Lubricants Must Be Formulated to Control Foam

Foam in oil may lead to mechanical damage.

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Brad Nelson
by Brad Nelson
September 30, 2022

Anytime a rotating assembly is submerged in an oil bath, air bubbles, aka foam, can form on the fluid surface. Lubricants must be formulated to control foam to reduce the risk of mechanical damage.

For example, the crankshaft in engines can create foam in motor oil and differentials can create foam in gear oil. In motorcycles, shared-sump transmissions, where the transmission and engine use the same oil, are good at creating foam.

Foam in hydraulic lifters can create valve train noise because the foam is made of air that compresses and creates lash in the valve train.

Foam can lead to poor component protection and ultimately mechanical damage. That’s why controlling foam is a critical piece of lubricant formulation.

A Host of Issues

There are several ways that foam causes problems:

  1. Heat — Foam heats to extreme temperatures under pressure, generating steam within the fluid. Then, foam compounds the issue by creating an insulating layer that prevents the heat from dissipating. Heat and water contamination limit the lubricant’s effectiveness.
  2. Wear — Because air is trapped inside the fluid, the fluid barrier is no longer impenetrable and wear-causing metal-to-metal contact can occur.
  3. Oxidation — The air trapped in foam promotes oxidation and shortens the service life of the fluid.

Hydraulic and other industrial applications face another issue. When hydraulic fluids foam, they become compressible and can make machinery inoperable or extremely inefficient. These heavy-duty lubricants require special formulations to prevent foaming.

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Friend or Foam

Controlling foam is not an easy task. Oil viscosity, contaminants, changes in surface tension and additives can all act as catalysts to the formation of foam.

Detergents and dispersants promote foaming and minimize the effectiveness of anti-foaming additives.

Anti-foam agents can stop foaming but require effective formulation to avoid trapping tiny bubbles within the lubricant.

Silicone additives are an example of how critical it is to get the formulation exactly right. Silicone compounds are widely used for their ability to reduce the surface tension of air bubbles. Reducing the surface tension causes the bubbles to break apart quickly and easily.

Silicone compounds in formulations of only a few parts per million can be extremely effective, however excess amounts can promote foaming.

Organic compounds can also decrease the number of small bubbles but require much higher concentrations than silicone.

Foaming Characteristics Test (ASTM D892)

A lubricant’s ability to resist foaming is measured with the Foaming Characteristics Test (ASTM D892). It measures the amount of initial foaming (in millimeters) contained within an agitated fluid and compares that value to the amount remaining after 10 minutes of settling time. Generally, the less foam remaining after a short amount of time, the better.

Built-In Resistance

AMSOIL synthetic oils are precisely formulated with anti-foam additives to resist the oxidation and acid formation that contribute to foam development. The suppression of foam improves mechanical performance and provides more reliable protection for your machines.

by Brad Nelson

Brad Nelson is a staff writer for AMSOIL. Outside of work he enjoys family adventures, wilderness exploration and riding/wrenching on vintage metric motorcycles.

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