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Marine propeller shaft materials & various coupling types

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 materials and couplings: 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.

The faces of flanged couplings (except where undercut in the centre area) are also smooth turned, with bolt holes carefully bored and reamered to give an accurate finish. Torque is transmitted by the friction between flanges and also through the shanks of the bolts. Each tightened bolt holds the flanges hard together in the area local to it. A circle of bolts is needed for a good all round grip. The design of flange couplings can be checked by formulae given in Lloyds or other classification society regulations.

Coupling bolts

The elongation of a bolt when tightened, causes a reduction in cross sectional area. The relationship between change in length and the change of cross sectional area is summarized by Poisson's Ratio. In a clearance bolt, this is not a problem, but with a normal fitted bolt, positive contact between the accurately machined bolt and the reamered hole is lost when the bolt is tightened.

An oversize bolt could of course be used and cooling of the shank probably with liquid nitrogen - would be necessary to cause contraction and reduction of cross sectional area before insertion. The effect of low temperature and the possibility of the steel becoming brittle as the result of the cooling must be considered.

Shaft coupling bolts are tightened to force the faces of the flanges together, so that friction between the faces will provide some proportion of the drive. However, fitted bolt shanks are also designed to take some load. A clearance bolt could provide the first requirement but not the second. A normal fitted bolt when tightened and subjected to a reduction in cross section, would also fail on the second count and probably be damaged by fretting. A tapered bolt (Figure 8,9) could be used instead of a conventional coupling bolt (Figure 8.8) to obtain a good fit and the required tightening.

Conventional coupling bolt
Figure : Conventional coupling bolt

Tapered coupling bolt
Figure : Tapered coupling bolt

The Pilgrim hydraulic bolt uses the principle embodied in Poisson's Ratio to provide a calculated and definite fitting force between bolt and hole. The bolt (Figure 8,10) is hollow and before being fitted is stretched with hydraulic pressure applied to an inserted rod from a pressure cylinder screwed to the bolt head.

Stretching makes the bolt diameter small enough for insertion into the hole, after which the nut is nipped up. Release of hydraulic pressure allows the bolt to shorten, so that (1) predetermined bolt load is produced and (2) diametrical re-expansion gives a good fit of the shank in the hole. These bolts, when used in flange couplings and flange mounted propellers, have the advantage that they are easily removed for inspection and maintenance.

Pilgrim type hydraulic coupling bolt
Figure :Pilgrim type hydraulic coupling bolt

Muff coupling

An alternative to the conventional flange couplings for the tailshaft, the muff coupling allows the shaft to be withdrawn outboard. The SKF coupling (Figure 8.11) consists basically of two steel sleeves. The thin inner sleeve has a bore slightly larger than the shaft diameter and its outer surface is tapered to match the taper on the bore of the outer sleeve. The nut and sealing ring close the annular space at the end of the sleeves.

When the coupling is in position, the outer sleeve is hydraulically driven on to the tapered inner sleeve. At the same time, oil is injected between the contact surfaces to separate them and thus overcome the friction between them. Oil for the operation is supplied by hand pumps; two for the forced lubrication and another hand or power pump for the driving oil pressure.

SKF (muff) coupling
Figure :SKF (muff) coupling

Surface pressure at least 12 kg/mrrv For 15000 hp (10000 kw) Shaft dia. 530 mm Ji dia. 0.89 mm Press-up 40 mm Length 1250 mm

When the outer sleeve has been driven on to a predetermined position, the forced lubrication pressure is released and drained. Oil pressure is maintained in the hydraulic space until the oil between the sleeves drains and normal friction is restored. After disconnecting hoses, plugs are fitted and rust preventive applied to protect exposed seatings. A sealing strip is pressed into the groove between the end of the sleeve and the nut.

The grip of the coupling is checked by measuring the diameter of the outer sleeve before and after tightening. The diameter increase should agree with the figure stamped on the sleeve.

To disconnect the coupling, oil pressure is brought to a set pressure in the hydraulic space. Then with the shafts supported, oil is forced between the sleeves. The outer sleeve slides off the inner at a rate controlled by release of the hydraulic oil pressure.

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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