Fighting Bearing Failures With Additive Chemistry

Stephen W. Eliot, Mobil Oil Corporation
Tags: bearing lubrication

Water contamination can cut bearing life by as much as 80%. Yet, when scrupulous maintenance practices and new oil additive technology are combined bearing life can be extended considerably.

Water is wonderful. You can drink it, bathe in it, grow trees with it and you need it for most manufacturing processes. Yet water remains a major challenge to the reliable operation of rolling element bearings. In fact, water contamination in lubricants can cut bearing life by as much as 80%! Because this can be such a costly problem, some industries have taken extreme measures to cut the rate of bearing failures. The effort has been largely successful. Used oil analysis shows that oil degradation, particulate contamination, and oxidation have been greatly reduced or nearly eliminated in some cases.

Yet despite vacuum dehydration, centrifuging, repairing steam leaks, and fighting water intrusion even with clean systems that boast 3-micron filtration, it is still incredibly difficult to keep water levels below 300 PPM in many lube systems. As a result water-induced bearing failures are still a major concern. What's remarkable is how little water it takes to cause so much grief.

Bearing Failures Reduced
These clean bearings show how the right additives fight the effects of water ingression.

The villain is hydrogen. When a water molecule passes through the bearing load zone, the enormous pressures liberate hydrogen ions. Through a combination of forces, hydrogen ions can migrate through microcracks into the metal below the raceway surface. This subsurface invasion causes embrittlement (sub-surface cracks) and sets the stage for fatigue failure.
The chemicals contained in various types of process waters compound the problem. For example, steam can carry boiler feedwater chemicals; water-based cleaners can introduce alkaline materials; and lubrication systems can pick up even harsher chemicals from white waters used in pulp mills.

Excessive water alone can create emulsions in oil that form deposits and further reduce lubrication effectiveness. When water and chemicals are combined, even trace amounts reduce lubricant demulsibility, corrosion protection, and antiwear capabilities.

This "hydrogen/chemical axis" invasion force is insidious. It may take several years for bearing failure to occur. But regardless of how long it takes, the design life of bearings is shortened considerably, and fatigue failures cause costly shutdowns. While oil analysis and vibration analysis programs can usually spot problems before catastrophic failures occur, downtime is still downtime, and it is costly no matter how well you anticipate it. Naturally, bearing manufacturers are concerned about the consequences of fatigue failure. They've conducted thorough studies that detail the negative effects of water on bearing life (Figure 1).
Many tests have shown that as little as 0.04% (400 PPM) water in lubricating oil can cut the fatigue life of tapered roller bearings by as much as 48%, compared with life expectancy achieved when the oil is dry.

There are three possible solutions to the problem, two of which have already been considered.

  • Keep the water out in the first place.
  • Remove it before it can do harm.
  • Change the water into something else.

Additive chemistry can slow the hydrogen/chemical invasion and extend bearing life significantly. It does so by chemically tying up water, preventing or greatly reducing the liberation of hydrogen ions as well as water's other negative effects.

In a recent series of laboratory bearing tests conducted by lubricant makers and bearing manufacturers, additives were shown to be important to blocking the adverse chemical reaction of water during those critical nano-seconds during which it is exposed to the extreme pressures (200,000 psi plus) in the load zone. So lubricant researchers, including Mobil's team at Paulsboro, New Jersey, developed a three-pronged additive approach to reduce damage caused by water:

  • Antacids neutralize hydrogen ions to prevent reaction in bearing steel microcracks;
  • Long-chain hydrocarbon compounds form a protective hydrophobic surface on bearing steels; and
  • Detergent additives sequester water molecules to keep them from adsorption on steel surfaces or migration into microcracks.

Different additives have been found to have different fatigue-prevention mechanisms, so combinations of additives appear to have a synergistic effect. As such, the composite bearing life extension gained by a combination of additives is greater than the sum of the life extensions from each additive alone.

Even as companies continue to improve their lubrication practices, fighting water-induced bearing failures will remain a priority. Since water levels are difficult to control and almost impossible to lower to a point where bearing life is not adversely affected, using new additive technology offers an important alternative defense.

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