An Analysis for Floating Bearings in a Turbocharger

A comprehensive analysis has been performed for floating bearings applied in a turbocharger. It is found that Couette power loss for a full-floating bearing (the floating ring rotates) decreases with increasing inner and outer clearances, while its Poiseuille power loss increases with increasing inner and outer film clearances. In comparison with a semi-floating bearing (the floating ring does not rotate), a full-floating bearing can reduce both Couette and Poiseuille power losses. However, floating bearing is found to have a smaller minimum film thickness for a given dynamic loading from rotor-dynamics. The total power loss reduction for typical full-floating bearings ranges from 13% to 27%, which matches well with some published experimental data. In general, the speed ratio increases with increasing outer film clearance, while it decreases with increasing inner film clearance because of shear stresses on the outer and inner film. The increasing radii ratio slows the floating ring and therefore decreases the cavitation effect in outer film, while the cavitation level in the inner film is not significantly changed. Meanwhile the minimum film thickness in the inner film increases with the increasing speed ratio. It is found that the speed ratio reduces from 0.43 to 0.31 as the shaft speed goes from 15,000 to 60,000 rpm, which is very close to published experimental observations. The behavior of self-excited oil whirl of floating bearings is also modeled and reported. It is found that the eccentricity in the inner film decays or develops much faster than the one in the outer film.