Section
4 Design and construction
4.1 Fatigue strength analysis
4.1.1 As an
alternative to the following requirements, a fatigue strength analysis
of components can be submitted indicating a factor of safety of 1,5
at the design loads, based on suitable fatigue failure criteria. The
effects of stress concentrations, material properties and operating
environment are to be taken into account.
4.2 Intermediate shafts
4.2.1 The diameter, d, of the intermediate
shaft is to be not less than that determined by the following formula:
where
k
|
= |
1,0 for shafts with integral coupling flanges complying with Vol 2, Pt 3, Ch 2, 4.8 Couplings and transitions of diameters
or with shrink fit couplings, see
Vol 2, Pt 3, Ch 2, 4.2 Intermediate shafts 4.2.4
|
= |
1,10 for shafts with keyways, in tapered or cylindrical connections,
where the fillet radii in the transverse section of the bottom of the keyway are
not less than 0,0125d, see
Vol 2, Pt 3, Ch 2, 4.2 Intermediate shafts 4.2.5
|
= |
1,10 for shafts with transverse or radial holes where the diameter of
the hole (d
h) does not exceed 0,3d
|
= |
1,20 for shafts with longitudinal slots see
Vol 2, Pt 3, Ch 2, 4.2 Intermediate shafts 4.2.6. |
F
|
= |
95 for turbine installations, electric propulsion installations and
engine installations with slip type couplings |
= |
100 for other engine installations |
P and R are defined in Vol 2, Pt 1, Ch 3, 4.3 Power ratings 4.3.1 (losses in gearboxes and bearings are to
be disregarded)
4.2.2 Beyond
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.
4.2.3 For
shafts with design features other than stated as above, the value
of k will be specially considered.
4.2.4 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 Vol 2, Pt 3, Ch 2, 4.8 Couplings and transitions of diameters.
4.2.5 Keyways
are in general not to be used in installations with a barred speed
range.
4.2.6 The application of k = 1,20 is limited to shafts with longitudinal
slots having a length of less than 0,8do and a width of greater than
0,15do and a diameter of central hole d
i of less than 0,7do, see
Vol 2, Pt 3, Ch 2, 4.5 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 1.3.1 Ck factors in Vol 2, Pt 5, Ch 1 Torsional Vibration, are valid for 1, 2 and 3 slots, i.e. with slots at 360,
180 and 120 degrees apart respectively.
4.3 Thrust shafts external to engines
4.3.1 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 Vol 2, Pt 3, Ch 2, 4.2 Intermediate shafts with
a k value of 1,10. Beyond 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, σu is
to be taken as the minimum tensile strength of the thrust shaft material,
in N/mm2. The fillet radius at the base of both sides of
the thrust collar is to be not less than 0,08 of the diameter of the
shaft at the collar.
4.4 Screwshafts and tube shafts
4.4.1 Screwshafts and tube shafts, (i.e. the shaft which passes through the
sterntube, but does not carry the propeller), made from carbon manganese steel and that
are exposed to seawater, are to be protected by a continuous liner composed of bronze or
another corrosion resistant material. Alternatively, the liner may be omitted provided
the shaft is arranged to run in an oil lubricated bush with an approved oil sealing
gland at the after end. Lengths of shafting between sterntubes and brackets, which are
readily visible when the ship is in dry dock, may be protected by coatings of an
approved type. Special consideration is to be given to liners composed of two or more
parts (non-continuous).
4.4.2 Means
for the protection of screwshafts and tubeshafts are not required
when the shafts are made of corrosion resistant material.
4.4.3 The diameter, d
p, of the protected forged steel 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:
where
k
|
= |
1,22 for a shaft carrying a keyless propeller fitted on a taper, or
where the propeller is attached to an integral flange, and where the shaft is
fitted with a continuous liner, a coating of an approved type, or is oil
lubricated and provided with an approved type of oil sealing gland |
= |
1,26 for a shaft carrying a keyed propeller and where the shaft is
fitted with a continuous liner, a coating of an approved type, or is oil
lubricated and provided with an approved type of oil sealing gland
P and
R are as defined in Vol 2, Pt 1, Ch 3, 4.3 Power ratings (losses in
gearboxes and bearings are to be disregarded)
|
4.4.4 The
diameter, d
p, of the screwshaft determined
in accordance with Vol 2, Pt 3, Ch 2, 4.4 Screwshafts and tube shafts 4.4.3 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.
