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Fighting Words: Gear Oils and Foam


Foaming gear oil not only creates a mess, it can lead to poor performance and early failure. Understanding the conditions that cause foam can keep it from spinning out of control.

Foaming in industrial gearboxes is one of the most frequent complaints that gear lubricant suppliers hear. Foam does not pump or circulate, and it reduces the effectiveness of the lubricant, resulting in accelerated gear wear and overheating. Foam can also create a safety hazard when it spills onto the floor. Its no wonder then that when foam occurs, the customer wants it fixed, and fast!

Although any gear lubricant can foam, the problem is most prevalent in more severe applications. According to lubricant consultant Angeline Card-is, foam is a big issue in wind turbines, where the high speeds and loads cause severe churning that can entrain air in the oil. Other problem applications are equipment where dirt and water are present such as cement, steel and paper mills, and food processing.

Regardless of the application, the savvy user can take some steps to prevent foam in gear oils. Industry experts offer their advice on how to avoid or cure the problem.

Ask a Few Questions

The first step in solving a foam problem in industrial gear oils is to ask a few pertinent questions. First, is foam really an issue, or is the concern more one of appearance? In fact, not all foam is a problem, says Cardis, president of Cardis Consulting LLC in Florence, N.J. If it breaks into islands within minutes of stopping the unit, foam is generally considered a cosmetic issue. While the user does not like to see it, it will not have an adverse effect on gearbox operation. However, if foam persists on the surface of the oil long after the unit has stopped, then it may cause operating problems.

Second, is oil is foaming out of the air vent? This is not only an operating problem, it is a safety hazard. However, the lack of foam at the air vent does not mean there is no problem. As noted by Mary Taylor, technical services director, industrial fluids, at Ultra Additives/Munzing in Bloomfield, N.J., many industrial gear oils foam without the end-user being aware of it. Since gearboxes typically are sealed, a foam problem can go undetected unless it spills out of the vent and onto the floor.

Third, does foam fill the sight glass, preventing a visual determination of oil level? If so, the problem may be an overfilled or underfilled sump, both of which can result in excessive air being churned into the oil. According to Mark DeBenedetto, chemist at Kluber Lubrication, Londonderry, N.H., improper oil level is one of the biggest causes of foam in industrial gear oils. The problem is more prevalent with a low oil level because the gears are more exposed to air.

Fourth, does the oil look creamy? A creamy or milky appearance is sometimes incorrectly attributed to foam while the real cause is more likely poor air release. The persistent entrained air makes the oil look creamy, as will emulsified water. Water can be removed, but there is no easy solution for poor air release. Excessive entrained air often results from improper gearbox design. The oil inlet or outlet may be located so as to allow excess air to be churned into the oil, or there may be an air leak in the inlet line. This condition also can persist if oil residence time in the reservoir is too short to allow air to be released. However, Klubers DeBenedetto notes that even a correctly designed gearbox may foam. Gearboxes operate under such a wide range of loads, speeds and environments that it is impossible to anticipate every condition that may cause air entrainment and foam.

Finally, does the oil look hazy or contain droplets (fish-eyes)? These conditions point to contamination as the main problem, rather than foam. The problem could be water, an incompatible lubricant, or a foam control additive that has separated from the oil. Bulk delivery tanks and repackaged containers can be a source of contaminants such as detergent solutions, water or solvents used for cleaning and flushing. Under these conditions, foam will not be eliminated until the contamination issue is resolved.

Know Your Oil

The first line of defense against foam in industrial gear drives is to use a quality lubricant. Antiscuff (extreme pressure) gear oils should meet the requirements of ANSI/AGMA 9005-E02, which specifies foam testing according to ASTM D892. These tests should give maximum Tendency/Stability result of 50mL/0mL for Sequences I, II, and III.

However, as Cardis notes, a good result in the ASTM D892 test – and the DIN 51566 method based on it – does not guarantee that the oil will not foam in service. In these tests, air is blown into an oil sample, and the time for the foam to dissipate is measured. Unfortunately, this test has no real correlation to the conditions that industrial gear oils encounter. It really amounts to a quality control check that indicates how long it takes an oil to dissipate entrained air.

If the goal is to simulate actual gear oil operating conditions, many of the experts interviewed for this article cited the Flender Foam Test, which European suppliers have adopted. In this test, a pair of gears rotates in the oil, mixing air into the sample. The test evaluates oil behavior with regard to air absorption, oil-air dispersion and surface foam. The foam rating is based on the volume increase (oil-air dispersion plus foam) one minute after stopping the test:

Up to 5 percent increase = Good

Up to 10 percent increase = Satisfactory

Up to 15 percent increase = Permissible

Over 15 percent increase = Excessive

This test has many advantages over the ASTM test, including:

The difference between foam and air entrainment can be clearly distinguished.

It can be run under varying conditions to simulate the field environment.

Contaminants can be added to determine their effect.

