A New Method of Measuring Mulfigrade Oil Shear Elasticity and Viscosity at High Shear Bates

Recent journal bearing oil film thickness data (1)* strongly suggest that, in order to develop minimum-viscosity multigrade oils for maximizing fuel economy, it is not enough to examine viscosity η alone, even at 150C at a shear rate ẏ = 10⁶ s⁻¹: it is essential also to examine a certain kind of shear elasticity, of which one measure is the first normal stress difference N₁ in shear flow. Within a group of isoviscous multigrade oils, N₁ values can differ significantly; for 10W-40 & 15W-40 oils, an average of 75% of the minimum oil film thickness can be attributed to N₁ (1). Here we describe (a) the method used in (1) to obtain high-ẏ N₁ data; (b) an increase in the N₁-measurement upper limit ẏ_max from 3 10⁵s⁻¹ to 10⁶s⁻¹ (where η = 10cP); & (c) an increase in the η-measurement ẏ_max to 5 10⁶s-ⁱ. This is close to the ẏ_max value found in (1) & is 5 times greater than that at present attainable with other commercially available viscometers.