The Subsynchronous Dynamic Behavior of Tilting-Pad Journal Bearings

Tilting-pad journal bearings are being increasingly used to avoid bearing instability problems. This paper describes the theory and results from a design program which has been developed for tilting-pad journal bearing calculations. The present approach is based on the numerical solution of the Reynolds equation using a one-dimensional finite element technique which considers temperature and viscosity changes and can also take into account the effect of turbulent flow. Computed results of dynamic stiffness and damping coefficients show very good agreement with data from the literature. The effects of various factors which may influence the bearing dynamic characteristics are discussed. For bearings with small preloads and operating at high Sommerfeld numbers, the effective damping at subsynchronous frequencies is considerably lower than that predicted for synchronous vibration. The stiffness is also affected by frequency. This frequency effect is attenuated by increased bearing preloads and offsets. Such factors have important consequences on the stability of high speed turbomachinery supported by tilting-pad journal bearings.