There are several important silent assumptions of bearing reliability. However, before I address these assumptions, some even more basic assumptions and statements of fact must be established.

While it might be a bit of a leap, I’m going to assume that the bearing is well-designed, well-manufactured, properly handled and stored, and finally, correctly selected for the intended application. With that said, we’re now ready to talk about those silent assumptions that are in the maintenance function’s domain.

These assumptions relate to the internal environment and duty cycle to which a bearing is exposed. Bearing manufacturers will frequently report that only a small percentage of bearings reach their fatigue limit (catalog life). According to one major supplier, typically only 10 percent.

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It takes only a small amount of water (less than 500 ppm) to substantially shorten the service life of rolling element bearings. There is a vast amount of research that supports this. Being a career-long crusader of clean and dry oil, I will certainly not argue the contrary. In fact, water’s destructive effects on bearings can easily reach or exceed that of particle contamination, depending on the conditions.

My theme for this column, therefore, is not about whether water imparts harm but rather how it does. Knowing how water attacks and causes damage helps in setting important dryness targets and also aids failure investigations post mortem. Further, when water contamination is unavoidable, understanding these water-induced failure modes can be valuable in the optimum selection of lubricants, bearings and seals for defensive purposes.

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You’ve heard the expression. There are only two things in life that are certain: death and taxes. Some have suggested that the same applies to machinery. We all know that if a machine generates a profit, taxes will be levied on that profit. But how about death? Is machine mortality also inevitable?

Let’s take a closer look. According to Massachusetts Institute of Technology professor emeritus and tribologist Ernest Rabinowicz, there are three things that cause machines to lose their usefulness (see Fig. 1): obsolescence, accidents and surface degradation. Without question, obsolescence is fundamental to the evolution of engineering and technology. The old must make way for the new. Yet some inventions have long life cycles, the grease fitting for example. Its design has changed little since Oscar Zerk invented it in the early 1920s, yet is still widely used today. The automobile, however, is dynamic and in constant flux. While the classic cars live on into perpetuity, most automobiles face practical obsolescence long before they are functionally inoperable.

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Water, once in an oil, is in constant search of a stable existence. _Unlike oil, the water molecule is polar, which greatly limits its ability to dissolve; and many additives have polar extremities which can markedly increase water solubility. Water may cling to hydrophilic metal surfaces or form a thin film around polar solid contaminants such as silica particles. If a dry air boundary exists, water molecules may simply choose to migrate out of the oil to the far more absorbent air interface. This migration can be accelerated if air and oil mix, Such as in splash lubricated and oil mist systems or any system where a stable fluid foam may exist.

If water molecules are unable to find polar compounds on which to attach, the oil is said to be saturated. Any additional water will create a supersaturated condition causing free water to be suspended or settle at the bottom of the sump. This supersaturation can also occur as a result of lower oil temperature.

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