ contact cycles. Micropitting is particularly common in ground spiralni zupčanici during the first 200–500 hours of operation (running-in), after which the surface roughness reduces, the EHL film re-establishes, and the micropitting arrests. If micropitting continues beyond the running-in period, it indicates an inadequate lubricant for the operating conditions.

Scuffing (Scoring) — Catastrophic Film Breakdown

Scuffing: Visual Characteristics and Rapid Progression

Scuffing is an adhesive wear mechanism in spiralni zupčanici that occurs when the EHL film collapses completely and the mating tooth flanks weld at asperity contacts under the flash temperature generated by sliding. The welds tear as the tooth surfaces move relative to each other, producing characteristic linear furrows in the sliding direction across the tooth flank. Scuffing in a spiralni zupčanik appears as:

• Lokacija: Tooth tip and tooth root (where sliding velocity is highest and EHL film is thinnest). The pitch line itself is typically free of scuffing because sliding velocity is zero at this point. This distinctive location pattern distinguishes scuffing from pitting.
• Appearance: Irregular, rough surface with directional scoring marks in the sliding direction. The surface appears torn rather than smoothly pitted. Transfer of material from one gear to the other is often visible — bright metallic transfer patches on one tooth surface, corresponding dark abraded zones on the mating surface.
• Napredak: Scuffing is rapid and catastrophic. Once initiated, the rough scuffed surface destroys the EHL film for all subsequent tooth contacts, accelerating scuffing across the full tooth face within hours. There is no “tolerable” level of scuffing — any evidence of scoring requires immediate machine shutdown and gear replacement.

Root Causes and Prevention

Scuffing occurs when the tooth flank temperature exceeds a critical threshold at which the lubricant film can no longer form. Contributing factors: incorrect lubricant viscosity grade (too low for the operating speed), gear oil at high temperature (above 90°C sump), insufficient EP additive content for the specific gear tooth material pair, very high pitch-line velocity combined with inadequate oil flow to the mesh, and insufficient running-in (new gears at full load from cold start). Prevention: use the correctly specified oil grade with verified EP additive content (confirmed by FZG scuffing test stage ≥ 12); ensure adequate oil flow rate; follow the manufacturer’s running-in procedure for new gears; monitor sump temperature continuously in high-speed applications above 25 m/s.

Tooth Root Fracture — Bending Fatigue vs Impact Fracture

Tooth root geometry — fractures initiate at the highest bending stress point in the root fillet. Bending fatigue fracture shows a progressive beach-mark crack growth zone; impact fracture shows a clean brittle fracture surface without beach marks

Bending Fatigue Fracture — Progressive Crack Growth

Bending fatigue fracture in a spiralni zupčanik tooth initiates at the tensile side of the tooth root fillet (the side from which the tooth bends under load) and propagates progressively with each load cycle. The fracture surface has characteristic “beach marks” — concentric curved lines visible to the naked eye that indicate the position of the crack front at different stages of propagation. The crack growth zone is typically smooth and concave; the final fracture zone (where the remaining tooth cross-section was insufficient to carry the load and failed instantaneously) is rough and may show chevron marks or river lines indicating direction of final fracture propagation. Beach marks identify a fatigue fracture unambiguously — they are present in fatigue and absent in impact fractures.

Impact Fracture — Single-Event Overload

Impact fracture in a spiralni zupčanik tooth occurs when a single load event exceeds the tooth’s ultimate bending strength — typically entry shock (rolling mill bite, crusher bite), hydraulic hammer blow, or dropped load. The fracture surface is uniformly rough with no beach marks; chevron patterns on the fracture surface indicate the direction of crack propagation from the root fillet. Impact fracture is more common in induction-hardened gears (where the shallow hardened zone over a tough core can absorb most shock loads) than in carburized gears (where the case-core boundary creates a potential stress concentration under impact).

Abrasive Wear — Gear Surface Damage from Oil Contamination

Abrasive wear in spiralni zupčanici removes material evenly from the tooth flanks — both addendum and dedendum, not concentrated at the pitch line as with pitting — through the cutting and gouging action of hard particles suspended in the gear oil. The worn surfaces show directional scratches in the sliding direction, and the tooth profile gradually deviates from the original involute as material is continuously removed. The gear analyser report of a worn gear shows increasing profile deviation over successive inspections.

Za spiralni zupčanik drives in dusty environments: maintain ISO cleanliness class at or below ISO 16/14/11 (per ISO 4406) through regular oil filtration (full-flow filter with 10 µm absolute rating) and oil sampling. In mining and outdoor environments where seal integrity is difficult to maintain, increase the filtration and oil sampling frequency — monthly for critical drives, quarterly for standard drives.

Failure Mode Diagnostic Quick Reference

When the Replacement Helical Gear Needs a Different Specification

The failure mode of the original spiralni zupčanik directly determines whether the replacement requires a specification change. Three failure modes almost always require a specification upgrade rather than a like-for-like replacement:

Korea Ever-Power — Failure Analysis and Replacement Specification

Korea Ever-Power’s failure analysis service accepts worn or failed spiralno rezani zupčanici for visual examination, gear analyser measurement, and metallographic section analysis (hardness traverse, case depth confirmation, grain flow assessment). The examination report identifies the failure mode, probable root cause, and whether the replacement gear requires a specification change (different material, accuracy class, lead crowning, or service factor). As a direct proizvođač spiralnih zupčanika, Korea Ever-Power can produce the replacement gear immediately after confirming the specification — without the additional delays of sending analysis results to a separate manufacturer.

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