The productivity of the conventional ring spinning process is currently limited by the frictional heat that occurs in the ring/traveler twisting system. In the framework of a fundamental research project from the German Research Foundation (DFG), the levitation principle of superconducting magnetic bearing (SMB) was implemented as a twisting element in order to eliminate the frictional problem and thus aim, at least, to double the productivity. A mathematical model of the dynamic yarn path has already been presented considering the friction free SMB system up to an angular spindle speed of 25,000 r.p.m. In this paper, the existing theoretical model, which was developed up to 25,000 r.p.m, was further modified considering the balloon control ring and yarn elasticity at a higher angular spindle speed, such as 50,000 r.p.m. The model was solved numerically using the RUNGE-KUTTA method. With this model, it is possible to estimate the yarn tension distribution and balloon form considering the above-mentioned parameters. The model established was further validated by comparing the yarn tension and balloon forms predicted with measured ones up to an angular spindle speed of 15,000 r.p.m in a ring spinning tester based on superconducting magnetic bearing.

The yarn tension and balloon form are the most important physical process parameters to characterize the dynamic yarn path in ring spinning. The present research work focuses on the in situ measurement of yarn tension in different regions of the yarn path in a developed turbo ring spinning tester with a friction-free superconducting magnetic bearing (SMB) twisting system and at an angular spindle speed of up to 50,000 rpm. The influence of different parameters, such as angular spindle speeds (15,000–50,000 rpm), yarn counts (15–40 tex) and balloon control ring (one or multiple), were evaluated to identify the influence of acting forces, for example, centrifugal forces. The effects of these process parameters were analyzed statistically using an analysis of variance. The yarn tension between the delivery rollers and the yarn guide was measured using a modified one-roller tensile yarn tension sensor. The yarn tension between the yarn guide and the SMB system was determined with an already existing optical approach at a higher angular spindle speed. As the highest yarn tension theoretically occurs between the SMB system and the cop, it was estimated in this region by measuring the coefficient of friction between the yarn and the yarn guide using the friction module of the constant tension tester equipment. The maximum balloon diameter was determined from the recorded balloon form between the yarn guide and the SMB system with respect to different angular spindle speeds. The results provide valuable information about the highest possible spinnable speed and enable a better understanding of the dynamic yarn path in the SMB spinning system.

Rotating superconducting bearings promise great potential in applications due to their frictionless operation. However, these bearings show a lower dynamic stiffness and damping coefficient compared to ball bearings. In this paper we studied a bearing consisting of a fixed YBCO ring and a rotating magnet above the superconductor. The influence of the magnet aspect ratio on the dynamic stiffness of the bearing was investigated in order to find an optimized size. To change the aspect ratio, we kept the inner diameter of the ring constant and reduced the outer diameter while increasing the ring height. In addition to these magnets, one magnet with a reduced cross-sectional area was studied. The aspect ratio selection was based on preliminary magnetic flux density simulations, which compared the magnetic flux density distribution and the potential radial force for different aspect ratios. To conduct the measurements, the field-cooled magnets were displaced in a lateral direction and then released, resulting in a damped oscillation. The dynamic stiffness constants were calculated for each bearing from the relation of three axis acceleration measurements for different field cooling heights. The comparison of the stiffness constants for the different bearings revealed an optimal aspect ratio for the given YBCO ring. This optimum is almost independent from the cooling height. The comparison between the two magnet rings with similar diameters and different heights was similar for the bearing characteristics at a low cooling height, whereas a significant reduction of stiffness was observed with a larger cooling distance. The difference is bigger for the magnet with a reduced height. The optimal aspect ratio as well as the stiffness dependence on the cross-sectional area was confirmed by simulations of the magnetic flux density distribution.

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