ASTM D6080 for defining the viscosity characteristics of hydraulic fluids
4. Summary of Practice
4.1 High VI hydraulic fluids often contain high molecular weight thickeners, called viscosity index (VI) improvers, which impart non-Newtonian characteristics to the fluid. These polymers may shear degrade with use, and reduce the in-service viscosity of the fluids.
4.2 This practice provides uniform guidelines for characterizing oils in terms of both their high and low temperature viscosities before and after exposure to high shear stress.
4.2.1 Since the performance of fluids at temperatures higher than 40°C is determined in the worst case, that is, most severe situation, by the sheared oil viscosity, the viscosity and viscosity index used to characterize fluids in this practice are those of the sheared fluid.
4.2.2 This practice classifies oils at low temperature by their new oil properties. Low temperature viscosities do not decrease greatly, if at all, with polymer shear degradation. Furthermore, this approach ensures that the fluid will be properly classified under the worst-case conditions, that is, when the fluid is new.
4.3 This practice may be used with either Newtonian or non-Newtonian hydraulic fluids. This provides the user with a more reasonable basis to compare fluids than previous practices.
5. Significance and Use
5.1 The purpose of this practice is to establish viscosity designations derived from viscosities measured by test methods which have a meaningful relationship to hydraulic fiuid performance. This permits lubricant suppliers, lubricant users, and equipment designers to have a uniform and common basis for designating, specifying, or selecting the viscosity characteristics of hydraulic fluids.
5.2 This practice is not intended to be a replacement for Classification D2422. Rather, it is an enhancement intended to provide a better description of the viscosity characteristics of lubricants used as hydraulic fluids.
5.3 This practice implies no evaluation of hydraulic oil quality other than its viscosity and shear stability under the conditions specified.
5.4 While it is not intended for other functional fluids, this practice may be useful in high-shear-stress applications where viscosity index (VI) improvers are used to extend the useful operating temperature range of the fluid.
5.5 This practice does not apply to other lubricants for which viscosity classification systems already exist, for example, SAE J300 for automotive engine oils and SAE J306 for axle and manual transmission lubricants.