2.5 Couplings

2.5.1 Flange couplings

1. Flange couplings of intermediate and thrust shafts and the flange of the forward coupling of the propeller shaft are to have a thickness not less than 0,2 times the rule diameter of the solid intermediate shaft and not less than the coupling bolt diameter calculated for a tensile strength equal to that of the corresponding shaft.
   The fillet radius at the base of solid forged flanges is to be not less than 0,08 times the actual shaft diameter.
   The fillet may be formed of multi-radii in such a way that the stress concentration factor will not be greater than that for a circular fillet with radius 0,08 times the actual shaft diameter.
   For non-solid forged flange couplings, the above fillet radius is not to cause a stress in the fillet higher than that caused in the solid forged flange as above.
   Fillets are to have a smooth finish and are not to be recessed in way of nuts and bolt heads.
2. Where the propeller is connected to an integral propeller shaft flange, the thickness of the flange is to be not less than 0,25 times the rule diameter of the aft part of the propeller shaft. The fillet radius at the base of the flange is to be not less than 0,125 times the actual diameter.
   The strength of coupling bolts of the propeller boss to the flange is to be equivalent to that of the aft part of the propeller shaft.
3. Non-solid forged flange couplings and associated keys are to be of a strength equivalent to that of the shaft.
   They are to be carefully fitted and shrunk on to the shafts, and the connection is to be such as to reliably resist the vibratory torque and astern pull.
4. For couplings of intermediate and thrust shafts and for the forward coupling of the propeller shaft having all fitted coupling bolts, the coupling bolt diameter in way of the joining faces of flanges is not to be less than the value d_B, in mm, given by the following formula:
   
   
   where:

   d

   :

   Rule diameter of solid intermediate shaft, in mm, taking into account the ice strengthening requirements of Pt E, Ch 8, Sec 3, where applicable

   nB

   :

   Number of fitted coupling bolts

   DC

   :

   Pitch circle diameter of coupling bolts, in mm

   Rm

   :

   Value of the minimum tensile strength of intermediate shaft material taken for calculation of d, in N/mm2

   RmB

   :

   Value of the minimum tensile strength of coupling bolt material, in N/mm2. Where, in compliance with [2.1.1], the use of a steel having RmB in excess of the limits specified in [2.1.4] is allowed for coupling bolts, the value of RmB to be introduced in the formula is not exceed the above limits.

5. Flange couplings with non-fitted coupling bolts may be accepted on the basis of the calculation of bolt tightening, bolt stress due to tightening, and assembly instructions.
   To this end, the torque based on friction between the mating surfaces of flanges is not to be less than 2,8 times the transmitted torque, assuming a friction coefficient for steel on steel of 0,18. In addition, the bolt stress due to tightening in way of the minimum cross-section is not to exceed 0,8 times the minimum yield strength (R_eH), or 0,2 proof stress (R_{p 0,2}), of the bolt material.
   Transmitted torque has the following meanings:
   • For main propulsion systems powered by diesel engines fitted with slip type or high elasticity couplings, by turbines or by electric motors: the mean transmitted torque corresponding to the maximum continuous power P and the relevant speed of rotation n, as defined under [2.2.2].
   • For main propulsion systems powered by diesel engines fitted with couplings other than those mentioned in (a): the mean torque above increased by 20% or by the torque due to torsional vibrations, whichever is the greater.
   The value 2,8 above may be reduced to 2,5 in the following cases:
   • ships having two or more main propulsion shafts
   • when the transmitted torque is obtained, for the whole functioning rotational speed range, as the sum of the nominal torque and the alternate torque due to the torsional vibrations, calculated as required in Ch 1, Sec 9.

2.5.2 Shrunk couplings
Non-integral couplings which are shrunk on the shaft by means of the oil pressure injection method or by other means may be accepted on the basis of the calculation of shrinking and induced stresses, and assembly instructions.
To this end, the force due to friction between the mating surfaces is not to be less than 2,8 times the total force due to the transmitted torque and thrust.
The value 2,8 above may be reduced to 2,5 in the cases specified under item e) of [2.5.1].
The values of 0,14 and 0,18 will be taken for the friction coefficient in the case of shrinking under oil pressure and dry shrink fitting, respectively.
In addition, the equivalent stress due to shrinkage determined by means of the von Mises-Hencky criterion in the points of maximum stress of the coupling is not to exceed 0,8 times the minimum yield strength (R_eH), or 0,2% proof stress (R_p0,2), of the material of the part concerned.
The transmitted torque is that defined under item e) of [2.5.1].
For the determination of the thrust, see Ch 1, Sec 8, [3.1.2].

2.5.3 Other couplings
Types of couplings other than those mentioned in [2.5.1] and [2.5.2] will be specially considered by the Society.

2.5.4 Flexible couplings

1. The scantlings of stiff parts of flexible couplings subjected to torque are to be in compliance with the requirements of Article [2].
2. For flexible components, the limits specified by the Manufacturer relevant to static and dynamic torque, speed of rotation and dissipated power are not to be exceeded.
3. Where all the engine power is transmitted through one flexible component only (ships with one propulsion engine and one shafting only), the flexible coupling is to be fitted with a torsional limit device or other suitable means to lock the coupling should the flexible component break.
   In stiff transmission conditions with the above locking device, a sufficiently wide speed range is to be provided, free from excessive torsional vibrations, such as to enable safe navigation and steering of the ship. As an alternative, a spare flexible element is to be provided on board.

2.5.5 Propeller shaft keys and keyways

1. Keyways on the propeller shaft cone are to have well rounded corners, with the forward end faired and preferably spooned, so as to minimize notch effects and stress concentrations.
   When these constructional features are intended to obtain an extension of the interval between surveys of the propeller shaft in accordance with the relevant provisions of Pt A, Ch 2, Sec 2, [5.5], they are to be in compliance with Fig 1.
   Different scantlings may be accepted, provided that at least the same reduction in stress concentration is ensured.
   The fillet radius at the bottom of the keyway is to be not less than 1,25% of the actual propeller shaft diameter at the large end of the cone.
   The edges of the key are to be rounded.
   The distance from the large end of the propeller shaft cone to the forward end of the key is to be not less than 20% of the actual propeller shaft diameter in way of the large end of the cone.
   Key securing screws are not to be located within the first one-third of the cone length from its large end; the edges of the holes are to be carefully faired.
2. The sectional area of the key subject to shear stress is to be not less than the value A, in mm², given by the following formula:
   
   
   where:

   d

   :

   Rule diameter, in mm, of the intermediate shaft calculated in compliance with the requirements of [2.2.2], assuming: Rm = 400 N/mm2

   dPM

   :

   Actual diameter of propeller shaft at mid-length of the key, in mm.

Figure 1 :  Details of forward end of propeller shaft keyway