December 2012

As the United States depletes its fossil fuel reserves, it is beginning to look for alternative energy sources, and wind power is a viable option. According to the United States Department of Energy, the U.S. has enough aggregate wind resources to generate electricity for every home and business in the nation.

When many Americans think of wind power, images of turbines dotting the California coastline or across open spaces in Texas and Iowa immediately come to mind.  Thanks to the efforts of educators in the Southeast, however, this region is becoming a leader in promoting the long-term benefits of wind energy and implementing it where viable resources exist.  With the support of their communities, academic institutions at all levels across the Southeast are developing a collaborative environment in which shared expertise and resources create a workforce of experts who are passionate about sustainable energy and its long-term benefits.

NC Solar Center Offers Thought Leadership, Supports Valuable Sustainability Research
North Carolina has valuable onshore wind resources, both in the mountains and at the coast.  The Department of Energy’s potential scenario for reaching 20% of the U.S. electricity needs with wind by 2030 includes North Carolina as one of only eight states with more than 10GW of wind energy capacity installed.  The reality of this scenario happening in North Carolina will be based on land-use decisions and policies for wind development over the next 20 years. Figure 1

That being said, North Carolina is preparing for the future by placing an emphasis on developing its renewable industry resources in the form of its workforce. The NC Solar Center in Raleigh has been an excellent resource for renewable energy education since it opened on North Carolina State University’s campus in 1988. The center’s original focus was solar energy, but it has since become a thought leader for clean energy in North Carolina.  The Solar Center provides research in four main areas: clean transportation, green building, clean power and efficiency, and renewable energy.

“Our mission is quite broad… we certainly have a technology bent, in that we’re trying to push these new technologies into the marketplace… but if we can’t make the economics work on these technologies and have a sustainable economy as a result, we’re never going to get there.  The Center is set up to bring together the pieces of the puzzle that you need to get to both of those issues in a comprehensive way,” said Steve Kalland, executive director of the NC Solar Center. 

In July 2010, the NC Solar Center announced the completion of its SkyStream 3.7 wind turbine, installed as part of the Wind for Schools program. The program, hosted by Wind Powering America, is a nationwide initiative of the U.S. Department of Energy’s Wind Program. Wind for Schools projects are supported in 11 states and more than 95 systems have been installed at host schools. Goals of the Wind for Schools project include introducing teachers and students to wind energy, equipping college juniors and seniors with an education in wind energy applications, and engaging American communities in wind energy applications, benefits and challenges. 
 
Baker Renewable Energy, a leading provider of renewable energy and clean building strategies for commercial, residential and institutional customers, installed the turbine on the center’s property.  The Solar Center’s turbine has a rated capacity of 2.4 kW, and it is used for a variety of educational purposes, ranging from K-12 education to workforce development to continuing education for experienced professionals.

“We believe this wind turbine will be an important demonstration tool, aiding in the real-world training of installation and service ‘best practices’ in a safe and controlled environment,” said Jason A. Epstein, executive vice president of Baker Renewable Energy.

The Solar Center is a strong advocate for the implementation of Science, Technology, Engineering and Mathematics (STEM) Education within schools. Data collected from the renewable technologies at the Solar Center is incorporated into lesson plans that utilize STEM concepts. This affords students the opportunity to develop skills learned in the classroom through real-world experiences.

It is also playing an active part in the development of the region’s workforce by offering the Renewable Energy Technologies Diploma Series. This is a non-degree professional development program that provides participants with an in-depth understanding of several renewable energy technologies, as well as residential green building principles and strategies.

Another course the Center offers is a Certificate in Renewable Energy Management. The goal of this program is to provide a sound foundation around how existing renewable energy technologies work.  It educates participants on the technology, policies and financial options available so they can make informed decisions as managers and businesspersons in the renewable energy industry.

“One of the beauties of the renewable energy industry is that it really helps to drive job creation at a lot of different levels,” Kalland said.

Virginia University Emerges as a Leader in Renewable Energy Education
Virginia is also looking to grow its green workforce and it has an excellent resource in James Madison University, located in Harrisonburg, Va. JMU recently installed a 7.5kW Bergey wind turbine on a 120-foot-tall tower as part of its Small Wind Training and Testing Facility, an educational initiative launched by the Virginia Center for Wind Energy (VCWE).  The system is capable of producing 10,000-12,000kWh of energy a year when operating at average annual wind speeds of 5 m/s, which is enough to power an average-sized house for a year.

