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Fluid-film bearings are widely used for supporting of various
rotating machines because of their favourable properties,
specifically low friction and wear and vibration-reducing
capabilities. However, a fluid-induced instability can develop
in a poorly designed bearing. A developed instability can be
easily detected performing fast Fourier transform (FFT),
because the instability is accompanied by large amplitudes
of motion with frequency ratio 0.45--0.49 of synchronous
(1X) vibrations. This article introduces a method which
helps to predict the instability before it is fully developed.
The method is derived with consideration of non-linear
hydrodynamic forces in the bearing, which are described by
the Reynolds Equation. The motion of a journal is modelled
employing flexible multi-body dynamics approaches and FEM-based methods for rotordynamics.

Highlights:

- There are three zones of interest with respect to
system stability: except a stable and an unstable zone there
is also a transient zone, where the instability is not fully
developed.

- Journal trajectory is studied in rotating coordinate system.

- Whereas trajectory in the stable and the unstable zone
is of the same pattern, trajectory in the transient zone differs
considerably (qualitative change of the rotor orbits).

- It is possible to detect the early developing instability
before it fully unfolds if rotor trajectory pattern changes are
monitored. However, it is not mathematically proved whether the
pattern changes occur in the transient zone of every rotor-stator system.