Executive Summary
Wind turbine bearings face unique lubrication challenges, including extreme operating conditions, difficult access and heavy and variable loads. As mentioned by many TLT readers, lubrication maintenance in general can be its own challenge, as it can be costly and requires expertise from the service technician. Some solutions to these challenges include implementing a condition monitoring program, using the correct lubricant and training technicians.

Q.1. Please describe the challenges involved in wind turbine main bearing lubrication and their impacts on turbine performance.

Difficulty accessing the gearbox. There are also shearing issues because of all the starts and stops.

As an outsider to the industry, I have no direct knowledge, but observationally, accessibility and temperature conditions would seem to be the most probable areas of concern. By the very nature of the physical constraints of construction, wind turbine bearings are difficult/time consuming to access on a routine basis and exposed to various temperature extremes. The first means maintenance of the lubricant requires additional resources and the second means a more aggressive degradation curve.
 
There are two key issues that affect the main bearing on a turbine. The first is moisture ingress in high humid operating environment. We frequently see this in our wind sites just onshore in eastern Ontario sites. The second challenge is maintenance. Most turbines are on a single annual maintenance schedule while others are on an annual and semi-annual schedule. Lubrication is one of the determining factors for how much maintenance we do with newer turbines that have autogreasers and other diagnostics reporting tools on a single annual maintenance. Autogreasers go empty without being refilled, and manual greasing is not done properly and frequently. Using tools like ultrasound grease guns is not possible as the turbine is locked out for technician access. In summary, moisture ingress and maintenance neglect are the top challenges. Turbines are robustly built.

Harsh operating conditions and variable loads, leading to premature failures that severely impact turbine performance and increase maintenance costs.

The constant back and forth of the “yawing” and “pitching” putting pressure and causing wear on the main bearing.

High and/or irregular load along with frequent stops and starts make it difficult to generate a consistent lubricant film.

Heavily loaded lubricated contacts in roller bearings (nonconformal surfaces) guiding the main rotor and in lubricated bearings (conformal surfaces) guiding the planetary gears. The surfaces are submitted to wear during low speed operation (wind entrainment).

Evolving regulatory requirements around additives, and the effect of altering ad-pack chemistry on existing bearings.

My opinion is that the biggest challenges are the wide ambient temperature swings combined with the difficulty in relubricating or maintaining an automatic relubrication system.

A couple of glaring challenges are obvious. Regardless of technology, the load demands are variable and difficult to control. Environmental variables are constantly changing as well. Second, service demands are extreme, and service techs have to be skilled in many areas which ae not necessarily related to the lubricant. If you have not scaled the 150 feet to the gondola you have not enjoyed the climb and the view.

Harsh conditions (extreme temperature, moisture), low speed and high load can cause premature bearing lubrication failure.

The main challenges are shock loading from changing wind direction and large torque on the bearing from misalignment of the wind direction and the axis of the bearing.

Some of the major challenges that I see with wind turbine lubrication are mainly around accessibility and predictive maintenance. Because they are so inaccessible (high), regular maintenance and monitoring is not done as often and thoroughly as it should, for such a critical piece of rotating equipment. Temperature swings and particulate ingression/generation are also some of the main contributing factors for less-than-optimal lubrication practices.

Remote location, very challenging access.



How do you primarily select lubricants for wind turbine main shaft bearings?
By base oil viscosity                                                         32%
As recommended by bearing supplier or lubricant supplier 65%
By comparing performance of lubricants (for example, lubricant film formation, wear, fretting, micropitting, etc.)                                                                                         52%
Based on an informal poll sent to 15,000 TLT readers. Total exceeds 100% because respondents were allowed to choose more than one answer.

1.) Poor accessibility for all sorts of maintenance and that poor access drives everything else. 2.) Extended relubrication intervals become an important dream. 3.) Vibration all along the drive chain.

The main challenge stems from the loading conditions under which such bearings operate. Consequently, lubrication conditions tend to deteriorate as the turbine undergoes unique wind and loading conditions.

From my point of view, main bearing lubrication in modern wind turbines is tricky: very low speeds with increasingly huge, oscillating loads, vibration and micro-movements (false brinelling), difficult grease distribution in massive bearings, long relubrication intervals and water/particle contamination which, as a whole, means higher friction and temperatures, loss of alignment, noise and vibration, accelerated wear/fatigue and early bearing failure.

The maintenance cost of the ball bearings is too high, and the ball bearings price is relatively high. Currently quite a few OEMs are investigating the application of slide bearings (hydrodynamic bearings) to replace the ball bearings on wind turbines.

Speaking strictly for Brazil, one of main challenges is correct lubrication. Many of the wind turbines in Brazil are running without automatic lubrication, and the manual lubrication intervals are normally only twice a year. Furthermore, OEMs specify most of the time grease “one size fits all,” meaning they recommend the same grease for wind farms located in Scandinavia and in tropical climates in Latin America.