Besides foam testing, Cardis recommends retaining a sample of every oil delivery to ensure consistent quality. While the sample may not be sufficient to run a foam test, visual inspection can provide information about the new oil condition. It also provides a way to compare oil quality from one delivery to another.

The samples should be checked for color, odor and phase separation or deposits, she advises. In particular, look for droplets or fish-eyes clinging to the glass. As noted above, these can be a sign of contaminated oil.

Foam Control Additives

High-quality EP industrial gear oils contain a number of different additives to help gears resist damage from seizing, wear, pitting and staining. Additives also improve oil oxidation and contamination resistance. However, these same additives also increase the tendency of an oil to foam because they are, in effect, impurities. Ultra Additives Taylor notes that detergent or surfactant additives are particularly troublesome because they have a higher surface tension than the oil, and they tend to surround and stabilize air bubbles.

To combat the foam-causing effects of the gear oil additive package, as well as the effects of water and contaminants, all industrial gear lubricants contain foam control additives. Some suppliers use the term antifoam for additives that keep foam from forming, and defoamer for additives that help collapse foam after it has formed.

Regardless of their function, foam control additives are polymeric materials of two basic types: silicone-containing, such as polysiloxanes; and non-silicone, such as polyacrylate and poly-methacrylate.

As explained by Arturo Cuellar, marketing specialist with Dow Corning Performance Chemicals, Midland, Mich., to be effective, foam control additives must be somewhat insoluble in oil and their surface tension must be less than the surface tension of the foaming medium. They also must be more surface active than the stabilizing surfactants in the system.

Their low surface tension enables foam control additives to wet the surface of the foam bubble, reduce its elasticity and penetrate the lamella. Once the additive penetrates the bubble, oil flows into it, breaks the bubble and releases the air.

According to Taylor, silicone materials are more effective than polyacrylic materials because they have lower surface tension and are more thermally stable. They also can be incorporated into the lubricant via different carriers, and at different particle sizes. Despite these advantages, however, certain industries such as automotive prohibit the use of silicones because they can cause fish-eyes in paint and interfere with adhesives and bonding agents.

Foam control additives are used at extremely low treat rates (2 to 50 ppm for silicones; 100 to 1000 ppm for non-silicones), and since they add only 1 cent to 4 cents per pound of finished lubricant, they represent a very small percentage of the total cost for a gear oil.

These treat rate recommendations are admittedly rather broad, and for good reason. As Dow Cornings Cuellar points out, although there is a theory and science behind the operation of foam control additives, oil viscosity and additive content make each foaming medium unique. Thats why an additive that works in one gear oil may not work in another, and also why an additive supplier may offer hundreds of different compounds.

To be effective, the foam control additive must be fully dispersed in the oil. If the dispersed particles are too large or too much is added, the additive can separate from the oil and settle to the bottom, or cling to the sides of the reservoir. As noted by Dave Oesterle, product manager, hydraulics and industrial gear, and Rob Profilet, commercial manager, hydraulics and industrial gear, of Lubrizol Corp., Wickliffe, Ohio, too much foam control is just as bad as too little. Studies show that there is an optimum treat level. Just enough additive leaves gaps so the bubble will collapse. Above this level, the additive completely surrounds the air bubble and effectively reinforces the structure, preventing collapse.

The sensitivity of an oil to additive level is the reason why most experts do not recommend tankside additions to control foam. Determining how much to add is not easy – typically only drops – and fully dispersing the additive in an operating gearbox is nearly impossible. In addition, tank-side additions usually involve opening the reservoir, which can introduce contaminants.

Stopping the Suds

A number of steps can be taken to prevent foaming in industrial gear oils. Many of them involve basic gearbox and oil circulation system design.

Lubrizols Oesterle and Profilet provide the following practical design guidelines:

Ensure that the return line and pump inlet line are below the fluid level (usually 2 inches, or 50 mm, from the tank bottom) to prevent aerating the oil.

Size the reservoir properly to give the oil sufficient time to dissipate air bubbles. A general sizing rule is a tank capacity two to three times the pump flow rate. Baffle plates also increase oil surface area, allowing the oil to deaerate and foam to dissipate.

Cardis adds the following suggestions:

Use desiccant air breathers on vents to prevent ingestion of airborne debris and moisture.

Take steps to prevent topping-up with the wrong lubricant.

Keep oil dry during storage, transfer and use.

Dont over or underfill the gearbox.

Monitor the unit for wear particles and oil condition, and take appropriate steps.

Finally, she cautions about the effect of filtration because of her experiences with wind turbine systems using multi-pass filtration. Foam control additives are chemically similar to and attracted by many filter media. Therefore, although the additive is smaller than the filter pores, it may still be drawn out of the oil and accumulate on the filter medium. As a result, using multipass filtration to produce a high level of cleanliness may result in a clean oil that foams.

To date, no standardized filterability tests are available for gear oil. Therefore, lubricant, additive and filter manufacturers must be aware of this potential problem when making recommendations.

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