“Schools are becoming thought leaders in wind energy education. JMU’s initiative to install a turbine on its campus is an example of the kind of momentum needed to push the developing renewable energy industry forward,” said John Matthews, president of Baker Renewable Energy. “This is definitely a great way to train people and get them excited.  By combining wind initiatives with homeruns like solar PV and solar thermal, JMU will quickly become one of the desired schools to go to for advanced renewable energy education in the region.”

The turbine project was funded by a grant from the state of Virginia, donations and a partnership with JMU’s Facilities Management.  This center is geared toward both students and companies in the state that may want to break into the wind industry.

VCWE conducts teaching, training, research and development on siting, safety, installation and operations, with the goal of cultivating a community educated in wind energy. This particular initiative will allow the organization to provide educational outreach about wind power development initiatives in Virginia to JMU students, area entrepreneurs and local K-12 schools. Professors can use the facility as a teaching tool geared toward student entrepreneurs who may be interested in wind power-oriented business. Such a curriculum, and the advancement of the wind industry in general, will help bring economic development, high environmental quality, and reliable and affordable energy to the Commonwealth.

“Our goal is to cultivate a community that is educated in wind energy; therefore, we need to inform decision-makers, members of the public and local students about wind power development initiatives in Virginia,” said Dr. Jonathan Miles, professor in the College of Integrated Science and Technology, and coordinator of the International Masters Program and director of the Virginia Center for Wind Energy at JMU.

Because of its expertise, Baker Renewable Energy was brought in to install the wind turbine.  A small solar array will also provide clean power to JMU’s College of Integrated Science and Technology library.  Sensitive instruments on the tower will measure wind flow to provide VCWE with data on area wind patterns.

“We are very proud to have been a part of the project team here at JMU,” Epstein said.  “In addition to providing continuous, clean energy and reducing utility costs, the turbine will also offer a hands-on educational opportunity for students who are interested in the renewable energy field, further supporting the industry in Virginia.”

Renewable Education Not Just for Universities Anymore
Jenny Christman, instructional technology resource teacher and veteran science teacher of 27 years, has proven to the Virginia community that renewable energy education isn’t just suitable at the university level.  For years, she had been searching for a way to educate students at Northumberland Middle and High School, in Heathsville, Va., about wind power. Christman had previously applied for grants to install a wind turbine, but had experienced little success.

When the school moved into a new facility in 2009, Clint Stables, the school’s superintendent, joined Christman in the push to bring sustainable energy onto the school’s campus and into the classrooms.
 

“We weren’t planning to power the whole school. The purpose behind this was educational. We wanted to be able to create real data that our students could incorporate into their math, science and computer classes,” Christman said.

As Christman continued to look for other grant opportunities, she came across the Wind for Schools program. In November of 2009, the school applied for a $20,000 stimulus grant from the Virginia Department of Mines, Minerals and Energy (DMME) to fund the turbine project. The Virginia DMME grant funding originated from the national American Recovery and Reinvestment Act, which was passed in 2009 to save and create jobs, and stimulate the economy.

Once funding for the project was approved, construction quickly began and by December 2010 the foundation had been poured. Baker Renewable Energy was enlisted to construct a 2.4kW SkyStream 3.7 wind turbine on the school’s property.

By February 2011, the turbine was completed, making Northumberland the first Virginia school to have a wind turbine installed on its campus under the Wind for Schools program. The turbine will be used to strengthen Northumberland’s STEM initiative.

“Nothing is isolated. Instruction can’t be isolated anymore, especially in the science, technology, math and computer areas, because they aren’t separated anymore,” Christman said. “They have to be integrated, and they have to be taught together. This seems like a really good opportunity for us to be able to jump into our first STEM initiative with both feet.”

Grassroots Effort Supports Turbine Installation at Virginia Middle School
In December 2011, after nearly two years of hard work, Henley Middle School, located in Crozet, Va., finally put the finishing touches on its Renewable Energy Resource Center, complete with a small wind turbine, a solar photovoltaic system and a solar hot water system. The school held a variety of fundraisers, including bake sales, golf tournaments and silent art auctions, to raise a significant portion of the funds needed for its renewable energy system.  A number of local organizations contributed the rest.

