Bearings that can provide the highest possible performance potential in a compact design are ideal for reducing the overall component size, weight, and manufacturing costs in wind turbines. This is particularly true and challenging of main-shaft bearings in offshore turbines, which must withstand harsh sea-salt conditions without fault.

Onshore wind developers and operators deserve enormous credit for their hard work and dedication. If the U.S. is to fully optimize wind capacity in the country, however, it is time to follow the UK and European markets. And this means pushing the offshore wind sector forward in America.

“There’s no end in sight for wind-turbine growth,” reads the lead of a recent investigation into the growing prevalence of wind power and larger turbines, globally. The report anticipates that the average rating of wind turbines worldwide will reach 2.8 MW by 2022, with significantly higher growth in the offshore market. In Europe, it is projected that the average offshore turbine could be 12 MW by 2024.

Typically, initial investments for offshore wind are bigger than onshore but marine projects offer greater generating capacity in the long run. Also, nearly half of the U.S. population lives in coastal areas, which means these regions have extremely high power needs. Offshore projects can leverage strong marine winds to cost-effectively supply electricity to these customers.

To maximize offshore capacity, larger, more powerful turbines are used with components that can withstand the harsh sea-salt conditions while providing reliable operation. However, such high-powered turbines are a challenge to conventional bearing designs. For this reason, offshore turbines rely more on direct-drive technology rather than gearboxes.

Main-shaft bearings are sill used routinely, however — and in just under half of offshore turbines, according to findings from MAKE Consulting. These bearings must endure higher loads, increased deflections, and slower rotational speeds than the ones found in smaller offshore turbines. The lubricants are exposed to saltwater, a corrosive element that may cause unwanted tribology conditions and premature bearing surface damage.

Ultimately, wind operators and turbine OEMs must understand which bearings perform reliably if they are to fully capitalize on offshore opportunities.

Maximizing offshore performance
The sheer size of offshore turbines — which are typically double the capacity of onshore machines — and extreme wind loads mean great demands are placed on a turbine’s gearbox and main-shaft bearings. The potential for costly failures in offshore wind turbines runs high. This means it is vital to choose high-quality components with reliable system designs for efficient operation and minimal downtime.

Ideally, a bearing’s geometry, clearances, and load capacity are custom-engineered for the application’s operational conditions and this is particularly important in offshore turbines.

Gearless, direct-drive technology (shown on left) may be the more reliable choice for offshore wind turbines, which must withstand fast wind speeds and harsh conditions. However, nearly half of all offshore turbines still use gearboxes (right) and require high-quality bearings that can handle high loads.

An improperly engineered bearing may cause too much pre-load, which may result in high stresses, high bearing temperatures, and a shorter life. Additionally, a bearing with too much clearance may experience excessive deflections, improper roller load sharing, higher stresses, misalignment, and edge loading, premature cage, or sliding damage. There’s a lot that can go wrong. (“Clearance” is the total distance that one bearing ring moves relative to another.)

Regardless of the form of damage, however, the result is the same: compromised performance of the turbine. Eventually, the bearing requires repair or replacement, leading to turbine downtime and lost operation. Technical expertise and product quality are necessary to successfully capitalize on the asset’s longevity and potential.

Offshore component repairs and replacements are also typically more costly than onshore ones. Replacing a damaged bearing in an offshore turbine requires a specialized O&M team with a transport vessel and the necessary equipment to correctly diagnose the problem. Additionally, it’s necessary to disassemble the turbine to repair the damage, which requires a costly offshore crane rental.

Weighing the options
Recently, tapered roller bearings have demonstrated desirable performance in a number of new wind-turbine applications when compared to spherical roller bearings, which are the conventional choice in most existing onshore wind applications. In fact, there are several 5-MW+ offshore turbine designs that now employ tapered roller main-shaft bearings.

Spherical bearings are composed of barrel-shaped rollers within a rounded (spherical) race, which behaves like a ball and socket joint (much like a hip or shoulder joint). The rollers in tapered roller bearings are shaped like a truncated cone and are fit within races that are angled (or tapered) to simultaneously support axial and radial loads.

Recently, tapered roller bearings have demonstrated desirable performance in a number of new wind-turbine applications when compared with conventional, spherical roller. Spherical bearings are composed of raceways that are rounded (spherical) on the inside axially. In tapered bearings, which are smaller in size, the rings and the rollers are tapered in the shape of truncated cones to simultaneously support axial and radial loads.

Tapered bearings can be sized smaller and offer an increased power density compared to spherical ones, reducing the overall cost of energy. An ability to properly carry thrust and radial loads typically means high performance in harsh conditions and unpredictable changes in wind speed and direction.

However, it is imperative that the specific demands of the application are considered first. This is because there is no one corrch for offshore applications, offshore wind is relatively uncharted territory in the United States.

For those wanting to take advantage of such project developments, it requires working with the right suppliers who can offer proven expertise on problem-solving and total system design to tackle the challenges of offshore wind.

Making the right choice
Dependable engineering and expertise are critical when selecting bearings capable of the performance required in offshore wind turbines. Here are a few additional tips.

• Reliability of the main shaft requires a bearing that can adequately withstand various loads from ever-changing winds. This feature is particularly critical as wind loads increase, such as in offshore applications.
• Bearings that offer the highest possible performance potential in a compact design are ideal for reducing the overall component size, weight, and manufacturing costs in wind turbines.
• Additional options are available to increase reliability and performance. For example, for onshore turbines, advanced diamond-like carbon (DLC) coatings are available that protect against micropitting and other surface damage.

