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

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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