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The “specific loading” of a bearing is a critical parameter for determining the load carrying capabilities of a bearing. The specific loading of a bearing is the ratio of the applied force to the cross-sectional area of the bearing. In a simple case, the applied force is equal to the rotor weight that is supported by the bearing. The cross-sectional area of a bearing is the axial length of the bearing surface multiplied by the diameter of the bearing bore. 

Reasonable loading for a fixed bore hydrodynamic bearing is in the range of 100 to 250 pounds per square inch. For journals that start rotating with a specific loading above this range and without lift oil, it is very likely that the Babbitt surface will be smeared or wiped.  Gear boxes typically start with light bearings loads and after reaching operating speed, transition to high bearing loads in the 400 psi range due to gear tooth separating forces. Such gear boxes can usually operate successfully with higher specific loading at normal operating speed.

Some heavy rotors supported by hydrodynamic bearings will run without issue when they are at normal operating speed but they require lift oil to separate the journal from the bearing thereby preventing damage when starting and stopping.  Lift oil also reduces the amount of power required to turn the rotor during slow speed operations, thus helping items such as turning gear motors.

A bearing with an excessive load, particularly with inadequate lube oil flow, will run hot and the Babbitt will become soft and smear and eventually fail. 

In summary, the sequence of failure of a bearing with excessive loading is

  1. An oil film with the pressure distribution required to lift the journal is not developed.  
  2. A thin oil film generates substantial heat which raises the bearing metal and journal temperatures.
  3. The heat causes the Babbitt to soften.
  4. Eventually, burnishing, smearing and wiping occurs resulting in bearing failure.

 

 

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Top 5 Problems that lead to Hydrodynamic Bearing Failure

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