For any lubricant to be useful it must remain stable while in use. For example, if equipment requires a specific viscosity for effective operation, the ability of a lubricant to retain its designed viscosity is one measure of stability. One of the elements that can break this stability is the natural stress or shear that occurs within a fluid during use. Lubricants must retain shear stability to remain effective at lubricating and protecting equipment.
Shear stability refers to a lubricant's ability to resist shear. Generally, shear occurs when one layer of a fluid begins to move in a direction different from another layer of that same fluid. For example, where two components are separated by a lubricant, such as a piston and cylinder, some of the lubricant film would naturally move in the direction of the piston. The lubricant layer in contact with the cylinder would begin to shear away from the lubricant layer in contact with the piston. This is known as the shear point. This resulting shear can reduce lubricant viscosity; loss of fluid viscosity can occur from conditions known as temporary or permanent shear.
Temporary shear occurs when long viscosity index improver molecules align themselves in the direction of the stress or flow. This alignment generates less resistance and allows a reduction in fluid viscosity. Yet, when the stress is removed, the molecules return to their random arrangement and the temporary loss in viscosity is recovered.
Permanent shear occurs when shear stress ruptures long molecules and converts them into shorter, lower-weight molecules. The shortened, lighter molecules offer less resistance to flow, which minimizes their ability to maintain viscosity.
Mechanical activity within an engine creates shearing forces that can negatively affect a lubricant's protective viscosity. Even base stocks that provide consistent viscosity through a wide temperature range (high natural viscosity index) are susceptible to shearing forces that reduce viscosity and load-carrying ability. Engines operating at high RPM and those that share a common oil sump with the transmission, like many motorcycles, experience high shear rates.
Viscosity Index improvers used in multi-viscosity oils can shear back when subjected to the combination of high operating temperatures and shearing actions found in modern engines. Permanent shearing of VI improvers can result in piston ring sticking (due to deposit formation), increased oil consumption and accelerated equipment wear. Some VI improvers are significantly more shear stable than others. Although the type of base stock used and the intended application determines the need for VI improvers, many synthetic stocks may not require them at all as they are naturally multi-grade.
Because VI improvers can be subject to shear conditions, formulating an oil using little or no VI improvers can be advantageous. In addition to the problems caused by shear stability, VI improvers' quality varies dramatically and cannot always be easily determined.
When comparing oils, small differences in shear stability indicate a significant drop in performance (all other things being equal).
The High Temperature/High Shear Test simulates shearing conditions at elevated temperatures. The viscosity of the oil is measured at 150