marine engineering

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Stern tube bearings -various type & functional guideline

Split stern bearings

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.

Glacier-Herbert stern bearing

In the Glacier-Herbert system (Figure 8.16) the two completely symmetrical bearing halves are flanged along the horizontal centre line and held together by bolts. The after end of the bearing carries a spherical support ring to which is bolted the outboard seal housing.

The spherical support ring rests in a carrier ring which is bolted to the after end of the stern frame boss. The forward end is supported by a circular diaphragm which is bolted to a flange provided in the stern frame casting. This diaphragm also acts as a carrier for the forward seal,

A series of axial bolts, fitted with Belleville washer packs to ensure virtually constant loading of these bolts and those securing the spherical seating ring, hold the diaphragm firmly in position. This arrangement permits sloped alignment of the bearing to give full support to the drooping tailshaft.


are used to hold the bearing positively in its final position. The arrangement is such that it allows for the differential expansion of the bearing and its housing without detracting from the rigidity of support at the forward end of the bearing.

The propeller shaft is flanged at the after end and the hub of the propeller is bolted to the flange. On the inboard side there is a shroud around the spigot projecting aft from the carrier ring. Two inflatable seals with individual air supplies are fitted in the periphery of the spigot. These can be inflated to provide a seal against inflow of water.

Solid propeller with hollow cylindrical boss and internal
flanged mounting
Figure :Solid propeller with hollow cylindrical boss and internal flanged mounting

Sealing the stern bearing space permits work to be carried out on the stern bearing and seals, without the necessity of drydocking. An alternative to using the inflatable seals, is to apply a sealing bandage around the small gap at the carrier ring flange.

The propeller shaft has two short rotating liners of chrome steel. The liner at the after end is bolted to the propeller shaft flange. The inboard liner is fixed by a clamping ring. These liners act as rubbing surfaces for the rubber lip seals. The coupling at the forward end of the tailshaft may be of the SKF oil injection type (i.e. muff coupling) described previously.

Ross-Turnbull split bearing

The Ross-Turnbull split stern bearing (Figure 8.17) has a bottom half bearing which is chocked on to two horizontal fore and aft machined surfaces within the stem frame boss. The whole bearing is held in position vertically by two 50 tonne Pilgrim type jacks, the chock thickness determining the bearing height. These jacks also hold the top half of the bearing in place. Lateral positioning of the bearing is by 30 tonne Pilgrim type jacks arranged on each side of the bearing.

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

A running track is arranged above the bearing to allow easy transport of the top half. Skids are provided below the bearing to provide easy transport of the bottom half. When removing the bearing bottom half, a jack is first placed underneath it to lift it free of its chocks. The chocks are removed and skates are placed under the bottom half bearing. With the chocks out, the assembly is lowered until the propeller rests on the shroud or support cradle, which is part of the stern frame boss.

Further lowering of the jacks, brings the bottom half away from the tailshaft until its weight is taken by the skates resting on the skids. The jacks are removed and the bottom half bearing is brought forward on the skates together with the seal face and the bellows section of the outboard seal.

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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