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Automatic lubrication of wind-turbine component — made possible with The Timken Company’s Groeneveld Twin — typically means costs savings and less turbine downtime due to repairs. In bearings and gears, the grease spreads out better across surfaces simply because automatic lubrication takes place while the turbine is in operation. There’s also little risk of blown out seals, thanks to the short intervals and small quantities of lubrication. In addition, it will form a grease collar or film over the surface and around pivot points, which means dirt and moisture are kept out.

Consider that most wind turbines experience some amount of lubrication leakage. These occurrences are relatively minimal for onshore turbines but can pose an environmental risk for offshore wind turbines. Undetected or excessive leakage can lead to a wide range of costly problems and repairs.

Additionally, water ingress can compromise lubricating greases and oils when bearing seals break down or are unsuitable for their intended task. Moisture is a strong contributor to white-etching cracking (cracks within the microstructure of a bearing), which lead to early failure of roller bearings.

Bearing seals should be given careful consideration at the wind-turbine design stage. The market offers a variety of options for meeting the extreme demands of main-shaft turbine bearings, including:

• Single lip seals: All-purpose seals available in a wide range of sizes that are suitable for most applications.
• Dual lip seals: Used for difficult sealing applications involving the separation of two fluids or exclusion of foreign materials.
• Single lip split seals: Engineered for ease of installation on large shafts (requires no costly teardown to replace seal).
• Bearing isolators: Keeps bearings protected from contaminants and debris in applications where long life is paramount.
• Protector seals: Used in highly contaminated operating environments to protect bearings on both rotating and stationary shafts.

Bearing seals are typically made from special elastomers and PTFE materials. Over time, the cumulative impact of abrasive forces caused by varying loads and speeds, temperature fluctuations, moisture, debris, and lubrication challenges, can drastically reduce seal performance in turbines. For these reasons, it is advisable to speak with a bearing expert to determine the optimal sealing arrangement for a given application.

A bearing isolator, for example, may employ a labyrinth seal design that uses an intricate pathway to exclude debris and retain lubrication. This can essentially eliminate seal torque, resulting in less frictional forces in the bearing.

Bearing seals play an integral role in maximizing turbine uptime and productivity. And while proper lubrication remains top of mind for wind-farm operators, more turbine owners are beginning to appreciate how seals can positively impact their bottom line

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Timken wind experts will be at WINDPOWER 2019, May 20 to 23 in Houston, to discuss the latest technologies for improving turbine performance while minimizing costly, unanticipated repairs that continue to impact operators in all parts of the world. (Booth1221).

“Timken’s extensive testing and development has pioneered stronger solutions for the wind industry,” said Rick Brooks, wind energy sales manager for Timken. “Our commitment to advancing wind energy has resulted in a broad array of power transmission products that have shown to reduce spending and drive down cost per kilowatt-hour for our customers.”

AWEA attendees can visit the Timken booth to learn about innovations for improving the life cycles of critical components in wind turbines, including:

Superior coupling technology. Timken imagined a better way to transmit torque between the gearbox and generator while protecting attached components from overload and stray currents. Next-generation Lovejoy couplings, bolted to an AeroTorque WindTC torque control, are now available for 2MW+ turbines. These innovative couplings integrate directly with the generator hub, include an anti-flail feature and use a composite fiberglass spacer for weight and cost efficiency.

A standard torque limiter offers protection against peak loads, primarily protecting the coupling from damage during torque spikes, while an AeroTorque-enabled coupling provides added advantages such as reducing peak torque by up to 40% and reducing torque oscillations up to 70% to extend gearbox life.

Together, Lovejoy and AeroTorque are part of a stronger technical solution for making reliable gearbox-to-generator connections.

Field-proven, wear-resistant mainshaft bearings. Recently, Timken analyzed a mainshaft spherical roller bearing that had seen seven years of demanding service. The bearing, which had been removed due to non-related issues, was one of the first to use a proprietary surface coating developed by Timken that greatly minimizes steel-to-steel contact.

Examination revealed little-to-no adhesive wear and no evidence of the bearing entering the next stages of damage. Timken estimates the bearing would have operated 15 to 20 years, giving wind energy producers a new option to consider when it comes to avoiding the pain of costly down-tower repairs.

Case-carburized bearings to resist cracking. White etching cracks (WECs) remain a leading cause of gearbox bearing failure. Timken offers carburized bearings that can better withstand WEC propagation compared to standard through-hardened bearings, which can result in more than double the product life in demanding applications.

Timken also provides wind farm operators the support they need to take greater control of their operation. In an industry that sees new challenges every day, H&N Wind Power Systems by Timken can keep you online from routine inspections to unexpected overhauls. The expert team at H&N has solved problems safely, professionally and efficiently since 2002, offering a full suite of up-tower services, including gearbox oil changes, bearing replacements, shaft repair and carbon brush replacements, as well as large corrective work including complete generator rebuilds.

It takes stronger products and people to reach new heights in wind energy, and at AWEA 2019, attendees can tap into both by getting to know Timken and its complete inventory of solutions for most turbine models. Onshore and offshore, Timken applies its expertise to the smallest and largest platforms, giving you access to a powerful combination of OEM-certified techs, experienced engineering teams and bearing and power transmission experts who can generate a positive ROI for your operation.

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