PSI - Issue 24

15

Vincenzo D’Addio et al. / Procedia Structural Integrity 24 (2019) 510–525 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

524

Comparing the natural frequencies with the frequency content of some loads acting on the system it can be observed that at the operating speed they are dangerously close to the first natural frequencies. This can be better understood if we take into account the Campbell diagram containing the Kr min and Kr max cases (the so-called band diagram) which is shown in Fig. 14: the lines corresponding to the frequencies of some asynchronous loads (3X, 3.1X and 3.9X in particular) are very close to potential critical regions at the operating speed (this means that possible resonance conditions can occur with high amplification factors) and it is confirmed by looking at the high magnitude of the forces on bearings (Fig. 15) obtained with the fully flexible multibody model, rotating unbalances (around 10 N) and sinusoidal forces (5 N amplitude) at the bearings. Since, tipically, this configuration does not satisfy the requirements related to the level of vibrations, noise emission, and durability of the bearings a modification in the design is mandatory.

Fig. 15. Magnitude of transmitted force on rotor side bearing with rotating unbalances and sinusoidal forces at the bearings.

While reaching the working point the system passes through critical speeds, but this does not involve significant amplification effects if it rapidly goes beyond. Finally the entire rotating system was simulated, inserting the detailed and defected bearing model for the rotor side support. Every time a rolling element hits the localized defect an impulse is produced so a series of impulses having repeated peaks with BPFO frequency can be observed.

Fig. 16. Magnitude of contact force and of force transmitted to the case at rotor side bearing.