4.4.5 The diameter of the portion of the screwshaft and tube shaft forward of the
length required by Vol 2, Pt 3, Ch 2, 4.4 Screwshafts and tube shafts 4.4.4 to the forward end of the forward sterntube seal is
to be determined in accordance with Vol 2, Pt 3, Ch 2, 4.4 Screwshafts and tube shafts 4.4.3 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
Vol 2, Pt 3, Ch 2, 4.8 Couplings and transitions of diameters.
4.4.6 Screwshafts which run in sterntubes and tubeshafts 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
Vol 2, Pt 3, Ch 2, 4.8 Couplings and transitions of diameters.
4.5 Hollow shafts
4.5.1 Where the thrust, intermediate, tube shafts
and screwshafts have central holes with a diameter greater than 0,4 times the outside
diameter, the equivalent diameter, d
e, of a solid shaft is not to be less than the Rule size, d, (of a
solid shaft), where d
e is given by:
where
d
o
|
= |
proposed outside diameter, in mm |
d
i
|
= |
diameter of central hole, in mm. |
4.5.2 Where the diameter of the central hole does not exceed 0,4 times the outside
diameter, the diameter is to be calculated in accordance with the appropriate
requirements for a solid shaft.
4.5.3 The design of hollow shaft arrangements is to be considered for the potential passage of
water between and into watertight compartments. The effectiveness of any sealing or
closing arrangement, which may be designed to be used in an emergency, is to be
demonstrated to at least the maximum head of water imposed by the vertical limit of
watertight integrity (Vol 1, Pt 3, Ch 2, 1.3 Watertight and weathertight integrity).
4.6 Cardan shafts
4.6.1 Cardan
shafts, used in installations having more than one propulsion shaftline,
are to be of an approved design, suitable for the designed operating
conditions including short term high power operation. Consideration
will be given to accepting the use of approved cardan shafts in single
propulsion unit applications if a complete spare interchangeable end
joint is to be provided on board.
4.6.2 Cardan
shaft ends are to be contained within substantial tubular guards that
also permit ready access for inspection and maintenance.
4.7 Coupling bolts
4.7.1 Close
tolerance fitted bolts transmitting shear are to have a diameter, d
b, at the flange joining faces of the couplings
not less than:
where
n
|
= |
number
of bolts in the coupling |
D
|
= |
pitch
circle diameter of bolts, in mm |
σu
|
= |
specified
minimum tensile strength of bolts, in N/mm2.
|
4.7.2 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 may be reduced by 5,2 per cent for craft classed exclusively
for smooth water service.
4.7.3 Where
dowels or expansion bolts are fitted to transmit torque in shear they
are to comply with the requirements of Vol 2, Pt 3, Ch 2, 4.7 Coupling bolts 4.7.1. 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.
4.7.4 The
minimum diameter of tap bolts or of bolts in clearance holes at the
joining faces of coupling flanges, pre-tensioned to 70 per cent of
the bolt material yield strength value, is not to be less than:
d
R
|
= |
1,348
|
where
d
R is taken
as the lesser of:
-
Mean of effective
(pitch) and minor diameters of the threads.
-
Bolt shank diameter
away from threads. (Not for waisted bolts which will be specially
considered.)
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 |
σ
y
|
= |
bolt material yield stress in N/mm2.
|
4.7.5 Consideration
will be given to those arrangements where the bolts are pre-tensioned
to loads other than 70 per cent of the material yield strength.
4.7.6 Where
clamp bolts are fitted they are to comply with the requirements of Vol 2, Pt 3, Ch 2, 4.7 Coupling bolts 4.7.4 and are to be installed, and
the bolt holes in the flanges correctly aligned, in accordance with
manufacturer's instructions.
