On the Vibrations of Horizontal Shaft Supported in Oil Lubricated Journal Bearings

Abstract

Based on a pressure solution for an infinitely long journal bearing clarified in the previous paper, the vibrations of a rotating shaft which is supported in oil lubricated journal bearings is investigated theoretically and experimentally. The results are as follows : 1) A system in which a rigid shaft is supported in oil lubricated journal bearings has in general an angular natural frequency slightly less than ω/2 (ω is angular velocity of rotating shaft). Although the free vibration with angular frequency about ω/2 is stable at low rotational speeds, it becomes self-excited when the rotational speed increases to a high enough value. 2) In the case of a flexible shaft, the system has three angular natural frequencies, one of which is slightly less than ω/2 and the others are nearly equal to Ω (Ω is a fundamental angular frequency of flexural vibration of the shaft). Only one of the vibrations with frequency Ω becomes unstable when the rotational speed increases above about twice the critical speed of the shaft.