Slow speeds and intermittent motion, not allowing the film to form due to hydrodynamic action, so the oils need to be fortified by suitable boundary additives. False brinelling and white etching crack issues need to be monitored closely.

Micro pitting, scuffing, fretting, corrosion and false brinelling. That causes premature failure, reliability issues and expensive repair cost.

White etching cracks (WEC) in race ways. 

Contamination, degradation and renewal of lubricants.

This does not pose a major problem, provided the correct lubricant is used according to the manufacturer’s instructions, or by ensuring proper lubrication based on environmental and operating conditions.

The relubrication cycle and the life span of the grease. Without providing the sufficient old film protection, this will impact the efficiency of the power generation.

If oil lubricated, then the general particle cleanliness and oil functionality during idling and startup.

The oils are exposed to critical conditions, especially in offshore installations. Oil aging and contamination are therefore a problem. Good oil diagnostics are therefore important for safe plant operation.

Wear, which impacts life of lubricant.
Q.2. What actions or solutions are needed to overcome these problems?

Installing condition monitoring sensors and opting for lube for life where possible.

Base oil and additive chemistry is going to be critical to provide the long term stability and resistance to degradation of any oil or grease in service in these conditions.

Proper sealing, seal inspection, autogreaser alert/monitoring, a tool to grease stationary bearing that takes the guess work out and maintenance training.

Improve lubrication system, high performance grease, real time monitoring systems, design of bearing.

A properly formulated bearing oil.



How do you handle wear particle contamination in wind turbine main shaft bearings and the resulting premature failures?
By periodic flushing                          52%
By using a proper and suitable grease 62%
By using a repair lubricant                           17%
Based on an informal poll sent to 15,000 TLT readers. Total exceeds 100% because respondents were allowed to choose more than one answer.

Improve grease quality and design the bearings to handle the proper load.

Proper design of contact surfaces, choice of materials and coatings, choice of lubricant and filtering.

Continued research into the actual causes of failures, and then programs and systems that combat those causes.

I think solutions have been explored to death. Regardless of the political blowback, if we want wind turbines to remain a part of the energy supply mix, we will have to include alternative equipment technologies necessary to keep the towers running: axillary heat and air conditioning, narrowing the run parameters, deicing.

Use full synthetic base oils that are less sensitive to temperature and more oxidation resistance.

Lubricants need to be developed specifically for main shaft wind turbine bearings. This is not an application where one lubricant can be used everywhere. The size of wind turbines may be a limiting factor.

Better online and real-time lubrication monitoring solutions could help to more accurately predict when the lubricant(s) health needs to be addressed, without physical sampling by a technician. Properly designed bearing protection (shielding) and insulation/temperature control could help prevent particulate ingression and major temperature swings (respectively), to further increase lubrication performance and remaining useful life (RUL).

Remote monitoring with a top class automatic lubrication system, dispensing the ultimate in lubricity is the best solution.
 
Realistic inclusion of actual operating conditions during wind turbine bearing analysis. All transient load cases should be thoroughly investigated.

To overcome main bearing lubrication problems in modern wind turbines, I suggest to focus on optimizing bearing design and alignment, selecting the right high-performance grease or oil with well-engineered automatic lubrication, robust sealing and contamination control, following smart operating strategies to minimize micro-movements and using condition-based monitoring—together reducing wear, increasing reliability and availability and cutting lifecycle costs.

1.) Lower the ball bearing costs and extend its life, so that the overall cost is competitive compared to the slide bearings. 2.) Research the bearing lubricant with longer life and better thermal properties.

More training for the service technicians and operations and maintenance staff. More after-sales service from lubricant manufacturers and suppliers.

Suitable formulation of oils to handle the lubrication issues, but careful and close monitoring for the other issues.

1.) Use automatic lubrication system. 2.) Optimize grease formulation. 3.) Implement grease flushing. 4.) Utilize elastohyrdrodynamic lubrication theory.

New metallurgies for bearing alloys and base oils with a much lower tendency to form WECs in raceways, like polyakyleneglycols.

Using sliding bearing to replace the rolling element bearings.

Measure the operating and ambient temperatures. This will help determine the best lubricant to use for optimal bearing performance.

Better performance grease formulation and possibly new design to reduce the stress loading.

A proper filtration system should be considered as well as how to assess the oil quality.

Online and offline oil diagnostics to detect the condition of the oil and any changes in it, enabling countermeasures to be taken in good time.

It’s an evolving problem to adjust additives.

Editor’s Note: Sounding Board is based on an informal poll sent to 15,000 TLT readers. Views expressed are those of the respondents and do not reflect the opinions of the Society of Tribologists and Lubrication Engineers. STLE does not vouch for the technical accuracy of opinions expressed in Sounding Board, nor does inclusion of a comment represent an endorsement of the technology by STLE.