There are many design, manufacturing and operational root causes for the formation of macropitting, and the failure mode could originate much earlier in the design life of a component. This can include material defects and material properties, or 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 as a result of 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 the 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 bearing steel which act as a stress concentration point and may form micro-cracks that progress to macropitting and possibly spalling. Inclusions are especially dangerous when they lay below the surface near a 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 failures.
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 larger areas on the bearing surfaces. Due to the release of hard material, macropitting can lead to additional damage in other gears and bearings. The sharp edges created by macropitting can also lead to higher contact stresses and damage to mating raceway or roller element surfaces. In most cases, progression of macropitting will continue until the bearing has failed functionally due to extreme material loss, seizing, or through-cracking.
|Visual inspection||✓✓||At the inspection port on the main bearing, where unaided visual access is possible, macropitting can usually be identified if present. However, other bearings can be difficult to see with the naked eye due to access limitation issues. Inspect filter magnet for evidence of liberated material.|
|Borescope inspection||✓✓✓||Borescope inspection may be required in order to obtain a suitable view of the bearing contact areas. For bearings with grease as a lubricant, significant cleaning of the bearing surfaces may be needed to properly diagnose macropitting on the contact surfaces.|
|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 bearing 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||✓✓||Gearbox 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||✓✓||Oil or grease samples will show metal debris content resulting from macropitting.|
In order to mitigate against the occurrence and progression of macropitting, the following steps should be taken:
- Design the bearings for minimum contact stress.
- Ensure high hardness for the component surfaces, such as that achieved by case hardening.
- Use super clean steel to minimise material inclusions. 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.
Despite the above points being well-known, in practice macropitting is relatively common in wind turbine gearboxes. Two common instances where Romax encounters this issue are:
- Uneven load sharing in 4-row planet bearings.
- Main bearings often develop macropitting while operating in boundary lubrication under high loads and relatively low rotational speeds. This can lead to significant contamination of the grease, which in turn leads to more damage, created at a faster rate. Romax has found that flushing the main bearing of contaminated grease can slow this process and extend main bearing life.