Effect of varnish on the performance and stability of journal bearings

Abstract

This paper is dedicated to characterizing the steady-state and dynamic behavior of journal bearings with the provision for varnishing. Degraded base oils and additives cause varnish at high temperatures. Once the varnishing initiates, the oil flow is restricted due to the reduction in film thickness, which leads to an increase in the operating temperature and further accelerated varnishing. The effect of the varnish on the bearing performances is predicted by solving the Reynolds equation for the pressure distributions with the mass conservation algorithm coupled with the energy equation through the viscosity. The solution of the Reynolds equation with the perturbations of the displacement and the velocity is used to calculate the stiffness and damping coefficients and determine the dynamic behavior of the bearing. At a given eccentricity ratio, the maximum pressure and temperature with the varnish become considerably higher than those without the varnish due to the reduced film thickness. Results are presented for both single and double axially grooved journal bearings. Results show that the maximum pressure decreases when the varnish size extends circumferentially at a given varnish thickness, but the maximum temperature still remains relatively constant. For both single and double-grooved bearings, the critical speed decreases considerably, leading to the reduction of the stability of the system.