What is blade bearing sub surface fatigue?
Blade bearing issues are becoming more prominent across wind farms, with the main cause being sub surface fatigue. This issue has become all too common in large megawatt wind turbines.
This damage can shorten blade bearing life to as little as one to two years. Some pitch bearings are particularly susceptible to sub surface fatigue, causing cracks and stress points. This is caused by high load, in this case, the weight of the blade. The most likely cause of failure for the bearing is cracking, originating from the axial drilled holes in the outer race.
https://material-properties.org/what-is-surface-fatigue-fatigue-wear-definition/
Subsurface fatigue is exactly what it sounds like; small cracks, generally around 5-6 microns deep, which cannot be seen with the naked eye. Subsurface fatigue is the first sign that a bearing is on its way to fatigue failure.
Over time, these microscopic cracks will eventually begin to connect, resulting in “advanced” subsurface fatigue.
https://material-properties.org/what-is-surface-fatigue-fatigue-wear-definition/
How is subsurface fatigue caused?
As subsurface fatigue progresses, the microscopic cracks eventually connect, and wide, thin chunks of metal start popping out of the race. At this point, the damage becomes palpable and audible. This is called spalling.
Once spalling occurs, it accelerates very quickly. The contact area becomes much larger, and the pressure becomes more volatile. As a result, it is difficult for the lubricant wedge to maintain its shape, and surfaces can no longer remain separate. The spalling then accelerates quite quickly.
Anything that affects the radial and axial load of the drivetrain accelerates this process, including the following:
- Imbalance
- Misalignment
- Broken bolts
- Loose bolts
- Broken welds
- Overloading
- Over-speeding
- Shock loading
Once the blade bearing has fully cracked, the damage can spread to the hub casting, causing large scale damage and asset downtime.
Reducing the level of sub surface fatigue
One long term solution is to install a compression strand retrofit. The purpose of a compression strand retrofit is to act as a compression strap, to reduce the overall tensile stress on the blade pitch bearings. The strand retrofit may also be applied to pitch bearings where bearing stiffness may be too low. The installation of such compression straps are known to reduce peak stress on the bearings by as much as 25%.
The way that a tensioning arrangement for a bearing ring works is by introducing a compressive force towards the rotational centre of the bearing ring, to prevent the bearing ring distorting as a shaft supported by the bearing turns. The tensioner arrangement includes a strap and a tensioner. The strap has a connector at each end, and tensioner has an element for connecting to the connectors. The tensioner also has an element for applying tension to the strap. In use, the strap is arranged around the bearing ring and a circumferential length of the strap in contact with the bearing ring is reduced, thereby applying compressive force to the bearing ring.
How to detect pitch angle defect?
While this wear and tear still cannot be seen or felt, the energy given off by the fatigue is ultra-high frequency, so ultrasound instrumentation picks it up very well. Vibration analysis is also helpful in identifying this phenomenon.
There are multiple technologies that can be used to detect sub surface fatigue, but no single technology can identify all the different types of sub surface fatigue. Ultrasonic analysis is the most inclusive and efficient technology for doing so as outlined below:
Ultrasonic analysis:
Ultrasonic inspection and monitoring of bearings is by far the most reliable method for detecting emerging bearing failure and conditions such as lack of lubrication. The ultrasonic warning appears prior to a rise in temperature or an increase in low frequency vibration levels. Ultrasonic inspection of bearings is useful in recognising:
- The beginning of fatigue failure.
- Brinelling of bearing surfaces.
- Flooding of or lack of lubricant.
Using ultrasonic analysis to predict when bearing failure will occur:
Certain ultrasonic analysers, such as the SDT340, allow you to measure and record the readings from a bearing. Static symbols are measured as decibels then trended against previous values as seen within the chart below. Dynamic signals are then recorded and analysed in a time and frequency domain for in depth analysis into fault conditions. This allows you to predict when a bearing will fail allowing you to plan accordingly.
https://sdtultrasound.com/products/sdt340/
Ultrasonic devices:
The superior option for ultrasonic analysis is the SDT340 from SDT Ultrasound solutions. Using the Ultranaylsis Suite 3 software the bearing readings can be correlated to give an effective prediction of when high level bearing failure will occur.
Conclusion
Seeing all issues that subsurface fatigue causes, it’s easy to see the business case for implementing a plan to detect and prevent it. As previously mentioned, ultrasound, oil analysis, vibration analysis, infrared, and motor circuit analysis are all helpful. No one inspection method can detect everything, but in conjunction, the technologies are very effective at finding the early warning signs of bearing failure.
Whilst there are many monitoring solutions on the market, these solutions are far more expensive and less effective than using a portable ultrasonic analyser. The suggested solution is to have ultrasonic analysis carried out on each of the turbines on a 6 months basis. This will allow for much earlier defect detection and the data can be correlated to give a fairly accurate estimate of when failure will occur, allowing the asset owner to act much earlier. If the rate of degradation is deemed to be to a high level, then the installation of a compression strand retrofit is advised as this can reduce peak stress on the bearings by as much as 25%.
How can Greensolver help?
Greensolver utilise the latest predictive technology, such as ultrasonic analysis and predictive condition monitoring, to identify sub surface fatigue on blade bearings. We can often identify issues before component failure can occur, allowing us to mitigate the associated asset downtime and repair costs. This can save asset owners large portions of revenue. For more information on predictive services that Greensolver provide, feel free to get in touch.
Written by Jake Mason, Engineering Manager
