Quality assurance measures during tightening of various bolts at Wendelstein 7-X

The central part of W7-X is the main magnet system, consisting of 50 non-planar and 20 planar superconducting coils which are fixed to a rigid central support structure. For further stiffening of the magnet system various inter-coil connection elements are used which are either welded or bolted to the coils. The bolts and screws from M20 to M90 are made from high-strength Inconel 718 as well as the tightening elements called Superbolts®. er to cope with the very large mechanical loads at 4 K due to the Lorentz-forces and to provide a secure connection, all bolts are pre-tensioned close to the elastic yield limit. Therefore the measurement of the preload must be done very carefully with an accuracy of better than 5%. This measurement is made by using the ultrasonic method. Since parameters like Youngʹs modulus, speed of sound, temperature, etc., influence this measurement; an individual calibration must be performed for each specific charge of material and each specific dimension. l attention must be paid to the short bolts with small grip length. Their stretching under full load often is only 1/10 mm. per will describe the procedure of tensioning, the structure of the database for the calibration data, the experiences with the reproducibility and accuracy of the measurement.