Macropitting – Gear failure

Causes of macropitting

Although macropitting results from contact fatigue, it would be wrong to assume it only emerges towards the end of the design life, after a given number of loading cycles, when the fatigue limit is reached. Rather, there are many design, manufacturing and operational root causes for the formation of macropitting much earlier in the design life of a component.

Causes can include material properties and defects, increases in the Hertzian contact stress due to surface defects, system misalignment, improper lubrication, or the gear tooth design. Metallurgical laboratory analysis is often required to conclusively determine the initiating factor, but they all originate from the two broad categories described below.

Surface initiated macropitting

Surface initiated macropitting may arise from pre-existing damage to the contact surface such as nicks, burrows, indentations or furrows. This damage itself can be from debris within the lubricant, or from manufacturing oversights such as grinding burn on case carburised components. It can also result from micropitting: small asperity sized cracks and pits which are a precursory Hertzian Fatigue failure mode to macropitting. The surface damage acts as a localised area of high stress which leads to a small crack being created and propagating down into the material, causing other cracks and eventually leading to a network of cracks which travel roughly parallel to tooth surface before returning to the surface and liberating material.

Subsurface initiated macropitting

Subsurface initiated macropitting is caused by non-metallic inclusions in the gear steel which act as a stress concentration point where micro-cracks may form and progress to macropitting and possibly spalling. Inclusions are especially dangerous when they lay below the surface near the depth of maximum Hertzian contact stress, or near the case-core boundary in case hardened components. Subsurface macropitting can also be initiated by porous voids, in a similar way as for inclusions. Metallurgical examination is generally required to determine if inclusions are the root cause of failure.

Appearance of macropitting

A macropit can be described as a crater, pit, pore or hole that develops on the active flank of a gear tooth. They are distinctive in appearance and easily identified as they tend to have sharp, angular edges.

As macropits form when multiple small fatigue cracks join and liberate material from the component, the bottom of a pit appears rough and reminiscent of potholes on roads. An initial macropit will become a localised point of high stress and further cracks will develop along the active gear tooth flank in the direction of sliding. This means that as spalling develops, the crater floor may exhibit beach marks (semi-circular lines akin to the marks left on a beach by the tide, which highlight the progression of multiple phases of successive cracks) as it propagates. A phenomenon called Point Surface Origin (PSO) macropitting is often indicated by growing from an initial point into an arrowhead shape in the direction of gear sliding contact.

Progression of macropitting

Once an initial macropit forms it will often progress as the edges of the crater act as a stress concentrator and promote the formation of additional subsurface micro-cracks. Over time these cracks will coalesce and liberate additional material, eventually covering a large area of the gear face. Due to the release of hard material, macropitting can lead to secondary damage in other gears and bearings. The sharp edges created by macropitting can also damage mating gear teeth.

Macropitting will generally continue to progress until the gear has failed. This is usually either due to a crack through a tooth, or substantial surface damage preventing proper load transmission.

Detectability

Discussion

To mitigate against the occurrence and progression of macropitting, the following steps should be taken:

• Design the components for minimum contact stress.
• Ensure high hardness for the component surfaces, such as that achieved by case hardening.
• Use super clean steel for gear teeth and bearings to reduce likelihood of material defects. ISO grade MQ or better is often specified.
• Keep lubricating oil very clean and ensure it is delivered to the components in sufficient quantity in order to prevent metal-to-metal contact and reduce indentation damage caused by debris resulting from abrasive wear or adhesive wear.

Severity rating

Rank 1 N/A

Detection:

Borescope, vibration, oil debris monitoring

Recommended action:

N/A

Rank 2 Single, isolated, macropit less than 1mm in axial direction. If due to redistribution of uneven loading by removal of asperities during run-in, it should arrest.

Detection:

Borescope, vibration, oil debris monitoring

Recommended action:

Run turbine, increase inspection frequency to once every 6 months. Carefully monitor ferrous particles in oil or grease and component vibration.

Rank 3 Isolated macropits covering less than 5% of contact area. No coalescence into spalling.

Detection:

Borescope, vibration, oil debris monitoring

Recommended action:

Run turbine, replace macropitted component as soon as possible to minimise consequential damage. If main bearing, consider grease flushing to prolong life.

Rank 4 Pitting covering greater than 5% of the contact area. Evidence of macropits joining to create spalling.

Detection:

Borescope, vibration, oil debris monitoring

Recommended action:

Stop turbine to minimise consequential damage and replace damaged components.