  Vibration peaks occurring at rational fractions of the fundamental rotating
critical speed, here named Local Resonances, facilitate cracked shaft detection
during machine shut-down. A modified Jeffcott-rotor on journal bearings
accounting for gravity effects and oscillating around nontrivial equilibrium
points is employed. Modal parameter selection allows this linear model to
represent first mode characteristics of real machines. Orbit evolution and
vibration patterns are analyzed, yielding useful results. Crack detection
results indicate that, instead of 1x and 2x components, analysis of the
remaining local resonances should have priority; this is due to crack-residual
imbalance interaction and to 2x multiple induced origins. Therefore, local
resonances and orbital evolution around 1/2, 1/3 and 1/4 of the critical speed
are emphasized for various crack-imbalance orientations.
