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Stern tube sealing arrangements- Marine propeller shaft Guideline

There are basically three sealing arrangements used for stern bearings. These are:
  1. Simple stuffing boxes filled with proprietary packing material.
  2. Lip seals, in which a number of flexible membranes in contact with the shaft, prevent the passage of fluid along the shaft.
  3. Radial face seals, in which a wear-resistant face fitted radially around the shaft, is in contact with similar faces fitted to the after bulkhead and to the after end of the stern tube. A spring system is necessary to keep the two faces in contact.

The Ross-Turnbull split stern bearing
Figure : The Ross-Turnbull split stern bearing

Figure 8,17 General arrangement of Ross-Turnbull Mark IV bearing (Ross Tumbull Ltd.)

Lip seals

A lip seal assembly (Figure 8.18) consists of a number of nitrile rubber rings of special cross-section (Figure 8.19) sandwiched between bronze rings. Each individual rubber lip seal is held in contact with a renewable sleeve fitted to the shaft by its elasticity and a garter spring. The rings are renewed by cutting and then vulcanizing the ends in situ. The Simplex stern tube (Figure 8.15) has a forward seal with two rings and an after seal with three rings.

A larger sketch of the after seal (Figure 8.18) shows how seals are built up from three basic assemblies namely the flange, intermediate, and cover rings and these parts can be used for either seal. It will be noticed that the garter spring holding the sealing ring against the shaft is located aft of the ring anchoring bulb in the case of both forward sealing rings.

In the case of the after seal the two outboard sealing rings have their garter springs located aft of the ring anchoring bulbs while the inboard ring has its garter spring located inboard of the anchoring bulb. Lip seals will accept misalignment but a floating ring design was introduced by one manufacturer.

In some instances four or more sealing rings are installed. These are arranged so that one ring does not normally run on the shaft liner. In the event of leakage from the working seals, adjustment is made to bring the reserve ring into play,

Lip seal assembly
Figure : Lip seal assembly

Nitrile rubber ring cross-section
Figure : Nitrile rubber ring cross-section

Radial face seal

An example of a radial face seal, is shown in the general arrangement (Figure 8,20) and a detailed sketch (Figure 8.21) of a Crane seal. One of the principal features of the design and construction of this type of seal is the split construction of all component parts. This facilitates installation, and subsequent inspection and maintenance.

The function of sealing against leakage around the shaft is effected by sustaining perfect mating contact between the opposing faces of the seal's seat which rotates with the shaft, and of the main seal unit which is stationary and clear of the shaft.

This mating contact of the seal faces, which are hydraulically balanced, is sustained by spring pressure and by the method of flexibly mounting the face of the main seal unit. The flexible member consists of a tough, but supple, reinforced bellows. Thus the main seal unit is able to accommodate the effects of hull deflection and vibration.

Example of a radial face seal
Figure :Example of a radial face seal (Deep Sea Seals Ltd.)

Detail of a Crane seal
Figure : Detail of a Crane seal

The bellows member is clear of the shaft, and its flexibility therefore cannot be impaired, as may happen when a flexible member is mounted on the shaft and hardens, seizes or becomes obstructed by a build-up of solids. The mechanical design principles also ensure continued sealing under fluctuating pressure conditions, i.e. changing draught.

An emergency sealing device can be incorporated into the design. The device, when inflated with air or liquid, forms a tight temporary seal around the shaft, enabling repairs to be made or a replacement seal fitted when the ship is afloat, without the shaft being drawn or drydocking being necessary,

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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