SEVERITY ESTIMATION OF CRACKED SHAFT VIBRATIONS WITHIN FLUID FILM BEARINGS

The equations of motion, with four degrees of freedom, taking into con sideration the flexibility, damping and cross coupling of the fluid fi lm bearings are derived for a cracked Jeffcott rotor supported on flui d film bearings. Dimensionless equations are developed for dynamic rad ial load, dynamic pressure developed in the fluid film bearings and co efficient of dissipation considering the journal vibrations in two har monics; bearing fluid film stiffness and damping coefficients. These a re applied to a cracked Jeffcott rotor supported on different types of bearings, i.e., cylindrical journal bearings, offset cylindrical bear ings, tilting pad journal bearings and three-lobe bearings. Based on t he allowable dynamic pressure developed in the fluid bearings, the sev erity of cracked shaft and allowable crack depths are estimated in thi s study. Measurement of dynamic pressure and dissipation for monitorin g the crack growth is suggested. However, 2x vibration is the best ind icator of cracks in the Shafts.