Bolting with regard to wind farms mostly concerns fastening the tower to the foundation and the turbine to the tower. The topic divides neatly into fasteners, accessories, and tooling.
Fasteners with tension indicators: Most bolts in a wind turbine are manufactured to OEM specs. The OEM also provides a torque value which the construction crew must apply to each bolt for a required level of tension. But because torque does not always correlate to bolt tension, bolt manufacturers have devised a range of devices for indicating tension.
One particular bolt design has an indicator cap on its head. Until the cap turns by hand with difficulty, the bolt is under tensioned. The device works by drilling out part of the bolt center and installing a tension indicator link to the cap. When tensioned, the bolt elongates, tightening the link and preventing the cap from rotating under finger pressure. Another design places a visual indicator in the bolt head. When the indicator is out of position, the bolt is under or over tensioned. The feature makes checking for tension a brief visual task.
Why torque control is a questionable way to achieve consistent pretension in structural bolts comes down to friction or what’s known as the k-factor. This is the relationship between the torque applied to a fastener assembly and the actual tension produced in the bolt. A short form of the relationship is:
Tension = Torque/kd where: k = k-factor, and d = diameter
K-factors vary from lot to lot, from bolt to bolt within a lot, and as a consequence of handling, storage, and lubrication. K-factors can range from 0.10 to over 0.20. Also, consider the variability of the torque tool. Manufacturers of tension indicators say it is likely that when using torque control to govern bolt tension that actual bolt tensions will vary ±40% or more.
Most industrial-fastener problems come from a bolt, cap screw, or spindle nut that has failed in service due to vibration, thermal cycling, or other harsh environment conditions. A few bolt manufacturers will design a prototype solutions and manufacture a locking system to accommodate load, corrosion, clearance, and material requirements. Customers, however, will have to test the prototypes to assure performance. Shaker or fatigue tests can often point to a best design when several are available. Users will be expected to specify the material, preload, and torque for their fastener applications.
A preassembled pair of washers provides a structural, self-locking fastener that prevents bolted joints from loosening by shock or vibration in high-stress applications. The device consists of two preassembled (glued for easier handling) washer-shaped pieces. Each has an inclined cam on one side and a series of ridges on the other. On installation, the cam sides are mated and placed between nut and joint material. Under vibration, the nut tries to rotate loose but, because the angle of the cams is greater than the pitch angle of the bolt thread, the interlocking cams and the non-slip ridges of the washer work together to create a jam which prevents loosening. The washer pair can also be used on bolts in either blind-holes or through-holes. In blind-holes, the washer pair is placed under the bolt head, while through-hole applications use the washers beneath a nut.
Filed Under: Uncategorized