Frank Hohmann / General Manager / ITH Bolting Technology
Proof that the devices work comes from more than 4,200 ultrasonic measurements made over a 40-month in-field analysis on wind-turbine towers. Their required assembly preloads were held within a close tolerance and consistently above the required preload of 70% of the load prescribed by the DASt 021, German steel construction guidelines.
The results indicate that connections bolted with IHF fasteners do not require the same frequent checks as do other conventional high-strength (HV) bolt connections. The recent fasteners are considered “maintenance-free,” a feature that trims significant costs from O&M work, especially from offshore wind turbines.
The accurate load measurements of the devices were based on:
- An accurate manufacturing process for the IHF Fasteners, which is a stress-optimized design, and
- The high system accuracy of the hydraulic-powered friction and torsion-free tensioning provided by the ITH bolt-tensioning cylinders.
It is difficult to apply pre-tensioning forces to standard HV bolt systems with high repeatability. HV bolts are tightened by torque alone, which is negatively affected by friction. These negative effects become even more severe for bolt sizes larger than M30. To address this issue, the IHF Stretch system and fasteners were designed to be nearly friction-free. The fasteners are ideal for wind turbines because tower-segment bolts are mainly exposed to tensile loads, whereas HV bolts were originally intended for applications exposed to shear loads.
The system works best where tolerances of the fasteners must be in a small scatter band. Also the clamp-length to bolt-length ratio must be in a certain range.
Where conventional HV systems fall short
Normally torqued HV connections in wind turbines are used for fastening the steel tower sections. There is high variation in the static and dynamic factors with every bolt, and there is no reliable system to calculate their values. What’s more, when torque alone is used to tension conventional HV bolts, it also applies bending and torsional stresses to the joint, which can lead to an overstressed bolt and possible connection failure. Equally flawed torque-angle systems make it possible to set a joining moment to an incorrect level. A low moment can lead to a gap between the two flanges and to a lower clamping load. The bolt tensioners, however, close the residual distance quickly and directly.
Because the HV Standard has origins in 1962, well before the recent wind industry, its design cannot meet current demands. DIN 6914, 6915, and 6916 are even older examples.
The norms and regulations were designed for bolt connections on bridges and other steel constructions, which were more or less designed to handle shearing loads and bearing stress. The HV Standard was extended from M36 to M48 bolts, then to M64 and afterward up to M72 bolts.
Same surface pressure with fewer parts and joints
In contrast to an HV set, the more recent bolt and nut set consists only of two components, the IHF Stretchbolt and Roundnut. Washers are forged into each bolt head and nut. This reduces the number of components. The incorrect mounting of the washers, a source of error, is completely eliminated. So are the friction factors that negatively affect torqueing methods.
In the tensioning system, the tensioning force is achieved by the axial elongation of the bolt with a bolt-tensioning cylinder. This hydraulic-powered, friction and torsion-free tightening leads to a high repeatability of the remaining tightening force of about ±2% (system accuracy). This results in an accurate and even tensioning load.
The system’s torque wrench simply overcomes the static and dynamic friction of the Roundnut and bolt surfaces (thread friction and head friction) and snugs the nut against the joint surface to maintain the tension applied by the hydraulic system.
Measurements for the 4,200+ bolts in the test were carried out with a mechanical length-measuring device and the USB – System (Ultrasonic Bolt System). This system integrates an ultra-sonic sensor into the frontal surface of the bolt which leads to high-accuracy measurements.
Field tests were organized with different major manufactures from the wind industry. All classes of wind turbines from 1.5, 2.5, and 6.0 MW were included in this test, along with all dimensions from M30 up to M64 bolts. All inspections and measurements proved that IHF is a maintenance-free bolt system.
A few more benefits
Because the variation of the hydraulic tensioning tools is low and the number of components is reduced by two, the IHF Stretch System achieves all the requirements of what we call a maintenance-free bolt connection.
In addition, by using a compact flange, manufacturing costs are further reduced. Mounting times are also reduced by using a faster tightening process. Flanges are tensioned much faster with ITH bolt tensioners instead of a torque wrench that requires fixing a reaction arm for every bolt. The time trimmed from the mounting operation can be up to 50% depending on bolt dimensions. This has been proven by practical experiences.
Lastly, the reaction surface of the bolt-tensioning cylinder is predefined at the flange surface. Hence, the low weight of the bolt tensioner allows for a safer operation than with nut runners and torque wrenches. The friction and torsion-free hydraulic stretching method with the ITH bolt tensioners is already used for bearing and gearbox-bolt applications, and foundation bolts. A longer-term analysis is still in process.
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