seals & bearings


adapted for a range of underwater repair activities. The principle of the system is the creation of a localised dry working environment, using a flexible structure


installed around


the area requiring work. The replacement of sterntube seals is a prime application but the system is also suitable for in-situ repairs of bow or azimuth thrusters. For newbuild vessels, there has been considerable pressure to adopt shaft and seal systems which are designed to entirely eliminate oil loss to the environment. The Thordon Compac bearing system uses sea water for lubrication in place of lubricating oil, eliminating oil loss problems. To facilitate this, the propeller shaft bearings are non-metallic and the seawater feed is fully treated to remove any suspended particles which could cause abrasive damage to the bearings. Water is taken directly from the sea, and suspended solids with a specific gravity of 1.2 or higher, and greater than 80 microns, are removed. It is then pumped through the propeller shaft bearings and returned to the sea. As the loss of sea water is of no environmental consequence, the need for an aft sterntube seal is eliminated. The full system provided by Thordon includes


shaft liners,


a water quality package, a forward seal and bearings. The bearings are manufactured from an elastomeric polymer alloy with the lower area of the bearing being smooth and the upper area incorporating groove features for flow of the water lubricant/coolant, promoting the formation of a hydrodynamic film between the shaft and bearing. To minimise corrosion and wear, Thordon has developed a shaft coating system which provides 10-year corrosion protection against seawater damage. This is a two-part epoxy coating which Thordon says is significantly more flexible than existing equivalent shaft coating products.


Another selling point is that it takes away the need for the storage, sampling and disposal of oil. Thordon also offers a 15-year bearing wear life guarantee on the Compac shaft bearing system, which is already installed on 600 vessels worldwide. The system is also available as a retrofit, to replace standard oil-lubricated metal- bearing-based designs. Icon Polymer Group, based in Retford, England and owned by Wyatt International, specialises in the supply of


materials


friction effect which has to be overcome in order to start the shaft turning. These characteristics enable pumps, for example, to be operated dry for a short priming period. The low stiction also allows the use of smaller motors for turning propeller shafts in drydock, saving significantly on equipment costs. The latest


water-lubricated bearings can


the business of Kobelco-Eagle, Japan, offers water-lubricated


sterntube bearings and also


be offered in a specially formulated nitrile compound that offers minimum and increased life in high-silt water conditions.


Many global companies manufacture both oil-lubricated and water-lubricated bearings and associated seal systems. Suppliers such as Kemel, renamed in 2010 and continuing


other sterntube associated equipment, plus a biodegradable oil for users of oil-lubricated systems wishing to reduce their environmental impact. Other industry developments include the acquisition, in 2011, of Cedervall of Sweden by the Wärtsilä Corp of Finland. Cedervall has built a strong reputation as a supplier of water and oil-lubricated sterntubes and sterntube bearings. The Trelleborg Group’s Orkot marine bearing range includes many types of fabric-reinforced composite bearings which are used in stern tube, rudder and hatch slide pads applications,


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for


water-lubricated bearings. Working with chemists at Aston University, has resulted in the development of a compound combining standard nitrile rubber with PTFE polymer – two materials that were previously incompatible. The new material provides both a low coefficient of friction and reduced stiction – the generalised term for the initial


46 I Marine Propulsion I February/March 2012


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www.mpropulsion.com


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