Stability and Unbalance Response of Large Turbine Bearings

Different bearing types frequently used in turbine practice are studied for both stability and unbalance response by linear and nonlinear analysis. The bearing configurations studies are: plain circular, three-axial groove, elliptical, three-lobe, and the three-pocket type. For the film force calculation, the Reynolds equation was solved by finite difference techniques using the effective viscosity concept. The bearing unbalance response was investigated for two values of out-of-balance loads: small unbalance which corresponds to an inaccurate balancing of rotors, and large unbalance representative of some emergency conditions in turbomachineries (blade loss, for example.) In the nonlinear analysis, the effect of energy dissipation in the film by the dynamic motion on the effective viscosity was investigated. The principal results show that all parameters which stabilize the bearing increase the bearing sensitivity to large unbalances. The bearings are ranked as follows in the order of increasing resistance to unbalance loading: three pockets, three-axial groove, offset three lobe, elliptic, circular. A quite different classification was obtained for the stability characteristics where the circular bearing was one of the least stable bearings.