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Damage in a Wind Turbine Gearbox: What am I looking at?

Wind turbine drivetrains are commonly inspected using borescopes (also called endoscopes) to determine the condition of the gears and bearings. The inspections may be scheduled due to a vibration warning, a controller fault, a noise heard from the ground or part of end of warranty activities. The borescope photos provide a visual account of the surface appearance and tangible evidence if a component is damaged. Much can be inferred into gearbox health and remaining useful life from the appearance of normal wear patterns. Inspections may show a failure mode, such as fatigue, fractures, wear or corrosion. When the damage is obvious, such as a missing gear tooth, the borescope photo is easy to understand.

However there is often a challenge in interpreting the less obvious borescope images that leave a question mark around decisions to continue with normal turbine operation versus planning for repairs.

Does the borescope image show a crack, or is that just a reflection?  Is that a pit or a dent or just a droplet of oil?

Pitting is a material fatigue failure mechanism that can be either micropitting, macropitting or a mixture of both. Let’s take a look at some borescope images and have a go at interpreting them.

 

An oil droplet is normal but can often appear like a pit or dent. A debris dent is secondary damage that may eventually serve as an initiation point for surface fatigue failure. You can see the original grinding marks in the base of the dent. A dent is also a clue that elsewhere in the gearbox a failing component is producing debris and additional searching with the borescope is needed to locate the debris source.

Today’s modern borescopes are actually videoscopes that save digital snapshots from the video, typically in the jpeg or bitmap format. Videoscope hardware continues to advance with increased light output, high image quality and improved field of view. They are not cheap. Despite these improvements there is often blurring of images from a camera tip fouled with oil or bearing surfaces that reflect adjacent features like a mirror. The magnification of the image makes it challenging to judge scale and some features just appear abstract, like a doctor reviewing an x-ray image with a patient. It is often hard to judge features that appear small, round and isolated on a bearing roller or raceway. Helpful assessment questions include:

  • Do the original grinding lines from manufacturing exist in the feature or are they interrupted?
  • Does the light reflected from the camera appear continuous or bent around the feature?
  • Is there a darker colour around the outer edges of the feature?
  • Does the inside of the feature appear shiny, dull, grainy or cratered?

All this being said, sometimes a feature in a borescope image is simply suspicious and unidentifiable. Categorize it as ‘unusual marking’ and recommend another inspection in 12 months.

Macropitting is a severe failure mode that will continue to progress until catastrophic failure, in a time frame that depends on the load, lubrication, material and number of cycles. It is sub-surface originated fatigue. Isolated regions of micropitting are concerning as they can progress to macropitting, but micropitting can also self-arrest and have no restrictions on gearbox operation for many years. Both these failure modes produce debris that may create debris dents when rolled over.

A great example of a poor borescope image. Ever been provided this in your inspection reports? Are those debris dents or is it just oil?

ONYX InSight offers training for wind turbine technicians and engineers. We come to your wind farm with a program of desk-based and hands-on, uptower inspection training.

Find out more on our Training page here >>

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