The formation of foam on the oil in an industrial gearbox should not be ignored. Most of the time you might see a few bubbles on the surface of the oil that dissipate quickly without destabilizing the oil level, in which case it is not a problem. However, if so much foam is forming that it rises to the top of the gearbox and ventilates through the breather, or if it lowers the oil level so much that it enters a pump inlet, those are causes for concern. Foaming in these cases can hinder gear protection and result in overheating.
So what can you do to control it? First, let’s look at why the foam is forming in the first place. Foam typically occurs in severe conditions, such as high-speed operation or dusty, dirty environments. Insufficient residence times or incorrect oil levels (too much or too little) can also contribute to foaming. Many industrial lubricants are formulated with a foam inhibitor additive, but excess contamination from water, dust, cleaning chemicals, greases or bacteria can compromise the inhibitor. Foam inhibitors can also be depleted in the filtration process. And because they are insoluble, they may separate from the oil and settle to the bottom of a container if stored too long. Contamination can also occur from mixing incompatible gear oils.
In the case of new gearboxes, corrosion inhibitors that are applied for storage and transport purposes are often known to cause foaming, which is why gearbox OEMs often recommend flushing before putting a new unit into service.
When you see excessive foam, you may be tempted to search for a solution using aftermarket products such as a foam suppressant. However, this is usually discouraged, since it may not solve the problem and may simply aggravate it. You’re better off identifying the root cause and remediating it – for example, controlling the ingression of contaminants that may be causing the problem.
As is often the case, prevention is the best cure for foaming. It begins with proper oil selection and storage. Avoid extreme temperatures that can negatively affect oil properties. Check drum or tote openings for accumulation of dust or humidity that can make its way into the oil.
When applying the lubricant in the gearbox, be sure to fill and maintain the proper oil level. Take measures to mitigate contamination, such as dedicated lines, pumps and oil cans, and take care whenever installing, topping off, filtering or dehydrating the oil. Desiccant air breathers are a popular tool to keep out particles and moisture, so long as they are properly maintained and replaced when needed.
If you’re concerned whether your gear oil may be susceptible to foam formation, you can pull a sample and send it to a lab for testing. In addition to identifying particle counts, the presence of water and other potential contaminants, a lab can perform the ASTM D892 test, which is the industry standard for rating an oil’s “foam performance,” meaning its ability to minimize foam formation within acceptable levels.
Some foam in an industrial gearbox is to be expected. Excessive foam is seldom caused by the lubricating oil itself. Most likely there are other factors involved. It pays to understand the contributors to foam formation so that when it appears, you can readily determine the likely cause and take steps to mitigate its impact.