Marine Water Lubricated Stern Tube Bearings: Design and Operation Problems

Faculty of Ocean Engineering and Ship Technology of Gdansk University of Technology (Poland) co-operates with ship industry and some well known manufactures of water lubricated bearings. In the last few years, three various test stands for research work with water lubricated main shaft bearings were designed and built. They were used for many different tests with various kinds of bearings. The testing process helped to understand and prove some phenomena. Water lubricated stern tube bearings are often used on modern ships. This environmentally friendly bearing is much cheaper than popular oil lubricated one. The difference in price is a result of simplicity of the bearing unit and lower material costs. Unfortunately, there were many serious breakdowns associated with this kind of bearing. Sometimes the wear process was very rapid. It was mainly a result of problems with stability of the hydrodynamic film, poor material properties or unfavorable working conditions. Theoretically, water lubricated stern tube bearings operate in hydrodynamic regime. Loads are low because of high bearing length to shaft diameter ratio (L/D). Sliding speed depends on engine type, main gear ratio and ship size. Generally, the speed could be even higher than 3 meters per second, which should be sufficient for hydrodynamic lubrication. So why are there problems with this kind of bearings? There are three main groups of reasons — design flaws, ship specific problems and negative influence of sea environment.