A Numerical Analysis of Optimum Air Journal Bearings

Abstract

The responses to changes of bearing length-to-diameter ratio and supply pressure of hybrid air journal bearings are investigated numerically for rotor dynamic instability. Different types of external pressure compensations, including multi-array of 1, 2, 3, 4, or 5-row orifice bearings and porous bearings, are analyzed to obtain more insight to optimum designs of journal bearings to improve the problem of whirl instability of rotor mass. The results show that the long porous bearings (L/D>1.0) in the higher rotation speeds (Λ>0.5) have higher threshold load capacities W before the onset of whirl instability and thus are more stable than orifice bearings. On the contrary, the short 5-row orifice bearings (L/D ≤ 1.0) are more stable than the porous bearing in the lower rotation speeds (0.1 ≤ Λ ≤ 0.5). The results also show that the change of supply pressure from Ps=2.0 to Ps=8.0 makes no difference to orifice bearing with whirl instability of rotor but that the porous bearing is the most stable for the lower supply pressure Ps=2.0 and becomes unstable as the supply pressure Ps increases in the higher rotation speeds (Λ>0.5).