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as well as in on-deck machinery. Orkot bearings, made in Rotherham, England, were selected for the rudders fitted to Mozah, the world’s largest LNG carrier, built by Samsung Heavy Industries in South Korea. Capable of carrying 266,000m3 of LNG, which its class society Lloyd’s Register says is almost 80 per cent more cargo than conventional ships, the 345m ship is fitted with a twin rudder system. The choice of Orkot TXMM material for the rudder stock and TLMM material on the pintle enables these rudders to operate without lubrication.


The TXMM material is an advanced synthetic composite which provides both long service life


and a low unlubricated friction The material


incorporates


co-efficient. solid lubricants,


and Trelleborg claims it has exceptional wear resistance, with virtually no swell in water and high-dimensional stability. Both bearing materials can also be used in conjunction with water and grease lubrication. Trelleborg also offers long service life water- lubricated designs for pollution-free propeller shaft bearings. “Service life of water-lubricated tail shaft bearings is influenced by bearing and liner wear and is extremely important on naval and merchant vessels where unscheduled docking


times are costly and affect fleet programmes,” says Barry Davies, general


An Orkot TXMM Mainshaft Bearing is made from machinable composite material


Survey & Inspection services UNDER WATER


Master Divers specializes in the provision of underwater survey and inspection services for ships, mooring systems, cables and a variety of other underwater structures. Underwater surveying is one of the core diving services we provide clients among a whole range of deliverables.


LR, ABS , GL, DNV, BV, NKK, KR, IRS Our equipment and services available for underwater survey and inspection include:


• Closed circuit colour television with diver voiceover recording facility. • Still photography (Digital). • Rudder pintle clearance.


• Propeller shaft poker gauge reading. • Underwater thickness gauging.


Master Divers (Pvt) Ltd. Maritime Centre, 234 / 2 Galle Road, Colombo - 03, Sri Lanka.


Email: hq@masterdivers.lk Phone : +94112 434564


www.masterdivers.lk Master Divers www.mpropulsion.com


Our experienced diving inspectors regularly carry out surveys approved by the following classification societies.


manager, Trelleborg Sealing Solutions. The vast majority of bearings used for sterntubes are still lubricated with oil and rely on the effectiveness of the sealing system to prevent oil leakage into the sea. Orkot TXMM bearing material offers a significant solution to this problem, particularly as the material has been developed to operate with unfiltered sea water. The Orkot TXMM segmented bearing system was also designed with a patented, removable locking system to allow easy removal of the bearing segments with the shaft and housing remaining in place in the vessel. This system is used on the UK Royal Navy Type 45 destroyer, the Queen Elizabeth class aircraft carriers and the Khareef class corvettes for the Royal Navy of Oman. The Orkot TXMM bearing uses two composite materials to give the required stiffness and self-lubricity for the low speed operation required on new generation vessels using direct electric drives. In these low shaft speed conditions, the normal hydrodynamic water film generated by shaft rotation is absent and trouble-free operation relies on the wear resistance and low friction factor of the bearing materials. While most bearings will generate a hydrodynamic film at higher speeds, the critical period for wear risk is when the water film is absent. Independent testing showed that the


wear rate for Naval Bronze and Inconel 625, both used on naval and other vessels, was extremely low when operating in sea water containing silica flour, as specified by the Royal Navy. A further important factor for wear is the alignment of the shaft and bearing, which may vary with shaft speeds due to propeller lift and other factors. This will affect the contact area and, therefore, the contact pressure on the bearing. Trelleborg Sealing Solutions developed


a


programme to assess the bearing pressure at static (cold) and running (warm) conditions and then modified the bearing by machining a slight taper. This allows elastic deformation


Marine Propulsion I February/March 2012 I 47


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