4.8 Couplings and transitions of
diameters
4.8.1 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 Vol 2, Pt 3, Ch 2, 4.7 Coupling bolts 4.7.1, and for this purpose
the minimum tensile strength of the bolts is to be taken as equivalent
to that of the shafts. For intermediate, 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 Vol 2, Pt 3, Ch 2, 4.2 Intermediate shafts 4.2.1
4.8.2 The
fillet radius at the base of the coupling flange, integral with the
shaft, is to be not less than 0,08 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.
4.8.3 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.
4.8.4 All couplings which are attached to shafts are to be of approved dimensions.
4.8.5 Where
couplings are separate from the shafts, provision is to be made to
resist the astern pull.
4.8.7 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.
4.9 Tooth couplings
4.9.1 The
contact stress, S
c, at the flanks of mating
teeth of a gear coupling is not to exceed that given in Table 2.4.2 Allowable S
c values , where
d
p
|
= |
pitch circle diameter of coupling teeth, in mm |
b
|
= |
tooth
face width, in mm |
z
|
= |
number
of teeth (per coupling half). |
Table 2.4.2 Allowable S
c values
Tooth material
surfacetreatment
|
Allowable S
c value N/mm2
|
Surface hardened teeth
|
19
|
Through hardened
teeth
|
11
|
4.9.2 Where
experience has shown that under similar operating and alignment conditions,
a higher tooth loading can be accommodated, full details are to be
submitted for consideration.
4.10 Flexible couplings
4.10.1 Details
of flexible couplings are to be submitted, together with the manufacturer’s
rating capacity, for the designed operating conditions including short-term
high power operation. Verification of coupling characteristics will
be required.
4.10.2 In
determining the allowable mean, maximum and vibratory torque ratings,
consideration of the mechanical properties of the selected elastic
element type in compression, shear and fatigue loading together with
heat absorption/ generation is to be given.
4.10.3 In
determining the allowable torque ratings of the steel spring couplings,
consideration of the material mechanical properties to withstand fatigue
loading, and overheating is to be given.
4.11 Interference fit assemblies
4.11.1 The
interference fit assembly is to have a capacity to transmit a torque
of S.T
max without slippage.
NOTE
For guidance purposes only, T
max = T
mean (1 + C)where
S
|
= |
2,0
for assemblies accessible from within the vessel |
= |
2,5 for assemblies not accessible from within
the vessel. |
Table 2.4.3 ‘C’ values for guidance
purposes
Coupling location
|
C
|
High speed shafting– I.C engine
driven
|
0,3
|
High speed shafting– Electric motor or
turbine driven
|
0,1
|
Low speed shafting– main or PTO stage
gearing
|
0,1
|
4.11.2 The
effect of any axial load acting on the assembly is to be considered.
4.11.3 The
resulting equivalent von Mises stress in the assembly is not to be
greater than the yield strength of the component material.
4.11.4 Reference
marks are to be provided on the adjacent surfaces of parts secured
by shrinkage alone.
4.12 Keys and keyways for propeller connections
4.12.1 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.
4.12.2 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. The omission of pins for keys for small
diameter shafts will be specially considered.
4.12.3 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.
4.12.4 The
effective sectional area of the key in shear, is to be not less than:
where
d
|
= |
diameter,
in mm, required for the intermediate shaft determined in accordance
with Vol 2, Pt 3, Ch 2, 4.2 Intermediate shafts, based on material
having a specified minimum tensile strength of 400 N/mm2 and k = 1
|
d
1
|
= |
diameter of shaft at mid-length of the key, in mm |
σ
u
|
= |
specified minimum tensile strength (UTS) of the key material,
N/mm2.
|
4.12.5 The
effective area in crushing of key, shaft or boss is to be not less
than:
where
σy
|
= |
yield
strength of key, shaft or boss material as appropriate, N/mm2.
|
4.13 Keys and keyways for inboard shaft connections
4.13.1 Round
ended keys are to be used and the keyways 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 shaft at the coupling.
The sharp edges at the top of the keyways are to be removed.
4.13.2 The
effective area of the key in shear, A, is to be not less
than:
where
d |
= |
diameter, in mm,
required for the intermediate shaft determined in accordance with Vol 2, Pt 3, Ch 2, 4.2 Intermediate shafts, based on material having a specified
minimum tensile strength of 400 N/mm2 and k =
1
|
d
1
|
= |
diameter of shaft at mid-length of the key, in mm |
σu
|
= |
specified
minimum tensile strength (UTS) of the key material, N/mm2.
|
Alternatively, consideration will be given to keys
conforming to the design requirements of a recognised National Standard.
