The rheology of oils during impact I. Mineral oils

Abstract

An apparatus has been constructed to subject a film of oil to high transient stresses and rates of shear by applying an impulsive load. The apparatus makes it possible to derive the values of the instantaneous viscosity of the oil through out the duration of the impact and to detect whether the oil exhibits viscoelastic behaviour in these conditions. Typical mineral lubricating oils do not exhibit elastic properties in the apparatus and their behaviour is explicable on the assumption that the viscosity depends on pressure and temperature in the manner observed in the usual types of viscometer. However, when only elementary calculations are made the viscosity appears to be abnormally low. To explain the behaviour adequately, it is necessary to consider the viscosity of the lubricant not only with respect to temperature and pressure but also from place to place and instant to instant through out the impact. When this is done, the theory provides a good description of the behaviour of the system up to the point at which the pressures cause significant deformation of the plates containing the oil film. Two other oils, each having an initial viscosity and pressure coefficient of viscosity well above the range of typical lubricating oils, have been observed to exhibit anomalous behaviour. The evidence is consistent with the supposition that they behave viscoelastically but further work would be required to prove the point.