Most bolts in a wind turbine are manufactured to OEM specs. OEMs also provide 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 tension, bolt manufacturers have devised a range of devices to more accurately tell of tension.
One particular bolt design has an indicator cap on its head. If the cap turns easily by hand, the bolt is under tensioned. When difficult to turn by hand, the bolt is at its proper tension, a value set during manufacturing. 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 can range from 0.10 to more than 0.20 and vary from lot to lot, from bolt to bolt within a lot, and as a consequence of storage and lubrication. 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.
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 nonslip 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.
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