Machinery Lubrication magazine published by Noria Corporation
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38 | January - February 2018 | www . machinerylubrication.com the bearing is exposed to an external environment that is exceptionally hot, this would indicate a root cause. If the temperature increase is from an interna l condition, then this would be a symptom with possible root causes such as excessive lubricant, lack of lubri- cant or misalignment. Regardless of the source of the hot running conditions, the heat will lead to increa sed lubric a nt ox idation, thermal degradation, additive deple- tion, viscosity changes and other failure modes. If the source of the higher temperatures is mechanical, this can be identified as part of the FMEA process. 5. Solid Contamination Solid contaminants can enter a system in a number of ways, including through a new lubricant, ingested from a headspace port or hatch, via defective seals, etc. e type of solid contaminants can vary depending on the source, but typical airborne dust/dirt will consist primarily of silica and alumina. E xc e ssive c ont a m inat ion w i l l result in lubrication failure, as the lubricant likely will not be able to overcome the various wear modes, like three-body abrasion. Addition- ally, if the contaminants are metal catalysts, they can contribute to lubricant degradation in the form of oxidation, particularly when in combination with water, higher temperatures and air. 6. Moisture Contamination Similar to solid contaminants, moisture can enter a system in many different ways, including through the headspace entry point, seals or new oil. When the headspace is humid, thermal cycles can cause moisture to escape the air, sweat onto surfaces and find its way into the oil through gravity. Moisture may exist in a lubricant as dissolved, emulsified or free water. Emulsified water has the most destructive potential in oil. Water is not a good lubricant, so when it displaces oil in a bearing's load zones, the water collapses, producing a lubri- cation failure and mechanica l wear. Water also contributes to oxidation and hydrolysis, with the lubricant undergoing permanent chemical degradation and addi- tive depletion. ese can lead to a lubrication failure by changing the lubricant's viscosity, removing additive functionality, and forming other contaminants, insolubles and acids. Of course, when considering the machine, water is the primary cause of rust. 7. Mixed Lubricant Topping up (if oil) or regreasing (if grease) a bearing with the wrong lubricant can drastically change the physical and chemical properties of the resulting lubricant mixture. Not only can factors like the wrong viscosity impact lubrication, but additives can also react nega- tively with each other, impeding their functionality. 8. Other Contaminants Depending on the machine type, bearings may be introduced to other process chemicals, blow-by contaminants, glycol, etc. Based on the type of contaminant, the lubricant can change chemically or physically, resulting in a lubri- cation failure. In conclusion, regardless of whether you have a lubricant- or contaminant-induced failure mech- anism, the result will either lead to lubrication failure modes or contribute directly to mechanical failure modes of the bearing. When multiple failure mechanisms are combined, there is greater potential for a lubricant failure. A machine FMEA performed on a failed bearing can frequently reveal mechanical wear signatures that indicate whether the failure was lubricant-related, although often the damage during the final stages of a catastrophic failure will destroy or overshadow the evidence of the failure's true root cause. In these cases, it usually is best to perform lubricant analysis (either grease or oil) to detect clues of the root cause, like a thermally degraded lubricant, abnormal levels of contam- inants, changes in viscosity, etc. When developing a conclusion, it also helps to include available mainte- nance records or condition monitoring data, such as vibration analysis, ther- mography or maintenance logs on relubrication and inspections. Again, just because a lubrica- tion failure has occurred doesn't necessarily mean the lubricant was insufficient in volume. Many fail- ures are associated with too much of something, like the lubricant's viscosity or amount. Moreover, if a contaminant is taking the place of the lubricant or disrupts the function of the lubricant, the contaminant will be the ultimate cause of the lubrication failure. ML References Troyer, D. (2000, May). "FMEA Process for Lubrication Failures." Practicing Oil Analysis. Bloch, H.P. (1999). Machinery Failure Analysis and Troubleshooting. Vol. 2. pp. 79. About the Author Bennett Fitch is a senior technical consultant with Noria Corporation. He is a mechanical engineer who holds a Machine Lubricant Analyst (MLA) Level III certification and a Machine Lubrication Techni- cian (MLT) Level II certification through the International Council for Machinery Lubrication (ICML). Contact Bennett at bfitch@noria. com to learn how Noria can help you implement effective lubrication strategies that prevent failure. LESSONS IN LUBRICATION