The Visco-elastic behaviour of lubricating oils under cyclic shearing stress

Abstract

The shear mechanical impedance of three base-stock lubricating oils has been measured as part of an investigation into the influence of relaxation behaviour on lubrication. The oils selected were of high, medium and low viscosity index. Measurements of the mechanical impedance have been made for frequencies of the cyclic shearing stress in the range 6 to 78 Me/s: it was found possible in effect to extend this frequency range by equivalent variation of the temperature and pressure of the oil under test. Thus, all the experimental data for a given oil are reduced to a single universal curve for each component of the complex shear modulus, G *. This information is then analyzed in terms of a relaxation spectrum. The practical and fundamental aspects of these results are discussed. In addition, results are given for a range of polydimethylsiloxane (silicone) liquids.