External unbalanced moments

The inertia forces originating from the unbalanced rotating and reciprocating masses of the engine create unbalanced external moments although the external forces are zero. Of these moments, only the 1st order (producing one cycle per revolution) and the 2nd order (two cycles per revolution) need to be considered, and then only for engines with a low number of cylinders. The inertia forces on engines with more than six cylinders tend, more or less, to neutralize themselves.

Countermeasures have to be taken if hull resonance occurs in the operating speed range, and if the vibration level leads to higher accelerations and/or velocities than the guidance values given by international standards or recommendations (for example, with reference to a special agreement between shipowner and yard). The natural frequency of the hull depends on its rigidity and distribution of masses, while the vibration level at resonance depends mainly on the magnitude of the external moment and the engine's position in relation to the vibration nodes of the ship.

1st order moments

These moments act in both vertical and horizontal directions, and are of the same magnitude for MAN B&W two-stroke engines with standard balancing. For engines with five cylinders or more, the 1st order moment is rarely of any significance to the ship but it can be of a disturbing magnitude in four-cylinder engines.

Resonance with a 1st order moment may occur for hull vibrations with two and/or three nodes. This resonance can be calculated with reasonable accuracy, and the calculation will show whether a compensator is necessary or not on four-cylinder engines. A resonance with the vertical moment for the two-node hull vibration can often be critical, whereas the resonance with the horizontal moment occurs at a higher speed than the nominal because of the higher natural frequency of the horizontal hull vibrations.

Four-cylinder MAN B&W MC two-stroke engines with bores from 500 mm to 980 mm are fitted as standard with adjustable counterweights (Figure 1.12). These can reduce the vertical moment to an insignificant

Figure 1.12 Four-cylinder engines of MAN B&W Diesel's 500 mm to 980 mm bore MC types are fitted with adjustable counterweights

value (although increasing, correspondingly, the horizontal moment) so this resonance is easily handled. A solution with zero horizontal moment is also available.

For smaller bore MAN B&W engines (S26MC, L35MC, S35MC, L42MC, S42MC and S46MC-C series) these adjustable counterweights can be ordered as an option.

In rare cases, where the 1st order moment will cause resonance with both the vertical and the horizontal hull vibration modes in the normal speed range of the engine, a 1st order compensator (Figure 1.13) can be introduced in the chain tightener wheel, reducing the 1st order moment to a harmless value. The compensator is an option and comprises two counter-rotating masses rotating at the same speed as the crankshaft.

Figure 1.13 1st order moment compensator (MAN B&W Diesel)

With a 1st order moment compensator fitted aft, the horizontal moment will decrease to between 0 and 30 per cent of the value stated in MAN B&W Diesel tables, depending on the position of the node. The 1st order vertical moment will decrease to around 30 per cent of the value stated in the tables.

Since resonance with both the vertical and the horizontal hull vibration mode is rare the standard engine is not prepared for the fitting of such compensators.

Seen from aft

Resulting horizontal compensating force

Resulting horizontal compensating force

Centrifugal force rotating with the crankshaft

Figure 1.13 1st order moment compensator (MAN B&W Diesel)

Centrifugal force rotating with the crankshaft

2nd order moments

The 2nd order moment acts only in the vertical direction and precautions need only be considered for four-, five- and six-cylinder engines. Resonance with the 2nd order moment may occur at hull vibrations with more than three nodes.

A 2nd order moment compensator comprises two counter-rotating masses running at twice the engine speed. Such compensators are not included in the basic extent of MAN B&W Diesel engine delivery.

Several solutions are available to cope with the 2nd order moment (Figure 1.14), from which the most efficient can be selected for the individual case:

• No compensators, if considered unnecessary on the basis of natural frequency, nodal point and size of 2nd order moment.

Moment compensator Aft end

Moment compensator Fore end

Moment compensator Aft end

Figure 1.14 2nd order moment compensators (MAN B&W Diesel)

Centreline crankshaft

Centreline crankshaft

Figure 1.14 2nd order moment compensators (MAN B&W Diesel)

• A compensator mounted on the aft end of the engine, driven by the main chain drive.

• A compensator mounted on the fore end, driven from the crankshaft through a separate chain drive.

• Compensators on both aft and fore end, completely eliminating the external 2nd order moment.

Experience has shown, MAN B&W Diesel reports, that ships of a size propelled by the S26MC, L/S35MC and L/S42MC engines are less sensitive to hull vibrations. Engine-mounted 2nd order compensators are therefore not applied on these smaller models.

A decision regarding the vibrational aspects and the possible use of compensators must be taken at the contract stage. If no experience is available from sisterships (which would be the best basis for deciding whether compensators are necessary or not) it is advisable for calculations to be made to determine which of the recommended solutions should be applied.

If compensator(s) are omitted the engine can be delivered prepared for their fitting at a later date. The decision for such preparation must also be taken at the contract stage. Measurements taken during the sea trial, or later in service with a fully loaded ship, will show whether compensator(s) have to be fitted or not.

If no calculations are available at the contract stage, MAN B&W Diesel advises the supply of the engine with a 2nd order moment compensator on the aft end and provision for fitting a compensator on the fore end.

If a decision is made not to use compensators and, furthermore, not to prepare the engine for their later fitting, an electrically driven compensator can be specified if annoying vibrations occur. Such a compensator is synchronised to the correct phase relative to the external force or moment and can neutralize the excitation. The compensator requires an extra seating to be fitted—preferably in the steering gear compartment where deflections are largest and the effect of the compensator will therefore be greatest.

The electrically driven compensator will not give rise to distorting stresses in the hull but it is more expensive than the engine-mounted compensators mentioned above. Good results are reported from the numerous compensators of this type in service (Figure 1.15).

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  • Simret
    What is 1st and 2nd order moment compensator in diesel engine?
    3 years ago

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