No Small Part of the Big Picture

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Wind turbines are awe-inspiring machines. It seems as if nothing could ever bring them down. A look from afar shows them as sleek and powerful. Close examination of the structure and the workings inside the nacelle reveal the thousands of components that all must come together and stay together for the great natural power to be harnessed.

Power generation from wind depends on bolts, studs, screws and nuts, from the structural bolting patterns at ground level all the way into the generator high in the air. An improperly designed joint, a substandard
screw, or even a loose nut can bring the entire operation to a stop. It can happen suddenly or take many months, going un – noticed until the final failure occurs. As the wind industry tries to get its footing in the United States, it seems like this is a good time to examine the critical nature of a component that has been dismissed as a “commodity” by too many buyers and engineers in the user community.

For many U.S. manufacturers, putting their components into turbines is something completely new. They are learning about metric components for the first time, this country having resisted metrication for decades now. It is the Europeans, for the most part, who have built the wind industry, and they rely on ISO standards. For fasteners, “grades” are now “property classes,” and dimensions are in millimeters, not inches. There are other things that differ in the ISO standards, and they should be reviewed carefully. It is time to learn or re-learn fastening, standards and beyond. There are three basic elements to a sound joint: good joint design, properly
assembly and maintenance of the joint, and consistent, high-quality fasteners. Proper joint design is science and art, and the engineer should keep in mind what has worked in the past for the application or similar
ones. This is not simple with a wind turbine component because the history in these applications is short. I would believe that particular attention in wind turbine joints should be given to preventing fatigue, knowing
that the turbine will be in a constant yet varying state of movement. That very condition with improperly preloaded joints would surely doom the joints to fatigue failure.

It may be useful to review the basic steps of bolted joint design. First, the basic geometry of the joint should be considered and the material of the clamped parts. Give thought to the number of axes in the joint. Make the initial selection of joint materials.

Second, calculate the magnitude, direction and introductory point of the external forces acting upon the joint, and whether they are static or dynamic. Determine the acting temperature range of the joint and coefficients
of thermal expansion of the mating parts. Next, select the material properties, size, drive style, and thread type of the fastener. Fasteners act like springs in storing energy to resist external forces that work to separate
clamped parts. Long, slender screws are the best choice. A 1:4 or even 1:6 diameterto-length ratio is desirable. Estimate the clamp load required to keep the joint functional. Develop a force diagram. Now calculate the mating material’s capacity to resist embedment while keeping in mind other setting force losses. Determine safety factor desirable for the joint. Now, the most important step: TEST the joint in application under realistic conditions. The second element of a sound joint, proper assembly and maintenance, also now ….

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