Study on Reduction of Oil Content in Porous Journal Bearings with Running Time and Effect of the Reduced Oil Content on Lubrication Characteristics

在实际应用中，多孔轴承通常仅由其结构内最初提供的油润滑。随着运行时间的延长，含油量的减少主要是由于泄漏造成的，油损导致摩擦系数等性能特性的变化。本文从理论上和实验上研究了多孔基质中含油量的降低及其对摩擦特性的影响。通过同时求解包含过滤项的修正雷诺方程和由达西定律导出的拉普拉斯方程，计算了油膜和多孔基质中的压力分布。采用单元法和流体体积法对这些方程进行离散化处理。VOF方法适用于含油量降低后渗透到多孔基质中的油与空气交界面区域的离散化。用逐次过松弛法求解了离散化后的差分方程。采用基于动量定理的积分动量方程和基于多孔介质中油损失率与轴向端通过轴承间隙的漏油流量平衡的连续性方程，给出了油膜压力的周向边界条件。采用小直径均匀填充的玻璃球作为多孔基体的模型轴承进行实验，观察随着运行时间的推移，油还原发生的区域及其模式向多孔基体扩散的情况。结果表明：(1)油膜减油发生在油膜范围尾端附近的位置。多孔基质中的油还原区随运行时间向径向和轴向旋转方向扩散，实验结果与数值分析结果定性一致。(2)计算得到的摩擦系数随着多孔基质含油量的减小而增大。(3)多孔基质中含油量的降低速率随着多孔基质渗透率的增大而增大。渗透率的影响是由实验结果和计算结果得到的。少

In actual applications, porous bearings are usually lubricated only by the oil initially provided within their structures. With running time, the oil content decreases mainly due to leakage and the oil loss yields variation of performance characteristics such as the frictional coefficient. In the present study, the reduction of oil content in the porous matrix and its effect on the frictional characteristics were investigated theoretically and experimentally. The pressure distributions in the oil film and the porous matrix were calculated by simultaneously solving the modified Reynolds equation including a so-called filter term and the Laplace equation derived from Darcy's law. These equations were made discrete using the cell method and the VOF (volume of fluid) method. The VOF method is applicable to discretization of the region including a boundary surface between oil and air which penetrates into the porous matrix with reduction of oil content. The difference equations derived from … More this discretization were solved by successive over-relaxation method. The circumferential boundary conditions for the oil-film pressure were given by the integral momentum equation based on the momentum theorem and the continuity equation based a balance of the oil -loss rate in the porous matrix and the oil leakage flow rate from the axial ends through the bearing clearance. Experiments were also conducted using a model bearing whose porous matrix was made of the packed glass spheres having small uniform diameter to observe the region where the oil reduction occurs and its pattern spreading into the porous matrix with running time.The results were summarized as follows :(1) The oil reduction occurs at the position near the trailing end of the oil-film extent. The oil reduced region in the porous matrix spreads to the radial direction and to the direction of the shaft rotation with running time, whose patterns obtained by the experiment and by the numerical analysis qualitatively agrees with each other.(2) The coefficient of friction obtained from the calculation increases with the reduction of oil content in the porous matrix.(3) The reduction rate of oil content in the porous matrix increases with the permeability of the porous matrix. This effect of the permeability is obtained both from the experimental results and from the computed results. Less