Bifurcations of a flexible rotor response in squeeze-film dampers without centering springs

Abstract

Abstract Squeeze-film dampers are often utilized in high-speed rotating machinery to provide additional external damping to the rotor-bearing system for the purpose of reducing the synchronous response of the rotor especially while traversing critical speeds, or to eliminate rotor instability problems. The application of these dampers are widely found in aircraft gas turbine engines that are usually mounted on rolling element bearings, which are known to provide almost negligible damping to the system. Although the squeeze-film damper is an inherently stable machine element, its operation at certain parameters may give rise to undesirable non-synchronous vibration. The effects of the design and operating parameters, namely the bearing parameter, B, gravity parameter, W, and mass ratio, α, on the bifurcations in the response of a flexible rotor supported by squeeze-film dampers without centering springs were examined using direct numerical integration. Specifically, the effects of these parameters on the onset speed of bifurcation and the extent of non-synchronous response of the rotor within the range of speed parameter, Ω, between 0.5 and 5.0 were determined. Numerical simulation results showed the occurrence of period-2, period-4 and quasi-periodic vibrations in the response of the rotor as the speed parameter, Ω, was varied from 0.5 to 5. The results further showed that increasing B resulted in the increase of the onset speed of bifurcation, and a decrease in the range of Ω where non-synchronous response was observed. With the exception of the case of W = 0.0, the increase of W was found to increase the onset speed of bifurcation and also the range of Ω where non-synchronous response was observed. The effect of increasing α resulted in a decrease in the range of Ω where non-synchronous response existed. The increase of α also caused the onset speed of bifurcation to increase, except for the case of B = 0.05, W = 0.0, where the onset speed of bifurcation was observed to decrease as α was increased. The numerical results presented in this work were obtained for an unbalance parameter, U, of 0.1, which is considered to be at the upper end of the acceptable range of balance quality specifications for practical rotor systems. The non-synchronous vibrations that were observed in the rotor response, for the range of practical design and operating parameters investigated in this work, are detrimental to the performance of rotating machinery as they cause alternating stresses in the rotor that may eventually lead to fatigue failure.