Clever bolt tells when it’s tensioned
September 19, 2011 by Windpower Engineering
Filed under Fastening-Joining for Wind Power, Mechanical Components, Turbine Design
The wind industry recognizes the importance of proper bolt tension. The acknowledgement will challenge some of the industry’s torque-tightening practices as it matures and adopts more sophisticated technology to improve reliability and reduce maintenance costs.
To start, let’s review current practices. For instance, bolts on turbines are often tightened in one of two ways: either by torque or by hydraulic tension tightening.
Torque is a rotational force applied to a lever and multiplied by its distance from the centre of the tightening end (fulcrum). Torque, measured in Nm or lbs-ft, rotates a bolt head or nut to tighten and stretch the bolt so it clamps two surfaces together. Hydraulic torque wrenches allow the operator to control the torque applied and are extensively used to tighten wind-turbine bolting.
One design for a tell-tale bolt comes from UK-based James Walker’s RotaBolt. A simple mechanism inside the bolt provides an indicator of the real load or tension. RotaBolt fasteners are guaranteed to consistently indicate within ±5% of the specified load regardless of environmental influences.
Although simple to use, the technique suffers from inaccuracy due to inconsistent friction. Even on lubricated threads and nuts, the level of friction is uncontrollable. This means there is a critical lack of tension control that can lead to bolt failure, and ultimately compromise joint integrity.
Hydraulic tension tightening is also used to tighten large diameter bolts on wind-turbine structures. A hydraulic ‘jack’ pulls the bolt axially. When the bolt is stretched by a specified amount, the nut inside the tensioner rises off the flange creating a gap between flange faces. The gap is closed by tightening the nut. Releasing the hydraulic pressure transfers tension to the bolt and flange.
If you can turn the indicator cap, the bolt is not tensioned.
The drawback here is that tension loss occurs when releasing the hydraulic pressure. It’s an effect called “load transfer relaxation.” To compensate for the loss, the hydraulic pressure originally applied is increased a bit, a “hydraulic overload value”. However, the amount of load-transfer relaxation is not accurately predicted and so varies from one bolt to another. Consequently, hydraulic tensioners are unable to control or measure the tension achieved for each bolt. The device is also unable to provide an in-service reliability check.
One way around the shortcomings of both tightening methods is with a bolt that indicates when its specified load or tension has been reached during tightening. The RotaBolt can be used in either torque or tension tightening. When this bolt is tightened, the cap locks at exactly the right amount of preload in the bolt. This allows for more control during tightening while ensuring that an OEM’s design criteria is met independent of the tightening system. It also allows checking the bolt’s status in service (if a cap on the bolt does not turn, it’s still tight) something traditional bolt retightening checks are unable to do.
Accuracy is not affected by friction, as in torque control, nor by load transfer errors, as in hydraulic tensioners. This bolt-tension solution can ensure joint integrity on wind turbines. Joint integrity, in turn, allows for greater reliability and the more efficient production of renewable energy.
Rod Corbett
Managing Director
James Walker RotaBolt
www.jameswalker.biz
WPE
Better Ways to Check Bolt Tension
October 15, 2009 by Brian Newcomb
Filed under Fastening-Joining for Wind Power, Featured Wind Power Articles, Maintenance
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.
::Windpower Engineering::
