High Speed Rotor Drop-Downs in Different Planetary Touch-Down Bearings Differing in the Number of Bearing Units

The design of touch-down bearings (TDB) for vertical, magnetically levitated outer rotor flywheels with high rotational speeds and operation under vacuum conditions is a challenging task. Conventional TDB are not suited for big diameters with high rotational speeds resulting in surface speeds of above 230 m/s. For such systems, the planetary design can be applied, consisting of several TDB units distributed circumferentially around the stator. This design has the advantages of a decoupling of the rotor diameter from the rolling element bearing diameter as well as a whirl-suppressing characteristic due to the polygonal-shaped clearance. The influence of the number of TDB units on the drop-down behavior is rarely investigated in the literature. However, it is expected that the number of TDB units influences the drop-down behavior and consequently the bearing service life heavily. Therefore, this paper investigates in simulations and experiments drop-downs with six, four, and three TDB units. The simulations are conducted with the simulation software ANEAS. The experiments are conducted with the TDB test rig, which is a robust test rig designed for rotor drop-downs in TDB. The results of both simulation and experiments indicate that the forces on the TDB are lower in configurations with less than six TDB units. However, with three units, the TDB was destroyed the fastest within less than twelve drop-downs.