Analyzing Foaming Tendency: Foam Test
The Foam Test measures the foaming tendency of a lubricant. According to this test, also referred to as ASTM D892, the tendency of oils to foam can be a serious problem in systems such as high-speed gearing, high-volume pumping, and splash lubrication. Inadequate lubrication, cavitation, and overflow loss of lubricant can lead to mechanical failure. This test evaluates
oils for such operating conditions.
Cavitation is the formation of air or vapor bubbles in the fluid due to lowering of pressure in a liquid, which then collapse (implode) in the higher-pressure regions of the oil system. The implosion can be powerful enough to create holes or pits — even in hardened metal — if the implosion occurs at the metal surface. This type of wear is most common in hydraulic pumps, especially those that have restricted suction inlets or are operating at high elevations.
Foaming is a fundamental physical property of a lubricating fluid. Foam can degrade the fluid’s life and performance as well as that of the equipment being lubricated. Even though foam performance often is a defined specification for the new fluid, it’s often ignored on used fluid. You need to understand the reasons for loss in foam control and the methods of controlling this property in a used fluid. A fluid’s foaming property is measured using ASTM D892, which measures foam by three sequences that differ only in testing temperature.
- Sequence I measures the foaming tendency and stability at 24°C (75°F).
- Sequence II uses 93.5°C (200°F).
- Sequence III uses the same conditions as Sequence I, except it’s performed on fluid that has just been measured in Sequence II.
The fluid sample from Sequence I isn’t used in Sequence II. The fluid sample used in Sequence II is carried into Sequence III.
The results are reported as two numbers for each sequence. For example: 20/0 means 20 milliliters of foam tendency was measured after 5 minutes of aeration followed by no foam stability (0 ml) after the 10 minute settling time. Most new oil specifications require 10 to 50 milliliters tendency maximum and 0 milliliters stability.