Integer Fractional Frequency Whirl in a Squeeze-Film-Damper Supported Rotor System

Abstract An interesting experimental vibration characteristic of a rotor system experiencing one-half-per-revolution subsynchronous response is presented. The tested rotor dynamic system is composed of an overhung rotor system supported in two axially preloaded ball-bearings with the forward bearing supported in an uncentered squeeze-film-damper (SFD). This has been referred to as fractional frequency motion [1] [2], forced subrotative speed dynamics [3], and subharmonic resonance [4] in the past. The sub-synchronous response is not driven by classic sources of instability like Alford’s force, labyrinth seals, nor internal rotor friction. The response is driven by imbalance when a non-linear planar-asymmetric supporting structuce is present. A planar-asymmetric support structure is one in which the load-deflection characteristic is more pronounced in one direction (e.g., the negative down direction) than the opposite direction (upwards). The important vibration characteristic of these systems is that a dominant component of the response occurs at exactly 1/N per revolution where N is an integer (N = 2, 3, etc.). A planar asymmetric condition is present in uncentered squeeze-film-damper (SFD) systems, especially ones in which the gravity load on the damper is important. Both past experiments and the present effort show the fractional frequency whirl occurs at a frequency consistent with a natural frequency of the rotor dynamic system. In this work, the integer fractional frequency whirl occurs at a frequency near the predicted lowest natural frequency of the linearized rotor dynamic system. When integer fractional frequency whirl occurs, one can think of the rotor group as “bouncing” under the influence of planar asymmetry every other revolution (N = 2) or every third revolution (N = 3), etc. A transient rotor dynamic analysis of the overhung system including gravity loading and imbalance does predict the sub-harmonic response at 1/2-per-revolution consistent with test. The simulation predicts the onset of the 0.5X tone at 6664 rpm vs. 6300 rpm in test, and predicts the 0.5X tone disappears at 14.3k rpm vs. 17.5k rpm in test. Analysis indicates the onset and disappearance of the 0.5X tone depends on the imbalance level, as well as the SFD clearance. Analysis correctly predicted that reducing the rotor imbalance would eliminate the fractional frequency whirl.