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The single largest source for the failure of Babbitted journal bearings is particulate matter in the lube oil. This is particularly true for machines with horizontal shafts. A hydrodynamic journal bearing depends on the shaft rotation to form an oil film. The viscosity of the lube oil that is dragged into the oil film under the journal creates a wedge-shaped film that has a pressure distribution that lifts the shaft off the bearing bore. Without rotation, there is no pressure distribution and no lift. At the beginning of the start-up phase or the end of the shutdown phase, the oil film has no measurable thickness.

When particulate matter in the lube oil is dragged into the thin oil film between the journal surface and the Babbitted surface of the bearing bore, and when a particle size is larger than the oil film thickness, the particulate is dragged all the way through the thin film or it goes part way through the film and then becomes embedded in the Babbitt. A volume of Babbitt is displaced that equals the volume of the embedded particulate matter. This can either cause the Babbitt to extrude at the axial ends of the bearing bore or it raises the Babbitt surface of the bore. When the rotating journal contacts the raised surface spots, a local wiping occurs. Small pieces of Babbitt are smeared over the adjacent Babbitt surface, or these Babbitt pieces break free and are carried away by the oil. The loose Babbitt pieces that are not carried away from the bearing continue to circulate in the bearing oil film, and when they enter another very thin film, they can get embedded in the Babbitted bearing surface, often causing a smear or a full-blown wipe.

There are many reasons why there may be particulate matter in the oil:

  1. There may be no filtering at all
  2. The filtering level of the filter element may have been specified improperly (too large).
  3. The filter element may be clogged so that oil by-passes the filter element.
  4. A by-pass valve may be set with a differential pressure too low so that oil bypasses the filter all the time or when the differential pressure exceeds the by-pass pressure set point.
  5. Particulate matter that settles in drain pipes may be shaken loose during periods of high vibration of the machine.
  6. Pipes made of carbon steel can rust on the inside due to water vapor. Almost all lube oil usually has some amount of water in it that is released as water vapor at elevated temperatures and condenses on the insides of any drain pipes above the level of the flowing drain oil. This water vapor with oxygen causes carbon steel drain pipe surfaces to form rust. The rust scale drops off and either accumulates in low spots in the drain pipes or accumulates in the oil reservoir.
  7. Weld beads and other heavy particles can remain in pipes after construction or repair and can move when the oil flows or vibration occurs.

 

 

Journal damaged by particulate matter

Tilting Pads damaged by particulate matter

The journal above was damaged by substantial particulate matter that became embedded in the Babbitt of the tilting pads. After the journal is damaged to the degree shown, the oil film is no longer supported making metal to metal contact possible. The result is that the journal will continue to erode the Babbitt on the pads. If the pads are refurbished to like-new condition and the damaged journal is not restored to a smooth surface, the pads will continue to be eroded. The only solution is to machine or hone the journal to a smooth like-new condition and to refurbish the pads at the same time.

 

  

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Top 5 Problems that lead to Hydrodynamic Bearing Failure

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