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Sea-water lubricated stern tube bearings

Stern tubes: The propeller shaft (or tailshaft) is supported in a stern tube bearing of one of a number of designs. The bearing, being at the end of the shaft, is affected by the overhanging weight of the propeller. The propeller mass pulls the outer end of the shaft down, so that there is a tendency for edge loading of the stern tube bearing to occur. The forward part of the propeller shaft is tilted upwards. Weardown of the bearing aggravates this misalignment and whirl due to weardown may give additional problems.

Sea-water lubricated stern tube bearings:

The traditional stern bearing (Figure 8.12) 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. The staves in the lower part of the bearing, are cut and fitted so that the end grain is vertical to give the longest possible life.

Staves in the upper part are cut with grain in the axial direction for economy. The staves are shaped with V or U grooves between them at the surface, to allow access for water. The grooves also accommodate any debris. As an alternative to wood, reinforced rubber or Tufhol can be used. Bearing length is equal to four times shaft diameter.

Stern tubes (Figure 8.13) are supported at the after end by the stern frame boss and at the forward end in the aft peak bulkhead. Their cast iron construction requires strong support in way of the bearing, from the stem frame boss.

A steel nut at the outboard end retains the tube in position, with its collar hard against the sternframe and the bearing section firm within the stem frame boss. Welded studs hold the forward flange against the aft peak bulkhead. Sea water, which enters at the after end or from the circulation system to cool and lubricate, is an electrolyte which will support galvanic corrosion.

(left) Rubber stave bearing (right) Lignum vitae bearing
Figure 8.12 (left) Rubber stave bearing (right) Lignum vitae bearing (Glacier Metal Co.)

Sea-water lubricated stern tube
Figure 8.13 Sea-water lubricated stern tube

Wastage of the vulnerable steel shaft is prevented by a shrunk-on bronze liner and rubber seal sandwiched between the propeller hub and the liner end. It is essential that the rubber has freedom to flow when nipped between the hub and liner.

Excessive weardown of bearing materials due to vibration or whirl, poor quality of work when rewooding, inferior materials, presence of sand/sediment in the water or propeller damage, could necessitate early rewooding. The life of the bearing for vessels with engines aft, and particularly tankers and ore carriers which spend long periods in ballast, has been short with rewooding being needed in perhaps eighteen months.

The centre of the stern-tube is connected to a sea-water service line which, together with ingress of water between the shaft and bush, provides the cooling and lubrication. A packed gland seals the forward end of the bearing and is adjusted to permit a slight trickle of water along the shaft and into the tunnel well where it is regularly removed with the bilge pump. Bearing clearances are liberal both to accommodate the swelling which occurs when the staves are immersed in water and to permit the essential flow of water through the bearing.

A large number of vessels with water-lubricated bearings are still in service and they continue to be installed.

Inspection of sea-water lubricated stern tubes and tailshaft During drydock inspection, bearing weardown is measured by poker gauge or by inserting a wedge between the shaft and bearing from the outside. The permissible wear is in the region of 912 mm on large diameter shafts.

The examination of the type of tailshaft described above requires removal of the propeller and inward withdrawal of the propeller shaft. The operation calls for the erection of staging, use of a large, suspended ram or tup for the spanner to slacken the nut and wedges to start the propeller. The nut remains on the thread after being slackened for safety reasons.

Accidents have been caused by the sudden loosening of propellers with no nut in place to act as a stop. Timber between the aft peak bulkhead and the flange at the forward end of the tailshaft, supports the shaft against the action of the wedges. The examination when a tailshaft has operated in a sea-water lubricated bearing and where the propeller Is keyed, may reveal (Figure 8.14) a number of defects. There is a potential for cracks in the keyway area but the likelihood of these occurring has been reduced by the employment of sled type keys, radiused corners within the keyway and spooning at the forward end.

Propeller shaft faults
Figure 8.14 Propeller shaft faults

A plain keyway milled in a shaft taper, is a weakening factor which allows deformation of the surface when push up is applied to the propeller and where there is any transmission of torque from the shaft via the key to the propeller hub. Torque causes a deformation which tends to open the keyway.

The rubber seal sandwiched by the propeller hub and protective bronze liner, prevents ingress of sea water which would act as an electrolyte to promote galvanic corrosion of the exposed part of the shaft.

A defective seal permits corrosion and wastage. Fretting of the steel shaft tends to occur beneath the forward end of the propeller hub or under the after end of the liner. Any pitting or marking of the shaft surface in the area (or notch) between the propeller hub and the bronze liner can initiate a fatigue or corrosion fatigue crack in this vulnerable area. (Shaft droop from the overhanging weight of the propeller, stretches the upper surface and compresses the lower, to produce alternating stress when the shaft is rotating. The imposed alternating effect likely to cause fatigue, is of a low frequency and high stress.)

The shrunk-on bronze liner, fitted to protect the steel shaft against black corrosion may itself be damaged by working conditions. Shaft whirl can lead to patches marked by cavitation erosion, scoring occurs due to the stern gland packing and liner cracking has sometimes penetrated through to cause corrosion cracking in the shaft.

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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