By Corey Bayles, Senior Product Engineer, Renewable Energy Applications, SKF USA
Extreme weather, unpredictable heavy loads, remote locations and designs for higher output are just a few of the operational challenges affecting wind turbines that can lead to unexpected bearing failures. Fortunately, wind turbine service life can be increased by upgrading design, components and technology. Upgrades can increase pitch bearing life by up to 10 years, improve turbine life and efficiency while reducing downtime — all at a fraction of the cost of a new turbine.
What makes pitch bearings unique
Slewing ring bearings connect the rotor hub (spinner) with the blade so it can be adjusted to the optimal angle for wind conditions. The blade is typically rotated by an internal/external spur gear or hydraulic actuator.
Designed for a lifespan of 20 years (approximately 175,000 hours), pitch bearings typically feature deep groove gothic arch raceways and maximized ball complement. Balls are evenly distributed by puck-like spacers or caged separators. A single-row four-point, or double-row eight-point, contact design provides exceptional load capacities, with bearing raceways that allow the balls to carry load from any direction simultaneously.
However, certain characteristics can pose challenges with bearing longevity. A typical pitch bearing might never rotate more than 90° its entire life, and heavy loads, combined with very fine (<5°) oscillation angles, can put a great deal of stress on pitch bearing components. They are also held stationary for long periods of time and constantly subjected to vibration, which rapidly degrades the lubricant and leads to adhesive wear.
The isolated location of many wind turbines, exposure to a wide range of weather conditions and the pitch bearing’s position atop the tower limit regular access and observation. Usually, pitch bearings are directly observed every six to 12 months during periodic maintenance, making it difficult to detect problems early. Additionally, the hollow cast-iron hub and composite blade are quite flexible and provide the bearings little support.
Why pitch bearings fail: lubrication
Unfortunately, the classic failure modes predicted by standard bearing calculation models (i.e., fatigue spalling and brinelling) are actually uncommon causes for pitch bearing failures. The main culprit is usually bad lubrication. Lubrication-induced failures include vibratory wear (false brinelling), corrosion, debris denting and surface-initiated fatigue. Separator damage, raceway flaking, split balls and bearing lockup can all be signs of a poorly lubricated pitch bearing. Many failures categorized as load-based might actually be the result of an issue that started with grease degradation.
In addition, more sophisticated pitch-control techniques designed to increase power production density result in heavier stress on the lubricant and bearing components. Dithering is a good term to describe the motion of bearings on turbines with active pitch control — continuous, rapid oscillations at extremely fine pitch angles. This type of operation is not only a foundation for recent turbine efficiency gains, but is also a catalyst for lubricant degradation and component wear.
Because wind turbines are subjected to harsh weather environments, lubrication practices must be designed to ensure maximum machine uptime with minimal maintenance. Proper grease selection is the first and most important step. Pitch bearing grease must resist water washout and contain a durable additive package that protects against high load and vibrations. Use of continuous-feed lubrication systems also enable sites to add or adjust grease-fill as necessary, without requiring technicians to climb.
Why pitch bearings fail: load and operation
While lubrication is the primary challenge, load failures are also an area of concern. Overloading usually happens because the bearing lacks rigid support from the hub assembly, leading to an imbalance where a fraction of the raceway carries most of the load. Load- and operation-induced failures include component fracture (rolling ball elements, ball separators, races), separator lockup and raceway core crushing. As noted above, these failures might also be exacerbated by lubrication conditions.
In a pitch bearing, the contact area between the ball and the raceway forms an elliptical shape that is centered over the race contact angle. Under heavy thrust or overturning loads, the contact ellipse can spill out of the physical limits of the raceway (truncation). The probability of contact truncation increases with the ratio of bearing diameter to thickness, or as external support decreases. Severe contact truncation produces stress risers that cause the path edges to break off or the balls to split in fragments.
Finally, calculations rely on a set of conditional assumptions that sometimes bear little resemblance to real life. A bearing in a clean room with new seals, fresh grease and mounted on a rigid, perfectly flat surface might last dozens of years. Unfortunately, real life is seldom neat, and industrial equipment (like a wind turbine) must perform where it is needed.
Bearing upgrade: increasing path surface area and strengthening rings
Although most pitch bearings fail in similar ways, the underlying causes can vary, and improvements must start with an understanding of that bearing’s unique issues. With the potential cost of downtime and bearing change-outs ranging into hundreds of thousands of dollars, it’s beneficial to work directly with a manufacturer that can offer a bearing replacement solution that will improve productivity and extend turbine life cycle. The most effective bearing upgrades mitigate edge loading, strengthen the races, address separator wear and prevent contamination — ultimately, this results in a more efficient bearing.
