06/06/2026 by Thomas Clark
In an industrial oil context, the term “foaming” refers to air bubbles accumulating on the lubricant’s surface.
A common problem that even cutting-edge products from Shell and Fuchs can encounter, it happens when air becomes mechanically trapped in the lubricant during service and is unable to separate and leave swiftly enough into the system’s sump or reservoir.
Foaming can lead to a wide range of unwanted outcomes including overheating, fluid loss, cavitation and, worst of all, compromised lubrication. Here, we explore the root causes of foaming in industrial oil.
Contamination
When impurities are present in oil, they reduce the lubricant’s surface tension substantially, letting bubbles form and become stable. Contaminants take many forms and moisture is among the most common. Even trace amounts of water infiltrating a hydraulic or gear oil can disrupt its natural attributes and result in foaming.
Particulates and solids like varnish, soot, dirt and other debris create “nucleation sites” that let tiny air bubbles collect and propagate more easily. However, grease and chemicals entering an oil through cross contamination can also cause foaming when they alter its surface chemistry.
Mechanical issues
Problems with the physical design of a mechanical system and its processes can quickly aerate the oils. For instance, excessive agitation from turbulence, high-speed or abrupt directional changes in systems can force air into the lubricant.
Suction and air leaks also cause foaming. Poor seals, loose fittings or faulty suction lines directly draw air into systems and the lubricants that serve them. Whether oil reservoirs are over- or under-filled, incorrectly maintaining oil levels also produces the right conditions for lubricant and air to mix.
Additive depletion
Industrial oils are often formulated with anti-foaming additives that have a silicone base, which can facilitate bubbles bursting quickly. However, over time, these additives naturally get used up and become filtered out, leaving lubricants at risk of foaming. Attempts to top up oil and extend its usefulness with products that include incompatible additives can make foaming even more severe.
Degradation
Finally, as an industrial oil’s base oil ages, it chemically deteriorates, becoming far more susceptible to foam issues. As it breaks down, it creates varnish and sludge that lower the lubricant’s surface tension, stabilising bubbles and promoting foaming.
To identify the root cause of lubricant foaming, accurate testing and analysis is critical.
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