Axial Oil Film Rupture in High Speed Journal Bearings and Its Effect on the Rotordynamic Coefficients.

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A test rig was designed and made to investigate axial film rupture in high speed journal bearings. Capacitance-type gap sensors installed in the test bearings were used to detect the film rupture. With this test rig, it was found that high speed operation of journal bearings resulted in the occurrence of axial film rupture.A theoretical model was derived to predict the behavior of the axial film rupture. This mathematical model is a corrected Reynolds Equation which incorporates the effect of centrifugal force acting on the libricating film which conventional lubrication theories assume to be negligibly small. Actually it is not small for high speed journal bearings due to high rotational operation. When the film pressure decreases due to the centrifugal force, resolved air come out and coalesce in the film, resulting in axial film rupture. The eccentricity ratio of the journal can be reasonably assumed to be nearly naught because of very high speed rotation of the journal, then the calulation domain can be reduced to a two dimensional plane defined by the film thickness and the bearing axis. The effects of various design variables, such as bearing length to diameter ratio, clearance ratio, supply oil pressure and so on, were obtained.The rotordynamic coefficients of the partially ruptured oil film were obtained for small displacements or small velocities of the journal at equilibrium in the bearing. The stability charts for a Jeffcott rotor supported in high speed floating bush bearings were analyzed, and this theoretical model were found to reasonably explain the vibration phenomena of actual machines with the bearings.