Ryan Bonner, Alcoa Fastening Systems & Rings
For most heavy equipment applications, design engineers have focused on a stable, consistently tight joint for long-term durability and vibration resistance. At the same time, torque has long been the standard by which the tightness, and maybe even the overall integrity, of a joint were measured. This conventional thinking was based on the idea that the more power applied to a nut and bolt assembly in the form of torque, the tighter, more secure, and vibration resistant the joint. Unfortunately, the belief that higher torque resulted in a more secure and durable joint is incorrect.
Torque, as it relates to fasteners, is the twisting force required to spin a nut along the thread of a bolt. The basic equation for torque is
where T = torque, ft.-lb; D = nominal diameter, in.; P = required clamp load or tension, lb; and K = coefficient of friction. The problem with this equation is that K is difficult to predict or measure because it is impacted by a wide range of variables such as surface texture, oil, rust, debris, type of thread, material, and even humidity.
Tension, P in the equation, stretches or elongates a bolt so it provides the clamp on the joint. Tension is also the load on the joint brought about by drawing the fastener components together. Critical to a joint, tension, is for the most part unrelated to torque. At best, torque is an indirect measurement of the tension applied to the bolt.
Eliminating vibration’s effect
The last line in the table above suggests there is a better fastener. Several design elements of HuckBolts – a swaged lockbolt – ensure their resistance to the effects of vibration. In conventional nut and bolt installations, gaps between the nut and bolt threads are universally encountered, providing the opportunity for loosening in high-vibration environments. By contrast, the Huck design, featuring a collar fully swaged into the locking grooves of the pin, creates no such problematic gap.
Instead of the deep threads necessary for a tolerance fit between conventional nuts and bolts, the HuckBolt pin requires only shallow locking grooves into which the collar is swaged. The design of the locking grooves allows for a much larger root radius, which contributes significantly to the fatigue strength – up to five times that of a conventional nut and bolt. Overall, this lockbolt design provides superior vibration resistance as indicated by the study in the table.
Because this fastening technology can withstand the effects of vibration without ever loosening, the fastener never needs inspecting, tightening, or re-torquing. The benefits that accrue from this ability to withstand the effects of vibration include:
- Eliminating a costly aspect of tower maintenance because there is no need to expend manpower and time to periodically inspect, tighten, and torque existing fasteners.
- A tower assembled with HuckBolts may never be off line for inspection and remediation of fasteners.
- Finally, vibration-resistant lockbolts almost eliminate the safety problems from a failed fastener.
What’s more, DIBt Institute has certified the 12, 14, 16 , and 20 mm, and 1 inch dia. bolts maintenance-free. A BobTail HuckBolt can be installed in as little as two seconds, based on a typical installation of a 5/8-in. Grade 8 fastener.
The Huckbolt at work
The BobTail line of fasteners, certified as “maintenance-free,” was selected by steel construction company Butzkies GmbH for a special lattice wind tower. Based on the positive results achieved using the bolts a pilot project, the company decided to use BobTails for fabrication of its innovative wind lattice towers. In an initial phase, three towers (each at hub height of 100m) will be built using the BobTail system.
In wind turbine towers, mechanical anchoring and fastening elements are exposed to extremely high vibration. The tension on the wind lattice tower increases significantly with hub height. Detlef Bengs, managing director of Butzkies Stahlbau explains this phenomena in terms of fastener performance: “No conventional threaded fastener is permanently resistant to extreme mechanical tension or vibration. Threaded fasteners loosen over time, which can result in considerable problems with a lattice wind tower. To comply with regulations for operational stability, threaded fasteners must be repeatedly checked, re-tightened and, if required, replaced. This issue can cost the operator large sums of money over the years. From the start, we wanted to avoid this problem with our tower. For this reason, we decided not to use conventional threaded fasteners, and alternatively to build our test plant using Huck BobTail fasteners.”
A “tower monitoring” pilot test was set up as a collaborative research project with the Rostock Applications Centre for Large Structures in Production Engineering of the Fraunhofer-Gesellschaft and the Business Development and Technology Transfer Corporation of Schleswig-Holstein.
The initial advantages of the BobTail system emerged during construction. The tower was erected on site in record time, mostly because BobTail fasteners can be installed about 75% faster than comparable conventional threaded fasteners. This speed of installation for the prototype saved about 14 hours.
In addition to Installation speed, the decisive added value was eliminating time-consuming and high-cost maintenance work, a factor that could cost almost €15,000 for a lattice wind tower at 130-m hub height. This potential cost is eliminated by the maintenance-free performance of BobTail fasteners. The Deutsches Institut für Bautechnik, DIBt also confirmed this special attribute by issuing the “National technical approval” (Number Z-14.4-591). By contrast, regular maintenance work is mandatory when conventional threaded fasteners are used in the fabrication of towers.
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