Section 5 Shaft alignment

5.1.1 Shaft alignment calculations are to be carried out for main propulsion shafting rotating at propeller speed, including 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.

5.2.1 Shaft alignment calculations are to be submitted to LR for approval for the following shafting systems:

1. all geared installations, where the screwshaft has a diameter of 300 mm or greater in way of the aftermost bearing, except for multiple input/single output geared installations, in which case all such installations will be submitted for approval;

2. Where prime movers in a direct drive installation or shaft line bearings are installed on resilient mountings.

5.2.2 The shaft alignment calculations are to take into account the:

1. thermal displacements of the bearings between cold static and hot dynamic machinery conditions;

2. buoyancy effect of the propeller immersion due to the ship's operating draughts;

3. effect of predicted hull deformations over the range of the ship’s operating draughts, where known;

4. effect of filling the aft peak ballast tank upon the bearing loads, where known;

5. 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;

6. propeller offset thrust effects

7. 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:

1. 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;

2. bearing influence coefficients and the deflection, slope, bending moment and shear force along the shaftline;

3. details of propeller offset thrust;

4. details of proposed slope-bore of the aftermost sterntube bearing, where applicable;

5. manufacturer's specified limits for bending moment and shear force at the shaft couplings of the gearbox/prime movers;

6. estimated bearing weardown rates for water or grease-lubricated sterntube bearings;

7. expected hull deformation effects and their origin, viz. whether finite element calculations or measured results from sister or similar ships have been used;

8. anticipated thermal rise of prime movers and gearing units between cold static and hot running conditions; and

9. manufacturer's allowable bearing loads.

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:

1. 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;

2. maximum permissible loads for the proposed bearing designs;

3. design bearing offsets from the straight line;

4. design gaps and sags;

5. location and loads for the temporary shaft supports;

6. expected relative slope of the shaft and the bearing in the aftermost sterntube bearing;

7. details of slope-bore of the aftermost sterntube bearing, where applied;

8. proposed bearing load measurement technique and its estimated accuracy;

9. jack correction factors for each bearing where the bearing load is measured using a specified jacking technique;

10. proposed shaft alignment acceptance criteria, including the tolerances; and

11. flexible coupling alignment criteria.

5.4.1 For main propulsion installations, the shafting is to be aligned to give, in all conditions of ship loading and machinery operation, bearing load distribution satisfying the requirements of Pt 5, Ch 6, 5.4 Design and installation criteria 5.4.2.

5.4.2 Design and installation of the shafting is to satisfy the following criteria:

1. The Builder is to position the bearings and construct the bearing seatings to minimize the effects of hull deflections under any of the ship's operating conditions with the aim of optimising the bearing load distribution.

2. Relative slope between the propeller shaft and the aftermost sterntube bearing is, in general, not to exceed 3 x 10^–4 rad.

3. Sterntube bearing loads are to satisfy the requirements of Pt 5, Ch 4, 3.12 Sternbushes and sterntube arrangement.

4. 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.

5. Bearings of synthetic material are to be verified as being within tolerance for ovalilty and straightness, circumferentially and longitudinally, after installation.

6. 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.

7. 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.

8. Equipment manufacturer's bearing loads are to be within the manufacturer's specified limits, i.e. prime movers, gearing.

9. Resulting shear forces and bending moments are to meet the equipment manufacturer's specified coupling conditions.

10. The manufacturer's radial, axial and angular alignment limits for the flexible couplings are to be maintained.

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 trials using an approved strain gauge technique.

5.6.1 Where the shafting system incorporates flexible couplings, the effects of such couplings on the various modes of vibration are to be considered, see Pt 5, Ch 6, 2 Torsional vibration, Pt 5, Ch 6, 3 Axial vibration and Pt 5, Ch 6, 4 Lateral vibration.