Reliability Engineering Snapshot TM

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

 

 

Lubrication - Case No. 95: Using Metal Color to Determine the Bearing Operating Temperature

 

Steel will change color when exposed to heat. This particular property can be used to advantage when evaluating lubrication performance.Example of Metal Discoloration Due to High Temperature

Part of lubrication failure analysis consists of determining how hot the equipment was running prior to failure. The phrase "failure due to a lack of lubrication" gets used quite frequently, and sometimes for the wrong reasons. The first words out of many foremen mouths are "The bearing was as black as the ace of spades. It couldn't have had enough oil!" Most of the time that statement is true, but sometimes it isn't true.

Most of the time when a bearing fails due to old age, it will come out looking spalled. The races and rolling elements will retain a dull metallic gray color, or they will be shiny in appearance. In either case, the lubricant still did its job.

However, there are many instances when some condition, other than the oil, changes and makes the bearing run hotter. It is at this time that having some tool to discern this change would be helpful. This is where observation of metal color is useful. The picture to the left shows the subtle changes in color of a bearing raceway. The mechanics used a "rosebud" (acetylene torch) to warm up the inner race in order to remove it. The increase in heat left permanent color changes to the inner race.

Roller Bearing Showing 400 to 450 F. Surface TemperatureThe same thing happens when a bearing begins to run hot. Long before the bearing housing is 2000 F. the contact surface temperatures between the rolling elements and the raceways are probably two to three times greater. The roller shown in the picture to the left was operating at a temperature between 4000 and 4500 F. It was taken out of service long before it failed. There are no signs of galling. The lubricant was still doing its job. Misalignment caused the rolling elements to be overloaded; see "Machine Design - Case No. 93 - Misalignment Overload of a Spherical Roller Bearing" article. The misalignment overloaded the lubricant's film strength. Greater surface contact forces between the rolling elements and the raceways meant more friction. This operating condition created additional heat that could not be dissipated quickly enough. The heat built up and discolored the metal.

The picture to the lower left shows a rolling element that had a large thrust load on it. How can that be discerned? The color difference is the key. The top half has a light straw color, while the bottom half is bluish black in color. This is about a 4000 F. temperature difference. The picture to the lower right shows a close up of the red oval in the lower left picture. The upper half surface is just as rough as the bottom half, the only difference between the two halves is the color. The uniformly rough surface indicates that the roller in the lower left picture had been operating for quite some time without any problems. At some time there was a change in the thrust load. As in the previous example, the additional loading overloaded the lubricant's film strength. The loss in film thickness generated more friction that resulted in a build up of additional heat. Discoloration of the metal was the result.

Roller Bearing With High Thrust Load Close Up of Roller Bearing Showing High Temperature at Surface
There are a lot of design parameters when it comes to troubleshooting lubrication problems, so it helps to be able to tell how hot the rolling elements got, and the heat profile. In the above two pictures there is a distinct possibility that the grease was not a high temperature grease. The roller was from a kiln trunnion assembly. Thrust loads are expected, within reason. However, depending upon the lubrication mechanic, someone may have pumped the bearing full of regular grease and not high temperature grease. The lower temperature grease would not have been able to withstand the higher operating temperature conditions, one of which is shown in the upper right picture.

 

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