Externally pressurised stern - gear

Abstract

Figures presented periodically by classification societies show, a major cause of ship casualty to be stem bearing failure. This is particularly the case for large fall bodied bulk carriers and tankers. Non-uniform wake distribution around the stem of vessels causes uneven and constantly varying loads upon the bearing. These loads are examined and it is concluded that they may cause lubricant film breakdown and seal failures. Examination of stem bearing failures generally confirms this view and several bearing case histories are presented Present designs of stem bearings are appraised with the conclusion that all recent advances in stem bearing design have been for the purpose of improving maintainability rather than reliability. Examination of the loading pattern and the basic design requirements of stem gear indicate that the provision of jacking oil would considerably improve stem gear reliability. This is particularly aimed at reducing the frequency of failure caused by turning gear operation. Experimental work in which oil film thickness measurements were taken on the aftermost bearing of a container ship is presented. This clearly demonstrates the problem of turning gear damage. Furthermore the shaft movements recorded at this plummer bearing show that considerable tallshaft lift (the full stem bearing clearance is taken up) occurs and that the tailskaft probably executes a closed loop under the action of the propeller. Design curves are given to show the lubricating oil pressures and quantities required. It is further postulated that, within the framework of existing classification society rules full hydrostatic lubrication would have even greater advantages. Design curves to a basis of shaft diameter are presented to enable clearance, pressure, lubricant flow, stiffness and basic dimensions to be derived by simple calculation. Experimental data on suitable materials are given and an overall material specification produced. Designs are given for three typical ship types and based upon these, cost comparisons made. Costs are such that an economic as well as technical case can be advanced for the use of hydrostatic stern bearings.