Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Lubrication - Case Study No. 7: Boundary Lubrication in a Bearing


A shiny surface on a bearing raceway is not a good sign. It actually means that the oil film thickness was minimal, and providing minimal protection. This condition is known as boundary lubrication.

To the left is a view of a cup and cone bearing with the rollers and cage removed. Taking the two pieces apart reveals the roller bearing surfaces. In the lower left hand picture, the pen points to the shiny surface, the reflection of which can be easily seen on the surface of the cone. The shiny surface ends just to the left of the pen; notice how the surface appears dull in appearance. In the lower right hand picture, again, the pen can be easily seen in the reflection on the surface of the cup.

What happened here was that as the rollers turned, the oil film provided just enough support to prevent general metal to metal contact between the rollers and the raceway; however, the rollers actually did contact the raceway by breaking off the highest asperities of the raceway surface. Continually breaking off the asperities polished and created the smooth surfaces shown below.

In lubrication shiny is not beautiful, dull is beautiful.

Bearing Cone - Boundary Lubrication Bearing Cup - Boundary Lubrication
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