4.14 Corrosion resistant liners on shafts
4.14.1 Liners
may be bronze, gunmetal, stainless steel or other approved alloy.
4.14.2 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:
where
t
|
= |
thickness
of the liner, in mm |
D
|
= |
diameter
of the screwshaft or tube shaft under the liner, in mm. |
4.14.3 The
thickness of a continuous liner between the bushes is to be not less
than 0,75t.
4.14.4 Continuous
liners are to be fabricated or cast in one piece.
4.14.5 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.
4.14.6 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.
4.14.7 Each
continuous liner or length of liner is to be tested by hydraulic pressure
to 2,0 bar after rough machining.
4.14.8 Liners
are to be carefully shrunk onto the shafts by hydraulic pressure.
Pins are not to be used to secure the liners.
4.14.9 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.
4.15 Intermediate bearings
4.16 Sternbushes and sterntube arrangements
4.16.1 Where
the sterntube or sternbushes are to be installed using a resin, of
an approved type, the following requirements are to be met:
-
Pouring and venting
holes are to be provided at opposite ends with the vent hole at the
highest point.
-
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.
-
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.
-
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.
4.16.2 The
length of the bearing in the sternbush next to and supporting the
propeller is to be as follows:
-
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.
-
For water lubricated bearings lined with two or more circumferentially
spaced sectors, of synthetic material, without axial grooves in the lower half,
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 2,0 times the
rule diameter of the saft in way of the bearing.
-
For oil lubricated bearings of synthetic material, 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.
-
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 the rule diameter of the shaft inway 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.
-
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.
-
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.
4.16.4 Sternbushes
are to be adequately secured in housings.
4.16.5 Forced water lubrication is to be provided for all bearings lined with
rubber or synthetic material. 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.
4.16.6 Oil
lubricated bearings of synthetic material are to be supplied finished
machined to design dimensions within a rigid tube. Means are to be
provided to prevent rotation of the lining within the tube during
operation.
4.16.7 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.
4.16.8 Oil
sealing glands must be capable of accommodating the effects of differential
expansion between hull and line of shafting for all sea temperatures
in the proposed area of operation. This requirement applies particularly
to those glands which span the gap and maintain oiltightness between
the sterntube and the propeller boss.
4.16.9 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 machinery space, see also
Vol 2, Pt 3, Ch 2, 5.1 Unattended machinery 5.1.1.
4.16.10 For oil lubricated bearings of synthetic material, the flow of lubricant is to be such
that overheating, under normal operating conditions, cannot occur.
4.16.11 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.
4.16.12 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.
4.16.13 Where
in-water surveys are required, means are to be provided for ascertaining
the clearance in the sternbush with the vessel afloat.
4.17 Vibration and alignment
4.18 Rope guards
4.18.1 Rope
guards that provide effective mechanical protection for all bearing
sealing arrangements are to be provided.
4.19 Shaft brake and locking arrangements
4.19.2 Shaft
brakes for safely and speedily slowing down propulsion shafting systems
are to be provided. Each shaft brake is to be capable of restraining
the shaft system whilst the ship is being manoeuvred at slow speed
and to hold the shaft while the shaft locking gear is being engaged.
Shaft brakes are to be capable of functioning in a compartment that
is flooded.
4.19.3 Means
of safely securing the shafting systems in position are to be provided
to permit the shaft to be locked in order to effect repairs whilst
the ship is at sea and operating at a speed to maintain steering capability
or not less than 7 knots, whichever is the greater.
4.20 Shaft seals in watertight bulkheads
4.21 Shaft grounding device
4.21.1 Where cathodic protection (CP) is utilised, a suitable shaft grounding device is to
be included to prevent the shaft from becoming electrically isolated from the ship’s
hull. A shaft potential monitoring device is to be included.
4.21.2 Shaft to hull voltages are to be monitored and recorded at regular intervals. An
alarm is to be provided at the main machinery control station when the maximum
allowable shaft to hull voltage is exceeded.
|