Journal bearing impedance descriptions for rotodynamic applications. Academic Article uri icon

abstract

  • Bearing impedance vectors are introduced for plain journal bearings which define the bearing reaction force components as a function of the bearing motion. Impedance descriptions are developed directly for the approximate Ocvirk (short) and Sommerfeld (long) bearing solutions. The impedence vector magnitude and the mobility vector magnitude of Booker are shown to be reciprocals. The transformation relationship between mobilities and impedence are derived and used to define impedance vectors for a number of existing mobility vectors including the finite-length mobility vectors developed by Moes. The attractiveness and utility of the impedance-vector formulation for transient simulation work is demonstrated by numerical examples for the Ocvirk 'PI', and '2PI' bearing impedances and the cavitation finite-length-bearing impedance. The examples presented demonstrate both bearings and squeeze-film damper application. A direct analytic method for deriving a complete set of (analytic) stiffness and damping coefficients from impedance descriptions is developed and demonstrated for the cavitation finite-length-bearing impedances. Analytic expressions are provided for all direct and cross-coupled stiffness and damping coefficients, and compared to previously developed numerical results. These coefficients are used for stability analysis of a rotor, supported in finite-length cavitating bearings. Onset-speed-of-instability results are presented as a function of the L/D ratio for a range of bearing numbers. Damping coefficients are also presented for finite-length squeeze-film dampers. (A)

author list (cited authors)

  • Childs, D., Moes, H., & van Leeuwen, H.