The importance of oil separation values

The Importance of Oil Separation Values
Storage stability is an important parameter in the quality of lubricating greases. While mild oil separation during long term storage is acceptable, heavy oil separation renders the grease unfit for use.
Oil separation, also called ‘bleeding’, is an indication of the thickener’s retention ability of the base oil, e.g. very low concentration of large network of fibres, or insufficient interaction between the base oil and the thickener or too high interaction inside the crystalline network leading to contraction and expulsion of oil.
High oil separation can lead to grease hardening. If such hardened grease is used for lubrication of bearings, it may lead to ineffective lubrication for the intended period with possibility of the residual thickener blocking rolling elements and causing irreversible damage. Hardening may also lead to clogging of grease ducts and impact functioning of grease relief valves.
Evaluation of the oil separation characteristics of the grease is therefore a key quality parameter. Many methods exist to evaluate oil separation tendency of the grease. The methods consists in depositing grease on a metallic sieve at prescribed temperature conditions and to evaluate, after a certain time, the quantity of oil separated. Additional pressure is applied on the top of the grease surface to mimic pressure exerted on grease in a stored drum.
DIN 51817 is a standard (German) for testing how much oil separates from lubricating grease under static, pressured conditions, simulating storage.

How the Test Works (Pressure Filtration Method)?
 Sample Preparation: A specific amount (e.g., 100 g.) of grease is placed in a cone with a 240-mesh wire cloth
 Loading: A metal weight (e.g., 100 g.) is placed on top of the grease sample
 Conditioning: The entire setup is placed in a temperature-controlled oven (typically 40°C) for a set time (e.g., 168 hours for standard, 18 hours for accelerated)
 Measurement: After the test, the separated oil collected in the beaker is weighed, and the oil loss is calculated as a percentage of the initial grease mass.
Why It’s Important (Oil Separation/Bleeding) ?
 Homogeneity: Grease should be a stable mix; excessive oil separation (bleeding) means it’s not.
 Performance Impact: Too much separated oil leads to grease hardening, potentially blocking grease paths, clogging ducts, and causing bearing failure.
 Quality Indicator: The test verifies the grease’s internal structure and stability for its intended use.
Related Standards DIN ISO 22285 (International version) and IP 121 (Institute of Petroleum) are similar standards for this test.