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A Few Different Perspectives on Wind Turbine Bearings

How we see things, very much forms our opinions!

As a wind turbine owner, how information is presented to you can drive your actions in very different directions.

Have a look at these gearbox borescope images of a 240/600 main bearing rolling element.  This is the large bearing that holds up the blades, rotor, and main shaft. This first image is captured with an older 6mm borescope using a macro lens (depth of field from 3mm to 20mm).

To the unfamiliar eye, this damage might appear to 1-2cm wide and highly progressed. It shouts, “failed” or “failing” and prompts calls to action to be seeking quotations and cranes or warranty claims.

Yet using the same borescope, with only a tip protector and no additional magnification from the lens, it appears very different (depth of field from 50mm to infinity).  Now our progressed failure is just a line on the surface of the roller, likely a standstill mark with micropitting, no need to panic!

Finally, this image taken with a digital camera, better represents the appearance of the axial line as seen with the naked eye (of course we have the luxury here of having the roller out of the bearing):

This damage will eventually progress in the rolling direction and require a main bearing replacement. Is that time right now? No. In fact it will likely take years to fail, but that largely depends on the condition of the rest of the bearing. Are there standstill marks on more rollers, possibly with more pitting progression?  What about the bottom dead center of the outer race? Is the grease contaminated with metal debris? In this case, flushing and replacing the grease may be the right action, prolonging the life by removing metal debris. Continue to monitor the bearing condition with vibration or annual inspections.

Here’s the takeaway: if the only image captured is the first one of this series, it’s possible that the severity of the damage could be classified as severe macropitting rather than a standstill mark showing minor micropitting.

Very different perspectives on the same issue.

Here are some inspection tips to increase the information you need to make the best O&M decisions:

  • Take multiple images that capture both the relative scale of the damage and the detailed topography of the surface.
  • Document the condition of the raceways and at least a few other rolling elements to be clearer on the overall component health.
  • Make sure your inspectors are trained to pick up the details and report thoroughly. Provide training on failure modes in wind turbines.
  • If possible, collate information with past inspections, temperature/vibration trends, and grease/oil sample results. View it all together for a broader perspective.

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