Orkot Marine Bearings: Engineering Manual - Trelleborg.com.au

Orkot ® Marine BearingsEngineering Manual for Rudder and Water Lubricated Propeller Shaft BearingsWater Lubricated SternTube BearingsThis section provides general advice on stern tubebearing design. It will also provide the informationrequired to allow the user to calculate a stern tubebearings dimensions before and after fitting.Page 25 shows a completed example of the Orkot ®Marine calculation for you to examine. Page 23contains a blank for you to complete your owncalculation. Alternatively please feel free to contactyour local agent, distributor or manufacturingplant and we will be happy to advise you of themachining sizes.Prior to carrying out any calculation, the followinginformation must be available:● Housing and shaft sizes with tolerancesBearing Design PressureWater lubricated propeller shaft bearings need tooperate hydro-dynamically, i.e the shaft speed shouldbe sufficient to generate a water film to separate theshaft from the bearing. The design of these bearings isgoverned by the length to shaft diameter ratio whichcan vary from 4:1 to 2:1 depending on the bearingposition and upon approval from the classificationsocieties. Most classification societies apply a ratio of2:1 for Orkot ® Marine grades. Full copies of thecertificates are available upon request or from ourweb site: www.orkotmarine.comMaterial SelectionIt should be noted that Orkot ® materials are notsuitable for use in high speed grease or oil lubricatedstern shaft systems.Orkot ® TLM Marine is suitable for the majority ofwater lubricated stern tube systems.Orkot ® TXM Marine is especially suitable for lowshaft velocities i.e. naval surface ship and submarinestern shafts.Shaft RequirementsOrkot ® Marine bearings can be used with mostrecognised shaft materials and is found to becompatible with gunmetal, phosphor bronze, Monel,Inconel ® 625, 18/8 stainless steel.If the bearing is subject to abrasive ingress,consideration should be given to the use of hardershaft liners or carbide coatings. Orkot ® Marine tends toimprove the surface finish during running. As with allbearings subject to the ingress of abrasive particles,life can be reduced. For the stern tube bearings of avessel operating under such conditions a filtered watersupply is recommended.A shaft surface finish of 0.8 micrometers or 32microinches Ra is required to reduce bedding in wear.DesignA number of bearing designs can be manufacturedfrom Orkot ® Marine grades for stern tube bearingapplications. The types in order of popularity areas follows:Multi Groove BearingThis conventional bearing design has equi-spacedaxial grooves to allow water to circulate and cool thebearing and to enable debris to pass through withoutcausing damage to the shaft or bearing. This designis suitable for most vessels where shaft velocities arenot high.Twin Groove BearingThis bearing has two large axial grooves at 90 degreesto the static shaft loading position and allows superiorhydrodynamic performance over a wide range of shaftvelocities. It was developed from testing Orkot ® TLMMarine bearings on a purpose designed shaft testingrig. The design allows for a hydrodynamic film todevelop at low shaft velocities reducing friction andwear. Good hydrodynamic performance is obtainedwith shaft velocities as low as 25 m/min. (82 ft/min).This design should be limited to a maximum 300 mm(11.8") diameter and may also be used for verticalshaft water pumps for marine use.Figure 4: Twin Groove BearingSpecial Designs0.4W W = 0.055d + 6 (.24")[d = shaft dia.]R=0.3dSpecial designs for naval applications that incorporatepartial arc segmental bearings mounted in split bronzehousings are also available.10