A Numerical Approach to the Thermal Effect on the Equivalent Dynamic Coefficients of Radial Bearings

摘要

To analyze a complete real machine, it can be convenient to divide the system into sub-systems, analyzing each sub-system individually, and then, assembling them together in the whole system. Many of these sub-systems can be found in an automotive engine, being the hydrodynamic bearing one of the most common mechanical components present in all kinds of power generation systems. Journal bearings are linking elements between parts with relative motion, and these linking elements must work to support radial loads with minimal friction and power loss. In 1925, Stodola realized that the bearing is not a rigid support, but it works like a set of springs and dashpots whose characteristics have an expressive effect on the dynamical behavior of the supported rotating shaft. Consequently, to represent the bearings by equivalent coefficients of stiffness and damping became the basis of the journal bearings study, since those coefficients can easily be inserted in a finite element model of rotating systems supported by rigid or flexible structures. However, the most general analyses for bearings in the automotive industry are based in isothermal approaches. Besides that, it is well known that thermal effects are very important in certain operational conditions and specific applications of journal bearings, since the viscosity, parameter that characterize the fluid film, decreases with the temperature increasing. The thermo-hydrodynamic (THD) effect modifies the geometric equilibrium position of the shaft inside the bearing, which can lead to an expressive change in the stiffness and damping coefficients, depending on the reference temperature of the fluid and the shaft, the boundary conditions applied to the model, and finally, the range of rotational speed of the shaft. Therefore, the analysis of the THD model effect on the equivalent coefficients of stiffness and damping, considering a journal bearing in an automotive engine application, has been developed and the results are compared to the classical hydrodynamic solution.