Studies in Lubrication IX. The Effect of the Pressure Variation of Viscosity on the Lubrication of Plane Sliders

Abstract

The Reynolds theory is applied to the calculation of the lubrication properties of plane sliders—thrust bearings—of infinite width provided with lubricants whose viscosities increase exponentially with the pressure. The friction coefficient, minimum film thickness, and lubricant flow were calculated both for fixed wedge angle and pivoted sliders. The effect of the viscosity variation with pressure is determined by the magnitude of the dimensionless product of the viscosity-pressure exponent and the bearing load per unit area. The analysis shows that for each choice of this product there will be a limiting position of the pivot line of the slider or of the equivalent Sommerfeld variable at which the film pressures and friction forces will become infinite, and beyond which it will be impossible to operate the slider. Moreover this product is shown to be limited by a maximum value equal to 2, which means that the absolute maximum load per unit area which can be carried by such slider systems is equal to twice the reciprocal of the viscosity-pressure exponent. Specific calculations on the friction properties of bearings operating with lubricants of different viscosity pressure exponents give curves of friction coefficient vs. load or Sommerfeld variable quite similar to those observed in practical tests. At high loads or low values of the Sommerfeld variable the friction coefficient curves split and follow the behavior generally interpreted in terms of oiliness and boundary lubrication phenomena.