Section 3 Design

3.1.1 The diameter, d, of the intermediate shaft is to be not less than determined by the following formula:

P and R are defined in Pt 5, Ch 1, 3.3 Power ratings (losses in gearboxes and bearings are to be disregarded)

After a length of 0,2d from the end of a keyway, transverse hole or radial hole and 0,3d from the end of a longitudinal slot, the diameter of the shaft may be gradually reduced to that determined with k = 1,0.

3.1.2 For shafts with design features other than stated in Pt 5, Ch 6, 3.1 Intermediate shafts 3.1.1, the value of k will be specially considered.

3.1.3 The Rule diameter of the intermediate shaft for engines, turbines and electric propelling motors may be reduced by 3,5 per cent for ships classed exclusively for smooth water service, and by 1,75 per cent for ships classed exclusively for service on the Great Lakes.

3.1.4 For shrink fit couplings k refers to the plain shaft section only. Where shafts may experience vibratory stresses close to the permissible stresses for continuous operation, an increase in diameter to the shrink fit diameter is to be provided, e.g. a diameter increase of 1 to 2 per cent and a blending radius as described in Pt 5, Ch 6, 3.7 Couplings and transitions of diameters.

3.1.5 Keyways are in general not to be used in installations with a barred speed range.

3.1.6 The application of k = 1,20 is limited to shafts with longitudinal slots having a length of less than 0,8d _o and a width greater than 0,15d _o and a diameter of central hole d_i of less than 0,7d _o see Pt 5, Ch 6, 3.6 Hollow shafts. The end rounding of the slot is not to be less than half the width. An edge rounding should preferably be avoided as this increases the stress concentration slightly. The values of c_K , see Table 8.2.1 Ck factors in Pt 5, Ch 8 Shaft Vibration and Alignment, are valid for 1, 2 and 3 slots, i.e. with slots at 360, 180 and 120 degrees apart respectively.

3.2.1 The diameter of the quill shaft is to be not less than given by the following formula:

where

P and R are as defined in Pt 5, Ch 1, 3.3 Power ratings.

3.3.1 Where there is only one pinion geared into the final wheel, or where there are two pinions which are set to subtend an angle at the centre of the shaft of less than 120 degrees, the diameter of the shaft at the final wheel and the adjacent journals is to be not less than 1,15 times that required for the intermediate shaft.

3.3.2 Where there are two pinions geared into the final wheel opposite, or nearly opposite, to each other, the diameter of the shaft at the final wheel and the adjacent journals is to be not less than 1,1 times that required for the intermediate shaft.

3.3.3 In both Pt 5, Ch 6, 3.3 Final gear wheel shafts 3.3.1 and Pt 5, Ch 6, 3.3 Final gear wheel shafts 3.3.2, abaft the journals, the shaft may be gradually tapered down to the diameter required for an intermediate shaft determined according to Pt 5, Ch 6, 3.1 Intermediate shafts, where σ_u is to be taken as the specified minimum tensile strength of the final wheel shaft material, in N/mm².

3.4.1 The diameter at the collars of the thrust shaft transmitting torque, or in way of the axial bearing where a roller bearing is used as a thrust bearing, is to be not less than that required for the intermediate shaft in accordance with Pt 5, Ch 6, 3.1 Intermediate shafts with a k value of 1,10. Outside a length equal to the thrust shaft diameter from the collars, the diameter may be tapered down to that required for the intermediate shaft with a k value of 1,0. For the purpose of the foregoing calculations, σ_u is to be taken as the minimum tensile strength of the thrust shaft material, in N/mm².

3.5.1 The diameter, d _p of the screwshaft immediately forward of the forward face of the propeller boss or, if applicable, the forward face of the screwshaft flange, is to be not less than determined by the following formula:

P and R are defined in Pt 5, Ch 1, 3.3 Power ratings, (losses in gearboxes and bearings are to be disregarded)

3.5.2 The diameter, d _p of the screwshaft determined in accordance with the formula in Pt 5, Ch 6, 3.5 Screwshafts and tube shafts 3.5.1 is to extend over a length not less than that to the forward edge of the bearing immediately forward of the propeller or 2,5d _p whichever is the greater.