In addition to the money the Henley community raised, the school received an additional $35,000 from Albemarle County Public Schools and a grant of $211,000 from the Virginia Department of Mines, Minerals and Energy to cover the remaining costs.

Leaders at Henley Middle School see the Renewable Energy Resource Center as an important opportunity to expand its curriculum on environmental studies. Data generated by the systems is being used as part of a new renewable energy lesson plan for students. Henley also hopes the Renewable Energy Resource Center will raise students’ awareness of careers within the renewable industry.

“The state’s grant talks about creating jobs in the renewable energy field and increasing the knowledge base of all Virginia citizens about solar and wind power technologies, “ said Dr. Patrick McLaughlin, principal of Henley Middle School.  “It’s our hope that this center will build greater interest and excitement among students for research and career paths in this field and will eventually help our residents realize the environmental and economic benefits of more efficient energy use. “ 

A 2.4kW SkyStream 3.7 wind turbine was installed on Henley’s campus as part of the Wind for Schools program with help from JMU seniors. Baker Renewable Energy was also on site to offer professional assistance. Data from the Renewable Energy Resource Center is collected through one dashboard and accessible to both the school and the public online.

Fulfilling a National Need for Wind Energy Education and Development
The use of wind power in the United States has expanded quickly over the last few years. To date, fourteen states have installed more than 1,000 MW of wind capacity, and a total of 36 states have now installed at least some utility-scale wind power.

Millions of clean energy jobs are expected to open up for American workers in the coming decade. The growing wind industry in the United States will require many trained, qualified workers to manufacture, construct, operate and maintain wind energy facilities as they come online. For this reason, it is imperative to educate today’s students on sustainable energy alternatives to develop greater interest in career paths within these industries.

Academic institutions throughout the Southeast are becoming increasingly sensitive to this need and are working to equip today’s students with a strong foundation of sustainable energy knowledge. They recognize that this information is vital for today’s youth to become informed and participatory citizens of society.

“Every part of our daily lives revolves around energy, yet we take it for granted,” Epstein said. “Therefore, we must provide our students with the opportunity to achieve a deeper understanding of the complex issues surrounding our energy resources by incorporating them into the academic curriculum.”

The Southeastern United States has the potential to become a significant player in the wind industry. The region’s offshore wind resource is compelling, and because of ongoing shifts in population, its electricity markets are some of largest and fastest growing on the East Coast. Because of the commitment of the academic institutions highlighted above, the Southeast has the ability to contribute to the wind industry, as much as wind resources allow, thereby boosting the region’s economy and sustainable workforce for years to come. 

Composites can be a designer’s dream — or a manufacturer’s nightmare. If you’re designing a one-of-a-kind Formula One race car, where performance is paramount and cost and manufacturability are of less concern, you can cut weight, modify, and adapt in creative ways. But if you’re designing a wind blade for competitive energy markets, over-customizing can result in a product that is extremely complex and difficult to produce. For a cost-sensitive product like a wind blade, you need to strike a design balance that accounts for both the subtleties of the physics and the realities of the factory floor.

As a 35-to 40-meter 1.5MW wind blade spins, it is subjected to a complicated collection of static and dynamic forces that vary along the blade from supporting root to tip. To account for these diverse loads, the several-hundred-ply-thick composite stack beneath the surface also varies — in material selection, number of plies, and overall thickness. The actual construction for each blade part depends on the specific structural characteristics required to ensure adequate strength and deliver optimal performance.

The root of a 1.5MW blade requires a thick, heavy construction to support the gravitational loads of the six- to seven-ton blade weight. The energy-capturing part of the blade must be rigid enough to prevent buckling from aerodynamic forces but light enough to maximize rotor speed. The box-shaped spar or I-beam with accompanying shear webs (which runs the length of the blade) requires a material and construction approach that provides increased bending stiffness. To create a successful design, the wind blade engineer must consider each of these structures and their functions.

Balancing Competing Design Demands
Given the variety of composites and the flexibility of stack design and layup, wind blade design solutions can be as complex as the loading scenarios. The dominant material for blades manufactured today is glass fiber reinforced polymer (GFRP). The more expensive carbon fiber reinforced polymer (CFRP), with its better strength-to-weight characteristics, is being used more frequently as blade length increases.

