Journal Bearings - Hydrodynamic or Fluid Film Lubrication

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Journal Bearings  - Hydrodynamic or Fluid Film Lubrication: Although not as obvious as plate or thrust bearings, the operation of journal or sleeve bearings is an example of hydrodynamic lubrication. When the journal is at rest, the weight of the journal squeezes out the oil film so that the journal rests on the bearing surface. As rotation starts, the journal has a tendency to roll up the side of the bearing. At the same time fluid adhering to the journal is drawn into the contact area. As the journal speed increases an oil wedge is formed. The pressure of the oil wedge increases until the journal is lifted off the bearing. The journal is not only lifted vertically, but is also pushed to the side by the pressure of the oil wedge.

The minimum fluid film thickness at full speed will occur at a point just to the left of center and not at the bottom of the bearing. In both the pivoting shoe thrust bearing and the horizontal journal bearing, the minimum thickness of the fluid film increases with an increase in fluid viscosity and surface speed and decreases with an increase in load.

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