What causes the fracture of gear teeth?
Fractures in gear teeth are often the result of a crack or series of cracks reaching multiple free surfaces and leading to material separation. Initially, cracks may form due to overload, fatigue failure, or chemical reactions that weaken material strength. Crack initiation often starts at stress concentration points, which may include material irregularities, inclusions, or existing gear damages like macropitting or indentations.
For example:
- Bending fatigue cracks typically originate at the surface, particularly around the gear root radius on the active flank, where tensile stress and the gear geometry are most strained.
- Contact flank fractures tend to start sub-surface at the core-to-surface interface due to plastic deformation and material strength variances introduced by heat treatment.
Appearance
Initially, a small crack will be observed leading away from the origin of the crack. For a bending fatigue crack on a gear tooth, the origin is usually at the surface of the root of the gear. For gear flank cracks, the origin will typically be around half way up the active flank of the gear, sub-surface. Cracks may also initiate from macropitting, indentations or other gear damage. As a fatigue crack propagates, it may leave a series of “beach marks” that correspond to positions where the crack growth was interrupted before starting again.
Once the fractured surfaces are visible, there are two types of fracture areas with different appearances:
- Ductile fracture area: Slow crack growth area characterised by a darker dull appearance with a smooth surface and appreciable plastic deformation. A lip or rim may be visible at the end of the fracture surface, on the opposite side to the crack initiation point.
- Brittle fracture area: Fast fracture as the part snaps characterised by a bright, shiny appearance with a rough surface and negligible plastic deformation. Macroscopic chevron marks may be visible, with the point of the chevron pointing towards the crack initiation point.
How fractures progress
Fracture progression follows three main stages:
- Crack initiation: Often seen in helical gears, cracks begin at stress points like surface notches or material inclusions.
- Crack propagation: Under tension or shear loading, cracks grow along grain boundaries, potentially causing plastic deformation and fatigue failure.
- Fracture: When a crack extends across the gear geometry or joins with other cracks, a part of the material separates, known as liberated tooth or gear fracture.
Cracks may grow in a ductile or brittle manner:
- Ductile fracture: Characterised by gradual crack growth with a dull surface and visible plastic deformation.
- Brittle fracture: Faster and marked by a shiny, rough appearance without significant plastic deformation. This type often appears in areas of high tensile stress within helical gears or other loaded components.
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Consequences of gear tooth fractures
Fractures in gear geometry lead to severe damage across gearbox systems, requiring careful monitoring to prevent cascading failures. Helical gears and others used in wind turbines, when fractured, can release material that may cause catastrophic damage to other components. Immediate shutdown and replacement of affected parts are often required to mitigate risks.
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Borescope
Monitor regularly for signs of progression or violent vibration and consider scheduling replacement soon. If persistent, improve mitigation against electrical erosion.
Visual, borescope, vibration
Shut down turbine, complete gearbox inspection and replace damaged components.