Within these two broad composite categories are a wide variety of fabrics and tapes with varying fiber orientations and resulting properties: zero-degree materials (with fibers running along the axis of the blade, from root to tip in the direction of the load) are used in the blade skins to control tip deflection; 45-degree materials (both positive and negative) are used to prevent twisting; and 90-degree materials (with fibers oriented around the circumference of the blade) are used sparingly to counter buckling effects. An expanding library of composites — Sandia National Laboratories’ Wind Energy Technology Department has a database of approximately 150 — gives the designer a wide variety of choices.

The composite engineer also has the almost limitless ability to add or delete laminate layers to either increase strength or shave off weight. But unlimited design flexibility has downsides as well. At every ply drop or add there is a potential structural weakness, and every transition introduces another layer of manufacturing complexity. At one extreme, a designer could create a uniformly thick blade, strong enough to weather any potential gust — but too heavy to generate maximum wattage. At the other, the design could be a jigsaw puzzle of laminate zones, customized to meet every subtlety of real-world loading — but a problematic and costly challenge to fabricate.

An ideal wind-blade design provides high performance at competitive cost while meeting the industry standard 20-year lifespan. To achieve this, designers must consider blade strength, stability (buckling), and stiffness (wing-tip deflection), as they have always done. But as blade length increases, they must also take weight into account to maximize performance. And to compete more effectively with other turbine manufacturers as well as established energy technologies, they must keep manufacturing process efficiency and cost in mind. To balance these competing criteria during design development, engineers working with high-performance composites are faced with numerous decisions.

Aerospace-Proven Optimization Software
The aircraft and space industries have wrestled with the challenges of composite design for years. Out of those efforts at NASA came a structural sizing and optimization software package, which has been renamed HyperSizer and commercialized by Collier Research Corporation as part of the space-agency’s technology transfer initiative.

HyperSizer evaluates complex composite and metallic designs and automatically searches for solutions that minimize weight while maximizing strength. Over the years, the software has been integral in the design and validation of high-profile, zero-failure space projects such as the NASA Ares I and V Launch Vehicles and the new Composite Crew Module. HyperSizer has also played a design role in commercial aircraft — such as Goodrich engine structures and Bombardier’s LearJet — and with experimental test craft, like Steve Fossett’s Virgin Atlantic GlobalFlyer (see Figure 1).

With mission-tested success in aerospace applications, HyperSizer’s crossover to the wind industry is a natural, since the design challenges of working with composites are similar in both industries. Collier has recently begun collaborating with Sandia National Laboratories’ Wind Energy Technology Department on the optimization of a new 13.2MW 100-meter blade to demonstrate the software’s capabilities. This project, like other Sandia turbine prototype efforts, is conceived to develop innovative technology that seeds commercial R&D and stimulates manufacturing advances.

Optimizing Designs for Manufacturability
On a wind blade design project of any scale, HyperSizer can be easily integrated with other widely used composite software design tools, including CAD programs (such as CATIA), composite design software (FiberSIM, for example), and a host of commercial finite element analysis codes (including Abaqus and Nastran).

A HyperSizer optimization starts with a baseline finite element model (FEM) and an FEA run to determine internal loads and deflections. The simulation results are imported into HyperSizer where the code conducts a trade study for a blade characteristic (such as laminate strength and stability), automatically surveying up to millions of design candidate constructions in a ply-by-ply, even finite-element-by-element process, to a multitude of failure criteria. From the many possible solutions, a design is then chosen and exported — along with its accompanying material properties — back into the FEA software where the model is rerun. This iterative loop, enabled by HyperFEA, is repeated as needed for additional characteristics, such as stiffness, until a final blade design is reached that meets a predetermined design target for strength, weight, performance, and cost. Because the optimization is automated, it eliminates costly, error-prone offline spreadsheets and manual calculations.

HyperSizer allows for an easy exchange of laminate specifications with FiberSIM and CATIA (see Figure 2). These software tools are good at modeling, displaying, and managing design data of complex composite parts, as well as automating manufacturing operations, such as cutting, fiber placement, and tape-laying.

