There are many possible conditions which could cause the lubrication film to be insufficient. Some examples include:
- Gear geometry errors, from poor design or operational wear.
- Insufficient lubricant selection, with inadequate anti-scuffing additives.
- Poor lubrication system design.
- Overheating. Even if there is a sufficient lubrication film, it may be vaporised by frictional heating.
- Gear misalignment, possibly resulting from bearing or housing deflections.
- Transient events, such as gusts or grid faults.
The severity of adhesive wear is determined by the scale of welding and tearing which takes place. This in turn is generally proportional to the load and distance over which sliding contact occurs.
Adhesive wear is a direct result of welding and tearing instigated by metal-to-metal contact of loaded components at high temperature.
As adhesive wear results from sliding contact, it forms at tooth tips and roots where such motion occurs. Furthermore, as material is transplanted from one gear flank to another, adhesive wear will be evident in corresponding areas on both.
While broadly similar, the appearance of adhesive wear varies with severity:
- Mild adhesive wear occurs on the micro-scale when discrete asperities weld and tear. In many cases the damage will not be visible to the naked eye, however close-up inspection will reveal small, dark furrows which represent weld-spots. Overall the appearance of the gear teeth will be largely unaffected.
- Moderate adhesive wear results from the welding and tearing of multiple but discrete asperities. It is visible to the naked eye, with scuffed, silver markings. It may occur in localised patches and could be mistaken for abrasive wear.
- Severe adhesive wear is frequently called scuffing, but is also known as smearing or galling. Material transfer happens on a macroscopic scale, as multiple adjacent asperities are welded and torn. This results in a raised patch of material being built-up and growing as it moves along the surface. Transferred material will be rough, torn and will mask machining marks. It may also appear “smeared” up and possibly over the top of a gear tooth. Torn areas are dark, jagged furrows.
Adhesive wear is not a fatigue phenomenon and can occur immediately. In contrast to contact fatigue failure modes such as micropitting and macropitting which take time to develop, adhesive wear can result from a single, short event. It may or may not be progressive.
- Mild adhesive wear during run-in is common. This will generally arrest as the asperities are worn down. As the importance of proper run-in procedure and replacing the lubricant afterward is well understood. Mild adhesive wear is not a cause for concern.
- Moderate adhesive wear will be progressive unless operating conditions are altered. Damaged areas are rough and torn and therefore act as an initiation point for damage progression. It may progress to severe adhesive wear, or to other failure modes such as micropitting, macropitting or abrasive wear.
- Severe adhesive wear, or scuffing, alone is not considered a failure mode with the potential for sudden and catastrophic failure. It does not progress as rapidly in gears as in bearings, and seizure is very unlikely. Rather, it acts as an enabler for failure by macropitting or fracture.
|Visual inspection||✓✓✓||Moderate and severe adhesive wear will be visible to the naked eye.|
|Borescope inspection||✓✓✓||Moderate and severe adhesive wear will be visible.|
|Vibration analysis||✓✓||Severe adhesive wear can result in violent vibrations which may be detectable from vibration data.|
|SCADA data||✓||SCADA does not aid detection of adhesive wear in gears.|
|Oil debris sensor||✓||Adhesive wear results in material transfer from one surface to another. In early stages, there is little release of debris into the lubricant.|
|Oil sample analysis||✓||Adhesive wear results in material transfer from one surface to another. In early stages, there is little release of debris into the lubricant.|
Proper run in of gears during the manufacturing load test is essential to eliminating adhesive wear. The gearbox lubrication system should be outfitted with a particulate counter during the test, and the gearbox run at 25% incremental load steps. At the beginning of each load step the particulate count will rise and then slowly fall as the asperities left over from gear tooth profile grinding are worn smooth.
Severe adhesive wear is not a common issue on wind turbine gears, as the above proper run-in procedure is well understood.
In addition to suitable run-in, adhesive wear can be mitigated against by using high hardness materials and ensuring sufficient lubrication with anti-scuffing additives.
Adhesive wear results from sliding motion, which is most likely to occur on high speed stage gears.