3.5.3 The diameter of the portion of the screwshaft and tube shaft, forward of the length required by Pt 5, Ch 6, 3.5 Screwshafts and tube shafts 3.5.2 to the forward end of the forward sterntube seal, is to be determined in accordance with the formula in Pt 5, Ch 6, 3.5 Screwshafts and tube shafts 3.5.1 with a k value of 1,15. The change of diameter from that determined with k = 1,22 or 1,26 to that determined with k = 1,15 should be gradual, see Pt 5, Ch 6, 3.7 Couplings and transitions of diameters.

3.5.4 Screwshafts which run in sterntubes and tube shafts may have the diameter forward of the forward sterntube seal gradually reduced to the diameter of the intermediate shaft. Abrupt changes in shaft section at the screwshaft/tube shaft to intermediate shaft couplings are to be avoided see Pt 5, Ch 6, 3.7 Couplings and transitions of diameters.

3.5.5 Unprotected screwshafts and tube shafts of corrosion-resistant material will be specially considered.

3.5.6 For shafts of non-corrosion-resistant materials which are exposed to sea-water, the diameter of the shaft is to be determined in accordance with the formula in Pt 5, Ch 6, 3.5 Screwshafts and tube shafts 3.5.1 with a k value of 1,26 and σ_u taken as 400 N/mm².

3.6.1 Where the thrust, intermediate and tube shafts and screwshafts have central holes, the outside diameters of the shafts are to be not less than given by the following formula:

However, where the diameter of the central hole does not exceed 0,4 times the outside diameter, no increase over Rule size need be provided.

3.7.1 The minimum thicknesses of the coupling flanges are to be equal to the diameters of the coupling bolts at the face of the couplings as required by Pt 5, Ch 6, 3.8 Coupling bolts and, for this purpose, the minimum tensile strength of the bolts is to be taken as equivalent to that of the shafts. For intermediate shafts, thrust shafts and the inboard end of the screwshaft, the thickness of the coupling flange is in no case to be less than 0,20 of the diameter of the intermediate shaft as required by Pt 5, Ch 6, 3.1 Intermediate shafts.

3.7.2 The fillet radius at the base of the coupling flange is to be not less than 0,08 of the diameter of the shaft at the coupling but, in the case of crankshafts, the fillet radius at the centre coupling flanges may be 0,05 of the diameter of the shaft at the coupling. The fillets are to have a smooth finish and are not to be recessed in way of nut and bolt heads.

3.7.3 Where the propeller is attached by means of a flange, the thickness of the flange is to be not less than 0,25 of the actual diameter of the adjacent part of the screwshaft. The fillet radius at the base of the coupling flange is to be not less than 0,125 of the diameter of the shaft at the coupling.

3.7.4 All couplings which are attached to shafts are to be of approved dimensions.

3.7.5 Where couplings are separate from the shafts, provision is to be made to resist the astern pull.

3.7.6 Where a coupling is shrunk on to the parallel portion of a shaft or is mounted on a slight taper, e.g. by means of the oil pressure injection method, full particulars of the coupling including the interference fit are to be submitted for special consideration.

3.7.7 Transitions of diameters are to be designed with either a smooth taper or a blending radius. In general, a blending radius equal to the change in diameter is recommended.

3.8.1 Close tolerance fitted bolts transmitting shear are to have a diameter, at the joining faces of the couplings not less than given by the following formula:

Diameter of coupling bolts =

3.8.2 At the joining faces of couplings, other than within the crankshaft and at the thrust shaft/crankshaft coupling, the Rule diameter of the coupling bolts defined inPt 5, Ch 6, 3.8 Coupling bolts 3.8.1 may be reduced by 5,2 per cent for ships classed exclusively for smooth water service, and 2,6 per cent for ships classed exclusively for service on the Great Lakes.

3.8.3 Where dowels or expansion bolts are fitted to transmit torque in shear they are to comply with the requirements of Pt 5, Ch 6, 3.8 Coupling bolts 3.8.1. The expansion bolts are to be installed, and the bolt holes in the flanges are to be correctly aligned, in accordance with manufacturer's instructions.

3.8.4 The minimum diameter of tap bolts or of bolts in clearance holes at the joining faces of coupling flanges, pretensioned to 70 per cent of the bolt material yield strength value, is not to be less than:

dR

=

where d_R is taken as the lesser of:

1. Mean of effective (pitch) and minor diameters of the threads.

2. Bolt shank diameter away from threads. (Not for waisted bolts which will be specially considered.)

   P and R are as defined in Pt 5, Ch 1, 3.3 Power ratings.