But to address composite stack composition in a detailed enough way to truly influence weight savings and manufacturability, HyperSizer can be used to complement FiberSIM and CATIA’s capabilities. It accomplishes this by providing detail on four diagnostic measures key to efficient composite manufacturing:

• The total number of plies dropped (the more ply drops you have, the more cutting, splicing, and lining up of plies are required on the shop floor)

• The simultaneous occurrence of ply drops and adds (which manufacturers try to avoid because it can introduce weakness in the stack)

• The alternation of ply drops with continuous plies, or interleaving (a feature that can also introduce weakness or a chance for defects)

• The number and pattern of laminate zones and transition boundaries (another factor contributing to manufacturing efficiency and cost)

As an illustration of the improvements that HyperSizer can make on these key manufacturability measures, consider the optimization of a sample wind blade design (see Figure 3). The total individual ply drops and adds for the design were reduced from 467 to 145 (69 percent). Simultaneous ply drops and adds were also dramatically reduced, from 105 to zero. In addition, the maximum number of ply drops not interleaved with continuous plies was reduced from 16 before the optimization (a construction described by engineers as equivalent to “falling off a cliff”) to two after using HyperSizer. And finally, laminate zones, shapes, and transitions were improved, making fabrication operations such as cutting, layup, assembly, and finishing more efficient for both touch-labor and automated manufacturing processes (see Figure 4).

The HyperSizer-optimized blade design was also significantly lighter, while meeting all loading scenarios and industry lifetime standards. In various test case optimizations, blade weight has been reduced as much as 20 to 25 percent, a savings equivalent to averages seen over many years in aerospace industry projects.

Impacting the Factory Floor, Changing the Energy Landscape
Simplifying manufacturing processes for wind blades — or any composite structure, for that matter — provides huge competitive benefits. In the wind industry, the standard is 24-hour turnaround in the mold. If designs are optimized for manufacturability, operators are ensured of getting the part out of the mold on schedule (or even developing ways to shorten the process) and getting the next one in (see Figure 5). Improved manufacturability means fewer processing steps, less machine time, and reduced blade cost.

The wind industry has made great strides over the last decade. Technology is improving at a rapid pace. Installed capacity is up. Prices are coming down. Optimizing composite designs has the potential to make additional significant contributions to wind’s growth. The results can be seen in blades that are lighter and deliver higher performance. More efficient manufacturing will contribute less expensive blades that can be produced more quickly. Taken together, these factors could help trigger the tipping point for wind and alter our view of the energy future. 

Wind turbine drivetrains must survive harsh, constantly varying loads throughout the lifetime of the turbine. The drivetrain transmits rotor torque to the generator, but in operation it is not just steady-state torque that the drivetrain experiences. In fact, it experiences a whole variety of events with rapidly changing torque and off-axis (non-torsional) loads, any of which can severely damage the machine, potentially leading to failure and very costly repairs.

Drivetrain Failures
In order to design a reliable drivetrain, it is crucial to first understand the damaging effect of transient loads such as emergency stops, start-ups, shut-downs, or gusty wind conditions. Figure 1 shows what can happen if this is not done correctly. Potential failure modes such as surface-initiated bearing pitting, gear tooth failures, and fretting fatigue on bearing rings can all occur if transient loads are not accurately considered at the design stage. In more severe cases, the gearbox housing or mounts can fail, potentially leading to catastrophic failure of the whole turbine.

Designing A Reliable Drivetrain
For wind turbine or drivetrain design, input loads are typically calculated using multi-body-dynamics simulation incorporating aeroelastic models and a representation of the turbine controller.  These loads simulation software tools are available from commercial and research organizations for the turbine-level load calculation.

Designing a new drivetrain poses a chicken-or-egg conundrum — how can rotor loads be calculated without first having a concept for the turbine and drivetrain? And, conversely, how can a turbine concept be defined without first knowing the loads? This is solved by defining loads in logical stages. For example:

• Concept loads — The turbine designer may assume these based on previous experience on turbines of similar type/power rating, or may be scaled up from experience on smaller turbines.

• Preliminary design loads — These loads are calculated based on initial concepts for the drivetrain, rotor, tower, electrical system, controller, etc.

• Detailed design loads — These loads are calculated using finalised concepts for all turbine components and are used for type certification and component certification.

