Bearings in tailspin
01 Jun 2000
Aurora?s untimely return to Southampton put tailshaft bearings in the headlines again. The tailshaft bearing problems causing Aurora to abandon its maiden voyage is the latest in a series of high profile failures. Twin propeller vessels have figured highly in the casualty list, and while Aurora has whitemetal bearings, others have been fitted with composite bearings. The reasons for these problems are unclear and could be different in each case. Also many in the cruise industry are switching to podded propulsion (see feature this issue). John Carlton from Lloyds? Register feels that twin-screw vessels pose particular problems for the bearings ? especially during a turn. This causes alterations in the loading in the sterntube bearings: the shaftline on the outside of the turn is pushed downwards into the bearing with a force much greater than when going straight ahead. The propeller on the inside of the turn has forces acting laterally and this results in the shaft trying to ?climb? the side of the bearing ? again with forces higher than during the straight ahead position. Phil Cumberlidge, sales director with composite bearing manufacturer Railko, says this is a known phenomenon on twin-screw vessels. To prevent the shaft rotating on the edge of the oil ways, it positions these at the 2 and 10 O?clock (against 3 and 9 O?clock on single screw). As to the increased forces, he says that the material is operating with a factor of safety approaching 100. As with the engine problems (The Motor Ship, April 2000), it could be that the sterntube bearing failures are the result of a problem rather than the cause. Many argue that the drive to make cheaper and lighter vessels has lead to more flexible hulls ? although class does not necessarily agree. What is evident is that the increase in speed for many merchant vessels means higher engine outputs that require larger diameter driveshafts. In bulk carriers, oil tankers and the like this means the driveshaft is short and stiff. In the past it could be that smaller diameter driveshafts would flex to accommodate hull deflection, for instance between laden and ballast conditions, but this may not happen with today?s larger shafts. In many ways cruise ships have the opposite problem, with what Mr Carlton calls "shaftlines like spaghetti." Tests by Lloyds showed that the ?A? bracket at the end of the sterntube on a cruiseship moved by 1mm when a load of 120t was applied. Forces pushing the ?inside? propeller sideways during turns can easily reach 100t (depending on speed, severity of turn, and the hull design) says Mr Carlton. It could be that this final degree of misalignment causes problems, and this is supported by the fact that many of the problems affect just one shaft. If there were an inherent fault with the bearings or the design this would cause problems for both shafts. On an 850mm shaft the clearance in a Railko bearing is 1.7mm over a length of 1.7m ? so even small misalignments can lead to edge loading. With whitemetal bearings the clearances are lower (around 1.3 ? 1.4mm for an 850mm shaft) but the length is similar. So if the hull flexes by a few tenths of a millimetre between the engine and the sterntube, problems could result and as the forward bearing is shorter, this is likely to manifest itself in the aft bearing. While all agree that during the normal operation of a ship a degree of misalignment is inevitable, they disagree about the abilities of whitemetal and composite bearings to accommodate this. The lower clearance with whitemetal means there is less tolerance for misalignment before metal to metal contact is inevitable. Smaller clearances are allowed by class because the whitemetal does not swell when in contact with seawater and it dissipates heat through the bearing wall ? unlike a composite material. Mr Cumberlidge says Railko bearing material will deform to accommodate a degree of misalignment. He says for a given degree of misalignment the contact area is five times longer with the Railko bearing than with whitemetal. Don Cassidy from Thordon says its material is even softer and so even better able to cope with misalignment. However, Danny Schleicher from IHC Lagersmit says you can scratch whitemetal bearings with your fingernails but not a composite. Adding that if edge loaded, the whitemetal will heat up and melt in that area to solidify further round the bearings. By this method the bearing will take up the correct alignment. Compounding the effect of misalignment is the effect it has on the hydrodynamic lubrication. Work done by LR shows that if the shaft and bearing surfaces are not parallel, the fluid film becomes wedge shape and the pressure distribution is in non-linear. This is especially the case with water lubrication because of the low viscosity. Mr Cumberlidge says tests by Shell in the mid 1970?s showed that the slightly ?fluffy? nature of the Railko bearing traps oil and allows the bearing to generate hydrodynamic lift at lower speeds. This may not be the case for water lubricated versions, admits Mr Cumberlidge. He says Railko bearings can operate outside of the hydrodynamic region (for limited periods) as oil is impregnated in the material itself. This makes it suitable for vessels like shuttle tankers holding station against a tide with a propeller speed of three or four rev/min ? this cannot be done with whitemetal. Mr Schleicher has the other view, saying that the smooth nature of whitemetal bearings promotes the formation of hydrodynamic lubrication, which it needs to run correctly. Where other factors are not causing stress on the sterntube bearing, either system can cope and many shipowner?s have good experience of whitemetal. The ultimate advocate for composites remains the fact that they are not damaged by seawater and if a sterntube seal fails, the vessel can continue to the next port under its own power. What is clear is that misalignment is becoming a bigger problem and can ruin a sterntube bearing whether it is composite or whitemetal. It may be that flexible couplings in the system could be a solution. As it is, P&O?s David Strawford sums up the current situation by saying "perhaps we are asking the bearing manufacturers to do the impossible."