Abstract

Water lubricated propeller shaft bearings are frequently employed on modern ships due to their advantages such as durability, simplicity and low price. Specific working conditions on ships cause shaft misalignment which often results in rapid wearing of bush and shaft. Shaft misalignment could be an effect of manufacturing or assembly faults but sometimes it appears due to hull or shaft deformation. The wearing process has influence on bearing hydrodynamic properties because of significant changes in bush geometry. In calculating pressure distribution and load capacity an EHL model was used. In addition, the bush wearing module described in previous works was applied. Calculations were conducted for bearings with length to diameter ratio of 2 (shaft diameter 100 mm and length 200 mm). This is a typical ratio for modern materials accepted by classifications societies for stern tube bearings. The assumption that minimum film thickness equals 5 µm was employed in calculating the maximum bearing hydrodynamic capacity. At every step of the proposed EHL+W model calculation are conducted for progressively increasing values of eccentricity. When the lubrication film thickness falls below the assumed 5 µm a layer of bush surface is removed simulating the wearing process. Frequently when significant shaft misalignment appears, the bearing is working in mixed lubrication regime. In such a situation, the wearing process which starts can progress especially rapidly for stiff composite materials with modules of elasticity equal to approximately 4000 MPa. This effect appears due to significant stress concentration on bush’s edge. The calculations model assumed that the edge load results in wear and change in bush geometry. Modification of the bearing interspace cause change in hydrodynamic properties of the bearing. The results of calculated bearing hydrodynamic capacity as a function of wear are presented in the paper.

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