Shafting Systems without a Forward Stern Tube Bearing: Advantages and Prevention of Failures

Abstract

In recent years shafting systems, particularly those powered by slow-speed diesel engines, have been designed with only one bearing supporting the propeller shaft (typically an aft stern tube bearing). Designs with only one stern tube bearing provide for a system with more flexibility, easier installation, and a reduction in cost. With no forward stern tube bearing, there are different alignment characteristics that must be considered. Of importance is the significant effect of the aft intermediate shaft bearing position on the relative shaft slope in-way-of the aft stern tube bearing. The lineshaft bearing can be offset sufficiently to cause excessive relative shaft slope in the bearing, resulting in poor load distribution, excessive bearing pressure and subsequent failure. To provide more assurance that this relative shaft slope is acceptable with the vessel afloat and the shafting completely assembled, the strain gauge alignment measurement technique can be used. This paper describes the advantages of having only one propeller shaft bearing, the measurement technique used to reduce the risk of bearing failure due to excessive pressures, and examples of the application of this technique.