- Not enough load on the bearing
- The rollers are too large
- There is not enough friction to maintain purely rolling motion
- Rollers accelerating upon entry into the load zone and then decelerating when leaving.
In all of these scenarios, the skidding motion will cause a drastic increase in temperatre. If the lubrication film is not thick enough, or is vapourised by the heat, metal-to-metal contact will occur. If the temperature from skididng is sufficiently high, asperities on the roller and raceway will momentarily weld as they come in to contact. As the roller and raceway surfaces move apart, these small welds are torn, and plastically deformed material transferred from one surface to the other. The markings this produces are known as adhesive wear.
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 the distance over which the roller has skid.
Adhesive wear is a direct result of the welding and tearing instigated by metal-to-metal contact of loaded components at high temperature.
The appearance of adhesive wear varies with its severity. Adhesive wear always occurs in the direction of sliding.
- 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. Damage tends to form in a line, which may be visible. Overall, however, the appearance of the bearing will be largely unaffected.
- Moderate adhesive wear is clearly identifiable as black-brown skid-marks. It results from the welding and tearing of multiple but discrete asperities. On close inspection, the surface appears plastically deformed and torn. Like mild adhesive wear, it is a micro-scale process, meaning it lacks the smeared appearance of severe adhesive wear. It can be seen on rollers, but primarily occurs at the start of the load-zone on raceways.
- 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 transferred material being built up and growing as it moves along the surface. This cascading, avalanching process gives severe adhesive wear its “smeared” appearance. The transferred material will be rough, torn and will mask machining marks. Often the transferred material is piled-up at the end of the wear patch, where roller skidding has ceased and rolling motion resumed.
Note: When attempting to identify adhesive wear visually, it is equally important to establish the absence of other, more distinct wear mechanisms such as abrasive wear, fretting corrosion and micropitting. Moderate adhesive wear in particular is easily mistaken for one of these.
As it is not a fatigue phenomenon adhesive wear may occur instantaneously.
- 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 concern.
- Moderate adhesive wear will be progressive unless operating conditions are altered. The skid-marked 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, can lead to catastrophic failure. Once the welding and tearing begins to bridge multiple asperities the damage becomes self-aggrevating and progresses rapidly. The amount of transferred material grows as it propagates along the surface. This material acts as a substantial stress concentrator, akin to a very large asperity. It may be substantial enough to produce a significant flat spot on the bearing roller. Eventually, it will cause failure by fracture, or in extreme cases, bearing seizure. Futhermore, the torn, flaked areas of removed material are susceptible to macropititng.
|Visual inspection||✓||Most bearings likely not visible without borescope.|
|Borescope inspection||✓✓✓||Moderate and severe adhesive wear on accessible bearings should be visible.|
|Vibration analysis||✓✓||Severe adhesive wear can result in violent vibrations which may be detectable from vibration data.|
|SCADA data||✓✓||Severe adhesive wear results in large temperature increase which may show up in SCADA data.|
|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.|
Adhesive wear highlights the difficulty of suitable bearing design for wind turbines: a low friction bearing surface is more efficient and helps prevent over-heating, but a high friction surface will help prevent adhesive wear from occurring. Another example is the size of the bearings: larger ones can withstand higher loads, but will have heavier rollers which could skid more and take more energy to get rolling again. Bearing sliding remains a common issue in wind turbines, but component failure from severe adhesive wear (scuffing) alone is relatively uncommon as other life-limiting failure modes are more prevelant.
Adhesive wear can be mitigated against by good bearing design which minimises skidding, uses materials with a high hardness and a sufficient lubricant with additives designed to prevent welding.