ABSTRACT
In rotordynamics, hydrodynamic bearings are vital components. Throughout history, several types of bearing geometries have been developed since the classical cylindrical bearings are susceptible to instability at high rotating speeds and/or low loads. However, it was found that preloaded segments in lobed geometries could postpone the instability threshold. Among the bearings with fixed geometry, elliptical and three-lobe bearings are the most used. The elliptical bearing consists of two circular arcs, being these centres located in the same line. Instead, the three-lobed bearing consists of three eccentric lobes, in which the centre of each lobe is equally spaced, often producing three wedges of hydrodynamic pressure. An important parameter that characterizes these bearings is the preload, defined by the ratio between the distance of the lobe’s centre of curvature and the bearing radial clearance. Another critical feature of hydrodynamic bearings is that, depending on the rotor operating conditions, the bearings may exhibit nonlinear behavior. In these cases, the dynamics of the oil film can no longer be represented by the classical linear theory. Therefore, there is a need to use nonlinear forces derived directly from the solution of Reynolds equation to model the bearings, resulting in a high computational effort, since this equation must be solved at each time instant. In this context, the goal of the present paper is to evaluate the influence of preload on elliptical and three-lobed bearings regarding the linearization of the hydrodynamic forces, defining when the linear model can, or cannot, satisfactorily represent the system. In addition, different rotor rotating speeds are evaluated to verify the influence of this operational parameter on the liner or nonlinear character of the hydrodynamic forces.
