Why does hydraulic fluid foam

This is the second of a multi-part series about air contamination of hydraulic fluid. Here's the official definition of foam in hydraulic fluid: clusters of air bubbles with a diameter greater than 1 mm in diameter about the thickness of a credit card or ID card on the surface of your hydraulic fluid.

It usually forms when hydraulic fluid is mixed with air from a free surface. As the air becomes trapped in the fluid it forms bubbles of air.

As those bubbles begin to rise to the surface of the hydraulic fluid they group together to form foam. And if foaming does lead to entrained air in your hydraulic fluid, then cavitation and pitting damage can result. And of these issues, lubrication problems and cavitation problems are the most serious when it comes to your final drive. Now let's talk about where these bubbles are most likely to form.

Anywhere there's a drop in system pressure can be a problem. For example, throttles and orifices can have pressure drops large enough for bubbles to form, as well as valves that open and close very quickly.

This can be especially problematic at the end of a pipe when a valve is suddenly opened. Pipe joints and branches can also lead to the formation of bubbles. And don't forget that when a pump suddenly stops it can cause shockwaves that can cause bubbles to form.

According to Causes and Solutions for Foaming in Oi l , there are several possibilities as to why your hydraulic fluid is foaming. They include:. Air enters a hydraulic system through the reservoir or through air leaks within the hydraulic system.

Air entering through the reservoir contributes to surface foaming on the oil. Good reservoir design and use of foam inhibitors usually eliminate surface foaming. Air entrainment is a dispersion of very small air bubbles in a hydraulic fluid. Oil under low pressure absorbs approximately 10 percent air by volume. Under high pressure, the percentage is even greater. When the fluid is depressurized, the air produces foam as it is released from solution.

This will quantify very fine contamination which can provide nucleation material. Ask your laboratory to run tests for foam tendency and foam stability. Foam tendency describes the amount of foam generated immediately after the fluid is agitated and aerated, and foam stability quantifies the amount of foam remaining 10 minutes after the cessation of aeration. This option should be requested for gear oils due to the nature of the defoamant chemistry commonly used in these fluids.

Cross contamination of the fluid by another lubricating fluid commonly contributes to foaming and other issues. Additive interferences prevent the defoamant from working properly. To check for this, analyze a sample of new oil from your storage for its elemental signature and compare it to a sample of used oil.

The additive signature of the used oil should be similar to that of the new oil, but slight differences due to additive depletion are likely.

Pay close attention to elements commonly found in additives calcium, magnesium, boron, molybdenum, phosphorus, sulfur, etc. Also, keep an eye out for elements in the used oil which might indicate grease contamination, if this is possible.

The use of Fourier transform infrared FTIR spectroscopy should also be considered, once again using samples of new oil and the used oil. Advanced analysis of the spectra is necessary. FTIR is particularly useful if fluids with different base stocks have been mixed. Hopefully your testing will have alerted you to some possible reasons for the excessive foaming of the oil.

In almost all cases, an oil change, or at least a partial drain and refill, will be required. If some type of contamination was the root cause, a flush will be required, too. This can become expensive for large-volume systems; so in certain cases, reconditioning of the lubricant may be considered. Be aware that this does not always work, and is likely to be a stay of execution rather than a pardon. Make sure you address the root of the problem before conducting the drain and flush.

For particle and water contamination, concentrate on managing contaminant ingress as far as possible before resorting to filtration.