Section
5 Shaft alignment
5.1 General
5.1.1 Shaft
alignment calculations are to be carried out for main propulsion shafting
rotating at propeller speed, including the crankshaft of direct drive
systems or the final reduction gear wheel on geared installations.
The Builder is to make available shaft alignment procedures detailing
the proposed alignment method and checks for these arrangements.
5.2 Particulars to be submitted for approval – Shaft alignment
calculations
5.2.1 Shaft
alignment calculations are to be submitted to LR for approval for
the following shafting systems:
-
All geared installations,
where the screwshaft has a diameter of 300 mm or greater in way of
the aftmost bearing.
-
All geared installations
with multiple input/single output, regardless of shaft diameter.
-
All direct drive
installations which incorporate three or fewer bearings supporting
the intermediate and screwshaft aft of the prime mover.
-
Where prime movers
in a direct drive installation or shaftline bearings are installed
on resilient mountings.
-
All systems where
the screwshaft has a diameter of 800 mm or greater in way of the aftmost
bearing.
5.2.2 The
shaft alignment calculations are to take into account the:
-
thermal displacements
of the bearings between cold static and hot dynamic machinery conditions;
-
buoyancy effect
of the propeller immersion due to the ship's operating draughts;
-
effect of predicted
hull deformations over the range of the ship's operating draughts,
where known;
-
effect of filling
the aft peak ballast tank upon the bearing loads, where known;
-
gear forces, where
appropriate, due to prime-mover engagement on multiple input/single
output installations. For multiple input systems, consideration is
to be given to each possible combination of inputs;
-
propeller offset
thrust effects;
-
maximum allowed
bearing weardown, for water or grease-lubricated sterntube bearings,
and its effect on the bearing loads.
5.2.3 The
shaft alignment calculations are to state the:
-
expected bearing
loads at light and normal ballast, fully loaded and any other draughts
deemed to be part of the ship's operating profile, for the machinery
in cold and hot, static and dynamic conditions;
-
bearing influence
coefficients and the deflection, slope, bending moment and shear force
along the shaftline;
-
details of propeller
offset thrust;
-
details of proposed
slope-bore of the aftermost sterntube bearing, where applicable;
-
manufacturer's
specified limits for bending moment and shear force at the shaft couplings
of the gearbox/prime movers;
-
estimated bearing
weardown rates for water or grease-lubricated sterntube bearings;
-
expected hull
deformation effects and their origin, viz. whether finite
element calculations or measured results from sister or similar ships
have been used;
-
anticipated thermal
rise of prime movers and gearing units between cold static and hot
running conditions; and
-
manufacturer's
allowable bearing loads.
5.3 Shaft alignment procedures
5.3.1 A shaft
alignment procedure is to be made available for review and for the
information of the attending surveyors for all main propulsion installations
detailing, as a minimum,
-
expected bearing
loads at light and normal ballast, fully loaded and any other draughts
deemed to be part of the ship's operating profile, for the machinery
in cold and hot, static and dynamic conditions;
-
maximum permissible
loads for the proposed bearing designs;
-
design bearing
offsets from the straight line;
-
design gaps and
sags;
-
location and loads
for the temporary shaft supports;
-
expected relative
slope of the shaft and the bearing in the aftermost sterntube bearing;
-
details of slope-bore
of the aftermost sterntube bearing, where applied;
-
proposed bearing
load measurement technique and its estimated accuracy;
-
jack correction
factors for each bearing where the bearing load is measured using
a specified jacking technique;
-
proposed shaft
alignment acceptance criteria, including the tolerances; and
-
flexible coupling
alignment criteria.
5.4 Design and installation criteria
5.4.2 Design
and installation of the shafting is to satisfy the following criteria:
-
The Builder is
to position the bearings and construct the bearing seatings to minimise
the effects of hull deflections under any of the ship's operating
conditions with the aim of optimising the bearing load distribution.
-
Relative slope
between the propeller shaft and the aftermost sterntube bearing is,
in general, not to exceed 3 x 10-4 rad in the static condition..
-
Sterntube bearing
loads are to satisfy the requirements of Ch 6,3.12.
-
Evidence is to
be provided to LR demonstrating that bearings of synthetic material
have been verified as being within the tolerance stated by the bearing
manufacturer for diameter, ovality, and straightness after installation.
-
Bearings of synthetic
material are to be verified as being within tolerance for ovalilty
and straightness, circumferentially and longitudinally, after installation.
-
The sterntube
forward bearing static load is to be sufficient to prevent unloading
in all static and dynamic operating conditions, including the transient
conditions experienced during manoeuvring turns and during operation
in heavy weather.
-
Intermediate shaft
bearings' loads are not to exceed 80 per cent of the bearing manufacturer's
allowable maximum load, for plain journal bearings, based on the bearing
projected area.
-
Equipment manufacturer’s
bearing loads are to be within the manufacturer's specified limits,
i.e. prime movers, gearing.
-
Resulting shear
forces and bending moments are to meet the equipment manufacturer's
specified coupling conditions.
-
The manufacturer's
radial, axial and angular alignment limits for the flexible couplings
are to be maintained.
5.5 Measurements
5.5.1 The
system bearing load measurements are to be carried out to verify that
the design loads have been achieved. In general the measurements will
be carried out by the jack-up measurement technique using calibrated
equipment.
5.5.2 For
the first vessel of a new design an agreed programme of static shaft
alignment measurements is to be carried out in order to verify that
the shafting has been installed in accordance with the design assumptions
and to verify the design assumptions in respect of the hull deflections
and the effects of machinery temperature changes. The programme is
to include static bearing load measurements in a number of selected
conditions. Depending on the ship type and the operational loading
conditions that are achievable prior to and during sea trials these
should include, where practicable, combinations of light ballast cold,
full ballast cold, full ballast hot and full draught hot with aft
peak tank empty and full.
5.5.3 For
vessels of an existing design or similar to an existing design where
evidence of satisfactory service experience is submitted for consideration
and for subsequent ships in a series a reduced set of measurements
may be accepted. In such cases the minimum set of measurements is
to be sufficient to verify that the shafting has been installed in
accordance with the design assumptions and are to include at least
one cold and one hot representative condition.
5.5.4 Where
calculations indicate that the system is sensitive to changes in alignment
under different service conditions, the shaft alignment is to be verified
by measurements during sea trials using an approved strain gauge technique.
5.6 Flexible couplings
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