marine engineering

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Propeller Shaft bearings - various type

Propeller shaft, tail shaft The aftermost section of the propulsion shafting in the stern tube in single screw ships and in the struts of multiple screw ships to which the propeller is fitted. Propulsion shafting constitutes a system of revolving rods that transmit power and motion from the main drive to the propeller. The shafting is supported by an appropriate number of bearings.

Shaft bearings : The intermediate shafting (Figure 8.2) between the tailshaft and main engine, gearbox or thrustblock may be supported in plain, tilting pad or roller bearings. The two former types usually have individual oil sumps, the oil being circulated by a collar and scraper device; roller bearings are grease lubricated. The individual oil sumps usually have cooling water coils or a simple cooling water chamber fitted. Cooling water is provided from a service main connected to the sea-water circulating system. The cooling water passes directly overboard.

propeller shaft system
Figure : Propeller shaft system

Usually for plain and tilting pad bearings, only a bottom bearing half is provided, the top acting purely as a cover. The aftermost plumber block however, always has a full bearing. This bearing and any bearing in the forward end of the stern tube, may be subject to negative loading.

Plain bearings

Any oil between a static shaft and plain journal bearing in which it rests, tends to be squeezed out so that there is metal to metal contact. At the start of the rotation the journal is inclined to roll up the bearing surface against the direction of rotation until friction slip occurs. Then, provided there is oil in the clearance space, this will cling to the moving surface and be dragged between the shaft and the bearing.

Shaft rotation, as it speeds up, continues to carry oil to the shaft underside so developing a film with sufficient pressure to hold the shaft clear of the bearing. The pressure build-up is related to speed of rotation, Thus oil delivered as the shaft turns at normal speed, will form a layer or film, separating shaft and bearing, and so prevent direct wear of metal to metal.

Fluid film pressure in plain bearing
Figure : Fluid film pressure in plain bearing

Pressure generated in the oil film, is most effective over about one third of the bearing area (Figure 8.3) because of oil loss at the bearing ends and peripherally. Load is supported and transmitted to the journal, mainly by the area where the film is generated.

Tilting pad bearings

Replacement of the ineffective side portions of the journal by pads capable of carrying load will considerably increase its capacity. Tilting pads based on those developed by Michell for thrust blocks (Figure 8.4a) are used for the purpose. Each pad tilts as oil is delivered to it, so that a wedge of oil is formed. The three pressure wedges give a larger total support area than that obtained with a plain bearing. The arrangement of pads in a bearing is shown in Figure 8.4b.

The tilt of the pads automatically adjusts to suit load, speed and oil viscosity. The wedge of oil gives a greater separation between shaft and bearing than does the oil film in a plain journal. The enhanced load capacity of a tilting pad design, permits the use of shorter length bearings or fewer bearings.

 Tilting pad bearing
Figure : Tilting pad bearing

Tilting pad shaft bearing (Michell Bearings Ltd)
Figure :Tilting pad shaft bearing (Michell Bearings Ltd)

Roller bearings

Roller bearings (Figure 8.5) are supplied in sizes to suit shafts up to the largest diameter. Flange couplings dictate that roller bearing races must be in two parts for fitting.

The section of the shaft where the split roller bearing is to be fitted, must be machined very accurately and with good finish. The two halves of the inner and outer races are fitted and held with clamping rings.

Adequate speed for build-up of fluid film pressure is vital for journal bearings. At low speeds there may be metal to metal contact with wear and damage. Friction at low rotational speeds, is high. Roller bearings are not dependent on speed for effective lubrication. Friction is low at all speeds. This makes them suitable for steam turbine installations and in ships where slow steaming may be necessary. Roller bearings, where fitted, are grease-lubricated.

Roller bearings
Figure : Roller bearings (courtesy of The Cooper Roller Bearing Company)

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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