The following factors have an influence on the assembly preload force FM :
Confirmation that a bolted joint is maintenance-free can be made using an appropriate measuring system - such as the USB system with Intellifast technology.
A large number of ultrasonic measurements on wind turbines over 10 years prove: IHF Fasteners are maintenance-free
The ETA-certified fasteners IHF Stretch Bolt, IHF Stud Bolt and IHF Round Nut have been specially designed for steel construction bolted joints subjected to tensile loads, such as those found on ring flanges of wind turbine tower segments. A long-term study based on ultrasonic measurements proves this: The measured residual preload forces are within a narrow tolerance band and are so precise that the bolted joints are installed according to a specified bolting process and do not need to undergo any further checks. For this purpose, online cloud ultrasonic-based measurement techniques are also used serially to provide verification. Based on this circumstance, the bolted joint is defined as "maintenance-free".
This means that IHF Fasteners require far fewer inspection intervals than conventional HV bolted joints, which can save considerable maintenance costs on bolted joints (especially in offshore applications).
The precise measurement results are a result of: the precise manufacturing of IHF Fasteners and their stress-optimized design as well as the very high system accuracy of the tightening method "hydraulic friction- and torsion-free tightening" with bolt tensioning cylinders.
A comparably high precision of the assembly pretensioning forces cannot be achieved when using conventional HV connections, which are pretensioned according to the friction-loaded torque method. In particular, this applies to bolted connections larger than M30. This optimization potential was efficiently exploited in the design of the IHF Fasteners. It was also taken into account that the bolts of the tower segments on wind turbines are not subjected to shear loads, but mainly to tensile loads.
If IHF Fasteners are prestressed hydraulically, friction- and torsion-free with bolt tensioning cylinders, one speaks of the IHF Bolting System.
Currently, conventional, torque-preloaded HV connections are mostly used for bolting tubular steel towers to wind turbines. The HV standard cannot fully meet this application due to its long history:
The normative origins go back to 1962 and even further (DIN 6914, 6915, 6916). In those standards, originally mainly bolted connections on bridges or steel structures were considered, which, however, were primarily exposed to shear loads or designed for perforated soffits. The HV standard was successively extended from M36 to M48, then to M64 and finally to M72.
The IHF Stretch Bolt, IHF Stud Bolt and IHF Round Nut were specifically designed for tensile loads. Unlike a conventional HV fitting, IHF fittings do not have washers. The washers are integrated into the bolt head and nut. This reduces the number of parting lines (see picture). It also eliminates a possible source of error due to incorrect installation of the washers.
In bolting technology, the tightening methods are divided into rotational and tensional methods. In the case of the tensioning method, the required assembly pretensioning force (FM) is applied by purely axial lengthening of the bolt by means of a bolt tensioning cylinder. This hydraulic friction- and torsion-free tightening of the bolts results in a very small variation of the assembly pretensioning force (FVM) of ± 1 %. This results in precise and uniform assembly pretensioning forces (FM).
In rotational processes (e.g. torque processes), the achievable assembly preload force (FM) is the result of the existing static and dynamic friction coefficients in the thread and on the contact surface (thread friction and head friction). These factors can vary so much from bolt to bolt that no valid basis for calculation can be found.
In addition to the inaccuracies in the achievable assembly preload (FM), the bolts also receive bending and torsional loads, which can lead to increased stress and, if overloaded, to failure of the bolted joint. It is not possible to use other methods - such as the rotation-angle method - because there is always a gap between the ring flanges and therefore the joining point cannot be precisely defined. By using bolt tensioning cylinders, the gap is closed quickly.
Conventional torque method: due to difficult to calculate thread and head friction, strong variations of up to 100% can occur when applying the pretensioning forces.
The measurement of the assembly preload force was carried out on the one hand using a mechanical length measuring method and on the other hand using the USB system (Ultrasonic Bolt System). With the USB system, an ultrasonic sensor is firmly integrated onto the face of the bolt head ("sputtering"), which ensures high accuracy of the measurement results.
The practical tests were carried out on behalf of and in cooperation with well-known wind turbine manufacturers on turbines of the 1.5 MW, 2.5 MW and 6.0 MW classes. Bolt dimensions from M30 to M72 were investigated. All tests carried out prove that the IHF Bolting System is a maintenance-free bolting system.
The main requirements at a glance:
The geometric tolerances of the fasteners used must lie within a small scatter band and the number of parting lines must be kept to a minimum. Furthermore, there should be a sufficient clamping length ratio. The innovative IHF Bolting System meets the requirements of a maintenance-free bolted joint, as the scatter of the assembly preload force is very small due to the use of pulling tools and the fastener can reduce the parting lines by two due to the integrated washers.
Besides the reduction in manufacturing costs due to more compact flange dimensions, assembly costs can also be reduced during initial installation due to a faster bolting process. Ring flanges clamped with bolt tensioning cylinders are bolted more quickly, as no counterholder for the torque wrench is required. Depending on the bolt dimension and number, the assembly time per ring flange can be reduced by up to 50%, as proven by practical results.
Compared with rotary screwdrivers or torque wrenches, bolt tensioning cylinders also offer lower weight and reliable positioning, as the support position of the bolt tensioning cylinder on the flange support surface is predefined. The hydraulic friction- and torsion-free pretensioning of bolts with bolt tensioning cylinders is already used in wind turbines for bearing and gearbox bolts, as well as for foundation bolts. The technology was developed over 40 years ago for applications on large-diameter antifriction bearings and for gas and steam turbines and has been continuously refined.
ITH is now carrying out the technology transfer for applications in general steel construction. This includes the fact that IHF Fasteners have received the European Technical Assessment (ETA) from the German Institute for Structural Engineering as well as certification from DNV.