High speed hydrodynamic/hydrostatic journal bearings have been widely used in various types of rotating machinery. In space applications, the use of cryogenic fluids as working lubricants has become significant. The primary goal of this paper is to model the nonlinearities that occur in a cryogenic, liquid oxygen (LO2) hydrodynamic journal bearing. They will be examined through bearing-fluid-film pressure distribution and linear and non-linear bearing stiffness and damping characteristics.
The numerical model that couples a variable property Reynolds' equation with the dynamics of the rotor is solved by means of a finite difference solution technique. The procedure involves an iterative scheme that solves the coupled Reynolds' and liquid oxygen property equations for fluid film pressure. The pressure curve is then integrated to calculate bearing support forces. Continuing the iteration loop a two-dimensional Newton-Raphson iteration method is used to locate the journal equilibrium position from which both linear and non-linear bearing stiffness and damping characteristics are evaluated using small perturbation techniques. Using a typical plain journal bearing as an example, we shall parametrically analyse the effects of input variables such as the lubricant inlet temperature, external load, angular rotational speed, and shaft/bearing axial misalignment on the axial and circumferential pressure distribution and the non-linear behaviour of the bearing stiffness and damping.
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