Performance prediction of journal bearing with fluid slip regions on part of lubricated surface

Abstract

Rotating machineries supported in journal bearings are operated at a rotational speed between the one where the minimum oil film thickness is ensured to prevent direct contact of the journal and bearing surfaces and the one where self-excited vibration occurs, while avoiding the critical speed of the rotor-bearing system. When a rotating shaft is supported in a cylindrical journal bearing, which has the simplest structure among journal bearings, the upper operational speed will be approximately twice the critical speed. In the present report, focusing on a rotating shaft supported in the bearing with fluid slip regions on part of the bearing surface, those speeds are numerically calculated by applying a hydrodynamic lubrication theory, then the effects of the fluid slip region on the range of stable operation are evaluated. It is found as follows; (1) the detachment speed of the journal from the bearing is not affected by whether or not the fluid slip regions are given, (2) the stability threshold speed is significantly enhanced when several fluid slip regions are given at intervals up to the apex of the bearing downstream from the direction of attitude angle at the detachment speed, and (3) the enhanced stability caused by the fluid slip region is attributed to a reduction in oscillating journal motion. The range of the safe operation of a rotating shaft can be significantly extended simply by giving fluid slip regions on a part of the bearing surface, without replacing complicated journal bearings or employing stabilization method such as adjusting the amount of oil supplied.