Bearing upgrade: separator rings and raceway geometry
Despite some theoretical advantages, the compromises necessary to account for manufacturing variation in continuous ring separators more than outweighs any of their benefits. With diameters regularly exceeding 2 m, it is virtually impossible to hold good shape and tolerance on a ring 5-mm thick. Gaps between the races must be enlarged to accommodate the ring, thereby reducing path contact area and increasing truncation. Rings also must be fabricated from mild steel, as high-strength alloys are not typically weldable. On the other hand, segment-style cages suffer none of these drawbacks and provide limited freedom of movement that can relieve loads that might rip apart single-piece rings.
For the paths, strict geometric dimensioning and tolerancing (GD&T) controls on the form, finish and spacing improves load sharing and balance. Nearer to perfect form means less friction, skidding and tight spots, thereby reducing internal wear and improving pitch system response and efficiency.
Bearing upgrade: upgrading seals
Pitch bearing seals play a dual role: protecting internal components from contamination and stopping lubricants from escaping into the environment. Unfortunately, seals are not completely effective; after all, a bearing cannot rotate if it is hermetically sealed. Common pitch bearing seals are hydrogenated nitrile butadiene rubber (HNBR), installed on a groove in one race with two seal lips that drag along the opposite race. This seal style wears quickly, rapidly degrades when exposed to UV and ozone, responds poorly to distortion and provides contaminants with a direct path to the bearing internals.
An “H-profile” seal design made from thermoplastic polyurethane (TPU) and installed on a labyrinth retention groove significantly improves seal effectiveness. This free-floating design is highly responsive and provides seal pressure even when deformed. It is less sensitive to ring deformation during operation, reducing grease leakage and water ingress to help improve robustness and reduce maintenance costs. In addition, TPU wear rate is a fraction of conventional rubber, extending effectiveness and replacement intervals.
Shear stress from heavy contact loads can penetrate beneath the surface and cause the softer core to yield, leading the hardened path to detach from the race (core crushing). To prevent this, the induction-hardened layer must penetrate deeply enough that steel strength exceeds the contact shear stress. In pitch bearings, structural deformations and heavy overturning loads mean that peak shear stress could occur at any point along the path surface. Therefore, it is imperative that the hardened layer be a uniform pattern and not diminish as it moves further from the design contact angle. A deep, uniform heat treatment greatly mitigates the effects of contact truncation.
Bearing upgrade: proper storage, packaging and handling
Since most bearings may have an extended shelf-life before installation, it is important to ensure they are stored and packaged to prevent degradation prior to use. Proper packaging can prevent corrosion and damage from shock, vibration and other hazards during transport. Packaging should include the application of a corrosion-preventative coating to mounting holes, wrapping bearings in volatile corrosion inhibitor (VCI) paper, packaging in vacuumed-sealed bags and individual crating (stacked in two-high sets).
Bearings should be left in their original packages until just before mounting to prevent exposure to contaminants, especially dirt, and should be handled with clean, dry hands and clean rags. Prior to installation, they should be placed on clean paper, kept covered and never be exposed to a dirty table or floor.
Customized solutions to extend turbine life
When turbines go offline because of maintenance issues or equipment failure, the high cost of repair crews and crane day rates can send costs soaring. Upgraded pitch bearing solutions can improve turbine life and efficiency and reduce downtime by:
- Increasing turbine reliability even in harsh environments
- Extending seal and bearing service life
- Reducing operation and maintenance costs
- Improving pitch control for increased performance
- Reducing installation and replacement time
Since bearing health is dependent on a variety of factors, it’s important to work with a manufacturer that can perform a failed bearing analysis and test the new solution using simulation programs to determine which upgrades may be necessary to mitigate risk of future failure. In addition, value-added services, such as condition monitoring and predictive maintenance, can further extend the service life of wind turbines well beyond their expected lifespans.
Corey Bayles is currently a senior product engineer for renewable energy applications for SKF USA. Corey graduated from the United States Military Academy at West Point in 2005 and was commissioned as a Second Lieutenant (Armor) in the U.S. Army. After two tours in Iraq, he left active duty and joined SKF (Kaydon Bearings) as a semiconductor market product engineer. Corey has nine years of experience in application and product design, and personally investigated dozens of failed pitch and yaw bearings. He frequently collaborates with owner/operators to improve pitch and yaw function and prevent unexpected turbine downtime. Corey and his family live in the Muskegon, Michigan, area.