Accurate turbine-level loads aren’t the only thing required for reliable drivetrain design. The methods for analyzing and considering these loads are critical. Slowly varying fatigue loads and static extreme loads are analyzed using standard methods (e.g. ISO 6336 for gear rating, ISO 281 for bearing rating, DIN 743 for shaft rating, etc.) in accordance with certification requirements. For these rating methods, it is essential to consider the behavior of the whole drivetrain, not just the gears and bearings in isolation. Previous studies have shown the importance of including structural flexibility and component-level deflections in these calculations [1]. A RomaxWIND simulation model like that shown in Figure 2 allows the whole drivetrain to be analyzed using a single model, incorporating detailed non-linear bearing models, advanced gear contact models, and flexible representations of shafts and housings. These methods have recently been applied successfully in the design and development of Romax’s latest 6MW Butterfly drivetrain platform, shown in Figure 3.

Transient Events
Although most wind turbines spend the majority of time running at rated load or at idle, it is widely recognised that damage can occur during short duration transient events such as start ups, shut downs and emergency stops. Figure 4 shows an example of the drivetrain high-speed shaft torque measured during an emergency stop. For events such as this, peak loads can be analysed using industry standard methods for gear and bearing rating but these only capture part of the story. Standard methods consider the effect of a static extreme load but do not consider in detail rapid changes in acceleration or gear impacts that occur during torque reversals. To understand the potentially damaging effect of these factors we must use more advanced gear analysis methods.

Detailed Drivetrain Time-Domain Simulation – When is it Not Needed?
The key property to consider in drivetrain time-domain simulation and load calculation is the rotational inertia of each drivetrain component. This is a measure of the component’s resistance to change in its state of motion. Components with small moments of inertia can generally be ignored from models used for load calculation but components with large moments of inertia must be considered.

A conventionally geared drivetrain is made up from a number of rotating parts, from the rotor through to the generator, as shown in Figure 5. When drivetrain loads and behavior are calculated during the design process, it is vital that the correct inertias are considered and this is especially important when considering gearbox loads during transient events, such as the emergency stop event shown in Figure 4.

Table 1 shows the contribution of each drivetrain component to the total moment of inertia for a 2MW wind turbine drivetrain. The key conclusion is that over 99% of the total drivetrain moment of inertia is from the rotor, generator, brake, and high-speed shaft coupling, with less than 1% of the total inertia coming from all other components. This means that it is reasonable to ignore the small inertias in the gearbox components (e.g. gears and planet carriers) when calculating turbine-level loads and behavior. In fact, this assumption is commonly made across the industry for turbine load calculation.

It is a common misunderstanding that high fidelity time-domain gearbox models are required for accurate load calculation. This is not true because the small inertias of components inside the gearbox are swamped by the large inertias from the rotor, generator, and high-speed shaft brake and coupling.

Detailed Drivetrain Time-Domain Simulation – When is it Needed?
Although accurate time-domain gearbox models are not required for turbine-level load calculation, there are special cases where time-domain gearbox models give us vital insight into stresses and failure modes inside the gearbox. An example of one such case is gear impacts that can occur during transient events with rapidly changing loads — such as an emergency stop or a grid fault.

For this type of analysis, the turbine-level loads are used as inputs for a smaller subsystem model – this could be a time-domain model of the gearbox, constructed as shown in Figure 6. This model is then used to analyze instantaneous gear stresses during transient events like the generator grid fault shown in Figure 7.

Conclusions
A single gearbox failure on a multi-megawatt turbine can cause costs in excess of $500,000. If this is multiplied over several wind farms, the wind turbine OEM’s exposure to risk during the warranty period is very high.

In order to reduce the risk of gearbox failures, it is essential that suitable simulation and analysis methods are used to understand drivetrain loads and durability at the design stage. The return on investment is clear. If an OEM can save just one gearbox failure by investing in state-of-the-art drivetrain analysis and simulation tools, then the future benefits are significant.  

References
• J. Coultate et. al., The Impact of Gearbox Housing and Planet Carrier Flexibility on Wind Turbine Gearbox Durability, European Wind Energy Conference 2009 proceedings, March 2009
• E. Bossanyi et. al., The Effect of Gearbox Flexibility on Wind Turbine Dynamics, paper by Romax Technology and Garrad Hassan, European Wind Energy Conference 2008 proceedings, March 2008

One of the most common, and most critical, components in any piece of machinery is the bolted joint. In wind turbines especially, nuts and bolts are found from ground level to the very top, and everywhere in between.

