From "Reliability-Centered Maintenance II"

Care should be taken not to confuse causes and effects when listing failure modes. This is a subtle mistake most often made by people who are new to the RCM process.

For example, one plant had some 200 gearboxes, all of the same design and all performing more or less the same function on the same type of equipment. Initially, the following failure modes were recorded for one of these gearboxes: gearbox bearings seize and gear teeth stripped.

These failure modes were listed to begin with because the people carrying out the review recalled that each failure had happened in the past to their knowledge (some of the gearboxes were twenty years old). The failures did not affect safety but they did affect production. So the implication was that it might be worth doing preventive tasks like 'check gear teeth for wear' or 'check gearbox for backlash', and 'check gearbox bearings for vibration'. However, further discussion revealed that both failures had occurred because the oil level had not been checked when it should have been, so the gearboxes had actually failed due to lack of oil. What is more, no-one could recall that any of the gearboxes had failed if they had been properly lubricated. As a result, the failure mode was eventually recorded as: gearbox fails due to lack of oil.

This underlined the importance of the obvious proactive task, which was to check the oil level periodically. (This is not to suggest that all gearboxes should be analyzed in this way. Some are much more complex or much more heavily loaded, and so are subject to a wider variety of failure modes. In other cases, the failure consequences may be much more severe, which would call for a more defensive view of failure possibilities.)

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"After seeing the same bearing fail several times, we had oil analysis tests done. The results appeared normal, but there was a small amount of water content. The oil doesn't appear to have water in it. Could this be the problem?"

Moisture in lubricating oils can have a devastating impact on component lifecycles. According to a major bearing supplier, it is possible to shorten the life of rolling element bearings by as much as 75 percent without ever knowing that moisture is in the oil, based on visual observation.

Water causes oxidation, acid formation, varnishing, sludging, foaming, viscosity problems (water first thickens and then thins the oil) and can cause an oil to become conductive. Water also creates conditions for corrosion to dramatically increase.

Water can be driven off oil by maintaining the right temperature, and through the use of absorbent media filters and vacuum dehydration. Industrial equipment that is frequently turned on and off is most susceptible to moisture from the atmosphere, particularly during the summer months when atmospheric moisture is at its peak.

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