Gear fracture is caused by a crack, or series of cracks joined together, reaching multiple free surfaces so that material is liberated. The first stage of a crack is crack initiation which is usually caused by overload, fatigue or chemical attack. The crack initiation point is usually at a stress concentration point, for example those from material inclusions, manufacturing irregularities or from previous gear damage such as macropitting or indentations. For a bending fatigue crack, the crack usually initiates on the surface at the gear root radius on the active flank side (tensile stress) as this is the point of maximum bending stress. For contact flank fracture, the crack usually initiates sub-surface at the point of maximum Hertzian shear stress or the interface between the core material and the case hardened exterior material due to heat treatment.
Crack propagation is caused by tensile or shear loading on the already cracked material. This can be in the form of fatigue where the crack propagates slightly for each loading cycle. It can also propagate due to overload, which is a fast propagation when the material can no longer sustain the loading condition and unstable crack growth occurs.
Fracture is caused when a crack propagates or joins with other cracks to completely separate part of the material, causing complete fracture or liberation of part of the gear.
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.
There are three progression stages typically associated with fracture:
- Crack initiation: Crack initiates due to a stress concentration, such as from a surface notch or material inclusions.
- Crack propagation: Crack grows along grain boundaries roughly in the direction perpendicular to the tensile load. This stage can be slow if due to fatigue or ductile fracture, or almost immediate when due to sudden overload or brittle fracture.
- Fracture: When the cracked material can no longer take the operating load and the crack reaches a free surface, or multiple cracks join, the component will fracture and part of the material will be liberated.
Once fracture has occurred, the liberated material can cause severe consequential damage to other gearbox components. In the case of a liberated gear tooth, this could result in lock-up and catastrophic gearbox failure.
|Visual inspection||✓✓||Careful visual inspection should be able to identify cracks on the parallel stage gears. Fractures would be apparent. For parallel stage gears with limited visibility, a borescope is likely required.|
|Borescope inspection||✓✓✓||Borescope inspection should be able to identify cracks and fracture.|
|Vibration analysis||✓✓||Fractured teeth can be, are frequently are, picked up by vibration analysis. Cracks however are difficult to identify and can progress quickly. Therefore, vibration analysis is often unable to give much, if any, lead time on a broken tooth. Rather it often picks it up only after the fracture has occurred.|
|SCADA data||✓||Oil temperature may increase if there is a severe gear fracture due to progression. However, it does not identify the problem component or failure mechanism.|
|Oil debris sensor||✓||Oil debris will likely increase if there is a gear fracture due to small pieces of material liberated during cracking and fracture. This may be detected using an oil debris sensor. However, it does not identify the problem component or failure mechanism.|
|Oil sample analysis||✓||Oil debris will likely increase if there is a gear fracture due to small pieces of material liberated during cracking and fracture. This may be detected during an oil sample analysis. However, it is unlikely to identify the exact problem component or failure mechanism.|
Fracture is a relatively common ultimate failure mode for gears. It is cause for serious concern and can result in significant turbine downtime and replacement costs. Gear tooth fracture not only damages the gear tooth in question: the liberated material can cause severe consequential damage to other gearbox components if the turbine continues to run. If a crack or fracture is identified, replacement should be scheduled. In some circumstances, it may be deemed possible to run the turbine de-rated for a short period, as long as the liberated material has been removed from gearbox. However, this should only follow a detailed engineering review, extreme care must be taken and the turbine closely monitored for any signs of progression.