It is closely related to the steady-running condition. This oscillating-film condition is of frequent occurrence in bearings in service. The motion of the journal axis results in both squeeze and angular swing of the film. The oscillating-film condition, (b), is obtained when a small amplitude of vibration is superimposed on the steady-running condition this vibration is sustained by a fluctuating force superimposed on the steady load. Figure 1.122 (a) illustrates the steady state film, with the film starting at the position of maximum clearance, (b) an oscillating film, (c) rotating trailing film, (d) rotating leading film and (e) rotating transition film. Figure 1.122 illustrates the steady running state film and other film regimes which may apply to the pressure bearing oil film with the journal vibrating. This is one of the sources of complication of the dynamics of turbine bearings. The way in which the oil film is redistributed depends on the character of the vibration. When a sufficiently large journal vibration is superimposed on the steady state running condition, changes in the oil film thickness and circumferential movement of the pressure bearing oil film take place, accompanying the movement of the journal.

The first step towards understanding the performance of a bearing under dynamic conditions is to know the condition of the bearing oil film in vibration as described by Smith, this is different from the steady state hydrodynamically-generated oil film noted earlier.

Steam turbine bearings