   F

   =

   2,5 where the flange connection is not accessible from within the ship

   =

   2,0 where the flange connection is accessible from within the ship

   C

   =

   ratio of vibratory/mean torque values at the rotational speed being considered

   D

   =

   pitch circle diameter of bolt holes, in mm

   Q

   =

   external load on bolt in N (+ve tensile load tending to separate flange, –ve)

   n

   =

   number of tap or clearance bolts

   σy

   =

   bolt material yield stress in N/mm2.

3.8.5 Consideration will be given to those arrangements where the bolts are pretensioned to loads other than 70 per cent of the material yield strength.

3.8.6 Where clamp bolts are fitted they are to comply with the requirements of Pt 5, Ch 6, 3.8 Coupling bolts 3.8.4 and are to be installed, and the bolt holes in the flanges correctly aligned, in accordance with manufacturer's instructions.

3.9.1 The thickness, t, of liners fitted on screwshafts or on tube shafts, in way of the bushes, is to be not less, when new, than given by the following formula:

t

=

mm

3.9.2 The thickness of a continuous liner between the bushes is to be not less than 0,75t.

3.9.3 Continuous liners should preferably be cast in one piece.

3.9.4 Where liners consist of two or more lengths, these are to be butt welded together. In general, the lead content of the gunmetal of each length forming a butt welded liner is not to exceed 0,5 per cent. The composition of the electrodes or filler rods is to be substantially lead-free.

3.9.5 The circumferential butt welds are to be of multi-run, full penetration type. Provision is to be made for contraction of the weld by arranging for a suitable length of the liner containing the weld, if possible about three times the shaft diameter, to be free of the shaft. To prevent damage to the surface of the shaft during welding, a strip of heat resisting material covered by a copper strip should be inserted between the shaft and the liner in way of the joint. Other methods for welding this joint may be accepted if approved. The welding is to be carried out by an approved method and to the Surveyor's satisfaction.

3.9.6 Each continuous liner or length of liner is to be tested by hydraulic pressure to 0,2 MPa after rough machining.

3.9.7 Liners are to be carefully shrunk on, or forced on, to the shafts by hydraulic pressure. Pins are not to be used to secure the liners.

3.9.8 Effective means are to be provided for preventing water from reaching the shaft at the part between the after end of the liner and the propeller boss.

3.10.1 Round ended or sled-runner ended keys are to be used, and the keyways in the propeller boss and cone of the screwshaft are to be provided with a smooth fillet at the bottom of the keyways. The radius of the fillet is to be at least 0,0125 of the diameter of the screwshaft at the top of the cone. The sharp edges at the top of the keyways are to be removed.

3.10.2 Two screwed pins are to be provided for securing the key in the keyway, and the forward pin is to be placed at least one-third of the length of the key from the end. The depth of the tapped holes for the screwed pins is not to exceed the pin diameter, and the edges of the holes are to be slightly bevelled.

3.10.3 The distance between the top of the cone and the forward end of the keyway is to be not less than 0,2 of the diameter of the screwshaft at the top of the cone.

3.10.4 The effective sectional area of the key in shear, is to be not less than mm²

3.11.1 For keyed and keyless propellers, see Pt 5, Ch 7 Propellers.

3.12.1 Where the sterntube or sternbushes are to be installed using a resin of an approved type, the following requirements are to be met:

1. Pouring and venting holes are to be provided at opposite ends with the vent hole at the highest point.
2. The minimum radial gap occupied by the resin is to be not less than 6 mm at any one point with a nominal resin thickness of 12 mm.
3. In the case of oil lubricated sterntube bearings, the arrangement of the oil grooves is to be such as to promote a positive circulation of oil in the bearing.
4. Provision is to be made for the remote measurement of the temperature at the aft end of the aft bearing, with indication and alarms at the control stations.

3.12.2  The length of the bearing in the sternbush next to and supporting the propeller is to be as follows:

1. For water lubricated bearings which are lined with lignum vitae, rubber composition or staves of synthetic material, the length is to be not less than 4,0 times the rule diameter of the shaft in way of the bearing.

