Spiral bevel gears are widely used in right-angle reducers because their curved teeth provide smooth conjugate motion, high contact ratio and good load capacity. In coal-mine scraper conveyor drives, however, the gear mesh can become an unintended path for stray electric current. When a potential difference exists across the meshing flanks, the lubricant film or local asperities may break down and an arc or spark can discharge through the contact. The result is electrical erosion: a field of small, smooth-edged pits that can rapidly progress to severe surface damage and gear failure.
Failure context and gearbox data
A reducer used on a coal-mine scraper conveyor was removed from service after abnormal vibration and noise developed during operation. The unit used a spiral bevel gear pair. Its main operating data are summarized in Table 1.
| Parameter | Value or condition |
|---|---|
| Application | Coal-mine scraper conveyor reducer |
| Motor power | 700 kW |
| Motor speed | 1450 r/min |
| Transmission ratio | 16.03 |
| High-speed shaft diameter | 100 mm |
| Observed symptom | Vibration and abnormal noise during operation |
| Damaged teeth | One tooth on each gear, forming a mating pair with large fracture areas |
Disassembly showed that only one tooth on the large bevel gear and one mating tooth on the small bevel gear had suffered large-scale fracture and spalling. The damaged areas on the two flanks were very similar in shape and could be matched to each other. The remaining teeth carried indentations that were consistent with contact against the raised ridges of the fractured teeth during subsequent rotation.
Site information indicated that welding had been carried out on a structure connected to the reducer while the reducer was stationary. The localized nature of the damage at the gear mesh is consistent with welding current passing through the gear pair rather than returning directly to the welding power source.
Electrical erosion mechanism and diagnostic features
GB/T 3481—1997, Gear tooth wear and damage terminology, defines electrical erosion of gears as the formation of many small, smooth-edged arc pits on the tooth flanks by arcs or sparks discharged across the meshing surfaces. Larger burned areas may also appear, sometimes with temper colors at their edges. Severe electrical erosion can remove material quickly and lead to premature gear failure.
Two common sources of gear electrical erosion are motor shaft currents and welding overcurrent. Shaft-current erosion has been studied extensively. Welding-current erosion is less frequently documented, but it can be equally destructive because the welding circuit may impose a large current across a stationary or slowly rotating mesh. Arcing can occur even when the gears are not running, provided that a sufficient potential difference exists and the contact path is not safely bypassed.
The morphology of electrical erosion differs from that of classical rolling-contact fatigue pitting. Electrical pits are typically circular or arc-shaped, with smooth edges and no surrounding fatigue cracks. In the present case, the combination of round pits, absence of crack networks, adhered weld spatter and matching fracture surfaces provides strong evidence of an electrical discharge origin.
Macroscopic fracture morphology
A fracture specimen was cut from the small spiral bevel gear for examination. The macroscopic fracture surface was complex. It contained deep and shallow depressions, polished edge regions, unpolished regions, ridge zones and areas with adhered weld spatter.
The fracture could be divided into an upper and a lower region. In the upper region, small weld spatter particles were clearly adhered to the tooth surface. In the lower region, no adhered spatter was visible; the original grinding marks from tooth-flank machining remained. This difference is important because it links the damage to an arc-welding event and shows that part of the original tooth surface was preserved away from the most severe erosion.
Microscopic evidence
A fracture specimen from the pinion was examined by electron microscopy at 15 marked locations. Selected observations are summarized in Table 2. The clearest electrical-erosion features were found near the edges of the fracture, where the original eroded surface was less disturbed by subsequent running. A central location also retained circular pits.
| Observation point | Location | Magnification | Observed morphology | Interpretation |
|---|---|---|---|---|
| 3 | Fracture surface | 2000× | Holes about 2 μm in diameter, superficially resembling dimples | Small electrical-erosion pits |
| 5 | Fracture edge | 200× | Highly porous, loose structure with polished areas | Excessive electrical erosion followed by surface polishing |
| 6 | Fracture edge | 1500× | Clear round holes 1–4 μm across, no cracks at edges; grinding marks visible | Typical electrical pits in an otherwise unworn surface |
| 8 | Fracture edge | 1000× | Round holes 1–5 μm across, no surrounding cracks; surface worn flat | Electrical erosion, not fatigue pitting |
| 10 | Fracture edge | 500× | Severely fragmented surface | Advanced or excessive electrical erosion |
| 13 | Central fracture region | 2000× | Fragmented running damage with circular holes | Electrical pits modified by subsequent operation |
At observation point 6, the tooth surface still showed fine grinding marks, indicating that this local area had not experienced significant running wear. The pits therefore retained their original electrical-erosion morphology. Energy-dispersive X-ray spectroscopy showed Cr and Fe at the edge of a pit, while Cr- and Fe-bearing oxides were detected inside the pit. This is consistent with exposed metal at the pit rim and oxidized material within the eroded cavity.
Composition and damage sequence
The evidence supports the following sequence. During welding on a connected structure, welding current passed through the stationary gear pair. At the tooth contact, the current concentrated at asperities or through local breakdown of the lubricant film, producing arcs and sparks. The resulting heat melted small volumes of metal and bonded parts of the two flanks together.
When the reducer was subsequently started, the bonded junctions were torn apart. Because the pinion and gear separated along the same bonded regions, their fracture surfaces were similar and could be matched. The contact patch then continued to run with differing local sliding velocities across the mesh. Repeated engagement gradually polished or removed pits in the central contact zone, while pits near the edge of the original erosion area survived in a more intact form. This explains why the most characteristic electrical pits were concentrated near the fracture edges.
The model does not account for every detail of the fracture, such as weld-spatter adhesion, local melting of weld droplets or irregularities in the torn junctions. Nevertheless, the combination of circular crack-free pits, oxidized pit interiors, adhered spatter, matching fracture surfaces and the known welding event makes the electrical-erosion diagnosis robust.
Conclusion and prevention
The failure of the spiral bevel gear pair in the scraper conveyor reducer was caused by welding overcurrent electrical erosion. Although only one mating tooth pair suffered large fracture areas, the resulting surface disruption was sufficient to disturb the smoothness of gear transmission, generate abnormal vibration and noise, and render the reducer unserviceable.
The primary preventive measure is to avoid welding on a reducer or on structures connected to it unless a validated current-diversion and grounding procedure is in place. The welding return path should be arranged so that current does not pass through gear meshes, bearings or other precision transmission elements. Where welding near an assembled reducer is unavoidable, the unit should be isolated from the welding circuit and the return connection should be made close to the weld. Post-weld inspection should include checks for abnormal vibration, noise and tooth-surface damage before the drive is returned to service.