Adhesive wear

Causes of adhesive wear

In bearings, the adhesive wear process generally arises from skidding or sliding. This happens due to various factors:

• Insufficient load on the bearing
• Rollers that are too large
• Insufficient friction to maintain rolling motion
• Rollers accelerating upon entry into the load zone and decelerating upon exit

These scenarios create friction and extreme temperatures. When the lubrication film becomes too thin or vaporises, metal-to-metal contact occurs. As the surfaces separate, welding and tearing lead to material transfer, producing adhesive wear marks.

The severity of adhesive wear depends on the scale of the welding and tearing process, which correlates with load and the distance over which the roller skids.

Appearance of adhesive wear

Adhesive wear always aligns with the direction of sliding. It can manifest in varying degrees of severity:

• Mild adhesive wear: Occurs at the micro-scale, and may not be visible. Under close inspection, small furrows and dark weld spots may be seen.
  Moderate adhesive wear: Identified by black-brown skid marks. It results from welding and tearing of multiple asperities, primarily occurring at the start of the load-zone on raceways.
  Severe adhesive wear: Known as scuffing, it features visible patches of transferred material, giving a smeared appearance. Galling and smearing are other terms for this severe form, where the rough surface is torn and material piles up at the end of the wear patch.

When diagnosing adhesive wear, it’s essential to rule out other wear types like abrasive wear, fretting corrosion, and micropitting.

Progression of adhesive wear

• Mild: Common during run-in and typically arrests once asperities are worn down.
  Moderate: This wear type progresses if conditions are not adjusted and may lead to other failures like micropitting, macropitting, or abrasive wear.
  Severe: Once severe wear occurs, the process accelerates and can lead to catastrophic failure, such as bearing seizure or fracture due to macropitting.

Detecting adhesive wear

Discussion on bearing design

Wind turbine bearings face challenges when balancing the need for low friction (to avoid overheating) and high friction (to prevent adhesive wear). For example, larger bearings may resist higher loads but are more prone to skidding due to their heavier rollers. While severe adhesive wear (or scuffing) is not common, addressing bearing sliding through design improvements can mitigate wear risks.

Good design choices, such as ensuring sufficient lubrication and using harder materials rather than softer materials, are key to preventing adhesive wear.

Severity rating

Rank 1 Light micro-scale markings from run-in. Superficial damage; normal operation can continue.

Detection:

Visual, borescope

Recommended action:

None – run turbine as normal.

Rank 2 Brown-black skid-marks with depth. Continue operation with monitoring and review lubrication.

Detection:

Visual, borescope

Recommended action:

Run turbine. Continue to monitor during routine inspections. Investigate suitability of lubricant and additives.

Rank 3 Severe scuffing; review for potential replacement.

Detection:

Visual, borescope, vibration, SCADA

Recommended action:

Review situation and either run, curtail or stop turbine in short term. Increase inspection frequency. Begin scheduling replacement

Rank 4 Evidence of large weld spots and macropitting. Immediate bearing replacement is necessary.

Detection:

Visual, borescope, vibration, SCADA

Recommended action:

Stop turbine immediately and schedule replacement. 1