Analysis of the effects of parallel and angular misalignment in hyperstatic rotors equipped with oil-film bearings

Misalignment is one of the most common sources of trouble of rotating machinery having couplings between the shafts. Ideal alignment is a chimera and the coupling flanges of the shafts are never ideally aligned, presenting angular and/or parallel misalignment (defined also as radial misalignment or offset). In particular, during the shaft rotation, if coupling misalignment between the shafts of a statically aligned line is excessive, a periodical change, of the load on the bearings in hyperstatic shaft-lines, occurs. If the rotating machine is equipped with oil-film bearings, the change of the loads on the bearings causes also the variation of their oil-film dynamic characteristics, i.e. damping and stiffness, and the complete system cannot be considered as linear. In the paper, this phenomenon is modelled accurately and analyzed by considering the simulated response of a misaligned rotor train in the time domain. A finite element model is used for the hyperstatic rotor, while bearing characteristics are calculated by integrating Reynolds equation (considering the actual type and dimensions of the bearings) as a function of the instantaneous load acting on the bearings, caused by the coupling misalignment. Nonlinear effects are highlighted and the spectral components of system response are analyzed, in order to give pertinent diagnostic information.