This month’s case study describes a very interesting failure of a gear shaft
R-Tech Materials received a gear shaft (Figure 1) from a gearbox used to drive a ball mill with the request to determine the cause of surface damage observed on the teeth.
Damage was evident at a similar position on the same face in the addendum area of the majority of the teeth. For many of the teeth, this damage was minimal, see Figure 2. For four of the teeth, the damage was more extensive, see Figures 3 and 4.
The damage observed was consistent with spalling which occurs when several pits join or when a crack runs parallel to the surface for some distance. Some of this damage was associated with through-wall cracking, the fracture surface of which exhibited multiple start marks and beach marks, see Figure 5. SEM examination of the area revealed the presence of striations across the surface, see Figure 6.
Metallographic examination in the area of the spalled surface showed the tooth to have been case hardened with a case depth of 1.5-1.7 mm, as expected. Multiple cracks were evident in the addendum area with small volumes of the surface close to becoming detached, see Figure 7.
Figure 7 Section through spalling
The features observed indicate that the damage observed is attributable to contact fatigue. Contact fatigue is a surface-pitting type failure commonly found in gears. Components that roll, or roll and slide, against each other under high contact pressure are susceptible to the development of surface pits or spalls. Spalling/pitting is a manifestation of metal fatigue from imposed cyclic contact stresses. Where gear teeth are subjected to significant levels of sliding contact in the area of the pitch line, initiation of contact fatigue cracks is found to be in the surface of the material. Fatigue cracks tend to nucleate at defects or inclusions within the microstructure. These cracks propagate at a shallow angle to the surface, and pits/spalls form when the cracks are connected to the surface by secondary cracks. If spalling is severe, the bending strength of the tooth may be decreased to the point where complete fracture of the tooth can occur. The rolling contact fatigue process is usually self-propagating through surface roughening, dynamic loads, and a decrease in the load-carrying area. The formation of dislocations and cracks, and the subsequent relative motion between fractured surfaces, gives rise to vibrations.
The damage was confined to a localised area of the gear shaft and located in the same area of each tooth, on the same side. This indicates that this position was exposed to the greatest stress which may suggest that one of the parts was misaligned. Misalignment will cause localised overloading of the gear which can then lead to spalling.
A failure analysis of this type is a valuable and informative tool to reduce the likelihood of a repeat incident which improves safety and productivity, reduces the risk of unplanned outages and enables evolution towards a better product.
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