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.
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.
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.
|Visual inspection||✓✓✓||Parallel stage gearing with macropitting will be visible to the naked eye: rotate the gearbox slowly and carefully and examine all visible gearing. Filter magnet for evidence of liberated material.|
|Borescope inspection||✓✓✓||A borescope is required to identify macropitting on the planet stage gears. If the gearbox employs a through hardened ring gear, look for tell-tale debris indentations that indicate large pieces of metal are being liberated by some component within the planet stage. If debris indentations are present a thorough inspection is required until the source of the indenting material is located.|
|Vibration analysis||✓✓✓||Vibration analysis is very effective for detecting macropitting of parallel stage gears and bearings. Planetary stage spalling is more difficult as the components rotate slowly and the vibrations may be damped by the multiple components that the signal must pass through on its way to the sensor.|
|SCADA data||✓✓||A rise in gearbox temperature may allow macropitting to be identified once it has reached a relatively progressed level, however this alone is unlikely to lead to a diagnosis.|
|Oil debris sensor||✓✓||Gear oil debris monitoring will give early warning of macropitting. A “burst” of particles will be counted every time the spalling progresses in size. Any alarm should be followed up with a thorough inspection to identify the source.|
|Oil sample analysis||✓✓||Debris generated from macropitting will be detectable from oil sample analysis.|
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.