Axial Oil Film Rupture in High Speed Journal Bearings and Its Effect on the Rotordynamic Coefficients.

为研究高速径向轴承的轴向油膜断裂，设计并制作了一台试验台。使用安装在测试轴承上的电容式间隙传感器来检测油膜破裂。在该试验台上，发现滑动轴承的高速运转导致了轴膜破裂的发生，建立了预测轴膜破裂行为的理论模型。这个数学模型是一个修正的雷诺方程，它包含了作用在振动膜上的离心力的影响，传统的润滑理论认为离心力的影响可以忽略不计。实际上，由于高速径向轴承的高旋转运行，这一点并不小。当气膜压力因离心力而降低时，溶解的空气出来并在气膜中凝聚，导致气膜轴向破裂。由于轴颈的高速旋转，轴颈的偏心比可以合理地假设为几乎为零，然后计算区域可以简化为由油膜厚度和轴承轴线定义的二维平面。得到了各种设计变量如轴承长径比、间隙比、供油压力等对油膜局部破裂的影响，得到了轴承平衡状态下轴颈小位移或小速度时部分破裂油膜的转动动力学系数。对高速浮动瓦轴承支承的Jeffcott转子的稳定性曲线进行了分析，发现该理论模型能够合理地解释有轴承的实际机械的振动现象。

A test rig was designed and made to investigate axial film rupture in high speed journal bearings. Capacitance-type gap sensors installed in the test bearings were used to detect the film rupture. With this test rig, it was found that high speed operation of journal bearings resulted in the occurrence of axial film rupture.A theoretical model was derived to predict the behavior of the axial film rupture. This mathematical model is a corrected Reynolds Equation which incorporates the effect of centrifugal force acting on the libricating film which conventional lubrication theories assume to be negligibly small. Actually it is not small for high speed journal bearings due to high rotational operation. When the film pressure decreases due to the centrifugal force, resolved air come out and coalesce in the film, resulting in axial film rupture. The eccentricity ratio of the journal can be reasonably assumed to be nearly naught because of very high speed rotation of the journal, then the calulation domain can be reduced to a two dimensional plane defined by the film thickness and the bearing axis. The effects of various design variables, such as bearing length to diameter ratio, clearance ratio, supply oil pressure and so on, were obtained.The rotordynamic coefficients of the partially ruptured oil film were obtained for small displacements or small velocities of the journal at equilibrium in the bearing. The stability charts for a Jeffcott rotor supported in high speed floating bush bearings were analyzed, and this theoretical model were found to reasonably explain the vibration phenomena of actual machines with the bearings.