Originally Posted by JAG
I'll repeat the key point already stated: it is due to temporary shear thinning...temporary viscosity loss at high shear rate. It regains its viscosity when the shear stress goes away. A Newtonian fluid does not have a shear thinning behavior since its viscosity does not vary with shear rate. To be Newtonian, polymer VIIs must NOT be present. Having "shear stable" VIIs does not change that. "Shear stable" VIIs refers to how well they avoid permanent viscosity loss, not temporary loss.
Following on from that JAG, I'll throw in some history... When oils were made with polymeric viscosity index improvers, it was found that they did not provide the "protection" that their grade should have indicated...studies ensued, including the one that generated the first of these charts, particularly interesting as they used an engine in which they access and tap into one main bearing, and supply the bearing with a constant supply pressure of a bunch of oils.
As JAG stated, Newtonian oils do not change the viscosity with shear rate. Oils with polymeric viscosity improvers "flatten" the VII with shear rate, and give an apparent viscosity in the bearing that is lower than the viscosity measured at lower shear rates.
I'll repeat the key point already stated: it is due to temporary shear thinning...temporary viscosity loss at high shear rate. It regains its viscosity when the shear stress goes away. A Newtonian fluid does not have a shear thinning behavior since its viscosity does not vary with shear rate. To be Newtonian, polymer VIIs must NOT be present. Having "shear stable" VIIs does not change that. "Shear stable" VIIs refers to how well they avoid permanent viscosity loss, not temporary loss.
Following on from that JAG, I'll throw in some history... When oils were made with polymeric viscosity index improvers, it was found that they did not provide the "protection" that their grade should have indicated...studies ensued, including the one that generated the first of these charts, particularly interesting as they used an engine in which they access and tap into one main bearing, and supply the bearing with a constant supply pressure of a bunch of oils.
As JAG stated, Newtonian oils do not change the viscosity with shear rate. Oils with polymeric viscosity improvers "flatten" the VII with shear rate, and give an apparent viscosity in the bearing that is lower than the viscosity measured at lower shear rates.