Wind turbine operations and maintenance procedures call for a 100% inspection of fastener tension after 500 hours of operation. This work is costly and can easily take a number of days to complete because each fastener is typically checked with a torque wrench to verify its proper tension.
Torque is an indirect indicator of fastener tension and bolted-joint clamp load. Numerous studies find that friction causes variations in tension accuracy by ±30%. Field tests, in fact, indicate the torque scatter to be as great as ±150%.
Fortunately, there is a faster and more accurate way to monitor tension and maintain joint clamp load. Fasteners with a “built-in tension-sensing device” can reduce these bolted joint inspections to just a few hours. Fixed maintenance cost is drastically reduced, while the accuracy of bolt tension narrows to within ±5%.
One built-in tension-sensing device is the RotaBolt, from EGC Enterprises. Fasteners with this modification provide a real-time indication of clamp load. A notable feature is that there is no need for sophisticated torque testing equipment. Users need only check the Rota-cap with a thumb and forefinger to assure themselves each fastener is properly tensioned.
It works like this: All bolts stretch or elongate when tightened. The RotaBolt takes advantage of the stretching with a gauge pin anchored inside
the fastener. When the fastener reaches proper clamp load the gauge pin pulls down on an internal disc called a Rota-load indicator and closes an air gap. The size of the air gap is proportionate to fastener elongation when properly tensioned. Closing the air gap prevents the cap from turning with finger pressure. This cap also seals out the environment and protects internal components.
Each of these fasteners is individually set to an application preload with an assured accuracy of ±5%. An installer knows he has reached the correct load when the cap no longer turns with finger-tip pressure. After installation, the load can be quickly checked at any time to assure ±5% design tension, regardless of environmental influences that can cause relaxation and clamp load loss in the bolted joint. When a cap is found to spin during fastener maintenance checks, simply retighten the fasteners to the point of cap lock-off.
The VLI or Visual Load Indicator provides another design, this one with a dial indicator at the end of the bolt or stud. A visual check of the dial indicator tells whether or not the bolt has reached its proper fastener load.
The straightforward advantages of VLI are that it gives maintenance personnel a quick visual tension check that can be performed at up to 30 feet away and greater distances with binoculars. If line of sight is interrupted, a remote camera can easily provide a closer eye. VLI also provides indication of tension overload as well as under load conditions. To check a VLI bolt, simply look at the indicator face. If the bright yellow tension indicator stripe is hidden on the black face, clamp load remains assured within ±5%. If the yellow tension indicator stripe appears, the fastener needs maintenance.
All fasteners are affected over time in service by temperature changes, joint relaxation, pressure fluctuations, and vibration. The Rotabolt and VLI indicate changes in bolted joint clamp load and integrity by monitoring the tension of each fastener. Adding these indicators to key wind-turbine fasteners significantly reduces scheduled time for maintenance and fixed costs while improving bolted joint safety and durability. It is no longer necessary to retighten every fastener.
What’s more, RotaBolt tension validating fasteners are approved by Germanischer Lloyd. To participate in a field validation trial of RotaBolt or VLI bolts, contact the company at (800) 342-0211.
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