A polished surface on a bearing raceway, shiny in appearance and able to reflect an image, is not a good sign. It indicates a boundary lubrication condition. The severity of the condition is proportional to the amount of polished surface area on the bearing as illustrated in Figure 1. In order for a boundary lubrication condition to exist, the oil film thickness must be minimal or nonexistent. This often means something has changed from the original system design, lubricant specification or operating conditions. Discovering this condition warrants a review of the present service load condition, lubricant specification and frequency of oil change.

Lubrication engineering consists of more than just designing an oil film between the rolling element and the raceway. An important aspect of lubrication engineering is to find a means for the oil to resist galling when metal-to-metal contact does occur. Metal-to-metal contact is anticipated in rolling element bearings, especially during shock loads, high load, stops and starts.

Figure 1

Protection during metal-to-metal contact can be accomplished by using wear control additives. The additives react with the mating surfaces at high contact temperature to form a protective layer that can be created by either adsorption into, or reaction with, the surface. As a result, the protective layer resists penetration and subsequent galling of the asperities, or high points, on the raceways as the rolling elements squeeze the oil film out from underneath.

Although the surfaces do not gall, they do rub. As a result, the surfaces become polished and will readily reflect an image (Figure 1). Although the oil is doing its job, it is a sign that the load is greater than the film strength of the oil and that demands upon the additive package may be higher than desired. When the additive package becomes depleted, smearing and galling will occur, followed by spalling and eventually failure.

The polished condition of the outer race is a good example of how boundary lubrication can be load-related as opposed to poor performance by the lubricant. The region circled in red identifies a transition between dull and shiny. The pen points to the exact location where the surface is highly polished, and can reflect the pen’s image. The reflection of the pen is missing to the left of the pen because boundary lubrication and metal-to-metal contact did not occur in this region. The entire inner raceway is polished because the raceway rotated through the load zone. The outer raceway is polished in a 120-degree zone because the raceway was stationary with respect to the load zone. In both cases, the output shaft was overpowering the oil film strength.

The inner and outer races are assembled together, minus the bearings and cage. The one pictured in Figure 2 was removed from a double reduction gear reducer. The output shaft, from which the bearing had been removed, failed due to torsional fatigue. The gear reducer was approximately seven years old and had approximately 260 million revolutions on the output shaft. The bearing was not the cause of the failure; it was intact and in good condition, aside from the polished raceway surfaces. However, the service life of the gear reducer fell well short of its expected lifespan.

Figure 2

Oil analysis indicated the machine was in good condition, and records indicated that it was the grade recommended by the gear reducer manufacturer. The next question asked was whether the gear reducer was indeed overloaded. Further investigation revealed that the gear reducer was overloaded; it operated at a lower output speed than the original design which increased the torque requirements.

Every component in the gear reducer was affected, including the lubricant. Greater operating loads not only caused the output shaft to fail in torsional fatigue, but the greater loads also overpowered the oil film on a consistent basis, and metal-to-metal contact was inevitable. This contributed to high surface polish in the load zone of the raceway. The fact that there was no smearing or spalling of either raceway is attributable to the performance and durability of the additive package in the lubricant.

Author Richard Adler has more than 25 years experience within the fields of maintenance and main-tenance engineering. He has worked for several companies in the petrochemical, oil refining, specialty chemical and pharmaceutical industries. Photos and articles about actual failure analysis events can be viewed on his Web site: www.RESnapshot.com.

Photos © 2001 R.H. Adler