Safety is paramount in these high flying applications, and utilizing a bolting method that tightens your nuts and bolts quickly, correctly, effectively, and safely, is priority number one. The unique challenges that a wind turbine presents make this a difficult task. In addition, an improperly bolted joint can result in damaged equipment, and frequent maintenance checks, all of which affect the bottom line.

Clamp Load and Other Considerations
Control of the clamp load in a bolted joint is vital. However, when faced with a tricky joint, sometimes the design engineer will not have an answer if asked about the clamp load. Torque calculations must always be based on the existing conditions that often are very vague. Unless all parameters are correct, the calculation will be unreliable. Here are a few parameters that should be considered:

• Thread condition of the fasteners
• Hardness of contact surface
• Material (steel, aluminium, copper, etc.)
• Extra friction from a locking fastener
• Extra friction from an adhesive
• Lubricant on the thread
• Type of bolt head (flanged, regular or serrated)
• Surface coating of the bolt
• New or reused fastener

During tightening, bolts are subjected to both tensile and torsional stress. The total stress in a bolt can be calculated using the formula:

Total stress =  

In order to maximize the desired tensile stress (σ_x) it is vital to minimize the torsional stress (τ_xy). Tensile stress (clamp load) is achieved when the bolt is axially elongated. Unwanted torsional stress (twisting) in bolts arises during tightening due to thread friction. High thread friction increases torsional stress and causes yielding at lower clamp load levels than normal.

A lubricant is necessary to minimize torsional stress. However, many commonly used bolt-locking systems are based on increased thread friction (deformed nuts, adhesives, etc). Minimizing thread friction and safely securing a joint has often been considered impossible. The use of locking systems that increase thread friction is the single most common reason why the full capacity of bolted joints is not utilized. The following example illustrates the problem.

Applying an adhesive significantly increases thread friction during tightening. The red graphs (Figure 1) show tightening of bolts with adhesive on the threads. Due to increased thread friction and torsional stress, only half as much clamp load is obtained before reaching the yield points (marked with x). When tightening the same bolts lubricated, which is illustrated by the green graphs, almost twice as much clamp load is obtained. The diagram clearly shows why low (and uniform) friction during tightening is necessary to ensure that the bolts’ full capacity can be used.

Under static load the achieved clamp load in a joint is maintained. However, bolted joints exposed to dynamic loads or vibrations are likely to gradually loosen. Even if some of the common bolt locking methods (such as serrated washers, adhesives or deformed nuts) work fairly well when the dynamic loads are light, wind turbine applications can see significant vibration and often need to be retightened and checked regularly.

Nord-Lock Wedge-Locking Washers
One option that is being used more and more in wind turbine bolting is the Nord-Lock washer. The Swedish company Nord-Lock produces this high-quality bolt securing system, consisting of a pair of pre-assembled washers. The washers have a cam angle “α”, which is greater than the thread pitch “β”. In addition, there are radial teeth on the opposite sides of the washers (Figure 2).

The washers are installed in pairs, cam face to cam face. When the bolt or nut is tightened the teeth grip and seat the mating surfaces. The washers are locked in place, allowing movement only across the face of the cams. Any loosening attempt of the bolt/nut to rotate loose is blocked by the wedge effect of the cams.

Nord-Lock has developed in-house laboratories where clients get the opportunity to put joints from their own applications to the test. In simulations of real-life conditions torque-load ratios are measured and Junker vibration tests are performed (Figure 3). In a Junker vibration test (meeting DIN 65151) bolted joints are subjected to transverse movements while a load cell continuously measures the bolt tension. The Junker test is used to compare different bolted joint configurations and is a first step in selecting the best technical solution to prevent bolt loosening. The Junker test is often considered a worst-case scenario and bolted joints performing well in this test normally function flawlessly in real life conditions. In Figure 3 you see that many commonly used bolt-securing devices show limited locking performance when exposed to vibration.

Bolted joints secured by NordLock lose some minimal initial preload due to normal settlements between the contact surfaces. The unique wedge-locking effect is verified by the increase in clamp load during untightening.

