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Foam
Table of Contents
Physical & Chemical Properties
Foaming is defined as the production and coalescence of gas bubbles on a lubricant surface. Foam may be a result of a variety of problems including air leaks, contamination, and over filling of sumps. Foaming can cause loss of oil out of a vent and serious operational problems in most lubricated systems. Excessive foam can starve bearings and pumps of liquid lubricant (pump cavitation) causing failure, and cause poor performance in hydraulic systems. The foaming characteristics of oil are measured by ASTM D-892. Using a calibrated porous stone, air is blown into the bottom of a graduated cylinder for a specified time. Immediately upon completion of the blowing period, the foam that has formed on the top of the oil is measured. Ten minutes after the completion of the blowing period, an additional measurement is made of the remaining foam as the foam retention characteristics of the oil. The results are reported in milliliters.
Most commercial lubricants contain foam inhibitors, as these inhibitors reduce foaming. A few ppm of silicone is commonly used (Reference 4).
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