ASTM D6022 Standard Practice for Calculation of Permanent Shear Stability Index
4. Summary of Practice
4.1 An index is calculated representing the change, due to shearing, in an additive's contribution to a lubricant's viscosity. A low index represents high resistance to permanent change.
4.2 Oils can be sheared by many means, including bench tests designed for that purpose, engine tests, and field service. A PSSI can be calculated for each. These indices can be used to compare the shearing severity of each test.
NOTE 1 - Some methods, especially engine tests and field service, may include conditions where other effects (for example, evaporative loss, oxidation, fuel dilution, soot accumulation, etc.) contribute to viscosity changes. The PSSI calculated from these types of service may not be representative of pure shearing.
4.2.1 ASTM tests commonly used to shear oils include Test Methods D2603, D3945, D5275, and D5621, among others.
4.2.2 Other standards organizations publish test methods which may be suitable for shearing oils.
4.2.3 An engine test, Test Method D5119, is also commonly used to shear oils and establish a PSSI for additives.
5. Significance and Use
5.1 Permanent Shear Stability Index (PSSI) is a measure of the loss of viscosity, due to shearing, contributed by a specified additive.
NOTE 2 - For example, a PSSI of 50 means the additive will lose 50 % of the viscosity it contributes to the finished oil.
5.2 The selection of appropriate base fluids and additive concentrations to be used in test oils is left to individual operators or companies. These choices will depend on the intended application for the additive.
NOTE 3 - PSSI may depend more strongly on base fluid, additive concentration, additive chemistry, and the presence of other additives for base fluids of unusual composition (for example, esters) or if additives outside the common range of chemistries and concentrations are used. Caution should be exercised when interpreting results from different sources.
