Foaming (400ml) is a common test (ASTM D892) performed on lubricating oils to determine their tendency to foam. Air is pumped through a porous stone (similar to an air stone used in household aquariums) to produce many fine bubbles. The process is split into 3 sequences – usually Sequence 1 is sufficient for most applications, but some applications require sequences 2 and 3 as well. In High temperature applications such as engines and transmissions there is a separate method for foaming often referred to as sequence 4.
- Sequence 1 –Air is pumped through the stone into the oil for 5 minutes at 24OSequence 2 –Air is pumped through the stone into the oil for 5 minutes at 93.5OSequence 3 – Perform sequence 2 first and then sequence 1.
- Sequence 4 – (ASTM D6082) This is a different method to standard foaming 3 sequences and is specific for engine and transmission oils. The analysis is performed at 150OThe result obtained on each sequence is quoted in millilitres of foam at the end of the test called the tendency to foam. Some of this foam will then collapse over time and at 10 minutes the foam is measured again. This foam still preset at 10 minutes is called stable foam. The result is expressed in tendency/stable e.g. 20/0 where there is 20ml of tendency to foam, but it had all collapsed before 10 minutes (0) leaving no stable foam.
Tested On: In service Turbines, gears, hydraulic oils and any new oil products. Sequence 4 is tested on engine/transmission applications.
Diagnostic significance: Foam is a poor lubricant. Additives such as detergent in the oil clean oxidation and sooty deposits, whilst anti-foam additives (typically silicone oil based) collapse foam bubbles. Imbalance in these additives can lead to foaming in a system. Stable foam is worse than foam tendency as it does not quickly collapse and enter load bearing zones such as bearings causing metal to metal contact and wear. Foam in general can cause leakage of oil from the surface by increasing the volume in the system and it is possible to have systems overflow with foam filling an entire room.
Typical Diagnostic limit: New oils typically have limits of <50ml tendency and 0ml stable foam, whilst in use applications tend to allow higher tendency up to 450ml and up to 10ml of stable foam.