Feasibility of nonlinear absorbers for transient vibration reduction

Torsional vibrations are a cause of severe damage to flexible couplings and gearboxes of a dredger drive line. In this paper the feasibility of three different vibration absorbers is discussed when reducing transient vibrations originating from sudden load changes. The classical linear absorber is compared to two nonlinear ones, one with a cubic spring (known as the nonlinear energy sink or NES) and one with a combination of a linear and a cubic spring (a Duffing type absorber). Both nonlinear absorbers succeed in achieving a multimodal vibration reduction, whereas the linear absorber can only mitigate a single vibration mode. The most important reduction is however obtained in the initial phase where the energy of one vibration mode is decreased through a beating phenomenon. A much slower energy reduction of the remaining vibration modes takes place after this initial phase. As a result, both the NES and the Duffing type absorber still need to be tuned to the most important mode, despite their ability to mitigate multiple modes

Changing the eigenfrequency spectrum using passive vibration absorbers

Using a substructure coupling technique the problem of assigning anti resonances is reformulated as a pole placement problem which can be analyzed in a more general framework. A possible approach to create these points of zero vibration is by attaching passive undamped vibration absorbers. As this approach is sensitive to changes in the excitation frequency, a robustness measure is proposed. Based on this measure, a better understanding is provided regarding the attachment location of the absorber and the absorber mass

Combination resonance in a multi-excited weakly nonlinear vibration absorber

It is established that oscillators with a weakly nonlinear spring, also resonates for excitations with a different frequency than its natural frequency. The studied type of resonance is called combination resonance, that occurs if the natural frequency approximately equals a sum of the excitation frequencies. This resonance can dissipate energy from several frequencies simultaneously. If a weakly nonlinear oscillator is used as an absorber on a main system, it can damps several vibration modes simultaneously. If combination resonance occurs in the absorber, it vibrates with the excited frequencies and its own natural frequency, which is higher as it is a sum of the excitation frequencies. This implies a potentially higher speed and dissipation. A tuning method is proposed to ensure this resonance in the weakly nonlinear absorbers. Additionally, a structural modification is added to the absorber, which increases the range of vibration energy of the main system that yields combination resonances. Simulations are performed that validate the theoretical analysis and tuning

Frequency response functions and modal parameters of a rotating system exhibiting rotating damping

In the analysis of the stability threshold speed caused by rotating damping in rotating machinery, there is a lack of experimental data. This stability threshold speed can be found theoretically by means of a linear speed dependent model. The accuracy of the model depends highly upon the linearity and especially on the damping type that has been chosen. In this paper, the theoretical model and the importance of the stability analysis is discussed together with an experiment to validate the model. A rotating shaft is used to extract frequency response functions at different speeds. The shaft is excited with an automated impact hammer and the response is measured by eddy current probes. From these frequency response functions, the poles are extracted and compared to the poles derived from the model. It is found that the imaginary part of the poles, or the Campbell diagram, agrees quite well. The decay rate plot shows a similar increase as from the model, but there seems to be an extra stabilizing effect that is not accounted for in the model

Instability due to internal damping of rotating shafts

Rotor internal damping has been indicated as one of the main causes of instability in rotating machinery for more than a century. However, the exact characterisation of this damping is still an unsolved research topic. Therefore, in this thesis the consequences of material damping in rotating shafts are examined more in depth. Two main steps are considered. Firstly, a finite element model of the beam, including viscous and hysteretic damping, is constructed. This model allows to calculate the threshold speed of instability and the resonance frequencies of a shaft. Furthermore it allows to vary the damping parameters and to compare the considered models giving an indication of the general relations between instability and damping properties. Secondly, an experimental approach should elucidate which model fits best for the physical damping. In general, the main purpose is to gain new insights into how the damping should really be modelled to have the most accurate and safe prediction of a designed rotor