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

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Orkot ® Marine BearingsEngineering Manual for Rudder and Water Lubricated Propeller Shaft BearingsMachining ToleranceThe following tables show the machining tolerancesOrkot ® Composites machine shop can achieve whenmachining stern tube bearings. These can besubstituted in the calculation with your own machiningtolerance should they differ from those stated.Warning: Please note that the Orkot ® MarineBearing calculation is set up as standard to calculaterudder bearing dimensions. As such the standardmachining tolerance figures given must be changedto those stated in this section.Once machining tolerance values have been selectedthey can be entered into the Marine BearingCalculation. All of the required element of thecalculation should now be in place and the finalfigures can now be calculated.Table 12: Recommended MachiningTolerance OD and ID for Orkot ®Propeller Shaft BearingsDiameter OD and ID Tolerance Bandmmmm1 - 100 0.10101 - 300 0.20301 - 500 0.30501 - 900 0.40Table 13Diameter OD and ID Tolerance Bandinchesinches0 - 4 0.0044 - 12 0.00812 - 20 0.01220 - 36 0.016Note:1. These tolerances are for guidance only, reducingthem will reduce the maximum clearance afterfitting. It is important that out of roundnesstolerance is maintained when chockingcompounds are used.2. Propeller shaft bearings are longer than rudderbearings; longer lengths require larger tolerances.13
Orkot ® Marine BearingsEngineering Manual for Rudder and Water Lubricated Propeller Shaft BearingsFitting MethodsOrkot ® Marine Bearings can be fitted using any one ofthe following methods:● Freeze fittingMethod 1:Method 2:Method 3:● Press fittingUsing liquid nitrogen(immersion method)Using liquid nitrogen(vapour method)Using dry ice and alcohol● BondingOur preferred method is to freeze fit using liquidnitrogen. However descriptions of all methods can befound in this section.Freeze FittingThis is a fast and efficient assembly method for anOrkot ® Marine bearing. The thermal properties of thematerial allow a good clearance between the bearingand housing when frozen and the material does notbecome brittle at cryogenic temperatures.Note:Extreme care should be taken when using liquidnitrogen to avoid severe burns. Adequate ventilationshould be provided because oxygen is depleted whengassing occurs in confined spaces, suppliers of theproducts will provide a data sheet advising on its use.Method 1:Using liquid nitrogen (immersion method)A suggested procedure for method 1 is as follows:1. Check the od of the bearing in at least 3 positionsaround the top, middle and bottom (a total of 9measurements) making a note of the largestfigure recorded.2. Check the id of the housing in at least 3 positionsaround the top, middle and bottom (a total of 9measurements) making a note of the smallestfigure recorded.3. Provide an insulated container capable ofwithstanding a temperature of -197°C (-320°F) andlarge enough to accommodate the bearing beingfitted with enough clearance on the od tofacilitate the insertion and removal of the bearing.4. Place the bearing inside the container and makeefforts to reduce its internal volume. This can bedone by sealing off un-used areas of the containeror filling any voids with rough cut timbers. Thiswill reduce the amount of liquid nitrogen that willbe required.5. Cover the bearing with the liquid nitrogen andmaintain this level for the duration of theprocedure. The nitrogen level will constantly dropas the liquid boils, turns to gas and escapes to theatmosphere. Use an insulated lid to cover thecontainer when possible. Once the liquid stopsboiling and settles down to a simmer then thebearing can be lifted slightly from the liquid andthe upper od measured to check for sufficient sizereduction. If this has not been achieved then thebearing can be returned to the liquid for 10 to 20minutes and then checked again6. Once sufficient clearance between the bearing andthe housing has been achieved then the bearingcan be removed from the nitrogen andtransported to the housing for fitting.7. The apparatus used to transport the bearing afterfreezing will need to be resistant to the cryogenictemperatures they will encounter (e.g. polyesterslings) and suitable to support the weightsinvolved.8. Ensure that the reduced bearing can be locatedquickly and easily. Once it comes into contact withany conductive surfaces the rate with which it willreturn to its original dimensions increases greatly.9. Slide the bearing into position, ensuring that it isheld there while its temperature normalises. Oncethe bearings surfaces have cleared of the ice thatforms on them during normalisation then anysupports can be removed.Method 2:Using liquid nitrogen (vapour method)Orkot ® bearings can be fitted without a metalcontainer by vaporising the liquid nitrogen within thebearing. This uses less liquid nitrogen and is thereforesafer and less expensive than the immersion method.Please note however that the rate of bearingcontraction is a lot lower using this method whencompared to the immersion method. As such thefreezing procedure will take a lot longerA suggested procedure for method 2 is as follows:1. Check the od of the bearing in at least 3 positionsaround the top, middle and bottom (a total of 9measurements) making a note of the largestfigure recorded.2. Check the id of the housing in at least 3 positionsaround the top, middle and bottom (a total of 9measurements) making a note of the smallestfigure recorded.3. Prepare a plywood disk with an od half waybetween that of the od and the id of the bearing.Drill a 15 / 20mm diameter hole in the middle ofthis disk.4. Place the bearing on a smooth, flat, none poroussurface. Seal the joint between this and the id ofthe bearing with silicone sealant.14
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