Design and certification of CFRP chopper disks for the NEAT II TOF spectrometer: A lesson learned

Chopper disks are commonly used in neutron Time-of-Flight spectroscopy. They are disks with one or more cut-outs rotating around an axis parallel to the neutron beam, reaching operational speeds of up to 366.66 Hz, correlating with accelerations of up to 200 000 g at the edge of the disk. The NEAT II TOF spectrometer utilizes a set of seven carbon fiber-reinforced polymer (CFRP) chopper disks with an outer diameter of 700 mm and different window configurations with a maximum window depth of 135 mm. A relatively thick (1.5 mm) absorber coating is required, together with a maximal operational speed of 333.33 Hz. When the Institute of Lightweight Structures (LLB) of the Faculty of Mechanical Engineering at Technical University of Munich (TUM) was asked to design and produce the set of disks for the instrument, the major challenge seemed to be that of achieving the operational speed in accordance with the given requirements. In particular, the large area with the boron coating, adding a non-structural weight in a highly loaded area, appeared to be the most challenging design parameter. A common cross section that could sustain the load for all the different window configurations, was manufactured for all the disks. A hub was designed which fitted the system and fulfilled the given requirements. Due to recurring vibrations at a rotational speed of about 260 Hz during the certification test, it proved impossible to achieve the required rotational speed with the original configuration, although all parts of the system seemed to work properly and no initial failure had occurred. This paper presents all the steps that led to the improved design of the system, consisting of disk and hub, which allowed the certification for the desired operational speed.