Sea-Water lubricated Stern-tube bearing :
The traditional stern bearing is water-lubricated and consists of a number of lignum vitae wooden staves held by bronze retaining strips, in a gunmetal bush. Lignum Vitae is a hardwood with good wear characteristics and is compatiable 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. Staves in the upper part are cut with grain in the axial direction for economy. The staves are shaped with V or U grooves between them at the surface, to allow access for water. The grooves also accommodate any debris.
Impregnated plastic resin compounds on plastic type bases have been used successfully in place of lignum-vitae. One such type is called Tufhol. This is a thermo-setting laminate produced from cotton fabric and phenolic resin as the main constituents. The fabric is impregnated with the resin and the layers of this impregnated material are pressurized under heat until the fabric laminations are bonded into one sheet. This material gives uniform density, hardness and swelling together with good wettability and low coefficient of friction plus an ultimate compressive strength (flatwise) approximately twice that of lignum-vitae. A coefficient of friction of 0.005 when water lubricated can be achieved and short-length diameter ratios with a greater continuous bearing surface could be designed at very high loadings with this material. A water supply to the inboard end of the bearing is essential and grease and oils should never be used.
Normally the bearing length is equal to four times the shaft diameter. Wastage of the vulnerable steel shaft due to sea water cooling and corrosion is prevented by a shrunk on bronze liner fitted on the shaft and rubber seal sandwitched between the propeller hub and liner end. It is essential that the rubber has freedom to flow when nipped between the hub and liner. During drydock inspection, bearing weardown is measured by-poker gauge or by inserting a wedge between the shaft and bearing from the outside. The permissible wear is in the region of 9-12 mm on large diameter shafts.
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