Shear Stability

Shear Stability ... one measure of how good an oil is ...

Shear Stability is a measure of the resistance of an oil to change in viscosity, caused by the oil being subjected to mechanical stress or shear.

The result of this mechanical stress is a reduction in viscosity, or thinning.

Multigrade engine oils, high Viscosity Index (HVI) Hydraulic fluids and certain gear oils are usually formulated using Polymeric thickeners (Viscosity Index Improvers) to give better Viscosity Index characteristics.

These oils viscosities vary inversely with the rate of shear to which they have been subjected to, that is, as the rate of shear increases, the viscosity of the oil decreases.

As the viscosity index improver itself is also subjected to shear, both permanent and temporary loss of viscosity can occur.

Viscosity decreases as long as the shear stress is maintained and in the case of temporary loss, the oil returns towards its original value as the shear stress is reduced.

In the case of permanent viscosity loss, shear stress causes breaking or shearing of the Polymer molecules resulting in destruction of the Polymeric viscosity index improver.

While all VI Improvers are subjected to shear stress, the quality of VI Improvers vary in their capability to resist permanent viscosity loss or destruction.

The shear stability of VI Improvers are evaluated using standard mechanical stress or shear testing laboratory equipment where in the VI improved oils viscosity is measured before and after being subjected to standard shear conditions.

The results are rated as a shear stability index (SSI) and the lower the value the better or more resistant is the VI Improver to mechanical stress or shear.

High quality multigrade and HVI oils are formulated using a VI Improver with a low SSI.