The stern tube is a hollow tube located at the lower stern part of the ship carrying the tail shaft and connecting it to the propeller,bearing for the tail shaft, lubrication arrangement and sealing arrangement. The propeller shaft or tail shaft is supported in a stern tube bearing. The bearing being at the end of the shaft is affected by the overhanging weight of the propeller shaft. The propeller mass pulls the outer end of the shaft down, so that there is a tendency for edge loading of the stern tube bearing to occur.
Seawater lubricated stern tube bearings
The traditional stern bearing is water lubricated and consist of a number of lignum vitae staves held by bronze retaining strips in a gunmetal bush. Lignum vitae is a hardwood with good wear characteristics and is compatible with water. The staves in the lower part of the bearing are cut and fitted so that the end grain is vertical to give the longest possible life. The staves in the upper part are cut with grain in the axial direction for economy. The staves are shaped with U or V grooves between them at the surface to allow access for water. The grooves also allow debris. As an alternative to wood, rubber or Tufnol can be used. Stern tubes are supported at the after end by the stern frame boss and at the forward end in the aft peak bulkhead. Their cast iron construction requires strong support in way of the bearing from the stern frame boss. Seawater which enters at the after end or from the circulation system to cool and lubricate is an electrolyte which will support galvanic corrosion. Wastage of the vulnerable steel shaft is prevented by a shrunk on bronze liner and rubber seal sandwiched between the propeller hub and the liner end. The center of the stern tube is connected to a sea water service line which, together with the ingress of water between the shaft and bush, provides the cooling and lubrication.
Oil lubricated stern tube
Progress from seawater to early oil lubricated stern tubes involved an exchange of the wooden bearing in its bronze sleeve for a white metal lined cast iron or bronze bush. Oil retention and exclusion of sea water necessitated the the fitting of of an external face type seal. The stuffing box was retained in many oil early oil lubricated stern tubes, at the inboard end. In oil lubricated bearings the shaft does not require a full length protective bronze sleeve. The later designs of oil lubricated stern tube are fitted in a stern frame with an elongated boss to provide better support for the white metal lined bearing. A minimum bearing length of two times the shaft diameter will ensure that the bearing load does not exceed 0.8 N/mm2. Oil pressure within the stern tube is maintained at approximately the same level as that of the surrounding sea water by a header tank.
Heat produced by friction will result in hardening and loss of elasticity of the rubber, should temperature of the seal material exceed 110 degree Celsius. Cooling at the outboard end is provided by the sea. Inboard seals unlike the outboard seals cannot dissipate heat to the surrounding water. Oil circulation aided by convection is arranged to maintain the low temperature of the seals at the inboard end. Connections for circulation are fitted top and bottom between the two inboard seals and the small local header tank.