Feasibility study of a sandwich chopper disc for a time of flight (TOF) spectrometer

The Institute of Lightweight Structures of the Faculty of Mechanical Engineering of the Technische Universität München has been designing and producing CFRP chopper discs for over a decade, specializing in designs of light discs with a high rotational speed. Chopper discs are commonly used in neutron “Time-of-Flight” (TOF) spectroscopy. They are discs with one or more apertures rotating around an axis parallel to the neutron beam, reaching operational speeds of up to 22,000 rpm. The driving design requirements are the operational speed and the maximum allowable weight. For instruments requiring a low operational speed, it is possible to design very light CFRP discs. An ultralight chopper disc, however, frequently implies instability: with its natural frequencies being very low, the disc is prone to oscillations during both acceleration to and deceleration from the operational speed. In contrast, a dynamically stable disc reacts directly, also in the case of an abrupt stop, and can be better controlled. Ideally, therefore, the natural frequency of the disc needs to be higher than its operational speed. This additional requirement makes it necessary to increase the weight of the disc. This paper presents the feasibility study of a sandwich chopper disc, with the aim of maximizing its natural frequency and reducing the weight penalty caused by a solid CFRP disc with the same natural frequency. The structural design of such a disc is presented as well as the manufacturing steps that lead to the manufacturing of a prototype.