If your plant has large equipment with accessible openings inside the gear box or reservoir, consider placing bar magnets inside the unit to capture damaging particles. Although magnetic plugs are good, they will not always capture all metal particles. We have found that between sampling, filter changes and filtering that bar magnets capture a large amount of wear debris that may accumulate from large equipment with pinions and bull gears. When a large amount of wear metal is found, we know that we may have a problem and can correct it before it causes more damage. (Spencer R. Anderson, PDM Tech, TLNA) More about magnetic filtration.

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Book Bits: Bearing Lubricant Selection

From the book "Rolling Bearings Handbook and Troubleshooting Guide"

When bearings must operate in a wide range of temperatures, use an oil that has the least viscosity change with changes in temperature - for example, an oil with a high Viscosity Index (VI). In many applications, pure mineral oils are most satisfactory. They should be free from contamination, and they should resist oxidation, gumming and deterioration by evaporation of the light distillates. Finally, they must not cause corrosion of any parts of the bearing during standing or operation.

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Question: Which type of oils should be tested to obtain baselines?

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"We are using a general-purpose lithium complex grease that is supposed to be good to 500°F, but whenever the bearing caps are removed we find a lot of varnish and hard residue. How do we know if the application is too hot for a grease?"

There are several points to consider in this question. First, what is high temperature? Bearing manufacturers begin to de-rate bearings at 300°F and above. One could consider this to be high-temperature if that is the steady-state condition.

Second, even though the grease thickener may withstand yet higher temperatures, the oil base stock and additives are not necessarily designed for high-temperature conditions, and may evaporate or oxidize rapidly.

Third, even though the product literature may show a dropping point at 500°F, that doesn't suggest that the grease will retain its properties through that temperature. A safe guideline is to consider the useful temperature limit for a grease to be 80 percent of the stated dropping point.

Finally, it is imperative to recalculate the relubrication frequencies whenever operating temperatures exceed 150°F. Above that temperature, the intervals will begin to drop appreciably.

Mike Johnson, Noria Corporation

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