Technical and Ecological Aspects of Water-lubricated Stern Tube Bearings

Abstract

A ship's propulsion system has a significant impact on the ship’s energy efficiency, environmental friendliness, reliability and safety. An indispensable part of a propulsion system with mechanical power transmission is a stern tube with bearings. During operation, the sliding bearings material is subjected to stresses caused by a force proportional to the total mass of the shaft and propeller. The reliability of stern tube bearings is particulary important for safety, and their durability has a significant impact on maintenance costs. Depending on the lubricant, there are bearings that are lubricated with oil or water. The permissible bearing load with hydrodynamic lubrication depends primarily on the material, the viscosity of the lubricant used and the shaft speed. Metals and their alloys with a low coefficient of friction are used for oil-lubricated bearings. Bearings lubricated with water are made of a special type of hardwood ("tree of life", lat. lignum vitae), rubber or various synthetic materials. The main advantages of oil-lubricated bearings are higher allowable load and durability than water-lubricated bearings. This paper analyses the technical and environmental aspects of the application of water-lubricated bearings whose main advantages are simplicity, better cooling and environmental protection.