High Shear Stress Behavior of Some Representative Lubricants

Shear stress independent behavior was observed for representative, synthetic, nonblended lubricants to about 4.8 × 10⁶ N/m² (700 psi) shear stress in high pressure viscometric measurements. This shear stress is of the same magnitude as the shear stress in sliding elastohydrodynamic contacts. It is shown that dissipation heating is the only mechanism of importance in the generation of the deviations from constant viscosity as measured with capillary tube viscometric methods. The Newtonian end corrections for the capillary tubes were found to be constant for the nonblended, liquid lubricants. Newtonian behavior will be expected of the fluids in a high shear lubrication situation. Shear induced, nonliquid behavior was found for the silicone lubricant at about 10⁶ N/m² and for the polymer-blended mineral oil at about 10⁴ N/m² at a relatively low pressure level. The observations might provide a key to an understanding of the generation of the anomalous low elastohydrodynamic film thickness as found with these lubricants. The polymer-blended oil showed shear thinning effects. The apparent viscosity was found to increase (∼30 percent) with increasing shear stress in the range of the second Newtonian viscosity level.