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Stern tube Lubrication systems- Oil-lubricated stern tubes

Oil-lubricated stern tubes

Progress from sea-water to early oil-lubricated stern tubes involved an exchange of the wooden bearing in its bronze sleeve for a white metal lined cast iron (or sometimes bronze) bush. Oil retention and exclusion of sea water necessitated the fitting of an external face type seal. The stuffing box was retained in many early oil-lubricated stern tubes, at the inboard end. In oil-lubricated bearings the shaft does not require a full length protective bronze sleeve.

Simplex type stern tube

The later designs of oil-lubricated stern tube (Figure 8.15) are fitted in a stern frame with an elongated boss to provide better support for the white metal lined bearing. A minimum bearing length of two times shaft diameter will ensure that bearing load does not exceed 0.8 N/mm2 (116 lbf/in2). The bearing bush is normally grey or nodular cast iron centrifugally lined with white metal. A typical analysis of white metal would be 3% copper, 7.5% antimony and the remainder tin. White metal thicknesses vary according to the classification society. Figures of 3.8 mm for a shaft of 300 mm diameter to 7.4 mm for a 900mm diameter shaft have been quoted, with bearing clearances of 0.51-0.63 mm and 1.53-1.89 mm respectively.



The forward part of the stern tube is fabricated and welded direct to the extension of the stern frame boss and into the aft peak bulkhead. Oil pressure within the stern tube is maintained at approximately the same level as that of the surrounding sea water by a header tank. Oil is contained within the Simplex type stem tube by lip seals. The elastic lip of each nitrile rubber seal, grips a rubbing surface provided by short chrome steel liners at outboard and inboard ends of the steel propeller shaft. The outboard liner additionally protects the steel shaft from sea-water contact and corrosion.

Heat produced by the friction will result in hardening and loss of elasticity of the rubber, should temperature of the seal material exceed 110C. Cooling at the outboard end is provided by the sea. Inboard seals, unlike those at the outboard end, cannot dissipate heat to the surrounding water. Oil circulation aided by convection, is arranged to maintain the low temperature of the seals at the inboard end. Connections for circulation, are fitted top and bottom between the two inboard seals and the small local header tank.

The chrome liners act as rubbing surfaces for the rubber lip seals but grooving from frictional wear has been a problem. The difficulty has been overcome by using a ceramic filler for the groove or alternatively a distance piece to displace axially the seal and ring assembly. Allowance must be made for the relative movement of shaft and stern tube due to differential expansion. New seals are fitted by cutting and vulcanizing in position.

Oil lubricated stern tube
Figure 8.15 Oil lubricated stern tube (Simplex type)

The static lubrication system for vessels with moderate changes in draught, have header tanks placed 23 m above the maximum load waterline. The small differential pressure ensures that water is excluded. The cooling of simple stern tubes, necessitates keeping the aft peak water level at least 1 rrt above the stern tube.

Tankers and other ships with large changes in draught, may be fitted with two oil header tanks (Figure 8,22) for either the fully loaded or ballast condition.

Single-bush bearing showing also a forced lubrication system
Figure 8.22 Single-bush bearing showing also a forced lubrication system (Glacier Metal Co.)


Hydrodynamic or hydrostatic lubrication

The requirement for steaming at a slow, economical speed during periods of high fuel prices (or for other reasons) gives a lower fluid film or hydrodynamic pressure in stem tubes, due to the slower speed. The possibility of bearing damage occurring prompted the installation of forced lubrication systems to provide a hydrostatic pressure which is independent of shaft speed. The supplied oil pressure gives adequate lift to separate shaft and bearing and an adequate oil flow for cooling.



Summarized below some of the basic procedure of marine propeller shaft :
  1. Propeller shaft materials and couplings

  2. The intermediate shafting and the propeller shaft for a fixed propeller are of solid forged ingot steel and usually with solid forged couplings. Shafts are machined all over but of a larger diameter and smooth turned in way of the bearings. ......

  3. Fixed pitch propeller

  4. The normal method of manufacture for a fixed pitch propeller, is to cast the blades integral with the boss and after inspection and marking, to machine the tapered bore and faces of the boss before the blades are profiled by hand with reference to datum grooves cut in the surfaces or with an electronically controlled profiling machine. ......

  5. Controllable pitch propeller

  6. Controllable pitch propellers are normally fitted to a flanged tailshaft as the operating mechanism is housed in the propeller boss. As its name implies, it is possible to alter the pitch of this type of propeller to change ship speed or to adjust to the prevailing resistance conditions. ......

  7. Propeller thrust block

  8. The main thrust block transfers forward or astern propeller thrust to the hull and limits axial movement of the shaft. Some axial clearance is essential to allow formation of an oil film in the wedge shape between the collar and the thrust pads ......

  9. Propeller shaft gears and clutches

  10. For medium-speed engine installations in large ships (as opposed to coasters or intermediate sized vessels) reduction gears are needed to permit engines and propellers to run at their best respective speeds. Their use also permits more than one engine to be coupled to the same propeller. Gearboxes are available from manufacturers in standard sizes. ......

  11. Propeller shaft check

  12. The intention of good alignment is to ensure that bearings are correctly loaded and that the shaft is not severely stressed. Alignment can be checked with conventional methods, employing light and targets, laser or measurements from a taut wire. ......

  13. Propeller shaft bearings check

  14. The intermediate shafting between the tailshaft and main engine, gearbox or thrustblock may be supported in plain, tilting pad or roller bearings. ......

  15. Oil lubricated stern tube

  16. Progress from sea-water to early oil-lubricated stern tubes involved an exchange of the wooden bearing in its bronze sleeve for a white metal lined cast iron (or sometimes bronze) bush. Oil retention and exclusion of sea water necessitated the fitting of an external face type seal. ......

  17. Water lubricated stern tube

  18. The traditional stern bearing is water-lubricated and consists of a number of lignum vitae staves held by bronze retaining strips, in a gunmetal bush. Lignum vitae is a hardwood with good wear characteristics and is compatible with water. ......

  19. Stern tube sealing arrangement

  20. There are basically three sealing arrangements used for stern bearings. These are: Simple stuffing boxes filled with proprietary packing material. Lip seals, in which a number of flexible membranes in contact with the shaft, prevent the passage of fluid along the shaft. & Radial face seals, in which a wear-resistant face fitted radially around the shaft, ......

  21. Stern tube bearings

  22. To avoid the necessity for drydocking when an examination of stern bearings amid tailshaft is needed, split stern bearings were developed. A suitable outboard sealing arrangement and design, permits the two halves of the bearing to be drawn into the ship, exposing the shaft and the white metal bearing. ......



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