2. For water lubricated bearings lined with two or more circumferentially spaced sectors of synthetic material, in which it can be shown that the sectors operate on hydrodynamic principles, the length of the bearing is to be such that the nominal bearing pressure will not exceed 0,55 MPa. The length of the bearing is to be not less than twice its 2,0 times the rule diameter of the shaft in way of the bearing.

3. For oil lubricated bearings of synthetic material the flow of lubricant is to be such that overheating, under normal operating conditions, cannot occur. The acceptable nominal bearing pressure will be considered upon application and is to be supported by the results of an agreed test programme. The length of the bearing is, in general, to be not less than 2,0 times the rule diameter of the shaft in way of the bearing. The nominal bearing pressure is not to exceed the maximum for which the synthetic material has been approved.

4. For bearings which are white-metal lined, oil lubricated and provided with an approved type of oil sealing gland, the length of the bearing is to be approximately 2,0 times the rule diameter of the shaft in way of the bearing and is to be such that the nominal bearing pressure will not exceed 0,8 MPa. The length of the bearing is to be not less than 1,5 times its diameter.

5. For bearings of cast iron and bronze which are oil lubricated and fitted with an approved oil sealing gland, the length of the bearing is, in general, to be not less than 4,0 times the rule diameter of the shaft in way of the bearing.

6. For bearings which are grease lubricated, the length of the bearing is to be not less than 4,0 times the rule diameter of the shaft in way of the bearing. Other lengths may be considered upon application, subject to the provision of suitable supporting in-service or testing evidence at relevant shaft pressures and velocities.

3.12.3 Forced water lubrication is to be provided for all bearings lined with rubber or synthetic material and for those bearings lined with lignum vitae where the shaft diameter is 380 mm or over. The supply of water may come from a circulating pump or other pressure source. Flow indicators are to be provided for the water service to the bearings. The water grooves in the bearings are to be of ample section and of a shape which will be little affected by weardown, particularly for bearings of synthetic material.

3.12.4 Bearings of synthetic material are to be supplied finished machined to design dimensions within a rigid bush. Means are to be provided to prevent rotation of the lining within the bush during operation.

3.12.5 Synthetic materials for application as stern tube bearings are to be approved in accordance with Rules for the Manufacture, Testing and Certification of Materials, July 2022, Ch 14, 2.13 Sterntube bearings

3.12.6 All sternbushes are to be adequately secured in the sterntube/housings.

3.12.7 The shut-off valve or cock controlling the supply of water is to be fitted direct to the after peak bulkhead, or to the sterntube where the water supply enters the sterntube forward of the bulkhead.

3.12.8 Oil sealing glands fitted in ships classed for unrestricted service must be capable of accommodating the effects of differential expansion between hull and line of shafting in sea temperatures ranging from arctic to tropical. This requirement applies particularly to those glands which span the gap and maintain oiltightness between the sterntube and the propeller boss.

3.12.9 Where a tank supplying lubricating oil to the sternbush is fitted, it is to be located above the load waterline and is to be provided with a low level alarm device in the engine room.

3.12.10 Where sternbush bearings are oil lubricated, provision is to be made for cooling the oil by maintaining water in the after peak tank above the level of the sterntube or by other approved means.

3.12.11 For oil lubricated bearings of synthetic material, the flow of lubricant is to be such that overheating, under normal operating conditions, cannot occur.

3.12.12 Means for ascertaining the temperature of the sternbush bearings are to be provided, e.g. monitoring of the temperature of the oil in the sterntube.

3.12.13 Where there is compliance with the terms of Pt 5, Ch 6, 3.12 Sternbushes and sterntube arrangements 3.12.2.(c) and Pt 5, Ch 6, 3.12 Sternbushes and sterntube arrangements 3.12.2.(d) to the Surveyor's satisfaction, a screwshaft will be assigned the notation OG in the Supplement to the Register Book for Periodical Survey purposes, see Pt 1, Ch 3 Periodical Survey Regulations.

3.12.14 Screwshafts which are grease lubricated are not eligible for the OG notation.

3.12.15 Where an *IWS (In-Water Survey) notation is to be assigned, see Pt 1, Ch 2, 2.3 Class notations (hull) 2.3.11, for water lubricated bearings means are to be provided for ascertaining the clearance in the sternbush with the vessel afloat.

3.13.1 For the requirements for torsional, axial and lateral vibration, and for alignment of the shafting, see Pt 5, Ch 8 Shaft Vibration and Alignment.