Thermal effects in hydrodynamic cylindrical bearings

Abstract

It’s known that a rotating machine, when in operation, is susceptible to vibrations, which occurs due to external excitations or to vibrations inherent from the machine operation, as the residual mass unbalanced excitations. If the rotating machine is supported by bearings with hydrodynamic lubrication, those are, consequently, susceptible to sub-syncronous vibrations due to a fluid-induced instability. The sub-syncronous vibrations, known as oil whirl/whip, can cause critical failures in the system, and consequent sudden stops and irreversible damages in the bearings. Through characterization of the oil film, by linearized stiffness and damping coefficients, it is possible to obtain an approximation to the threshold of instability. The hydrodynamic lubrication’s classical theory applies the constant viscosity condition to calculate the dynamic coefficients. Nevertheless, when the bearing is under operation, viscous fluid shear occurs, resulting in the increasing of the lubricant temperature, influencing the dynamic behavior of the entire rotational system. This paper presents a comparative analysis of the dynamic behavior, regarding the threshold of instability, considering the Lund critical mass and the logarithmic decrement theories for the classical hydrodynamic model and the thermohydrodynamic model.