For wind turbines, which require regular service and maintenance, this is the optimum solution. The bolted joints are easily assembled and disassembled and no special tools are needed. Because Nord-Lock washers use tension instead of friction to secure bolted joints, the locking function is not affected by lubrication. The use of a good lubricant is recommended in order to reduce torsional stress, minimize clamp load deviation, and to protect against corrosion. Since the clamp load deviation is very low when tightening lubricated fasteners, bolted joints can always be safely locked at the highest possible preload level.

X-Series Washers
An enhanced version of the Nord-Lock washer, called the X-series, introduces a spring effect which can be especially beneficial in critical wind turbine applications. The wedge effect prevents bolt loosening, and the spring effect compensates for loss of preload due to settlement that can occur due to thick surface coatings, materials like polymers and composites, soft metals, multiple clamped parts, and other design issues.

How They Work
After the fastener is tightened, the X-series washer flattens and the serrations engage the contact surfaces. The cam angle being greater than the thread pitch, the wedge-locking effect prevents rotation of the fastener. After tightening, the joint settles and the fastener sinks into the surface material. The spring effect in the washer counteracts the slackening movement of the bolt, minimizing loss of preload on the joint (Figure 4). This is the first multi-functional wedge-locking washer and is of particular interest in wind turbine bolting applications.

Large Diameters
When dealing with larger size bolting diameters, one of the problems that arise is that the amount of torque required to attain a given preload increases exponentially as the diameter increases. This means that special tooling is usually required to achieve the required preload.
 
In the case of wind turbines, this means getting some expensive and heavy tooling into some pretty interesting places. These methods can also cause problems with joint integrity or do not provide consistent bolt load across the bolted surface. The conventional hex nut and bolt have been around so long we often don’t give them a second thought. However, there is another interesting bolting technology being put to good use by many wind farms.

Multi-Jackbolt Tensioners
An alternative to using high-powered tooling on existing nuts and bolts is the Superbolt multi-jackbolt tensioner (MJT). Instead of turning a hex nut or bolt to obtain preload, MJTs use a number of smaller jackbolts (Figure 5). The benefit is that any size MJT tensioner can be installed and removed using a hand held electric/air tool or torquewrench. This is a huge advantage in wind turbine applications where location can make it difficult to utilize heavy tooling. Worker safety is increased, and expensive tooling is eliminated.

How MJTs Work
Each tensioner consists of a hardened washer, a nut body and jackbolts. The jackbolts are threaded through the round nut body or bolt head, and thrust against the hardened washer to stretch the stud/bolt and generate clamping force (Figure 6).

The torque needed to tighten each jackbolt is small and easy to obtain, thus enabling the use of small hand tools. The amount of preload achieved is directly proportional to the size and number of jackbolts used (Figure 7).

Advantages
Besides the increased worker safety and tooling benefits, MJT tensioners also help to improve the entire bolting system. Design engineers in particular find this technology of high interest, and many OEM’s utilize MJTs to improve their equipment design.

Torquing conventional hex nuts and bolts causes twisting of the main bolt. This can add unwanted internal stresses and reduce the load capacity. Superbolt tensioners tighten in pure tension, so there is no torsion on the main bolt. Also, since the main thread does not slide under load, no thread galling or stud seizure occurs.

Another problem with hex nuts and bolts is that there is a high stress concentration found on the first few threads. By comparison, the jackbolts in the MJT system create hoop stresses in the nut body, so the nut flexes out slightly at the bottom and in slightly at the top (Figure 8). This flexing distributes the load throughout the entire thread engagement, and also adds elasticity, which can make short, stubby bolts much less likely to break due to fatigue stress.

One perceived downside with the MJT system, however, is the fact that there are multiple jackbolts to tighten. You would think the multiple jackbolts would result in longer installation and removal times. However, the MJT system has been proven to reduce installation and removal times. For example, a 1-1/2” tensioner can be tightened in about 1 minute and 29 seconds and a 2” tensioner would be around 1 minute and 33 seconds.

In conclusion, if you are experiencing bolting problems on one of the many wind turbine bolting applications, or want to make the bolting process easier and safer for your workers, these two technologies are being used with success and could be a viable alternative for you.