Hydrodynamic Conditions in Plain Bearings

This figure illustrates how lubrication works in a plain bearing. The figure is showing a plain bearing which is supporting a shaft. This is a highly exaggerated figure, designed to illustrate some key points. Notice that under stationary conditions, the shaft is touching the bearing. Also note how the lubricating oil surrounds the shaft.

Press the “Start Rotation” button. Notice that the shaft starts to rotate. If the lubricating oil were not present, the shaft will score the bearing and chew it up within seconds.

Notice what happens when the shaft begins to rotate. It rises a little bit and has no contact with the bearing. As the shaft rotates, it creates a hydrodynamic wedge underneath it. The oil wedge pushes the shaft up and holds it in place. Under ideal conditions, this is a frictionless operation.

Every time you start a piece of equipment having a shaft similar to what is shown here, you cause slight damage to the bearing – because in the beginning the shaft is in contact with the bearing. It takes a few revolutions to develop the hydrodynamic wedge.