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
Filed Under: Components, Turbines
