The rheology of multigrade oils at elevated pressures

A capillary rheometer is adapted to permit measurements of the rheological properties of lubricants at elevated pressures (up to 80 MPa). The pressure drop in the capillary section of the rheometer is used to provide shear viscosity data and the 'entrance pressure loss' in the contraction flow before the capillary section is used to estimate the extensional viscosity, employing ideas pioneered by Cogswell and Binding. Experimental results on single grade oils (which are near-Newtonian in response) are used to validate procedures, and attention is then given to two multigrade oils in the 15W/40 category, both in a virgin and a degraded state. A substantial pressure dependence of the Trouton ratio for the virgin oils is demonstrated, and this is shown to be greatly reduced in the degraded samples. The relevance of the data to the performance of multigrade oils in practical journal-bearing situations is emphasised. (C)1999 Elsevier Science B.V. All rights reserved.