Natural frequencies and modes shapes of two rigid bladed discs on the shaft

The dynamic behavior of a rotor consisting of two bladed discs on a solid shaft is considered. The effect of shaft flexibility on the dynamic characteristics of the bladed discs and the coupling effects between the shaft and bladed disc modes are investigated. Results presented for various cases with differing blade flexibility show clearly the coupling effects in a bladed disc-shaft system. Interference diagrams are developed, from which the dynamic behavior of a system can be predicted for differing flexibility relationships between the solid shafts and the bladed discs. In this study, the global rotating mode shapes of flexible bladed disc-shaft assemblies have been calculated. Rotational effects such as centrifugal stiffening have been accounted for, and all the possible couplings between the flexible parts have been allowed. Gyroscopic effects have been included in the shaft with two discs. Calculated natural frequencies obtained from the blade, shaft, bladed disc and shaft with two discs have been checked to determine resonance conditions and coupling effects. The calculations have shown the influence of the shaft on the natural frequencies of the bladed discs up to one nodal diameter frequencies. The torsional frequency of the shaft with two discs is coupled with the zero nodal diameter modes of the single bladed discs. The bending modes of the shaft are coupled with one nodal diameter modes of the bladed discs. It is shown that including the shaft in the bladed discs model is important from a designer's point of view and can change the spectrum of frequencies considerably.